Should not the 5AT be a more radical design incorporating more innvotive features?

In Feb 2009, the following question was asked by a reader in Minnesota, USA:

“Why, when you are from the country that rbulleally brought innovation to steam design still persist thinking inside the box? Bulleid went outside the box with the Leader project, and yes with so many new innovations it was not a perfect success. It did prove out many things and he went on to do the Turf Burner that proved practical. Leader absolved. We have so many new ways of fabricating today and sealants that the enclosed gearbox could be practical. The engine was operable from both ends and visibility was equal to modern diesels. Powder coal fired, with computer control and a water tube boiler could help some of Leaders other difficulties. Why was no consideration given to Bulleids last designs?”

Chris Newman offered the following response:

“Your question is an interesting one in that to some extent it answers itself. However I believe that both your question and my response warrant posting on the FAQ pages of the 5AT website since it is similar to questions that has been asked by many other people over the years.

When I say your question answers itself, the story of Bulleid’s Leader class is a case study in what can happen (and has happened all too often in the history of locomotive development) when too many novel and untried concepts are combined into a new design. The Leader Class was brilliant on paper but a disaster in reality, and deserved the fate that befell it. I think it’s fair to say that its fundamental faults (the unworkable conditions for the fireman and the sleeve valves spring immediately to mind) could not have been corrected without beginning again from scratch. Sure, it might be possible to begin again using 21st century materials and technologies to build a new and better Leader, but what guarantee could we or anyone offer that it would work?

If we are ever going to get the 5AT built, we are somehow going to have to convince an investor (or investors) to risk several million pounds in the design and construction of a “demonstrator” unit that will prove the performance and reliability claims that we make and hopefully, in so doing, awaken renewed interest in the possibilities for further development of steam traction. What chance would we have of attracting an investor to part with his money if we had to point back to Bulleid’s Leader Class as the inspiration behind our plan? The answer is obvious.

So you are quite right, that the 5AT breaks no new ground in terms of its basic design. It retains the classic Stephensonian format that successfully powered the world’s railways over their first 125 years of existence. And on that basis alone, we can guarantee that the basic concept is not just sound, but well-and-truly proven. Notwithstanding, the design incorporates an integrated package of technical advancements that will ensure a level of performance and reliability that were undreamed of (beyond the borders of France) in the heyday of steam. Yet with only a few minor exceptions each of those advancements has been successfully tried and proven in locomotives such as Wardale’s Red Devil and Porta’s modified Rio Turbio Santa Fes.

The last thing the steam movement needs if it is to re-launch itself as a modern technology is technical failure brought about by trying to achieve too much too quickly. Indeed one of Wardale’s principal aims when he prepared the specifications for the 5AT was that it must perform as predicted “out of the box” with an absolute minimum of teething troubles. If it fails to do so, he will regard it as a failure, which is likely how it would also be regarded by the press and general public. Indeed it would probably become the butt of jokes of the sort that prevailed in the UK in the 1950s when images of steam were adopted by cartoonists as representative of the (then) inefficient, slow and rather decrepit railway system that the country inherited after WW2.

Besides all of that, there is a secondary reason for retaining the Stephensonian arrangement, and that is its intrinsic appeal – the very foundation on which our interest and your interest in steam traction is based, and the reason why the more disinterested “general public” still enjoys the sight and sound of a steam locomotive. The 5AT is expected to earn its keep in the operation of tour trains, taking over from “classic” steam as and when it become too slow, too unreliable and too expensive to maintain for continued operation on the UK (and/or European) high speed densely trafficked modern rail systems. If the 5AT were to look like Bulleid’s Leader, it would have no more appeal than a diesel to the average “man in the street” on whom tour train organizers rely to fill their seats.

Incidentally, it may reassure you to know that within our group we have some very talented engineers who have come up with ideas for future developments that are in many ways more radical than Bulleid’s Leader, and yet of sounder concept. I won’t dwell on these now but mention it only to reassure you that we are not rivetted in the past. The 5AT may look old fashioned to some, but our thinking is very much towards the future.”

The following is taken from Wardale’s response to an article by Bob Butrims published in Locomotives International #56 which itself included a response to the artlcle “What Could You Do With £1.7 Million? David Wardale Answers” as published in edition 55 of the same magazine.  Bob’s response included the comments that: “there is the problem with ash disposal (in narrow fireboxes)”, and that “any firebox that goes down between the frames is a nightmare for maintenance when you get boiler leaks down low”:

“Coming to the comments on my proposal, some of them unfortunately show how prejudices might creep in. For example the idea of a high speed 4-6-0 is criticised on the grounds of riding quality. Like Bob, I have travelled many thousands of km on locomotives, both with and without trailing trucks, in a fair number of countries, and certainly the worst riding in a lateral plane were the Chinese QJ Class 2-10-2s. It is not the presence or absence of a trailing truck (which is undesirable non-adhesive weight, not there for stability reasons but due to the large fireboxes necessary in former locomotives), which makes a locomotive stable, but the whole design of its chassis. Thus it is possible to have a perfectly riding 4-6-0 and a badly riding 4-6-2, and practical examples of both certainly existed. Against Bob’s experience one can, for example, point to the GWR King and Castle class 4-6-0s which by all accounts rode like Rolls Royces, so much so that the crews complained of the riding of the Britannia 4-6-2s when they were introduced. And as eminent a person as André Chapelon proposed 4-6-0s for 200 km/h operation, (see book reviews – Ed.), which he was unlikely to have done if they were inherently unstable. Thus experience of imperfectly designed 4-6-0s should not be used to condemn this type as such. The chassis of the proposed 4-6-0 can and would be designed for very stable riding, roller side control of the leading bogie, roller bearings, self-adjusting wedges, and manganese steel liners and mechanical lubrication of the relevant components (or “dry” lowwear materials), enabling riding quality to be maintained from shopping to shopping, as was actually the case with SAR locomotives so fitted.

Concerning comments made on firebox design, with coal firing using the GPCS (Gas Producer Combustion System), there is no need to “clean the fire” in the conventional sense, but merely shaking the grate on the move to discharge ash into the ashpan, for which a narrow firebox between the frames is more suitable than a wide firebox when the latter, as was usual, does not have a self-emptying ashpan (one of the problems with 3450). Higher thermal efficiency translates into reduced fuel consumption, so that the simple dumping of accumulated ash from the ashpan would be required at relatively infrequent intervals. But as explained in my original article, the type of fuel is optional, and the preliminary performance calculations have in fact been made for diesel fuel (gas oil). Although there is currently some difference of opinion regarding the merits of deep narrow fireboxes and shallow wide ones for oil firing, it is the case that in the past the former has been preferred for locomotives specifically designed to burn only oil fuel. Bob would prefer wide ones which “suit what we do very well”, but he has not mentioned the oily deposits, building up to 6 inches in 5,000 km, which have to be laboriously scraped off the lower firebox walls of No. 711 – another maintenance nightmare which would have to faced every 4 days at the level of utilisation of the former New York Central 4-6-4s!

With modern design and construction methods and water treatment, there would be no leakage from the firebox, therefore having a firebox partly between the frames would not give rise to any maintenance inconvenience. A deep narrow firebox is not a matter of going back to the past, but is rather based on the not-negligible advantages of such a design, as recognised by Chapelon.”

The following comes from a “Summary Specification” drafted by Wardale on 17 Apr 2001:

“The power of the 4-6-0 proposal fully utilizes the available adhesion, therefore any larger locomotive designed for higher power will have to have more coupled axles. A 4-6-2 of higher power would not have adequate adhesive weight, and an 8-coupled locomotive with driving wheel diameter large enough for the envisaged speed would complicate the design process, be more expensive to manufacture, and almost certainly reduce availability. For higher power the preferred wheel arrangement would be 4-8-0, which has no precedent on Britain’s railways.”

Note from Alan Fozard: “Because there is no precedent for a 4-8-0 on British railways, it would be more difficult to get the rail/safety authorities to give it type/vehicle approval. The 5AT design scores because it is being evolved from the proven BR 5MT design”.

The following answer is taken from Wardale’s response to an unpublished letter from Bryan Attewell written in response to Wardale’s two-part article “Steaming into the Future” published in Steam Railway magazines #272 and #273 (see ‘Articles and Letters’ page of this website):

“Regarding the designations “mixed traffic” and “express” we should not forget that for the larger BR Standards (i.e. those with 6′ 2″ coupled wheels) “mixed traffic” meant what it said, and certainly included express duties (the Britannias were used on little else). It was the improvement in front-end design which allowed latter-day steam locomotives to become ‘maids of all work’ – a colleague during my BR days was only partly joking when remarking that for the schedules of the late 1950’s only two classes were required – 0-6-0 diesel shunters and 9F’s. Even with 1950’s design 6′ 2″ wheels were no barrier to 90 mph running, and with the 5AT they will be no barrier to 100+ mph.”

In fact we recognize that it will be impossible to gain approval to run any faster than 75mph in its initial period of life.  We would hope that lifting of the limit to 90 mph would be granted after the locomotive has proved itself safe when running at 75mph for some (unknown) period of time.  Whether approval would be granted to increase the limit to 110 or 115 mph is obviously an unknown at this stage.  But as Wardale pointed out in a recent letter: “The whole project stands on the ability of the 5AT to run steam trains significantly faster than at present, thereby avoiding them running for long stretches on slow lines, as Tornado has had to do.  And what when the slow lines get faster and more congested, which is sure to happen? Without realizing its speed potential, the rationale of the 5AT simply falls away.”


he following answer is taken from Wardale’s response to Leonard Staghurst whose letter was published in Steam Railway #277 (copied below):

“Unfortunately Mr. Staghurst’s comments betray his lack of knowledge, not only of what is possible with steam traction but also of what has already been achieved. Consider the following.

ItemSAR 26 Class 4-8-45AT 4-6-0
Type of LocomotiveRebuild of 1950's designNew design
Quality of Thermodynamic DesignRestricted by structural limitations of existing designState of the art
Number of Cylinders22
Simple or CompoundSimpleSimple
Valves / Valve GearPiston / WalschaertsPiston / Walschaerts
Boiler Pressure (lb/in2)225305
FuelCoalGas oil or deisel
Ash Content of Fuel15%0
Fuel Calorific Value (Btu/lb)12,00018,400
Engine Weight - Excluding Tender (tons)12380
Maximum Indicated h.p.5060 (peak of measured i.h.p. curve)3460 (calculated)
i.h.p / ton of Engine Weight41.143.3

The above figures, which are amply supported by other data, show eloquently enough that the calculated power capacity of the 5AT is perfectly realistic, and I regard achieving it as a formality (and surpassing it a probability) – even on this planet! Likewise all factors concerning thermal efficiency, except the target combustion efficiency at maximum evaporation, which I have acknowledged will be a difficult problem. But that does not mean impossible, simply that very good engineering will be required to achieve it.”

Note: The subject of 5AT Reliability is also described in the 5AT Features section of this website.

The following summary of 5AT “reliability factors” was compiled by David Wardale in March 2005.Design features leading to a much higher reliability of the 5AT Class steam locomotive
compared to First Generation Steam (FGS) Locomotives.

The advanced technology 5AT Class 4-6-0 steam locomotive is being designed to have a performance meeting the requirements of the modernised UK rail network. The 5AT will also be an exceptionally reliable form of steam traction. The factors which will enable the 5AT to provide much higher reliability than FGS are shown below

A. General Points:

  • More accurate design (i.e. replacing the empirical design used in FGS with one more closely based on whatever theory is applicable to the component concerned – as largely done in the Fundamental Design Calculations);
  • Better materials;
  • Better lubrication;
  • Use of advances in Engineering Science. Engineering science has advanced considerably since the last FGS locomotives were designed and techniques such as finite element stress analysis and fatigue analysis – not available to FGS designers – will be employed where relevant;
  • Replacement of bolted or riveted connections by welded ones where possible, eliminating the possibility of things becoming loose;
  • The simplicity of the concept (two cylinder simple loco). This minimises the number of moving components compared to a multi-cylindered engine. In particular there are no inaccessible components;
  • The use of Association of American Railroads’ (AAR) rules where appropriate. AAR rules are generally considered to be the most robust design rules where empirical methods have to be used;
  • Starting afresh with a new design, any features of FGS that gave persistent problems can be looked at again and the fault designed out;
  • Where mandated by Railway Group Standards design will be, of necessity, “state-of –the art” (e.g. brake control system).

B. Specific Points:

  • Roller Bearings. Roller bearings will be used on all major joints (e.g. axles, crankpins, connecting rod small end and valve gear). Roller bearings are more reliable than plain bearings, require “no field attention” and have effectively zero wear, and hence prevent vibration
  • Self adjusting wedges at all driving and coupled axleboxes – eliminate axlebox-frame gaps, hence pounding and vibration;
  • Generally improved valve and cylinder tribology, greatly reducing wear of affected components;
  • ‘State-of-the-art’ air sanding, reducing slip risk;
  • Oil firing – eliminates the need for fire and ashpan cleaning, eliminates performance uncertainties caused by variable coal quality, vagaries of the firebed and variable levels of firemen’s skill and endurance;
  • Sophisticated exhaust system – eliminates indifferent steaming caused by inadequate combustion air supply (a common problem with FGS);
  • All welded boiler – eliminates problems caused by riveted seams and screwed stays, especially no possibility of leakage and caustic embrittlement;
  • Superior firebox stay design, reducing incidence of fractured stays;
  • Elimination of lineside fire-risk because of oil firing, which means the locomotive can be used in all weathers;
  • Compensated springing which reduces the risk of spring breakage;
  • High capacity tender and efficient locomotive give long ranges on a tender full of fuel and water minimising the need for en-route fuelling and watering;
  • Franklin-type engine-tender buffer and drawgear eliminates ‘stamping’ and vibrations at this point;
  • Good riding qualities and high vehicular stability eliminate vibration caused by poor (rough) riding;
  • Robust horn stays, minimising risk of frame cracking at top corners of horns;
  • Use of ‘drop-type’ firebox fusible plugs which are safer than the usual lead filled plug;
  • Use of corrosion-resistant (copper bearing) steel for such items as the tender superstructure and smokebox will minimise corrosion;
  • Optimum boiler-frame connections (slide bearing at the barrel-frame and expansion plate at the firebox-frame connections) tie the boiler better to the frame and therefore better brace the transverse forces on the mainframe;
  • Geared roller centring of the leading bogie eliminates the possibility of bogie guide-spring breakage;
  • Clasp brakes eliminate axle and axle bearing loads due to braking forces.

Note: The above list does not mention the savings in maintenance costs that can be achieved through the use of Porta’s water treatment system, as proposed for use on the 5AT. Porta’s boiler water treatment involves the use of:

  • very high levels of alkalinity/totally dissolved solids (TDS) to reduce corrosion and to promote the formation of a highly mobile sludge that inhibits scale formation;
  • powerful polyamide anti-foam to control foaming normally associated with high TDS and thus to prevent the damaging carry-over of water and impurities (priming) that is caused by foaming;
  • tannins to remove oxygen from the water (thereby reducing corrosion) and to prevent caustic embrittlement;
  • phosphate to prevent scaling formation in the tender tank and feedwater system.

Proper use of the system can reduce the frequency of boiler washouts to 6 months or longer even when using the hardest water, and can effectively eliminate water-side corrosion of boiler plates, assisted by the build-up of magnetite on their surfaces. The system has been successfully used and demonstrated by Shaun McMahon at FCAF in Argentina and by the Kirkless Light Railway in the UK. The treatment is now being commercialised by Martyn Bane – see http://www.portatreatment.com/.

Note: The subject of 5AT reliability is also discussed briefly in the 5AT Features section of this website.

In Sept 2009, David Wardale wrote a lengthy letter in response to a question about the thermal efficiency of the 5AT in comparison with other “modern steam” designs.  He wrote as follows:

  • FDC 1.3.F.[114]: Drawbar thermal efficiency of the 5AT at maximum drawbar power = 11.4% Representative best corresponding figure, simple expansion FGS (BR 7MT) = 7.7%. Increase = 48%. This is correct.
  • From “The Red Devil …” Page 501: My estimate for maximum possible drawbar thermal efficiency, SGS. = 16.3%. Page 492: Representative best corresponding figure, simple expansion FGS (BR 7MT) = 9.2%. Increase = 77%. This is also correct.  Here the SGS figure assumes compound, the FGS is for simple, but the comparison is taken as valid because globally FGS compounding did not find favour, whereas it may be assumed that it can be successfully applied to future SGS (but for various reasons not to the 5AT.)  If you don’t like that then consider that FGS compounds peaked at about 10% drawbar thermal efficiency which would give a SGS/FGS drawbar thermal efficiency increase of 63%. This is also correct.  And Porta estimated maximum SGS drawbar thermal efficiency as 17 – 18% (“The Red Devil …” page 501, footnote 2) which would give a SGS/FGS drawbar thermal efficiency increase of 70 – 80% based on all-compound, or 85 – 96% based on compound-SGS/simple-FGS. If you prefer Porta’s estimate to mine, all these figures are correct.
  • From “The Red Devil …” Pages 217-8: Increase in drawbar thermal efficiency for the SAR 26 class over the 25NC class (i.e. modified FGS over standard FGS) at maximum power for the 25NC class = 150%. This is also correct.
  • Drawbar thermal efficiency of Stephenson’s ‘Rocket’ running light engine = 0%. Drawbar thermal efficiency of state-of-the-art 3-phase AC electric locomotives running light engine = 0%. For this case drawbar thermal efficiency increase from about 200 years of traction development = 0%. This is also correct.

Conclusion: 1. So many different figures, and they are all correct – for the particular conditions of each comparison. 2. Therefore thermal efficiency figures cannot be arbitrarily stated without giving the exact basis of the comparison. 3. The absolute maximum 5AT drawbar thermal efficiency cannot be deduced from the FDCs. Nor is it readily calculable and cannot therefore be stated. It will most probably be less than my ‘Red Devil’ estimate of 16.3% for various reasons such as the large tender/engine mass ratio and being optimized for fairly high speed.

See also 5AT Thermal Efficiency and Thermal Efficiency pages of this website.


The following answer is taken from Wardale’s response to an unpublished letter from Bryan Attewell written in response to Wardale’s two-part article “Steaming into the Future” published in Steam Railway magazines #272 and #273 (see ‘Articles and Letters’ page of this website):

“Mr Attewell would like a larger boiler, but what counts most for steam production is the effectiveness of the exhaust and combustion systems, and all a larger boiler would really do is spoil that most important parameter – vision ahead from the cab.”

The following comes from a “Is it just a Phantom” – David Wardale’s response to Herr Ebel published in Lok Report in March 2001:

“The proposed working pressure of 21 bar is less than the maximum successfully used in America, and the normal nature of train operation will give a mean boiler stress significantly less than the nominal maximum figure of 116 kg m-2h-1. Experience shows that the mechanical well-being of a boiler is more a question of good detail design (much of which originated in Germany) and water side cleanliness than of pressure or loading. Thus a high pressure boiler designed to accommodate high thermal loading without excessive mechanical stress, and which is kept scale and corrosion free by the kind of water treatment developed by Porta, will give less trouble than a badly-designed boiler operating at low load and pressure but which is allowed to scale up.”

In Feb 2017, Chris Newman wrote to David Wardale to ask if there was any justification in the suggestion that the predicted evaporation rate for the 5AT of 17,000 kg/h might be unrealstically high, particularly with coal-firing.

He pointed out that in lines [197] and [198] of FDC 1.3F a maximum specific burning rate for the 5AT is given as 709 kg/m2/h, or some 11% higher than the maximum sustained rate achieved with the Red Devil.  However when comparing evaporation rates, the figure of 6,376 kg/m2/h for the 5AT is some 36% higher than that for the Red Devil (4,658 kg/m2/h).  Similarly, the evaporation per unit of coal burned appears to be about 21% higher for the 5AT than for the Red Devil.

Wardale replied as follows:

“The following figures are taken directly or calculated from the FDCs or The Red Devil … Consider:

  1. Fuel lower calorific values for 5AT taken as: gas oil / diesel fuel = 42.9 MJ/kg, coal = 30 MJ/kg.
  2. Average upper calorific value of coals used for 3450 tests = 27.5 MJ/kg.  The higher values for the 5AT fuels will increase the evaporation rate per m2 of grate area compared to 3450.
  3. The 5AT has a combustion air preheater for a combustion air temperature of 100 °C.
  4. 3450 has no combustion air preheater.
  5. The provision of a combustion air preheater on the 5AT will increase the evaporation rate per m2 of grate area compared to 3450.
  6. The feedwater heater heat transfer area of the 5AT per m2 of grate = 18.27 m2 / 2.67 m2 = 6.84 m2/ m2.
  7. The feedwater heater heat transfer area of 3450 per m2 of grate = 13.30 m2 / 6.44 m2 = 2.065 m2 / m2.
  8. The feedwater heater heat transfer area per m2 of grate of the 5AT is thus 3.3 times larger than for 3450.
  9. The temperature of the feedwater entering the boiler of the 5AT will therefore be significantly higher than on 3450 (calculated as 110.5 °C with clean tubes, 3450 best measured value = 102 °C with clean tubes, therefore 5AT calculated value seems conservative.)
  10. The higher feedwater temperature on the 5AT will increase the evaporation rate per m2 of grate area compared to 3450.
  11. The boiler evaporative heating surface area per m2 of grate of the 5AT is 56.25 m2 / m2.
  12. The boiler evaporative heating surface area per m2 of grate of 3450 is 44.78 m2 / m2.
  13. The 5AT thus has 26% more evaporative heating surface area per m2 of grate than 3450.
  14. The higher boiler evaporative heating surface area per m2 of grate of the 5AT will increase the evaporation rate per m2 of grate area compared to 3450.
  15. You have 4 reasons above why the 5AT would have a higher evaporation rate per m2 of grate area compared to 3450. There are also more esoteric ones, but the above is enough I think.
  16. All individual factors such as the above are insignificant compared to the fact that the integrated calculations predict what the evaporation will be taking all factors into account. Those who dispute the calculated figure should be asked to prove what they are saying by equally comprehensive calculations, or otherwise shut up.
  17. But there is a far easier way of showing the 5AT evaporation is an entirely realistic figure.  The 1934 Chapelon rebuilds, the 240-700 Class, achieved a sustained evaporation of 23,500 kg/h with a narrow, deep grate area of 3.76 m2 (Chapelon pages 75 – 76), i.e. 6,250 kg / m2 of grate per hour.  This with coal, without the gas producer system, with hand firing, with no combustion air preheater, and a steam temperature not much lower than that of the 5AT.  Your 5AT figure is 2% higher than this.  Is this an “unrealistic” increase for over 80 years of technical progress?  [Note:  I have always expected that this figure would be increased in practice, i.e. the calculations are conservative.]”

Q: One very common traditional means of delivering feed water to a boiler was through a clack-valve on the back plate and along a pipe past the firebox to the front end of the boiler. Does this arrangement act as a basic form of preheating as the feed water passes the firebox end of the boiler?

Dave Wardale’s response: Not in the thermodynamic sense that preheating is understood, i.e. as reclamation of otherwise waste heat, as the heating effect in this case comes from the surrounding boiler water/steam before this has done any work in the cylinders. It is therefore merely a part of the boiler evaporative heat transfer. The reason for such an arrangement was probably to precipitate scale on the feed-pipe walls, the pipe presumably being fairly easy to remove and descale (or descale in-situ by removing the clack-valve).

Note: this type of feed delivery would probably not be ideal with Porta-type feed-water treatment.

Stuart McIntosh from Australia commented: “Higher boiler pressure must mean a smaller (lighter) piston disc and less unbalanced mass and reduced hammer blow. Efficiency would also be increased.”

Chris Newman’s replied to say: “Higher boiler pressure would provide all sorts of benefits, not least being higher efficiency. And you’re right too that it would mean smaller cylinders, pistons and hammer blow.

Wardale chose to be conservative with his boiler pressure for two related reasons:

  1. He wanted the design of the 5AT to be based on tried and proven technology, and 2100 kPa (310 psi) was commonly used in the USA;
  2. He wanted to avoid frightening the safety/approvals authorities by increasing the pressure higher than what had been accepted practice in the industry. He felt that it will be hard enough to get the 5AT approved without adding to the difficulties by adding extra “risks” (as a higher boiler pressure would likely be perceived).

If we can ever get the 5AT off the drawing board and onto the rails, then for certain the next step will be the development of the design to boost its performance further.”

Fuels and Combustion

David Wardale has proposed that the 5AT be fueled with “gas oil“, saying it was “equivalent to diesel“. In fact, gas oil is diesel fuel but without the dyes which show it has had the road tax paid.

Note – the possibilities of using “greener” fuels in the 5AT are discussed in other sections of this website (with minor duplication of content) as follows:

On page 83 of his book, Wardale quotes a 19th century writer, saying:  “We were trying to keep out of trouble by preventing smoke, but soon found that the prevention of smoke and the saving of fuel did not agree. If you prevented smoke, you burned more fuel.”

Chris Newman asked why this should be so when smoke emission was indicative of carbon carry-over and therefore wasting of fuel.

Wardale answered as follows:

“Because smoke prevention was achieved by high excess air, hence high smokebox gas loss (and is to this day on the new SLM rank tanks, which operate at far too high an excess air coefficient).  This loss of heat can be far greater than that carried away in soot (smoke).”

In a 2001 letter to Newman, Wardale qualified this statement saying:

“The excess air in the SLM locos is too high for best efficiency, and is thus to dilute the pollutants. It is not a question of a difference of opinion between myself and Roger Waller. We both know that excess air should be kept to a minimum for best boiler efficiency but he sees the diluting of pollutants as a more worthy goal.  I would go for both.”

In 2005, the question was asked as to whether consideration had been given to firing the 5AT with “bio-mass”, ethanol or other fuel that is more environmentally friendly than gas oil, since this might offer cash benefits through government grants.

Chris Newman responded as follows:

This is a very valid question that I have been asked by several people recently, and one that I have responded to with the following response. (Note: my opinions on the matter have not been endorsed by Dave Wardale, so I offer them tentatively):

“The question is very pertinent in this day and age, but it has not as yet been looked at seriously for the 5AT project. [Note: this issue has since been studied in detail – see Review of Carbon Neutral Fuels with Potential for Use in Modern Steam Locomotives by Brian McCammon.]

Certainly L.D. Porta saw a great future for steam technology through the burning of biomass – in fact the only letter that I ever received from him shortly before he died included the words: “May I venture to say that after the first 5AT loco runs, there will be an avalanche of steam loco buildings. But let me say that the cardinal point is to make them run on biomass, a matter of which I started to have experience as apprentice fireman in 1940 when I was 18: invaluable experience!”

The reason why biomass hasn’t been given serious consideration so far is because of the perception that to be commercially successful the 5AT will (a) need to run in a diesel traction environment where refuelling with diesel fuel will require no special infrastructure, and (b) it will have to be highly reliable. The latter requires that all (or practically all) the technology that is used on the locomotive is tried-and-proven, which biomass fuelling (and the technology for feeding vast quantities of the stuff into the loco’s firebox) would not be. In the project’s early days, consideration was given to fuelling the loco with LPG to make it more environmentally friendly, but this idea was abandoned because of perceived dangers (and perceived difficulties with safety authorities) associated with transporting a large volume of flammable gas in a high pressure container at high speed immediately behind the locomotive.

John Johnston in the USA has put forward the idea of fuelling a locomotive directly with corn (avoiding the cost of converting the starch into ethanol) and is planning his own project to design and build an environmentally friendly locomotive in miniature (see http://www.greenloco.com/)

As to seeking government grants, the only comment that I can make (and I make on the basis of hearsay only) is that the hurdles imposed by the government on any organization applying for grants are so great as to often render the effort unwarranted. Still, I’m sure grant money could be sought if we had a sufficiently robust commercial proposal, but at present our focus is aimed at the simpler (but still very difficult) task of putting together a strong enough plan based on “known” technology, to attract private finance.

If the recent rises in oil prices continue, then “alternative” fuel options are going to become more and more attractive. As I say though, methane suddenly has great appeal to me but it is an idea that I’ve not pressed on the others in the 5AT planning organization because this is not the time for radical new ideas. Planning an oil-burning 5AT is radical enough as it is! The same applies to biomass and other fuels. Certainly though, once the 5AT has demonstrated the possibilities for steam technology in the modern world, then there will probably be a rapid demand for experimentation with alternative fuels.”

Note: In 1987 L.D. Porta presented a paper titled “The Contribution of a New Steam Motive Power to an Oil-less World” at an International Seminar on Railway Technology in Mexico, July 1987.  The English translation not published but a 4MB PDF version transcribed from the original typescript is downloadable from this website.

Dr. David Smith, who works for boiler manufacturers Doosan-Babcock, offers the following additional observation:

“Oil firing is enough of a technological step in the first instance. As an aside there is a lot of work being done on co-firing biomass with coal in UK power stations at the moment. I remain skeptical about biomass in a locomotive boiler though (or rather I think it would take a considerable amount of development work) where there is enough difficulty keeping relatively large lumps of coal on the grate, never mind small (therefore light) granular biomass materials (GPCS or no GPCS!!). A  possibility is to go for pulverized fuel combustion – but how would you pulverize the fuel? – what fineness is required to get complete combustion in a locomotive firebox and so on?…..Plus – there can be a lot of moisture in some of these fuels (I saw 50% quoted for bagasse) which means a big latent heat loss.”  (Note – see FAQ page on coal fuel for further comments on pulverized coal firing.)

As mentioned in the Fuels page under the Environment section of this website, one obvious advantage that steam traction offers is its ability to burn almost any combustable fuel, including renewable fuels like wood. These possibilities are further discussed in several papers downloadable from this website, including:

In May 2008, John Tasker wrote to ask: “With the current price of oil and the re-opening of some coal mines, has any thought been given to coal-fired alternative mentioned in the design spec for the 5AT?”.

Chris Newman offered the following response:

What you say about the effects of oil price rises is quite correct. And yes, consideration is being given to the use of coal as an alternative steam locomotive fuel. No serious thought has yet been given to burning coal in the 5AT, we have for some time been looking at the possibility of developing a freight haulage version of the 5AT specifically for coal haulage duties that would burn coal as its fuel.

I have done quite a lot of work over the last four years in developing and putting forward economic arguments in favour of steam traction wherever coal and labour costs are low. In fact I presented a paper on the subject at the conference on modern steam traction at York in December 2006, and have developed my cost models quite a lot further since then.

As for the 5AT, it is still difficult to consider coal as a fuel when the rest of the railway is using gas oil. Whilst ever that situation exists then logistically gas oil will remain the preferred choice, bearing in mind that the 5AT is intended to operate in the modern railway environment. Of course it will not be able to compete with diesel traction in terms of fuel consumption, which is why it is intended to operate only in the tour train market in which steam traction has particular appeal. Furthermore, the use of oil as fuel brings with it several subsidiary advantages including ease of firing, reliable combustion and steam generation, zero spark emissions, no tube abrasion, no ash disposal, no smokebox cleaning, and greater operating range.

Lump coal is a nasty fuel that is difficult to fire, unreliable in its combustion, creates ash, emits char that erodes tubes, fills up smokeboxes and throws out sparks. GPCS combustion certainly reduces these problems but it does not eliminate them whereas the use of pulverized coal would, which is presumably why power stations all prefer to use pulverized coal these days.  The technology was successfully developed and tested in steam locomotives in Germany and Australia and the UK the 1940s and 50s, so presumably it can be resurrected for use again. Certainly it would be the best option for the 5AT if/when oil prices become so high that the railways have to think about alternatives to diesel traction. In the meantime, if we can develop a market for steam traction for coal haulage in developing countries, then this would be where pulverized coal combustion technology should be tried and perfected.  (See note below for further comments on pulverized coal.)

A worrying but little-known fact is that even coal is not an inexhaustible fuel.  Whilst “peak oil” is just probably around the corner (if it’s not already past), “peak coal” is not so many years away..  Indeed Brian McCammon reports that peak coal may be only 17 years away, and if he’s right then coal is not going to be a panacea for an oil-depleted world. Our children are going to be facing some difficult times in the future, and more than likely we will witness them ourselves. Nuclear power will inevitably have to take up some of the demand for power, but I’m not optimistic that renewable fuel and power sources will be able to fill the gap. Food-based bio-fuels may keep rich countries’ SUVs going, but they are not going to help the poor and the starving. The best option that I can see is “fuel from waste” but the UK and other governments seem to be astonishingly slow in promoting it.

There are copies of several papers on the Links and References page of the 5AT website, including Chris Newman’s 2008 CORE 2008 paper “Feasibility of Steam Traction of Coal Haulage in Developing Countries” and his 2006 York paper “Traction Cost Comparisons for Indonesian Coal Haulage“; Brian McCammon’s 2007 paper “Review of Carbon Neutral Fuels with Potential for Use in Modern Steam Locomotives“.  In addition, a reference to “Coal – Resources and Future Production” was published by the Energy Watch Group in March 2007″

Note: Pulverized coal combustion could offer significant advantages over lump coal combustion. For instance, it would eliminate the “grate limit” phenomenum and problems associated with clinker formation, spark emissions and ash disposal. Stoking could also be automated much more easily making one-man operation a real possibility. Effectively it would have all the advantages that oil-firing confers. The technology was in fact used successfully in Australia, Germany and the UK in the 1920s, 30s and 50s, as described in Chapter VI of a book titled “Brown Coal” by H. Herman (one-time State. Director of Geological Survey) published by the State Electricity Commission of Victoria, Australia in 1952. The chapter titled “Brown Coal Dust Firing for Locomotives” is reproduced here.

Steam Flow and Exhaust

Dave Wardale offered the following responses to questions put to him relating to his Fundamental Design Calculations for the exhaust system [FDC 12].  (See also Wardale’s Notes on FDC 12.)

Blower nozzles: I had always imagined that blower steam was simply exhausted up the chimney through the blast pipe when no exhaust steam was available to create a draught. I noticed on plates 52 and 53 of your book “Red Devil and Other Tales from the Age of Steam” that 3450’s Lempor system had separate (very small diameter) blower nozzles spaced around the outside of the Lempor nozzles. I presume they are much smaller because the steam pressure delivered to the blower is much higher than that coming through the Lempor nozzles. Do they have separate mixing chambers? And does the presence of blower steam in the chimney cause any interference with the exhaust steam when the locomotive running? Answer: Blower steam has no separate mixing chamber; it is fed into the chimney where mixing occurs. Blower nozzles are smaller because of higher steam pressure and low vacuum requirements, see FDC 12 pages 13/14. If the blower is used when steaming, it merely adds to the input kinetic energy hence gives higher gas pumping – the blower could be turned on to augment the exhaust steam if an engine was steaming very badly.

Lempor Nozzles: With reference to the same photos and the accompanying note that the badly worn Lempor nozzles of 3450 had produced no noticeable reduction in draught, can one deduce that the geometry of the nozzles is not as critical as might otherwise be assumed? Answer: Yes, Lempor nozzle shape does not seem to be so critical – see “The Red Devil” page 475 point (i).

Kordina: I had to re-read “The Red Devil” to figure out just what the Kordina is for. Do I understand correctly that its purpose is to reduce the diameter of each steam exhaust duct before they join, so that throttling occurs in the duct rather than at the blast nozzle which can cause exhaust steam from one cylinder to blow back down the opposite cylinder’s exhaust duct? Answer: It should be expressed that the Kordina is to create a region of low pressure and high velocity at the exhaust passage junction so that steam does not back-flow into the opposite cylinder during release.

Swirl Plates: I am still vague about the purpose of the swirl plates at the end of the Kordina in 3450, but deduce that they are introduced to improve the air-gas mixing in as the steam enters the mixing chamber. If so, what effect does swirling have? Is it simply that “swirling” is conducive to “mixing” therefore the addition of swirl plates improves mixing? Is the effect quantifiable? Answer: Swirl was given for the reason you say, to improve steam-gas mixing downstream of the blast nozzles (Porta’s suggestion) see “The Red Devil” bottom of page 153: but this was not beneficial with multiple jet nozzles, see “The Red Devil” page 476 point (v). The figures for FDC 12 include the proposed 5AT Kordina, which is a new design.

Note: Wardale has also commented about the 5AT exhaust system at much greater length in correspondence with Jos Koopmans that covered elsewhere in these FAQs.

In Oct 2007, John Tasker wrote to ask if steam jacketed cylinder had been considered for the 5AT. He suggests that when a loco is stationary a steam jacket would help reduce thermal losses when starting from cold. Dave Wardale offered the following response:

“Cylinder steam jacket. This has been well explained by Porta. It was rejected on the 5AT due to the difficulty of actually making a steam jacket that is effective enough to warrant the complexity – and the reduction in thickness of insulation it would result in where the latter is restricted (e.g. by the moving structure gauge). Remember it is insulation that stops heat loss, not a steam jacket – the latter just transfers any loss from cylinder steam to steam jacket steam, but it is a loss that has to be made good by the boiler in either case. One possibility is to use the valve and cylinder liner cooling passages (if fitted) as a steam jacket whenever the throttle is shut, but I doubt if it’s worth the effort. It was considered for the QJ and rejected – see “The Red Devil” page 441. Another is to have a cylinder warming valve under the driver’s control – or simply crack open the throttle – admitting a small amount of steam to warm up the cylinders before starting. This was used in China at very low ambient temperatures, and helps to some extent to reduce the high steam demand when starting from cold. Cylinder insulation has been covered in FDC 6 [226] – [228].”

Q: Are large (i.e. huge) diameter steam pipes necessary? For example, a smaller steam pipe, provided it has smooth bends, will have less surface area for thermal losses. If the effective delivery of steam to the cylinders is of concern, then can it be solved by having large steam chests or using receivers as on KLR’s Hawk?

Dave Wardale’s response: Yes. Thermal losses from steam pipes are very small whatever the pipe size due to small temperature differences (zero for the main pipe and at most 100oC in the smokebox). On the 5AT, once the steam pipes leave the smokebox, they enlarge considerably to form part of the steam chest volume, which should equal the piston swept-volume (not so easy to achieve). Due to the “high saddle” design there is no part of the steam pipes exposed to the air – they are either within the smokebox or within the (heavily lagged) cylinders. The pressure drop through the pipe is influenced by the pipe-length/equivalent-diameter ratio, as well as by bends. The full equation for pipe friction flow in a most useful form is given in FDC 11.3 item [140]. Minimizing this pressure drop through the live steam piping is of the utmost importance (= internal streamlining – Chapelon!).


A question was asked (by Chris Newman) as to how can it be deduced from Fig 50 (in Wardale’s book The Red Devil and Other Tales from the Age of Steam) that there was scope for an increase in exhaust lap for valves with longer steam lap?

The actual wording referred to appears on page 170 of the book, where it says:  “Fig 50 suggests that there was ample scope for further increase in the exhaust lap with valves having a long steam lap”.  Fig 50 is reproduced below:

Wardale answered as follows:

“Because with longer steam lap the exhaust opening is much larger than for the standard engine, so more exhaust lap can be used without restricting the exhaust opening to below that of the standard engine.  The need for more exhaust lap when the exhaust is improved is because the compression starts from a lower pressure and is generally inadequate, as proved by 3450’s indicator diagrams.”

See also the reference to Exhaust Lap on the Valves and Valve Gear page of this website.

Letter from J.J.G. Koopmans to Dave Wardale dated 21 July 2005:

21 July 2005©

Dear Mr. Wardale,

We have exchanged some correspondence in the past. As you may, or may not, remember, I have been busy writing a Ph.D. thesis at the University of Sheffield on the history of steam locomotive front-end development with an added goal to update theory.

As this is nearing its final date, I would like to give you an update on the results.

  • Concerning the Lempor theory by the late Mr Porter (sic). This is rejected by the supervisors of the University. One of the reasons being that it is based on the historical concept of “Shock loss” which is not part of present day theory of fluid dynamics anymore.
  • Concerning a replacement theory, a brief outline of a present day dimensionless approach to the chimney shapes concerned is attached. If there would be a course on this specific aspect of fluid dynamics, this is the way it would be taught to the University students.

Concerning the diffuser chimney part of the theory, there will he a problem. The Eu(ler) numbers calculated from the theory are the theoretical upper limits possible from a specific dimensioned layout. As far as I am aware, you have given some details of the 5AT front-end to Mr Peter Mintoft last year and he discussed this with me. From the requested vacuum of 5200 Pa and the steam flux of some 12800 kg/hr the Eu number requested can be calculated at 0.055. If the numbers are used in the Lempor equation to calculate dimensions, the orifice area of each Lempor would be 0.008 m, which you seem to confirm on the 5AT website. The Lempor mixing chamber diameter would be around 319 mm and the Lempor exit diameter some 573 mm. These numbers seem to be of the same order as those shown on the very small drawing of the website of Martyn Bane.

However, if these dimensions are used in the equation for a parallel chimney with a diffuser, as shown in Section 3 of the Appendix, the theoretical upper Eu limit can be calculated. This is only of the order of 0.045. This is less than you requested. A calculation for the SAR 26 type Lempor shows these numbers to be the other way round, requested 0.0475, theoretically possible 0.056, so it worked.

My firm conclusion is therefore that, based on the numbers supplied last year, which may have changed in the mean time, the steaming rate for the 5AT cannot be sustained by the Lempor system.

These conclusions are at present, only known by Mr Peter Mintoft, my supervisors at Sheffield and myself. From the 15th of September onwards, they will be known to the external examiners and, as I am preparing a trade edition of the thesis, to the peer reviewers.

Yours sincerely
J.J.G. Koopmans

Response from Dave Wardale to Jos Koopmans:

26 July 2005

Dear Mr. Koopmans,

Thank you for your letter of 21st July, to which my comments are as follows:

  • It is accepted that there is rarely a ‘last word’ to anything (the basic geometry of wheels seems to be an exception) and that it is therefore quite possible that current ‘state of the art’ ejector pump engineering knowledge may be an improvement over Porta’s own theories, such improvement being in the nature of a refinement. As a matter of interest, I myself queried the mathematics of the Lempor theory with Porta when I first became acquainted with it some 30 years ago, but did not receive a satisfactory explanation, and left it at that.
  • Notwithstanding the above, or any ‘rejection’ of the Lempor theory by academics, it is an engineering fact that locomotive exhausts designed according to Porta’s work have proved more successful than any others. This practical experience is not to be dismissed.
  • Practical experience has also shown that exhaust systems which have treated mixing ‘shock’ losses in the classical way have proved superior to those which have not done so. The former includes the Giesl ejector, the design of which is specifically aimed at minimising mixing losses. Whatever doubts one may have on the theory and assumptions on which the Giesl ejector is based, when correctly proportioned it has given good results at modest flow rates, its limitation at high flow rates being no doubt due to the high exit kinetic energy loss that is inseparable from a single chimney on a large locomotive (refer to page 473 of my book). This practical experience of the benefit of minimising mixing shock losses is also not to be dismissed.
  • The currently preferred exhaust dimensions for the 5AT are in fact not to the Lempor theory but to the 1972 work of Kentfield and Barnes (The Prediction of the Optimum Performance of Ejectors. Proc. I. Mech. E., Vol. 186, London, 1972). This was one of a number of alternative works given to me by Porta, showing that he was open-minded to alternatives to his own theories. In fact it gives results which are remarkably similar to his own, which I think you would agree is quite significant and which implies that any rejection of the Lempor theory is also a rejection of this other more recent work, coming from a totally different branch of engineering. In the case of the 5AT an exhaust designed according to Kentfield and Barnes is marginally superior in terms of blast nozzle tip area to the true Lempor and has therefore been preferred, designated as a ‘modified Lempor’.
  • I am confident that the 5AT exhaust will perform as predicted, because the established, service-proven theories say it will, and also because its dimensions are similar by both the two above-mentioned methods of calculation (i.e. they tend to confirm each other). Calculations of this nature, especially those for the input data, are not 100% accurate, and therefore do not preclude the possible need to optimise dimensions during tuning-up, as has always been common practice with locomotive exhausts (see (13) below).
  • That parameters for the 5AT exhaust may differ from those of 3450’s does not surprise me, as by classical criteria the former is much closer to the ideal than the latter, which was subject to extreme height limitation for the mixture flow at maximum evaporation which in turn made it of necessity quite far from optimum proportions, and not to be taken as a ‘model’ design.
  • The basic Euler No. is as you give it, but equating this to an expression containing terms from exhaust geometry is not necessarily rigorously accurate, as assumptions have to be made that are not necessarily true. I cannot confirm the correctness of your equations, the validity of which will need practical confirmation.
  • Your analysis takes a value of blast nozzle tip area, An, without any consideration of how this area is constituted. You yourself have in the past put great emphasis on the value of multiple blast nozzles, yet this factor is completely absent in your Euler No. analysis. Although your letter states that ‘The Eu(ler) numbers calculated from the theory are the theoretical upper limits possible from a specific dimensioned layout’, the layout of the blast nozzles is in fact not specified. You may say this is not relevant for finding ‘the upper limit’, but consider that it is an established fact that, other things being equal in a properly proportioned exhaust, multiple nozzles do improve performance, i.e. give the same vacuum for a larger blast nozzle tip area, meaning with lower (time-average) exhaust steam velocity. This acts to increase the ‘requested’ Euler No. whilst at the same time it may, depending on the numerical values for the various interesting parameters, reduce the ‘theoretical upper Euler No. limit’ for the exhaust design, principally by virtue of an increase in Rn (it does actually do this for the 5AT Lempor). This somewhat curious phenomenon implies that according to this theory multiple nozzles would act to make an exhaust less suitable for its target, i.e. reduce exhaust system performance, which is the opposite of what actually occurs.
  • The key to understanding this paradox appears to be the nature of the flow through a locomotive exhaust, which is not steady, as assumed in your work, but pulsating. Furthermore, most steam (and gas) flow is during release, at a (continuously varying) pressure which initially depends on the cylinder pressure at the time the valves open to exhaust. This being the case, all factors in your Euler No. equations, except those fixed by the exhaust dimensions, are liable to be different from the steady-flow values during release, which is when most of the gas pumping work takes place. Steady flow equations are therefore only an approximation for what actually occurs in a locomotive exhaust.
  • A further factor that does not appear to be accounted for by your theory is the advantage taken of the pressure ratio across the blast nozzle being greater than critical during release, therefore giving supersonic exhaust steam velocity if the nozzles are designed to achieve this. Again, the effect of this will be to alter various parameters from their steady state values.
  • The above points out basic limitations in the Euler analysis as you have presented it. It is true that other work, such as that of Porta, has been based on steady flow, but service experience under the actual flow conditions of a locomotive exhaust has proved satisfactory. Your ideas will have to be put to a similar practical test before they can be accepted. It can also be pointed out that those assessing your work must not only be knowledgeable in ejector pump design but also in the precise nature of the pulsating flow through a locomotive exhaust. It is quite possible that they are not – unless you are brave enough to tell them!
  • As a matter of interest, Porta’s theories also make no allowance for multiple blast nozzles, but the work of Kentfield and Barnes does.
  • The above has implications for any assessment of the present 5AT exhaust design on the basis of your Euler No. analysis. We can say that it is premature to jump to any conclusions about the performance of this exhaust on the basis of theory which, as I have explained above, must be regarded as only an approximation to what actually happens. For the record, quick calculations based on the actual 5AT true Lempor exhaust data (which is slightly different from what you assume) show a smaller difference between ‘requested Euler No.’ and ‘theoretical upper Euler No. limit’ than given in your letter, i.e. 0,052 versus 0,047, which, even if it were valid, would only translate into a small rise (approximately 10%) in back pressure being required to equalise the numbers, to be simply achieved by changing exhaust nozzle area during tuning up, whilst of course retaining the Lempor system as such (you have implied that the Lempor system per se would not be adequate). There would be a greater shortfall with the modified Lempor to Kentfield and Barnes, but all this is considered rather academic at this stage given the apparent limitations of the Euler No. analysis as an applicable criterion for the case in question, as pointed out above. If the theory can be made more rigorous, to account for the factors introduced by pulsating flow, and / or its validity demonstrated in actual practice, then it could be considered further at the detail design stage.
  • The open question is always whether an alternative ejector can achieve the same gas pumping work with lower exhaust steam kinetic energy, i.e. with a larger blast nozzle tip area. You will recall that at the time of the 5AT exhaust design you were invited, through Mr. Mintoft, to design an alternative exhaust according to your ideas, for the same service parameters and dimensional limitations, for comparison purposes. This you were unable to do. No analysis, however erudite it may appear, is useful to a design engineer unless it can be used for design. Perhaps you may be able to put forward a detailed alternative once your thesis is complete, which would be welcomed, but until you can design such an alternative exhaust, showing a significantly larger blast nozzle tip area than that already designed, and back up your proposal with practical proof that it will work as predicted, the ‘modified Lempor’ will have to stand, and for the present time we had better leave it at that.

Yours sincerely,
D. Wardale


In December 2006 Dave Wardale prepared a 14 page response to Jos Koopmans’ thesis printed in book form under the title “The Fire Burns Much Better … “.  Wardale’s response is available in PDF format and can be downloaded here.


In September 2013, Jos Koopmans published a 27 page response to Wardale on his website at http://www.thefireburnsmuchbetter.nl/, a copy of which can also be found here.


In October 2013, David Wardale’s wrote a 8 page response to Koopmans a copy of which has been posted onto this website at Wardale’s request. It can be downloaded in PDF form by clicking here.

The following answer is taken from Wardale’s response to a letter from Angus Eickhoff, published in Steam Railway #276 (see ‘Articles and Letters’ page of this website):

“Compound expansion for the 5AT has been considered – and rejected. The arguments for and against compounding are too complex to air here, but I am firmly convinced that for a high-speed locomotive such as the 5AT, simple expansion – using all the cylinder refinements that are now possible, but which are not common knowledge – is the right choice. Mr. Eickhoff states that the compound has left the simple to catch up – well, I believe the 5AT will show that it has now fully caught up, and indeed probably surpassed the compound for power generation at high speed.”

The following is taken from the 5AT technical specifications Item 16 (Par. 5.1.): “The proposed design will define ‘state of the art’ for 2-cylinder simple locomotives, and may serve as a reference level to which the performance of all other types of locomotive can be compared. Analysis has shown no net advantage for the 3-cylinder simple type compared to 2-cylinder simple, however the 3-cylinder option remains open if the dynamic augment of a 2-cylinder design at the proposed maximum speed is unacceptable to Railtrack. Whilst the 3-cylinder compound type, with one h.p. and two l.p. cylinders and with the l.p. cranks set at 90º, might offer somewhat better thermal performance, more especially at lower speed, an overall thermal improvement sufficient to justify the extra design complexity and higher manufacturing cost cannot be guaranteed. The limited l.p. cylinder volume possible within the British moving structure gauge, with a conventional layout of the cylinders, is an important limiting factor on compound design and performance.”

See also the page on Simple & Compound Expansion in the Modern Steam section of this website.

Motion, Balancing and Valve Gear

The effects of “hammer blow” are discussed in the response to the FAQ relating to perceived benefits of a 3-cylinder arrangement.  However they are covered in more detail in the Notes on FDC 8 where Wardale’s conclusions are presented as follows:

The 5AT locomotive can be satisfactorily balanced for 200 km/h operation by all criteria given in these calculations. The key to this is the utmost lightness of the reciprocating parts.

For 200 km/h the maximum permissible dynamic augment limits the amount of the reciprocating mass which can be balanced to some 19.7%, this fraction being quite small even for the lightweight components concerned. Conversely, according to the criteria of FDC 8 line items [39]/[44] and [46]/[47], the reciprocating parts need no balancing at all. However the present recommendation is that the reciprocating balance be set at FDC 8 line items [100] – [101], which will give the least unbalanced forces on the locomotive’s structure at a dynamic augment no greater than that of the present BR 5MT at speed. If a lower maximum speed, V, is mandated than 200 km/h, the reciprocating balance can be increased accordingly, in the ratio of (200/V)2.

It should be noted that these calculations have mostly been based on ‘worst case’ conditions, e.g. design speed (200 km/h) which is greater than the maximum continuous operating speed (180 km/h), an engine-tender mass which is less than the minimum which will occur in practice, due to the necessity of always running with a reserve of fuel and water in the tender, and so on. Therefore, even under the most extreme conditions likely to be found in service, the unbalanced forces acting on the locomotive, the dynamic augments on the track, and the amplitudes of vibrations will all be less than given by these calculations, i.e. the riding of the locomotive will be smoother and its impact on the track less severe.

Note: With 30 inch piston stroke and 6′ 2″ wheels, the piston speed at the max. design speed of 200 km/h (125 mph) will be 23.8 m/s or 24.8 m/s with worn tyres.
David Wardale responds as follows:

  • Whatever the mean piston speed, a piston always moves slowly close to the end of its stroke and is in fact stationary, relative to the cylinder, at dead centre. Cylinder liners are known to wear more at the ends than the centre. These 2 factors are linked. Near the ends of the stroke there is too little velocity for hydro-dynamic lubrication to occur – with any piston speed. Conversely where the piston is moving fast, near the centre of the stroke, hydrodynamic lubrication does occur and wear is less. The high velocity of the 5AT piston in this zone will merely accentuate the trend to good hydrodynamic lubrication and low wear near the centre of the stroke, whilst doing nothing to make it worse at the end of the stroke where the velocity is always at or near zero.
  • The piston head does not touch the liner, therefore contact is limited to the piston rings and liner.
  • The rings are “barrelled” on their outside diameter and can to a limited extent tilt in their grooves, both of which favour full hydrodynamic lubrication.
  • The rings are very elastic, therefore their pressure on the liners is low when steam pressure is low, e.g. when drifting.
  • Drifting has to be with a limited amount of drifting steam which carries away friction-generated heat which would otherwise build up during high-speed drifting.
  • The rings and liners are perlitic chromium cast iron of high quality, which gave excellent results in 3450 (Red Devil“).
  • Cylinder lubrication will be such as to guarantee oil reaching the cylinder liner walls and not simply blown to exhaust which all too often happened in the past.
  • Cylinder oil may have colloidal graphite added, which greatly reduces friction and wear, or perhaps synthetic oil specifically for the conditions in a locomotive’s cylinders and valves may be used.
  • Drifting steam ensures no abrasive particles are carried into the cylinders from the blast nozzles, because of no vacuum occurring in the cylinders. (Note: There will in any case be minimal such particles would at any rate be minimal with oil firing). Likewise antifoam ensures no abrasive scale particles can form in the steam in the superheater, because no water carrying dissolved solids leaves the boiler.
  • The reciprocating masses are supported by the crosshead/slidebar bearing and tail rod, which are easy to lubricate as they are not subject to steam temperature (and the piston etc. mass is also very low for reasons of reducing the reciprocating masses, therefore low bearing pressures).

The above 10 reasons are given to indicate why no lubrication problem is expected on the 5AT despite its high piston speed (and it is not so much higher than what has been achieved before, e.g. the N & W J Class 4-8-4s apparently ran regularly at 90 mph with 70 inch drivers giving the same piston speed that the 5AT would have at 97mph – see note below). The key, apart from the fundamental item No. 1 in the list, is the taking of so many factors (e.g. light piston, use of a tail rod, flexible rings, drifting steam etc.) all of which act to improve lubrication. If you didn’t have all these factors coming together you would have trouble if for any reason (e.g. oil starvation or excessive bearing pressure) full hydrodynamic lubrication (i.e. the forming of a proper load-bearing oil film) were not present during the entire piston stroke. That this may well have happened with first-generation locomotives is why some idea of limiting piston speed probably developed, but, e.g. like using 450oC steam (where 400oC was thought the limit in the past), such previously considered limitations no longer apply where proper means are taken to overcome them.

Note: According to Philip Atkins in his book “Dropping the Fire”, page 24, one of the Class Js attained a speed of 110 mph whilst experimentally running on the Pennsylvania Railroad in 1944 with half worn (5ft 8½ inch) tyres.  And of course the boiler pressure of the Js in final form was 300 psi.  The 110mph speed reached by a J is further corroborated in David P. Morgan’s book “Steam’s Finest Hour” p61: “As for the J, need anything more be said than that a comparatively low-drivered 4-8-4 proved herself not only equal to the mountains but also capable of whipping a 1025-ton test passenger train up to 110 miles per hour across the Virginia swamps”.

There has been much debate over the years since this project was initiated as to whether Wardale was mistaken in adopting piston valves driven by Walschaerts valve gear on the 5AT instead of British Caprotti valves driven by rotary cams. The issue is discussed in the FAQ pages and two or three letters on the subject appear in the Articles and Letters pages of this website.

In September 2009, Wardale received a lengthy letter raising both old and new debates on this thorny subject. Wardale’s reply in 28 separate points is reproduced on this website at his suggestion.  It may be viewed in both html (website) format and in PDF (printable) format.

Wardale’s defense of the use of piston valves on the 5AT is quite lengthy but it makes stirring reading.  It remains to be seen whether it ends the debate, but his closing comments suggest that he will not take it up again unless presented with detailed calculations that disprove his conclusions.

Wardale’s responses cover 7 pages of this website, the first of which can be found here.

The following comes from a “Is it just a Phantom” – David Wardale’s response to Herr Ebel published in Lok Report in March 2001:

“It should be noted that needle roller bearings were an integral part of the American Baker valve gear, and are extensively used on Walschaerts valve gear on Chinese locomotives, operating successfully in a rather harsh environment.”

David has also offered the following response to the same question: “Baker valve gear is primarily rejected because it has too many pin joints (more than Walschaerts, check for yourself).  I suspect it is also heavier.  Obviously opinion was divided even in America on the relative value of Walschaerts and Baker as some of the better railways (e.g. the Pennsylvania) preferred the former to the end.  The only advantage of Baker that I know of is elimination of the sliding link/dieblock, and as you rightly surmise that problem is dealt with by mechanical lubrication as on the SAR 25 Class.”

There are many people (including many with great knowledge of the subject) who challenge Wardale’s choice of Walschaerts valve for the 5AT, believing that British Caprotti (as used on the BR 8P Pacific No 71000 Duke of Gloucester) would have been a far more appropriate choice because it offers larger port openings and zero back pressure when coasting.

Wardale strongly defends his choice and has explained his reasons in several letters, some of which are included in the page titled “Why Walschaerts Valve GearWhy Walschaerts valve gear?”.  However a more comprehensive reply is recorded in a letter from David Wardale to John Duncan which is transcribed over 7 pages titled “Piston Valves vs. Caprotti Valves – The Final Discussion?” and in PDF format in Wardale responses to Caprotti proposals 3 Sept 2009.

The following answer is taken from Wardale’s response to an unpublished letter from Bryan Attewell written in response to Wardale’s two-part article “Steaming into the Future” published in Steam Railway magazines #272 and #273:

“Mr Attewell criticises the choice of 2 cylinders for high speed, but the 2-cylinder locomotive cannot be simply condemned without fully exploring its possibilities. The acceptability of a 2-cylinder engine hinges on the issue of balancing, as I have acknowledged in my article. Although Mr. Attewell invokes Newton’s laws to support his case, it is in fact these same laws which show us how a 2-cylinder engine can be made acceptable. Newton tells us that force = mass x acceleration, so (i) the reciprocating masses cause forces and (ii) these forces act on a mass (the locomotive) to produce accelerations (i.e. vibrations). Considering item (i), force is proportional to the mass of the reciprocating parts, which in the BR 5MT are far from being as light as they should be (notwithstanding the LNER type crosshead, a design which is not amenable to lighter construction by using aluminium). A reasonable yardstick is the mass of the reciprocating parts per ton of piston thrust, so let us consider the following:

Piston thrust,
(metric) tons
Mass of reciprocating parts per cylinder, lb. Mass of reciprocating parts in lb. per ton of piston thrust
BR 5MT 28.9 826 28.6
Burlington RR 4-6-4 with Timken lightweight parts 55.7 944

This illustrates the extent to which the reciprocating parts can be lightened, correspondingly reducing the forces they exert when in motion. But this is only half the story, the other half being the engine-tender drawgear. Using an American-type coupling incorporating a friction-damped radial buffer allows the tender mass to be added to that of the engine in absorbing fore-and-aft forces, which from Newton’s equation proportionally reduces the resultant accelerations (vibrations) of the engine, which is the important parameter. Put simply, if we can roughly halve the reciprocating masses and double the mass of the locomotive resisting fore-and-aft forces (by incorporating the mass of the tender) then the balancing problem is solved, eliminating the objections to a 2-cylinder machine for high speed. The key is optimum design of the reciprocating parts, and it can be revealed that the pistons, piston rings, piston rods and piston tail rods for the 5AT are already fully specified, the total mass of this assembly being only 200 lb., with a stress-based fatigue life equal to the full expected life of the locomotive. It would therefore appear that the target of 550 lb. for the total mass of the reciprocating parts per cylinder is going to be bettered.

Mr. Attewell’s other arguments in support of 3 cylinders are rather unconvincing. Adhesion is a complex subject – suffice it to say that there is little practical evidence (especially in Britain!) to suggest that 3-cylinder engines have superior adhesion. And at the kind of running speeds the 5AT would normally be used at, the damping of combustion gas flow through the boiler tubes produces fairly uniform draught on the fire whatever the number of cylinders, as I can attest from having taken thousands of draught readings. Of much greater importance is to dampen the draught peaks due to over-rapid release of steam from the cylinders, and I would remind Mr. Attewell that this problem destroyed the performance of the 3-cylinder “Duke of Cloucester”, but not that of the 2-cylinder Caprotti Class 5’s!

Readers have occasionally asked whether consideration was given to applying variable lead to the Walschaerts valve gear on the 5AT.

In fact, Wardale did give thought to this and he discusses it in FDC 5 (line 34) where he wrote:

Varying lead with cut-off is possible by

  1. shortening the eccentric crank length or
  2. slotting the combination lever top and suspending the front of the radius rod from a link held in one arm of a crank, and by raising or lowering it varying the effective length of the combination lever dimension between the radius rod and valve spindle joints.

The first method is simple but gives a lead which diminishes continually from FFG to FBG, i.e. it gives maximum lead in FFG, which hinders starting and is therefore potentially a significant drawback.

In the second method, by having the crank cam-operated and linking the cam rotation to cut-off, any reasonable desired variation of lead with cut-off (such as zero lead in full gear, then high lead in the long to mid cut-off range, decreasing for shorter cut-offs) could be arranged. This system is potentially advantageous, but it would be more so on a freight locomotive required to make difficult starts and habitually working in long cut-off, as did, for example, the D. & R.G.W. Railway 4-6-6-4’s to which it was applied, and whilst mentioned here it is not proposed to use it on the 5AT due to the not inconsiderable extra design complexity involved.

Chris Newman posed the question: “Why were piston tail rods dropped from latterday designs?”

Wardale responded as follows:

“Without a tail rod the weight of the piston is taken by its head or by the rings if these are of lipped type.  They were dropped because head and linear wear was found to be tolerable (by former standards) without them, and because the poor rod packings tended to leak.  But they were never done away with in, for example, German practice (they had much better packings!)”

Wardale strongly defends his choice of Walchaerts valve gear and piston valves, and has explained his reasons in several letters, some of which are included in the page titled “Why Walschaerts Valve Gear?”.  However a more comprehensive reply is recorded in a letter from David Wardale to John Duncan which is transcribed over 7 pages titled “Piston Valves vs. Caprotti Valves – The Final Discussion?” and in PDF format in Wardale’s letter dated 3rd Sept 2009.

In his letter to Wardale, John Duncan refers to letters by Bryan Attewell and Angus Eickhoff relating to Wardale’s article “Steaming into the Future” published in Steam Railway magazine in 2002.  These letters are referred to below.

The following answer is taken from Wardale’s response to an unpublished letter from Bryan Attewell written in 2002 in response to Wardale’s two-part article “Steaming into the Future” published in Steam Railway magazines #272 and #273:

“Caprotti valve gear is expensive specialist equipment, so piston valves and Walschaerts valve gear have been chosen because with Porta refinements they give a performance of the same standard and at much lower capital cost. Indicator diagrams from “The Red Devil” – which the 5AT will greatly improve on – have proved that. Can Mr. Attewell produce figures to show that poppet valve gear requires “considerably less” power to drive than Porta-type lightweight piston valves driven by Walschaerts gear running in needle roller bearings?”

Wardale offered further comments in response to a letter from Angus Eickhoff published in Steam Railway #276 in Oct 2002:

“The question of Walschaerts versus Caprotti valve gear has been partly dealt with in the reply to Mr. Attewell. To answer Mr. Eickhoff’s points, the contribution of Walschaerts valve gear to the balancing issue is negligible as most of its inertia forces are out of phase with those of the main reciprocating masses, and the maximum acceleration (i.e. inertia force per unit mass) of even ultra long travel valves is only some 30% of that of the pistons.

Although altering the valve events by changing the cams may be convenient on an “experimental machine”, the terms of the project mean that the design of the 5AT is in no way intended to be experimental.

Chapelon’s reservations about piston valves are no longer valid. Porta’s invention of valve liner cooling allows higher steam temperatures to be used without lubrication difficulties, and inertia forces are kept at manageable levels by the very lightweight construction made possible by designing the valves according to stress analysis rather than empirical rules. It is also worth noting that Chapelon’s final masterpiece, the 242A-1, as well as his aborted 152P design, had piston valves driven by Walschaerts gear. No recourse to poppet valves needs to be considered before the possibilities of piston valves have been fully explored – which is what the 5AT will do.”

16 Mar 2003: Wardale offers further comment on the issue in his Fundamental Design Calcs for the main crank pins for the locomotive. In FDC 3 Item 137, he states:

“Because bending stress due to inertia load at maximum speed is greater than maximum allowable crankpin fibre stress, cushioning must be provided to relieve the inertia load on the main crankpin. Note that this is one reason to reject Caprotti valve gear as finally applied to BR locomotives, as the valves were arranged to drop from their seats during drifting thus providing a full by-pass from one end of the cylinder to the other, precluding the build-up of cushioning steam pressure. The required cushioning steam pressure at dead centre is now calculated for the worst case condition, i.e. maximum speed with minimum coupled wheel tyre thickness, this pressure being therefore suitable for all lower coupled wheel rotational speeds …..”

Wardale goes on to state: “…. other reasons are:

  • cost;
  • it is specialist equipment;
  • less than 100% certainty about steam tightness and good flow coefficients past the double-beat poppet valves;
  • the fact that the piston valves for the 5AT will in all respects (e.g. steam flow, lightness, lubrication, wear, resistance to high steam temperatures, resistance to steam leakage) be greatly superior to the general level of piston valves in the past with which Caprotti valves have been compared;
  • Walschaerts gear mounted on needle roller bearings and with mechanical lubrication of the dieblock will be extremely wear-resistant and hence maintenance-free;
  • lastly (from the point of view of the project’s aim, not least), Walschaerts gear is aesthetically more attractive.”

23rd March 2003: In response to the question “Why not make the crank-pin bigger?”, Wardale responded with the following:

“Why not make the crankpin bigger? Because if we do, the mass of everything associated with it – the crankpin itself, the roller bearings, connecting rod big-end, coupling rod eye, and seals will all get bigger too. As crankpin stress is inversely proportional to the diameter cubed and mass directly proportional to diameter squared (roughly), equilibrium will always be reached at slightly lower stress as diameter is increased, but remembering that the whole lot has to be balanced and because we can keep the original diameter (FDC.3 item 17, similar to BR 5MT design) by using alloy steel, my judgement is to use that option. Alloy steel for the main crankpin is not an unusual thing – for example the modified QJ would have had to have it, as the crankpin could be made no bigger due to clearance restrictions (on small-wheeled engines there is always a problem with gauge clearance of the big-end when the crank is at the bottom, although this does not apply to the 5AT).”