Piston vs. Caprotti Valves 2
Piston vs. Caprotti Valves – The Final Discussion? Part 2
Note: The full text of this correspondence can be downloaded in PDF format under the title:
Wardale responses to Caprotti proposals 3 Sept 2009.
John Duncan’s comment #8 relates to the first paragraph of Wardale’s reply to a 2002 letter from Angus Eickhoff in which Wardale wrote:
“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.”
John Duncan’s Comment 8
Inside admission piston valves travel in the same direction as the piston and I agree it could be that the maximum acceleration of the 5AT piston valves is probably only 30% of the pistons. What concerns me is the length of the stroke on the two cylinders of the 5AT at 800mm (31.5″). The longest stroke locomotives on BR were the GWR two cylinder locomotives of same wheel arrangement as the proposed 5AT, at 729mm (30″). Even with lightweight motion I doubt permission would be granted to run above 75 mph.
Wardale’s Response: If balancing is his concern, the FDC’s show acceptable balancing can be achieved whilst keeping wheel-rail dynamic augment (hammer blow) at 200 km/h to no more than that of the BR5 at 75 mph (120 km/h). In fact, they show that the reciprocating parts do not require any balancing at all, by the latest American criteria, which would give zero balance weight dynamic augment. Therefore from a balancing aspect the 5AT is perfectly suitable for 200 km/h running. Whether or not this would be allowed is obviously beyond our control.
John Duncan’s comment #9 relates to the second paragraph of Wardale’s reply to a 2002 letter from Angus Eickhoff in which Wardale wrote:
“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.”
John Duncan’s Comment 9
The last two ex. LMS class `5’s 44686 & 44687, the BR class `5’s 73125 to 73154 and the 3 Cylinder, 4-6-2, 71000 were fitted with the standard camboxes and poppet valves, all interchangeable. What Angus meant was you could design the cam profiles and valve sizes for the 5AT performance you require. In time it could be computer controlled, the cut-off & throttle controlled to produce the most efficient out-put.
Wardale’s Response: Partly answered by (3) above. Using the 5AT data from the FDC’s, the late Prof. Hall’s Perwal program predicts that the 5AT as proposed will meet its performance target, i.e. its piston valve steam distribution delivers the target performance. Which is what is required. Please let’s forget about “computer control”: to quote from Bulleid, “it’s all very well, but it doesn’t sound much like a steam locomotive, does it?” In general the throttle is to be fully open when steaming and the cut-off set to maintain the schedule. That’s the driver’s job, and it is not too difficult.
John Duncan’s comment #10 relates to the third paragraph of Wardale’s reply to a 2002 letter from Angus Eickhoff in which Wardale wrote:
“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.”
John Duncan’s Comment 10
If the 5AT speeds proposed for service on Network Rails tracks of 113 mph service speed and a maximum of 125 mph, then piston valves are not suitable. Only poppet valves are suitable for high speed working. As regards the French 4-8-4 242.A1 rebuild of the Etat Railways 240.101, a 1932 3-cylinder simple with Renaud rotary cam poppet valve gear, the rebuild was started in 1943 and completed in 1946 with Willoteaux double admission and exhaust piston valves as fore-runner of French steam locomotive development. In the case of the 4-6-4 with 7′ 2″ driving wheels, it was to have Lentz concentric-type oscillating cam poppet valves. The remaining two designs, the 4-8-4 with 6′ 4″ driving wheels and the 2-10-4 with 5′ 4″ driving wheels were to have Willoteaux piston valves fabricated in steel plate to minimize weight. The high pressure valves were two sets under the high pressure cylinder of 8.5″ diameter (similar to the 5AT). The low pressure valves were to be 17″ diameter, weighing 242.5 lbs plus the piston valve rod weighing 40 lbs, total of 282.5 lbs. To quote Chapelon’s book ‘La Locomotive a Vapeur’, the ‘Addenda to the 1952 Edition’:
“For the other three high powered types the use of piston valves rather than poppet valves was dictated largely by workshop and running shed requirements, many of these having experience with piston valves only. However, whilst suitably designed piston valves – Willoteaux with large steam passages – were thermodynamically equal to poppet valves, in practical terms where inertia limitations are important, the poppet valve is far Superior because of its much lighter weight (up to 15 / 20 times less than a piston valve) and much smaller displacement (one fifth to one eighth the travel of a piston valve). Furthermore, poppet valves are superior in their ability to withstand steam temperatures of over 400 degrees for prolonged periods, in which conditions the lubrication of piston valves may be difficult and result in premature wear of piston rings and valve liners with inevitable consequences for steam tightness.” – See A. Chapelon, ‘La Locomotive 242-A.1’ Revue Generale des Chemins de Fer, December 1947.
Wardale’s Response: Chapelon’s reservations about piston valves are no longer valid because when he made them he had no knowledge of Porta’s advances to piston valve and cylinder tribology design. “Only poppet valves are suitable for high speed working.” Completely false. I will remind you that the A4 world speed record holder had piston valves. Yes, the middle valve over-travelled at high speed, but this was not intrinsic to piston valves nor Walschaerts gear, but to the Gresley gear. (The problem of driving the inside valve of a 3-cylinder engine is neatly overcome on the 5AT by not having an inside cylinder, so any advantage of Caprotti gear in this regard is irrelevant.) And the only steam locomotives ever required to sustain 100 mph working in regular service to maintain the schedules, the Milwaukee 4-4-2s and 4-6-4s and the DR 05 Class 4-6-4s, engines of the last two of which were, I think, recorded at 125 mph on test, i.e. only one mph less than Mallard, all had piston valves, of the ‘first generation’ variety, not the much superior Porta type. So your assertion that “only poppet valves are suitable for high speed working” is simply not true, in fact almost the opposite, it could be said that it is poppet valves that have yet to prove themselves suitable for high speed (which does not mean they could not do so.) For example, the possibility of the cut-off cam follower leaving the cam and thereby lengthening cut off at high speed is possible. This possibility is calculable. In the absence of any calculations, you have made an unsubstantiated claim. As far as Walschaerts gear is concerned, it has been proven to work at very high speeds – the above-given examples – but this does not obviate the necessity of a full deflection analysis being carried on the 5AT valve gear, in addition to stress analysis, as part of the detail design, to ascertain what, if any, deviation from the nominal valve motion will occur at very high speed – a meaty task for whoever has to do it. No 3-figure speed has been recorded by steam in this country with a poppet valve locomotive, to my knowledge: all those classes which have (Al, A2, A3, A4, MN, King, Duchess…) had piston valves. The highest speed I am aware of with a poppet valve locomotive is 177 km/h (110.6 mph) during SNCF electric loco pantograph tests in 1956, with a very light load (220 equivalent tons), by a 231E Class with Walschaerts-driven (again!) oscillating cam poppet valves. Chapelon’s comments, when analyzed, have no merit for the 5AT. “Inertia limitations”: Porta-type valves may well be lighter than anything Chapelon envisaged. At maximum design speed (200 km/h) with coupled wheel tyres at minimum size and using a cut-off of 26%, which is a transitory overload case as this cut-off at this speed would be beyond the boiler’s continuous steaming capacity (i.e. absolute worst conditions that might be briefly encountered) the instantaneous peak inertia load per 5AT valve and spindle assembly is calculated as 9,234 N (2,075 lbf) (FDC 5.(250)), which is a quite acceptable figure, this because of careful design to limit the weight of the valves. As the valve acceleration is proportional to the locomotive speed squared, at the maximum continuous operating speed (180 km/h) this falls to 7,480 N (1,680 lbf), and at the maximum speed at which 26% cut-off could actually be sustained according to FDC 1.3 (113 km/h) to 2,948 N (662 lbf). The result is a valve spindle of only 28 mm diameter to take the highest load at an acceptable stress level (FDC 4.(420)), a very modest figure which would not be possible if the valve inertia load were a ‘limiting factor’. The examples quoted above of high-speed engines with piston valves are the practical proof that piston valve inertia is no barrier to high speed. “Ability to withstand steam temperatures of over 400 degrees”: Chapelon had not foreseen valve liner cooling, which eliminates his objection to piston valves at high temperature. Porta has shown that what counts is liner rubbing surface temperature, not steam temperature. The low wear rates of 3450’s valve components (point (2) earlier) were recorded with steam temperatures up to 450′ C (842 °F).