Enhanced Piston Valve Events

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Enhanced Piston Valve Events

This page covers briefly a number of topics related to the enhancement of valve events to improve a locomotive’s performance. These come under the following headings:

A separate page covering valves and valve gear can be found in the Technical Terms section of this website.

Valve Events

Valve Events have been defined elsewhere as the four defining points in the cylinder power cycle – viz: (1) Admission; (2) Cut-off; (3) Release and (4) Exhaust Closure. These four points, in turn, define the intervening periods of Admission, Expansion, Release and Compression as shown on the idealised Indicator Diagram (below).

The same page also defines the terms Lap and Lead and describes the purpose of each.

The design of valves and the timing and nature of their associated “events” have a strong influence on a locomotive’s performance through their association with triangular losses and incomplete expansion losses in the cylinders. Thus any modifications to valve and/or their events should be aimed at improving the steam flow through the valve, most especially by maximizing the area available for steam flow is vital, as is streamlining of the path that the steam is to take.

Specific examples of improvements that can be adopted (some with relative ease) include:

Associated changes that will help to improve valve performance include:

  • Increasing the size of the steamchest so as to minimise the steamchest pressure loss during admission.
  • Improving the exhaust system to minimise back-pressure (remember that when back pressure is lower, the steam’s specific volume is higher, so a larger volume of steam has to pass through the valve)
  • Optimising clearance volume.

Most of the above are to some extent interlinked – for instance increasing the steam lap usually necessitates increasing valve travel; and streamlining valve heads and rings usually involves making the events ring-controlled.

Any alterations to valve events should take into account the nature of the work that the locomotive is intended to undertake. For instance

  • Where a locomotive is to be used for high speed operation, then increasing both lead and steam lap is likely to be beneficial. (Note: Increasing lap is likely to require changes to the valve gear geometry to increase valve travel.)
  • Where a locomotive is to be used for shunting or heavy haulage, then a shorter lead may be appropriate.
  • Where a locomotive is required to deliver high power outputs at high wheel rotation rates, then variable lead may be advantageous.
  • Where a locomotive is to be operated mainly in one direction, then the valve gear geometry might be arranged to optimise the indicator diagram when running in that direction.
  • Where a locomotive (e.g. a tank engine) is to be operated in both directions, then the valve gear geometry should be optimised so that it produces similar indicator diagrams in both directions.
  • If the exhaust system is producing so much draught that it diisturbs the firebed, it may be advantageous to increase the exhaust lap to reduce the cylinder pressure at release. (Note: increasing exhaust lap will also advance the point of Closure and thus increase steam compression.)

 

Optimising Lead

Porta’s views on Lead

The subject of Lead is discussed in depth by L.D. Porta in his paper titled “Notes on the Optimum Value of Lead in Steam Locomotives” in which he summarizes the optimum value of lead as “that which gives the least inlet pressure drop”. In generally this occurs when the compression line of the Indicator Diagram reaching the steamchest pressure when the piston reaches “dead centre”. He illustrates this with the diagram below:

Fixed Lead

Walschaerts valve gear is commonly described as having a fixed lead. In fact it is fixed only in linear terms, in that the linear position of the valve remains fixed regardless of cut-off when the piston is at dead centre. However when looked at in relation to the crank rotation angle, it will be discovered that as cut-off increases, the lead decreases in angular terms as may be seen in the diagram below (adapted from Fig 46 of The Red Devil). In other words, at any given rotation speed, the time available for steam to enter the cylinder in advance of the piston reaching its dead-centre position, reduces as cut-off increases. This is desirable for starting purposes, but may not be optimal for hauling heavy loads at high speed.

Variable Lead

In the same paper, Porta mentions the idea of introducing variable lead to Walschaerts valve gear by shorting the eccentric crank. Wardale refers to this in FDC 5 line 34, saying: “Variable lead can be applied to Walchaerts valve gear by (i) shortening the eccentric crank length or (ii) 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.”

Introducing variable lead using Porta’s shortened eccentric crank has the effect of increasing “angular lead” at long cut-offs thereby reducing a locomotive’s starting capability but increasing its ability to haul heavy loads at high speed – i.e. produce high power at high speed.

According to Wardale, the slotted combination lever and suspended radius rod concept was adopted by the Denver and Rio Grande Western Railroad on their L-105 4-6-6-4 Mallet locomotives. He describes its purpose as follows: “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”.

 

Ring-Control of Valve Events

In most locomotives, the opening and closing of the ports to admit steam to the cylinder and to exhaust steam from the cylinder, occurs when the edge land (the rib that holds the valve ring in place) passes over the edge of the port. This is termed valve-control (or land-control) of the valve events.

One of Porta’s many innovations was to introduce the concept of ring-control in association with the streamlining of the corners, edges and surfaces that the steam has to negotiate its way around as it passes from the steamchest and into the cylinder, and on its way back out to the exhaust system. The concept is illustrated below, in this case comparing control of admission steam with the (traditional) edge land and with (Porta’s) ring control.

Two important observations may be made from these diagrams:

  1. Ring control provides a much more precise timing for the event. In the case of land control, steam will begin to find its way into the cylinder before the ring passes over the edge of the port, leading to wire-drawing and “triangular” loss. With ring control, admission to steam is almost instantaneous and the steam flow much more streamlined (hence much lower triangular losses).
  2. The edge land on the admission end of the valve head can be very thin because the steam pressure serves to keep the ring away from the land thereby reducing (or eliminating) intertial ring pressure on the land. Thicker lands are required at the exhaust ends of the valves (as shown on the similar diagrams on the Streamlining page).
  3. The right-hand diagram illustrates Porta’s recommendation that the bottom of the valve head rubs on the liner. See Piston Valve Design page for further explanation.

The above diagrams illustrate the principles of streamlining which are covered in more detail on a separate page.

 

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