The British Caprotti Valve Gear

In 1911, Arturo (Arthur) Caprotti, a student of automobile engineering at Milan University, was told by his tutor that the steam railway locomotive was THE most inefficient machine. The reasons were:

Steam does its work through expansion in the cylinders of the locomotive and when exhausted has greater volume than when admitted, yet the design of the universally accepted piston valve (and similar systems) meant that steam had to be both admitted and exhausted through valve ports of the same size.

Piston valve heads were fixed in position on their rod, compromising valve events.

The Walschaerts gear, most commonly used to operate these valves, did not permit any advance or retard of engine "timing".

On the other hand, the motor car engine used poppet valves, which were totally separate, with inlet and exhaust valves of different sizes.

Caprotti transferred the principle of the poppet valve from the motor car to the steam railway engine:

Instead of a simple single seat poppet valve, where petrol/air vapour or exhaust gases passes by the sides, he invented a hollow (spoked) double seat valve, where steam not only passed by the two seats but also went down or up through the centre of the valve as well. His invention was much more efficient than its motor car equivalent.

Instead of a camshaft, with fixed cams on it (to operate the valves), which gave fixed valve events, he invented an assembly with two inlet cams, which could be moved (mechanically) in relation to each other, giving variable inlet valve actuation and one exhaust cam, giving fixed valve events.

His invention was brilliant but flawed. The British Development of the valve gear, brought major changes in the poppet valves themselves, and in Daniels, Chief Engineer for Associated Locomotive Equipment, changed the 1950 Tom camshaft design to include two exhaust cams instead of one, which could be moved (mechanically) in relation to each other, like the inlet cams, thus achieving complete variable valve actuation.

Item 1

Exhaust valve from 71000

Item 2

The cage in which it operates

Item 3

Exhaust valve from a contemporary diesel locomotive (Class 47)

Item 4

Exhaust valve from a Rover motor car (3500 V8)

HOW VARIABLE VALVE ACTUATION WORKS

Steam engines do not have mechanical gears; instead "gearing" is achieved by shutting off the supply of steam at an earlier and earlier point in the piston stroke. This is called "cut-off". Starting with a train, the steam cut-off of THE DUKE will be at 83% of the piston stroke and the rest of the work will be done by expansion. Once the train has gathered enough speed the cut-off will be progressively reduced, with more of the work being done by expansion. The shortest workable cut-off is 3%. "DUKE OF GLOUCESTER" is the only engine that will run at such a low cut-off.

As the Duke gathers speed and the driver shortens the cut-off, the variable valve actuation makes the inlet valves close sooner in the cycle. Subsequently, after the cut-off has been shortened still further, the exhaust valves begin to open sooner in the cycle, giving rapid acceleration and great power. Cylinder efficiency is 86% of the Rankine cycle (that which is theoretically possible).

Ironically, in this decade, some motor car manufacturers, including Mercedes and Honda, have succeeded in applying some of the British Caprotti principles to new camshaft designs in motor car engines, albeit electronically controlled and hydraulically induced.

The new Jaguar 3 litre AJ-V6 engine features what is described as a - "sophisticated camshaft phasing system, also under the control of the engine management module, further helps to OPTIMISE TORQUE output, particularly at mid and high engine speeds. Engine torque output is optimised by advancing or retarding the opening and closing of the inlet valves at different engine speeds. This benefits fuel economy, emissions control and engine idle quality, as well as performance.

Perhaps now they should all be seeking to imitate some of the other British Caprotti design features?

Left: The British Caprotti Camshaft Assembly

Kind permission has been given by The National Railway Museum for the exhibit shown right (original centre cylinder, cambox and reversing gear), which was exhibited at The Science Museum in Kensington until 1997, to be loaned to The West Somerset Railway Association. This will be exhibited at the Bishops Lydeard Visitor Centre, West Somerset Railway, from the beginning of the 2002 season.

Right: The sectioned original centre cylinder on display at the Science Museum, Kensington in 1978.

This is now known as the Caprotti Demonstration Gear.

The Caprotti Demonstration Gear