The German Diesel-Pneumatic Locomotive.

Updated: 12 July 2010
Better pics
More info
Back to Home PageBack to The Museum
The astute among you will have recognised that this is not strictly a steam locomotive, even if it looks rather like one because of the external driving cylinders. However, there was a little steam involved...

This 4-6-4 diesel-pneumatic loco was designed to solve the problem of power transmission between a diesel engine and the wheels. Time has shown that diesel-electric is the way to go, but in earlier years it was by no means obvious that dragging around a heavy generator and lots of electric motors and associated control equipment was a good idea.

Built in 1929, V3201 was the first high-performance Diesel loco on the Deutsch ReichsBahn. It used the MAN Lo6 Vu 45/42 engine, originally developed for use in U-boats. (surprise, surprise) It was a 6-cylinder 1000/1200hp engine direct-coupled to a MAN 2-cylinder double-acting single-stage air compressor. Air was delivered at 7 Bar. (102 psi) The axle loading was 18 tons.

Above: The internals of the V3201. The big block in the middle is the diesel engine; to the right of it is the compressor. In this picture the front of the locomotive is at the right.

It is a fundamental problem with pneumatic power transmission that a lot of energy is released as heat during compression. Usually this has to be dissipated to stop the compressor overheating, and even without this the compressed air usually cools naturally before reaching the point where it is applied, making the overall process very inefficient. This locomotive tackled the problem by heating the air further after it left the compressor, using the diesel exhaust in long tubular heat exchangers, and this hot air then drove pistons exactly as if it was steam. I have so far no figures on the air temperature when it reached the cylinders, but I would imagine the lubrication required some thought; steam is not so bad as condensation provides a measure of inherent lubrication, but the hot air would have been very dry indeed.

Despite this measure to make the air hot when it reached the cylinders, it was still necessary to cool the compressor cylinders. This was done by water injection directly into the cylinders, presumably introducing the need to carry around and refill tanks of water. There was no water-jacketing at all, which may have been a mistake. Thus there was a certain small proportion of steam in the working fluid. There appear to be two radiators at the front of the chassis, and I suspect (but do not know) that the upper one may have been an oil-cooler.

Above: V3201 from the other side. The long tubes are the exhaust heat-exchangers. On the left is the diesel engine radiator. In this picture the front end is at the left.

On 10th April 1931, the British journal The Railway Gazette reported that the locomotive had successfully completed twelve months of testing by November 1930, during which it exceeded its design speed by 25 mph for long periods. (The design speed is not currently known) On stripping it down there was no significant scaling due to the compressor water injection. It was reported that the officials of the Deutsche Reichsbahn were thoroughly satisfied with the trial results, and that the locomotive was being transferred to the hilly district around Stuttgart, and put into service for further test observations.

Nonetheless, it appears not to have been a total success; five years after it was delivered to the DRG, it was taken out of service, and the design was not repeated. Investigations are proceeding, but at the moment the only suggestion found as to why is that the compressor was prone to overheating despite the water injection. Presumably if the water supply ran out the engine had to stop immediately.

Above: V3201 with its casing in place. Note plenty of windows to the engine compartment.

You will be glad to know this remarkable locomotive is not wholly forgotten. Marklin have produced what appears to be a limited edition model:

Above: Marklin model V3201

Back to Home PageTop of this pageBack to The Loco Index