The Uniflow engine

The first uniflow-engine dates from somewhere 1830. The Uniflow engine was made a commercial success in 1909 by professor Johann Stumpff in Charlottenburg. Drawing of a uniflow-engineDuring WW2 the design was finalised in America and used in several steam-ships.

The drawing shows the engine. The engine is very simple, has little parts but has a comparable efficiency to a modern triple. So it is as efficient, but much cheaper than any other reciprocating engine. The uniflow is by far the most economical reciprocating steam-engine ever.

The piston is very long, almost as long as half of the cilinder. In the middle of the cilinder there is a row of ports, connected to the condensor. Inlet of fresh steam is by the valves in the head of the cilinder. After inlet there is the obvious expansion with the appurtenant drop of temperature. The large ports in the middle of the cilinder ensure that the steam is rapidly sucked in by the condensor.

The valve gear used on these engines was mostly the Paxman-Lentz gear.

Because of the large expansion the steam is cooled down strongly. However, this cold steam does not effect the inletvalve and the fresh steam does not effect the outletports. The steam always moves in one and the same direction uniflow from inlet to outletports (uniflow!). The temperature at the head and at the middle of the cilinder are constant so risks of condensation and re-evaporating are eliminated and there is no need of multiple cilinders like in a compound-engine.

The construction of the engine permits a very small clearance-space and together with the applying of superheated steam the efficiency of the engine is therefore very high. The construction- and maintenance-cost are low. Because of this very small clearance starting of the engine is dangerous. If a little water is left in the cilinder the compression might cause a serious water-hammer, breaking the cilinder head. therefore extra clearance-spaces are located in the head. When starting or stopping this spaces have to be put into communication with the cilinder. To get the desired high efficiency this chambers have to be disconnected when a sufficient vacuum is reached.


On the other hand the strong compression raises the temperature practically to the temperature of the fresh steam so that the fresh steam meets no cold surfaces.


 The uniflow is a very promising design, but when this engine came on the market the internal combustion engines were already taking over, so it was too late.

 short uniflow-engine

Some interesting adaptations were made, for instance Robey developed automated extra clearance by auxiliary exhaust valves, operated by the vacuum. A loss of vacuum automatically opened these valves, securing a much safer operation.

A dutch mr Brouwer invented the short-build uniflow. In stead of the ports in the middle it operated with extra valves. The engine was much shorter, but had extra parts, so the engine used less floorspace, but was more expensive to maintain. The dutch firm Stork has built several of them.