The Heinkel HeS 40 was an experimental constant-volume jet engine

Jet engine

A jet engine is a reaction engine that discharges a fast moving jet to generate thrust by jet propulsion and in accordance with Newton's laws of motion. This broad definition of jet engines includes turbojets, turbofans, rockets, ramjets, pulse jets...

 designed by Adoph Müller's team at Heinkel

Heinkel

Heinkel Flugzeugwerke was a German aircraft manufacturing company founded by and named after Ernst Heinkel. It is noted for producing bomber aircraft for the Luftwaffe in World War II and for important contributions to high-speed flight.-History:...

 starting some time in 1940 or 41. It was based on the mechanical layout of the HeS 30

Heinkel HeS 30

-Further reading:*German Jet Engine and Gas Turbine Development 1930-1945, Antony Kay, Airlife Publishing, 2002....

, but replaced the conventional flame cans with oversized ones including large poppet valve

Poppet valve

A poppet valve is a valve consisting of a hole, usually round or oval, and a tapered plug, usually a disk shape on the end of a shaft also called a valve stem. The shaft guides the plug portion by sliding through a valve guide...

s that sealed off the chambers during firing. Constant-volume combustion, similar to the Otto cycle

Otto cycle

An Otto cycle is an idealized thermodynamic cycle which describes the functioning of a typical reciprocating piston engine, the thermodynamic cycle most commonly found in automobile engines....

 used in most piston engines, is considerably more fuel efficient than the constant-pressure combustion used in a typical jet engine.

Design

The design was based on the HeS 30 not only to make parts more readily available as well as to make direct comparisons between the two easier. The main changes were to reduce the compression ratio of the compressor to about 2:1 (from 2.8:1), and add the new combustion chambers. The new chambers were considerably larger than the originals, forcing a reduction in the number from ten to six burners. The valve stems projected forward into streamlined fairings in the intake area behind the compressor.

The operational cycle of the engine is somewhat similar to a conventional six-cylinder engine. Slightly compressed air, similar to an automobile equipped with a turbocharger

Turbocharger

A turbocharger, or turbo , from the Greek "τύρβη" is a centrifugal compressor powered by a turbine that is driven by an engine's exhaust gases. Its benefit lies with the compressor increasing the mass of air entering the engine , thereby resulting in greater performance...

, was channeled into the cylinders in turn, closed off with the poppet valves, and then burned. By the time the combustion was complete the pressure in the flame cans would be much higher, although the actual compression ratio

Compression ratio

The 'compression ratio' of an internal-combustion engine or external combustion engine is a value that represents the ratio of the volume of its combustion chamber from its largest capacity to its smallest capacity...

is not specified. The hot gas then blew through a turbine to extract power, instead of forcing a piston to move. Although there would be some loss of charge during the burning period, and thus the design would be less efficient than the true Otto cycle, it would nevertheless be somewhat more efficient than a traditional jet engine, at the cost of some complexity.

History

It appears the HeS 40 was never built, and remained a paper design. Nevertheless, work on the design was ended by 1942, by which point the HeS 30 was making good progress.

Further reading

• German Jet Engine and Gas Turbine Development 1930-1945, Antony Kay, Airlife Publishing, 2002