Heinkel HeS 40
The Heinkel HeS 40 (HeS - Heinkel Strahltriebwerk) was an experimental constant-volume jet engine designed by Adoph Müller's team at Heinkel starting some time in 1940 or 41. It was based on the mechanical layout of the HeS 30, but replaced the conventional flame cans with oversized ones including large poppet valves that sealed off the chambers during firing. Constant-volume combustion, similar to the Otto cycle used in most piston engines, is considerably more fuel efficient than the constant-pressure combustion used in a typical jet engine.
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.
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.