Concept
A compound engine is an engine that has more than one stage for recovering energy from the same working fluid, with the exhaust from the first stage passing through the second stage, and in some cases then on to another subsequent stage or even stages. Originally invented as a means of making steam engines more efficient, the compounding of engines by use of several stages has also been used on internal combustion engines and continues to have niche markets there. The stages of a compound engine may be either of differing or of similar technologies, for example: In a turbo-compound engine, the exhaust gas from the cylinders passes through a turbine, the two stages being dissimilar. In a compound steam locomotive, the steam passes from the high-pressure cylinder or cylinders to the low-pressure cylinder or cylinders, the two stages being similar. In a triple-expansion steam engine, the steam passes through three successive cylinders of increasing size and decreasing pressure. Such engines were the most common marine engines in the golden age of steam. These examples and compound turbines are the main but not the only uses of compounding in engines, see below. A compound engine uses several stages to produce its output. Not all engines that use multiple stages are called compound engines. In particular, if an engine uses a later stage purely to extract energy from the exhaust for some other purpose, and notably for turbo charging, is not called a compound engine. Similarly, proposed engines that use a free piston engine to drive a turbine would not be called compound engines, as only the second stage produces output power. However, if a turbo compound engine is also supercharged by feeding some of the shaft power back to the supercharger, as in some aircraft engines, it is still a compound engine. Usage of the terms supercharged and turbosupercharged has varied with time, for example the makers of the Wright R-3350 Duplex-Cyclone compound engine described it at the time as turbosupercharged.
