Furnace air engines

Open Cycle Closed cycle Description
Direct heating Furnace air engines These are air engines in which the air is heated within a closed furnace so that the working fluid is heated directly.
The engine works on an open cycle.
At the beginning of each cycle fresh air is taken from the atmosphere and, after being heated (directly) and having done work, it is rejected outside of the engine.

Examples of furnace air engines:
- The Wenham heated-air engine,
- The Hock & Martin engine,
- The Bénier engine.
Indirect heating

Inventors of furnace air engines

Year Inventor Country Engine page
1807 Cayley England Cayley's air engine
1829 Arnott England Arnott's air engine
1837 Cayley England Cayley air engine
1845 Gordon England Gordon's fumific impeller
1861 Shaw America Shaw furnace air engine
1862 Roper America Roper's caloric beam engine
1868 Wenham England Wenham's heated-air engine

Engine characteristics

Furnace engines are also open engine but having closed firing, in which the air is also used as working fluid and also for combustion. The air mixes with the heating gases and feeds the working cylinder. Obviously the heater room must be airtight. A such they avoid the most important drawback of the external fired open engine: the important heat loss that goes through the chimney.