[War Department Technical Manual & Aircraft Engines] Page: 2

                                
TM    1-405
2-3                           AIR CORPS
2. Conversion of heat into mechanical energy.-nternal-
combustion engines are of a class of prime movers known as "heat en-
gines," that is, they convert heat energy into useful mechanical energy
through a process of combustion (fig. 1). A mixture of fuel and air
in proper proportion. after it has been compressed to a comparatively
high pressure, is burned within the cylinder. The sudden increase in
pressure. due to combustion, causes the piston to move against the load
and deliver mechanical energy to the engine crankshaft. The fuel
must be vaporized, or in a gaseous state, when used in an internal-
1. Crankshaft.                     4. Fuel and air mixture before compression.
2. Cylinder.                       5. Fuel and air mixture compressed.
3. Piston.                         6. Igniter or spark plug.
FIGURE 1.--Method of converting heat energy to mechanical energy in an internal-
combustion engine.
combustion engine and must be mixed with the proper proportion of
air in order to burn properly. The igniting of the gas is of the ut-
most, importance. The various methods of obtaining and timing the
ignition spark and the proper regulation and adjustment of the differ-
ent parts of the power plant are vital elements in the operation of a
conventional aircraft engine.
3. Engine cycles.-a. In order to operate continuously and de-
liver power, the engine must go through a routine of operations, each
act being performed over and over in the same sequence. Each of
these operations is known as an event, and a series of events is known

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