The high power density of miniature reciprocating engines makes them
attractive portable power sources. Although such engines are available
commercially, they have extremely low efficiency due to high heat losses
and poor control of the combustion process. Ignition timing control is a
particular challenge because the fuel/air mixture is compression heated,
with supplemental initiation from a hot catalytic glow plug. This paper
describes the combustion process in a 5 cc displacement, single-cylinder,
four-stroke engine and evaluates its relationship to engine performance.
Measurements of in-cylinder pressure and luminosity as a function of crank
angle and ignition timing quantify cyclic variations. Maximum brake
power is measured to be 195 W, which corresponds to a power density of
approximately 700 W/kg. Pressure analyses show that pumping and
friction losses are significant. Luminosity data reveals that a secondary
burn can occur on the intake stroke of the engine, and cyclic variability in both pressure and ignition analyses reveal an issue of repeatability that is likely caused by the poor mixing and inconsistent fuel flow into the engine.