MINIATURIZATION LIMITS OF SMALL IC ENGINES

Considerable effort has been devoted in recent years to building miniature heat engines as battery replacements and prime-movers for micro-air vehicle propulsion. Simple thermodynamic analyses show that as a heat engine is miniaturized, it becomes less thermodynamically efficient. A critical length scale exists at which losses outstrip power production and cycle efficiency goes to zero. The objectives of this research are to identify a minimum practical length scale for two-stroke piston engines and investigate the processes responsible for setting this limit. The performance of 7 engines weighing 15g to 500g is studied using a specially designed dynamometer. Peak power outputs have been measured at 8-278W with peak efficiencies ranging from 3-9%. A scaling analysis similar to one established for conventional scale engines shows the minimum displacement for a 'practical' IC engine to be between 0.5 and 1 cc.