Performance Evaluation of a Mini I.C. Engine

In this work, a 7.45 cc capacity glow plug based two-stroke engine for mini aircraft applications was evaluated for its performance, emissions and combustion. It uses a fuel containing 65% methanol, 25% castor oil and 10% nitromethane by volume. Since test rigs are not readily available for such small engines, a reaction type test bed with low friction linear and rolling element bearings was developed and used successfully. The propeller of the engine acted as the load and also the flywheel. Pressure time diagrams were recorded using a small piezoelectric pressure transducer. Tests were conducted at two different throttle positions and at various equivalence ratios.

The brake thermal efficiency was generally in the range of 4 to 17.5% depending on the equivalence ratio and throttle position. IMEP was between 2 and 4 bar. It was found that only a part of the castor oil that was supplied participated in the combustion process. The most suitable equivalence ratios based on torque and thermal efficiency were about 1.3 at full throttle and 1.7 at 50% throttle. HC emissions were found to vary from 4000 8000 ppm whereas CO emissions varied from 0.3-6% at full throttle. At very rich or too lean mixtures, considerable misfire and alternative cycle firing was observed. The coefficient of variation in peak pressure reached the lowest value at the most suitable equivalence ratio where the torque was also the highest.