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Increased thermal efficiency for spark ignition engines is studied experimentally using computer simulation of a three phase heat release model. Various compression ratios, combined with advanced multiple-direct-injection techniques, are researched in an effort to extend the effectiveness of current engine designs beyond their present knock-limited compression ratio. A new ideal engine cycle is presented to demonstrate the conditions appropriate for use of 88-98 octane fuel and spark ignition with an 18:1 compression ratio. The value of isothermal combustion to achieve over 42% brake thermal efficiency is presented.

A computer program (PISTON) has been developed for evaluating air and exhaust flow parameters within the cylinder and the exhaust manifold of single or multi-cylinder reciprocating engines. The PISTON Code has been experimentally verified to accurately determine such 1) air and exhaust flows (including volumetric efficiency, trapped mass, and blowthrough), 2) piston pumping horsepower, and 3) cylinder and exhaust stack pressures, temperatures, and horsepowers (both instantaneous and average), A turbocompounded system, including a blowdown exhaust stack designed using the PISTON Code, increased the engine BHP output of the Army’s 525 CID VHO diesel engine by over 25%. This program serves as a useful tool for designing reciprocating engines for optimum performance, i.e. increased airflow, reduced pumping horsepower, and/or increased exhaust energy utilization.

A turbocharger, incorporating bleed flow from the compressor to the turbine, was developed towards the objective of improving the lug performance of the Army's VHO diesel engine, without the complexity of variable geometry diffuser vanes. An existing VHO engine turbocharger was redesigned and modified for bleed flow testing. The bench and engine test results indicated that the configuration bleeding from both the diffuser hub and the diffuser vane suction surface gave over 75% improvement in baseline engine lug torque. In addition, the bleed flow turbocharger gave improved engine BSFC, with a measured value of less than .39 lb/bhp hr. The lug torque improvements were obtained while maintaining rated engine power. The results of this program indicate that the bleed flow turbocharger can give significant improvements in lug engine performance without the complexity of variable geometry diffuser vanes.