Analysis and Design of a Lightweight High Specific Power Two Stroke Polygon Engine

Abstract

Current trends in engine design have pushed the state of the art regarding high powertoweight ratio gasoline engines. Newly developed engine systems have a powertoweight ratio near 1 hp per pound. The engine configuration presented herein makes it possible to package a large number of power producing pistons in a small volume, resulting in a powertoweight ratio near 2 hp per pound, which has never before been realized in a production engine. The analysis and design of a lightweight twostroke 6sided inplane polygon engine having a geometric compression ratio of 15.0, an actual compression ratio of 8.8, and a piston speed of 3500 ft/min are presented in this investigation. Typical results show that for a hexagonal engine with 2 in. diameter pistons and 1.25 in. stroke, a single piston displacement is 7.85 cubic in., while the total engine displacement is 47. 1 cubic in. Full power at 12,960 rpm at an air flow rate of 353 cubic feet per minute affords 0.444 cubic ft/min/hp for specific power. For an efficiency of 21%, the blower power is 168 hp. Our airflow analysis shows that the power of the engine does not depend on the number of pistons, but rather on the volume of the gasair mixture which passes through the engine. System level engineering of power output, kinematic modeling, airflow modeling, efficiency, scavenging predictions, crankshaft sizing, and weight estimates are presented.