Optimisation of Fuel Consumption of High Specific Output Two-Stroke SI Engines 2003-26-0023

Vehicles Research and Development Establishment (VRDE) has developed two-stroke, air-cooled, horizontally-opposed, simultaneously firing, gasoline high specific output twin-cylinder engine developing 21 hp and four-cylinder engine developing 37 hp with modular concept for Remotely Piloted Vehicle (RPV). High specific output requirement entails some problems associated with high RPM potential, reed valve induction, high compression ratio, plated cylinders, cylinder ports, crankcase induction, air-cooling etc.

This paper describes and lists the technology incorporated for optimization of fuel consumption on these engines. The six-petal reed valve induction provided in these engines which works independently on the basis of engine demand not only ensures steady and smooth engine performance throughout the entire range of speed but also prevents back-flow of mixture during crankcase compression. Further, the diaphragm-type carburetors overcome flooding and wastage of fuel and continue to deliver best performance even at engine tilt condition. Also, the ‘horizontally-opposed’ configuration with two cylinders sharing the common crankcase chamber and minimum dead space with-in increases the delivery ratio. The one-piece crank pin cage produces lesser centrifugal forces as a function of speed and hence smaller size for a given force. Further more, the lesser friction heat resulting from a smaller centrifugal force permits lesser oil to fuel ratio. In addition, the loop-scavenged cylinder unit, with its five transfer ports and dual exhaust port is of novel design, by which the engine assures greatly improved scavenging efficiency at all speeds and reduces fuel short-circuiting to the minimum. The flat-top piston reduces heat losses to the piston and has more uniform heat distribution across the head. Maintaining the optimum clearance between the piston and cylinder and selection of common material for the piston and cylinder as well as coating with common material on piston ring and cylinder assists in uniform and reduced rate of wear and hence long life. The ultra-light capacitor-discharge ignition system gives consistent engine performance over longer duration leading to better fuel average. Performance of so designed engines is covered in this paper.