Variable crank length for variable compression ratio (vcr) engine

摘要

Energy economy and pollution factor highly influence the global development. An engine is a device which converts the chemical energy of fuel into thermal energy and into mechanical energy. About 35% of total chemical energy is converted to useful crankshaft work, and remaining fuel energy is carried away by exhaust and coolant. In a conventional reciprocating engine operates at constant compression ratio and fuel consumption increases in higher engine speed. It was found that, the efficiency increases by increasing compression ratio (CR), and is limited by knocking and thermal stress of combustion chamber of an engine. On other hand, efficiency decreases with increasing speed of the engine. In view of this, a device is considered which can change compression ratio with respect to engine speed. The centrifugal force on crank revolution is utilized to change the crank length and corresponding change in the compression ratio of the engine. Thus more speed of the engine provides increased compression ratio and corresponding more efficiency. The maximum and minimum compression ratios are achieved automatically according to engine speed change. The CR is controlled by expansion spring fitted sliding block in the slotted crank, which is coupled with big end of connecting rod. Thus variable engine speed and corresponding variable ratio (VCR) promise enhanced thermal efficiency and low fuel consumption. It was theoretically analyzed through computer simulation that, 3 mm variation of crank length provides around 7% increment in thermal efficiency and 8% of lower brake specific fuel consumption for 13 CR at 6000 engine revolution (RPM).