The Effect of Lean Mixture Combustion and Compression Ratio in Turbocharged Gasoline Engine

Turbocharged lean combustion was realized by using a multi-spark ignition device. The turbocharged lean operation in air-fuel ratio of 21:1 got the same torque level as the stoichiometric air-fuel ratio operation in naturally aspirated engine. For the turbocharged operation, NO and CO emissions remarkably reduced about 80 per cent and 85 per cent respectively and unburned HC increased about 30 per cent in condition of air-fuel ratio of 21:1 compared with stoichiometric air-fuel ratio.

When compression ratio was increased, brake power and brake thermal efficiency increased and NO, CO and HC in exhaust emissions also increased.

Combustion stability was estimated by using the coefficient of variation in indicated mean effective pressure. COVimep decreased when compression ratio was increased, but that value exceeded 10 per cent in low speed range. Accordingly, one of the important problems to be resolved in lean combustion is combustion stability in low speed range.