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In order to study the potentiality of the modification of the combustion rate for NOx formation control in the spark ignition (SI) engine, the authors first developed a new mathematical model by assuming the stepped gas temperature gradient in the cylinder. The predicted results from this new mathematical model show good coincidence with the experimental data. Second, the authors discuss the effects of the modification of the combustion rate on NOx formation using the new mathematical model. It was concluded that NOx formation in the premixed SI engine would be essentially determined by the specific fuel consumption only, regardless of any modification of the engine combustion rate.

The potentiality of direct injection (DI) diesel engines for passenger cars has been examined by comparing the characteristics of fuel consumption, exhaust emissions and noize levels between a turbocharged DI diesel engine and a turbocharged IDI diesel engine with the same displacement, 4 cylinders and 2 liters. It was observed that improved fuel consumption was obtained as the engine load increased, namely, 10 - 15% in the higher load range and 5 - 10% in the partial load range. In comparison to the IDI engine, the exhaust emissions of the DI engine tended to contain two or three times higher NOx and HC, and also about 30% higher particulates. Further, the noise levels of the DI engine were approximately 2 - 4 dB (A) higher than those of the IDI engine.