Improving NOx and Fuel Economy for Mixture Injected SI Engine with EGR

A large quantity of recirculated exhaust gas is used to reduce NOx emissions and improve fuel economy at the same time. The effect of exhaust gas recirculation (EGR) was investigated under the stoichiometric and lean operating conditions and compared with the effect of lean operation without EGR. A mixture injected SI engine that has a mechanically driven mixture injection valve installed was prepared. In this engine, it is possible to charge combustible mixture independently from combustion air and recirculated exhaust gas introduced from intake port in order to stratify the mixture.

The effect of the EGR ratio on NOx emissions and fuel consumption was measured under the stoichiometric and lean operating conditions. Due to the mixture distribution controlled by the mixture injection, a large quantity of recirculated exhaust gas could be introduced into the combustion chamber under the stoichiometric air/fuel ratio. The limit of EGR ratio was 48 %. The capability of introducing a large quantity of recirculated exhaust gas resulted in much lowered NOx emissions and reduced fuel consumption. Fuel economy was most improved under lean operating condition (A/ F=20) with EGR, and NOx emissions were further reduced under stoichiometric operating conditions by using a large quantity of the recirculated exhaust gas. Combustion characteristics of the mixture injected SI engine using EGR under stoichiometric operating conditions was compared with that of a conventional lean burn engine without EGR. The improving fuel economy is almost same level for a conventional lean burn engine and NOx emissions level were, more further, reduced by a three-way catalyst.