Abstract
Keywords - direct injection, stratified charge, variable valve timing, EGR, lean NOx catalytic converter
Abstract - The authors developed an ultra lean direct- injection stratified charge engine employing a center-injection system in which a high-pressure fuel injector is located in the center of the combustion chamber and the fuel spray is vertically injected into the cylinder toward the piston bowl. Stratification is controlled by the fuel spray characteristics and the piston bowl shape which are calibrated using CFD simulation and in-cylinder ana lysis. A VTEC mechanism is employed to control the burn rate with in-cylinder swirl gas motion, which is generated by one- intake-valve deactivation. Optimization of the mixture preparation and combustion through calibrations of the piston bowl, the fuel spray characteristics of the high-pressure injector and the in-cylinder gas motion enabled stable combustion with an ultra lean air-fuel ratio of 65. As a result, this engine has significantly improved fuel economy and emissions. In addition, higher full load performance is attained by applying i-VTEC mechanism, which consists of VTEC as a cam shifter for four- valve operation and a cam-phaser to obtain higher volumetric efficiency. In order to meet the rather stringent exhaust emission category in Japan, the authors apply the ultra lean combustion system to mass EGR during lean operation, so that the engine-out NOx emission is reduced by 90%. Furthermore, a newly developed lean NOx catalytic converter system is adopted and a conversion efficiency over 95% is obtained. The integration of the combustion control technology and a highly efficient lean NOx catalytic converter system enables the vehicle with a lean burn direct injection engine to become the first one to achieve the emissions category of 25% of the level stipulated by the Japanese 2000 emissions regulations and obtain ULEV certification.
The new engine is installed on a mini van for the 2004 Japanese market model and has enabled the vehicle to achieve fuel economy of 15.0 km/L in the Japanese 10-15 driving cycles, which is at the top of its class. On the other hand, a driving pleasure is realized as well because of a full load performance with a rated power of 115kW at 6300rpm and maximum torque of 188Nm at 4600rpm.