Application of a New Combustion Concept to Direct Injection Gasoline Engine

Abstract

A direct injection (DI) gasoline engine having a new stratified charge combustion system has been developed. This new combustion process (NCP) was achieved by a fan-shaped fuel spray and a combustion chamber with a shell-shaped cavity in the piston. Compared with the current Toyota D-4 engine, wider engine operating area with stratified combustion and higher output performance were obtained without a swirl control valve (SCV) and a helical port. This report presents the results of combustion analyses to optimize fuel spray characteristics and piston cavity shapes. Two factors were found to be important for achieving stable stratified combustion. The first is to create a ball-shaped uniform mixture cloud in the vicinity of the spark plug. The optimum ball-shaped mixture cloud is produced with a fuel spray having early breakup characteristics and uniform distribution, and a suitable side wall shape in the piston cavity to avoid the dispersion of the mixture. The second factor is to reduce the over-lean area in the piston cavity. A compact shell-shaped cavity was designed for this purpose. The resulting flame propagation improves combustion stability. The effect of tumble motion was also investigated. The best combustion characteristics were obtained at weak tumble motion condition. Measurement results of incylinder flow by LDV showed that the fan-shaped fuel spray produces sufficient turbulence strength without extra intake air flow systems. The application of the NCP to an actual 3L in-line 6 DI gasoline engine showed that in addition to satisfying Japanese regulations which start in from 2000, a fuel economy gain of more than 20% was obtained. INTRODUCTION Increasing the fuel economy of automotive engines is very important for saving energy and improving the global environment. DI gasoline engines which are able to achieve both better fuel economy and output power simultaneously, have been considered for many years as a possible solution for these problems [1-3]. MITSUBISHI was the first to introduce gasoline DI engines in the market in1996, and Toyota launched the D-4 engine later in the same year [4]. The main features of the D-4 stratified charge combustion system are: 1. Piston with involute shaped concave combustion chamber; 2. High pressure swirl fuel injector; 3. Intake system with a helical port and a swirl control valve (SCV). This combustion system achieves a wide range of stratified combustion, but the output power is partially deteriorated by the helical port at high speed conditions. We have been conducting research and development of a new combustion system jointly with Toyota Central R&D Labs,Inc. to improve the previous system. This joint R&D resulted in a new combustion process (NCP), which realizes both wider range of stratified combustion and excellent homogeneous combustion without an extra intake air flow controlling system. This paper discusses the required characteristics of the fuel spray and piston cavity shapes that are the main elements of the NCP, and also presents the performance characteristics of an actual engine in which the NCP was utilized.