HOMOGENEOUS CHARGE COMPRESSION IGNITION COMBUSTION CONTROL BY CNG DIRECT INJECTION

Homogenous Charge Compression Ignition (HCCI) is a combustion process that emits very low nitrogen oxides and has high thermal efficiency. It is one of the few solutions on hand that looks very promising to address the issues on the atmospheric air pollution and depleting fossil fuel resources exacerbated by increasing energy consumption of the world. However, currently there is no established means for HCCI combustion control and it has high HC and CO emissions. In this project, CNG direct injection was proposed as a tool for HCCI combustion control. Proportion of gasoline and CNG flow rates and degree of stratification of CNG were identified as potential parameters for HCCI combustion control. Role of CNG direct injection on HCCI combustion control and corresponding effects on performance and emission characteristics were experimentally investigated.
The studies were carried out on a single cylinder, CNG direct injection (CNG DI) research engine. A gasoline fuel injection system and an intake air heater were fitted to the engine to operate the engine with dual fuels and in HCCI mode. Compression ignition combustion of homogeneously premixed charge of gasoline was achieved by heating the intake air and CNG was directly injected. CNG stratification was achieved by direct injection on a specially designed piston with a groove on its crown. The degree of stratification was varied by changing the start of CNG injection. Early injection (300° BTDC) created homogeneous mixtures and stratified mixtures were obtained by retarding the injection timing. High degrees of stratification were obtained by injecting at 80° and 120° BTDC, that is, after the closure of intake valves (132° BTDC). To study the effect of fuel proportions, CNG injection rate was varied at constant equivalence ratio of gasoline (φg) at 0.20 to 0.26. The effects of CNG stratification were studied by changing the injection timing form 300° to 80° BTDC and all experiments were repeated at different engine speeds ranging from 1200 to 2100 rpm.

It was observed that heat released by gasoline HCCI combustion resulted in the subsequent combustion of CNG and the engine load could be increased by varying the CNG injection rate. Proportions of gasoline and CNG and degree of stratification of CNG were found to be effective means of combustion control within certain limits of engine load and HC and CO emissions could be significantly reduced.