Analysis of Intake Charge Temperature and EGR Stratification Effects on HCCI Combustion

Abstract

HCCI combustion is characterized by a near homogeneous composition and temperature mixture of air, fuel and post-combustion products. HCCI related research work around the world defines different strategies for achieving HCCI combustion based on intake charge characteristics, fuel type, and fuel and residual gas mixing methodologies. Engine specific characteristics that defined our HCCI combustion research can be listed as follows: - Fuel injection was always performed upstream of the intake port to ensure that a fully homogeneous air and fuel mixture charge was delivered into the engine. ? Low levels of in-cylinder residual gas were achieved by a late EVC timing and a minimum of NVO for the purposes of mitigating the in-cylinder temperature, air-fuel ratio and diluent stratification effects imposed by the trapped residual gas. ? Externally added EGR helped achieve HCCI combustion within the desired limits of combustion variability and ringing index, and also allowed running the engine fully unthrottled. ? Cylinder head split port and dual intake surge tanks / dual intake runners made possible it to inject two fully independent charges into the engine. ? Fuel type used to run these experiments was isooctane. This fuel was chosen due to its characteristics as a single stage ignition fuel. A very specific intake port and intake system setup has been designed and installed in a single cylinder engine to analyze and characterize the effects of imposed intake charge temperature and composition stratification on combustion and emission metrics. The characteristics of this particular engine intake port and intake system setup allowed us to establish different sorts of charge stratification, such as thermal and composition stratification. The main objective of this research work was to conduct HCCI combustion and emission studies through the isolation of combined effects imposed by the stratified charge. Thermal and composition intake charge stratification effects could be imposed by applying separately diluent or charge composition and thermal stratification effects throughout the intake charge. The list of combined effects within the stratified charge was detailed as follows: local air-fuel ratio effects, thermal charge gradient effects caused by differences in charge specific heat ratios, diluent stratification and fuel number density non uniform spatial distributions. Experimental results have shown that: - Combustion phasing is sensitive to thermal and diluent stratification effects. ? The combustion event was a unique function of Combustion Phasing CA50. - With thermal charge stratification, combustion advances at a fixed intake charge temperature. - With diluent stratification, combined effects take place to modify ignition timing. - Stratified charge effects such as A/F, Non-uniform Spatial Distribution of Mixture Gamma, Fuel and Diluent are all interconnected. Combustion phasing is strongly sensitive to each one of these individual effects.