EMISSION-STUDY OF HOMOGENEOUS CHARGE COMPRESSION IGNITION ENGINE

Abstract

The potential and limitations of the Homogeneous Charge Compression Ignition (HCCI) engine concept has been experimentally investigated. The operating range in terms of usable air/fuel ratio and engine load and the emission characteristics have been studied. A single cylinder Kirloskar AV-1 water cooed diesel engine was used for the tests. For reducing the knocking sensitivity it was required to reduce compression ratio to 11:1 from 16:1. The experimental work was limited to the study of HCCI emission at constant speed operation. The HCCI combustion process is very different compared to the Spark Ignition and Compression Ignition (Diesel) combustion processes. HCCI lacks normal flame propagation; instead the entire charge is gradually consumed in a non-flame mode almost at the same time.HCCI has the potential to form very low emissions of nitrogen oxides (NOx). It is possible to operate the engine with almost zero NOx with high rate of EGR. The reason for the low NOx potential is that the HCCI concept has the capability to use ultra lean premixed mixtures, resulting in a low and uniform combustion temperature in the combustion chamber. The emissions of unburned hydrocarbons (HC) was found to be two to three times higher compared to existing diesel engine operation for a given engine load. The main source of unburned hydrocarbons is crevices, primarily the piston top land crevice. The emission of carbon monoxide (CO) is very dependent on the temperature history during the combustion period. CO is quite high compared to existing diesel engine operation. HCCI combustion results in high smoke with heavier fuels, like diesel fuel. Smoke formation is very sensitive to the mixing of fuel and air during compression stroke. Very high smoke was generated at high load. The HCCI engine concept has superior potential for achieving high part load fuel conversion efficiency. This is due to the combination of small pumping losses, high compression ratio as compared to spark ignition engine but in experimental study it was found that the fuel consumption was high as compared to existing diesel engine operation. With the present test engine the best thermal efficiency at part load conditions was about 25%. 91