HCCI Combustion - Engine Operation and Emission Characteristics
Abstract: The potential and limitations of the Homogeneous Charge Compression Ignition (HCCI) engine concept has been experimentally investigated. The operation range, in terms of usable air/fuel ratio and engine load, and the emission characteristics have been studied. An 1.6 litre single cylinder engine based on a heavy duty six cylinder Volvo, was used for the tests. As follows the experimental work was limited to study HCCI combustion at low speed operation in a heavy duty sized engine. 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. As almost the entire charge is involved in the combustion process at the same time, the global heat release rate is primary determined by the local heat release rate. This means that the heat release rate is controlled by the grade of charge dilution. Normally, the entire charge is consumed in 5 – 20 crank angle degrees. Due to its nature, HCCI has the potential to generate very low emissions of nitrogen oxides (NOx). It is possible to operate the engine with almost zero NOx. 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) proved to be two to three times higher compared to spark ignition operation for a given engine load. The main source of unburned hydrocarbons are crevices, primary the piston topland crevice. Wall effects and bulk quenching proved to be less important. If the total crevice volume is minimized and the engine is operated rich enough, say below lambda = 2.5, very low HC levels are possible to obtain. The emission of carbon monoxide (CO) is very dependent on the temperature history during the combustion period. Close to the rich limit and/or with early timing, CO is quite low, lower than for SI operation. HCCI combustion can be considered as smokeless when using high volatility fuels (light fuels like gasoline). With heavier fuels, like diesel fuel, smoke is very sensitive to the mixture preparation. With port injection of diesel fuel very much smoke was generated in some cases. The combination of low charge temperature and port injection of diesel fuel, resulted in smoke levels of 1 – 4 BSN units. 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 and short combustion period. With the present test engine, the best obtained indicated efficiency at part load conditions was about 45 %. The requirement of highly diluted mixtures limits the attainable engine load. With the present test engine the maximum IMEP was around 5 bar at unthrottled operation (naturally aspirated). However, by applying supercharging via an external air compressor, 16 bar of IMEP has been obtained. Besides the limited power density, the lack of direct ignition timing control is maybe the major disadvantage of the HCCI concept. A feedback signal from some sort of combustion sensor is necessary in order to contol the ignition timing via an indirect method. Possible control strategies are through Variable Compression Ratio (VCR), dual fuels or internal EGR. Turbulence and combustion chamber geometry proved to play an important role for HCCI combustion. Higher turbulence resultet in reduced heat release rate.
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