Characteristics of Smokeless Low Temperature Diesel Combustion in Various Fuel-Air Mixing and Expansion of Operating Load Range

The characteristics of smokeless low temperature diesel combustion in various fuel-air mixing was investigated by engine tests with high rates of cooled exhaust gas recirculation (EGR), three compression ratios, and fuels of various cetane numbers, as well as by computational fluid dynamics (CFD) simulation of the in-cylinder distributions of mixture concentration and temperature. The results show that besides combustion temperature, fuel-air mixing is also vital to efficient, smokeless, and low NOx diesel combustion. Smokeless and low NOx diesel combustion can be realized even with insufficient fuel-air mixing as long as the combustion temperature is sufficiently low. However low combustion temperature and insufficient oxygen due to ultra-high EGR cause very high UHC and CO emissions, and a severe deterioration in combustion efficiency. While smoke is influenced by combustion temperature, it depends strongly upon the premixing time from the end of fuel injection to the onset of ignition. With the premixing time longer than a critical value, smokeless combustion is realized regardless of intake oxygen and combustion temperature. The UHC and CO emissions show a stronger dependence upon the intake oxygen than the premixing time. Therefore, to achieve efficient, smokeless, and low NOx combustion at higher loads, promotion of fuel-air mixing at relatively higher intake oxygen concentration is necessary. A strategy of using low cetane number fuels and low compression ratios is proposed to expand the operating load range of smokeless low temperature combustion.