Abstract
Experiments and a CFD analysis have shown that the charge mixture in a premixed charge compression (PCCI) engine with direct in-cylinder injection early in the compression stroke is still heterogeneous even at the compression end. Although improvements in homogeneity do not always result in low emissions, elimination of regions having a near stoichiometric mixture ratio, where NO_X formation is significant, is essential to realize ultra low emissions. Fuel volatility has a strong influence on the mixture formation process in a direct injection PCCI engine. Direct injection of a low volatility fuel, such as diesel fuel, early in the compression stroke results in adhesion of unevaporated fuel on the cylinder liner wall. This undesirable condition results when the fuel distillation temperature is higher than the temperatures of the in-cylinder gas and cylinder liner wall. Fuel adhering to the wall is also drawn into the crankcase where it dilutes the lubricant oil. If the quantity of fuel wall-wetting becomes large, lubricating oil dilution becomes a serious problem in practical applications. It may be possible to improve both mixture formation and homogeneity, and decrease wall-wetting by using higher volatility fuels with distillation temperatures lower than the in-cylinder gas temperature early in the compression stroke. This research addressed the potential for improvements in early direct injection type PCCI combustion with a higher volatility fuel, experimentally and computationally. The combustion and emissions in a PCCI engine with a higher volatility fuel and with ordinary diesel fuel were compared to establish the effect of distillation temperature, and the mixture formation processes of the early stage injection were analyzed with a CFD model. A normal heptane + isooctane blended fuel with ignitability similar to diesel fuel in PCCI operation was used as the higher volatility fuel. The quantity of fuel lost to lubricant oil under several fuel injection conditions was measured for the normal heptane + isooctane blend and for the ordinary diesel fuel by determining fuel components in the used lubricant oil. The experimental results showed that the deterioration in thermal efficiency that occurs with advanced injection timings with ordinary diesel fuel could be eliminated with the higher volatility fuel without significantly altering the THC and CO emissions. With early injection timings, the rate of heat release with diesel fuel is smaller than with higher volatility fuels. This result suggests that with diesel fuel there is significant fuel adhesion to the cylinder liner wall and also absorption into the lubricating oil. The CFD analysis showed that the normal heptane + isooctane blend fuel improves the homogeneity of the mixture, probably due to the higher volatility. The analysis of components in the used lubricant oil showed that a remarkable quantity of diesel fuel, corresponding to 7% of the total energy supply, is lost to the lubricating oil when the fuel spray is allowed to impinge directly on the cylinder liner wall. The higher volatility fuel effectively reduces fuel adhesion on the cylinder liner wall and prevents fuel absorption into the lubricating oil even with early injection timings when the fuel is allowed to impinge directly onto the cylinder liner.
