Isolating the Effects of Fuel Chemistry on Combustion Phasing in an HCCI Engine and the Potential of Fuel Stratification for Ignition Control 2004-01-0557

An investigation has been conducted to determine the relative magnitude of the various factors that cause changes in combustion phasing (or required intake temperature) with changes in fueling rate in HCCI engines. These factors include: fuel autoignition chemistry and thermodynamic properties (referred to as fuel chemistry), combustion duration, wall temperatures, residuals, and heat/cooling during induction. Based on the insight gained from these results, the potential of fuel stratification to control combustion phasing was also investigated.

The experiments were conducted in a single-cylinder HCCI engine at 1200 rpm using a GDI-type fuel injector. Engine operation was altered in a series of steps to suppress each of the factors affecting combustion phasing with changes in fueling rate, leaving only the effect of fuel chemistry. This involved the use of two novel techniques: 1) alternate-firing operation to remove changes in wall temperature and residuals; and 2) a method for determining the effective intake temperature to remove the effect of heating/cooling during induction.

Three fuels were examined. Iso-octane was found to have only a small change in autoignition chemistry with fueling rate; gasoline had a change just slightly larger than iso-octane; and PRF80 had a large change, due to its significant cool-flame chemistry. Comparison of the data with chemical-kinetic modeling showed that the detailed iso-octane mechanism matches the trends well, but that the detailed PRF mechanism does not. The experimental results indicate that engine management becomes more complicated for fuels with cool-flame chemistry. For PRF80, combustion phasing changes immediately with changes in fueling, whereas sudden changes in fueling have little effect on the combustion phasing for iso-octane or gasoline. However, the results also show that the potential for ignition control by fuel stratification is much larger for PRF80. Stratification significantly and rapidly shifts combustion phasing with PRF80, but not with iso-octane. Charge stratification was also found to be effective for improving combustion efficiency at low-load conditions.