Effects of Air/Fuel Ratios and EGR Rates on HCCI Combustion of n-heptane, a Diesel Type Fuel

The effects of Air/Fuel (A/F) ratios and Exhaust Gas Re-Circulation (EGR) rates on Homogeneous Charge Compression Ignition (HCCI) combustion of n-heptane have been experimentally investigated. The experiments were carried out in a single-cylinder, 4-stroke and variable compression-ratio engine equipped with a port fuel injector. Investigations concentrate on the HCCI combustion of n-heptane at different A/F ratios, EGR rates and their effects on knock limit, engine load, combustion variability, and engine-out emissions such as NOx, CO, and unburned HC. Variations of auto-ignition timings and combustion durations in the two-stage combustion process are analyzed in detail. Results show that HCCI combustion with a diesel type fuel can be implemented at room temperature with a conventional diesel engine compression-ratio. However, its knock limit occurs at very high A/F ratios, although high EGR rates can be tolerated. It was also found that auto-ignition timings or start time of both the low- and high-temperature combustion stages of diesel HCCI combustion are very sensitive to EGR rates. But combustion durations dominantly depend on the A/F ratios. Since the combustion temperature is very low, NOx emissions is at near zero ppm level among all attainable operating regions. However, HC and CO emissions are high and increase with incomplete combustion caused by misfire.