Influence of Engine Speed on HCCI Combustion Characteristics using Dual-Stage Autoignition Fuels 2009-01-1107
Homogeneous Charge Compression Ignition (HCCI) combustion characteristics of dual-stage autoignition fuels were examined over the speed range of 600 to 1700 rpm using a Cooperative Fuels Research (CFR) engine. A fuel vaporizer was used to preheat and partially vaporize the fuel inside the intake plenum. The air and fuel were well-mixed prior to entering the cylinder.
Since low temperature heat release (LTHR) is known to be an important factor that affects HCCI combustion of fuels that exhibit dual-stage autoignition behavior, a detailed heat release analyses were performed on both time and crank angle bases. At the lower and upper speeds, the operating ranges were compared as a function of air/fuel ratio (AFR) and exhaust gas recirculation (EGR) from the knocking to misfiring limits.
The AFR-EGR operating region was more limited at 1700 rpm than at 900 rpm for the commercial ULSD fuel. Combustion stability was problematic at higher engine speeds. Reduced LTHR at higher engine speeds was one source of the combustion instability. LTHR timing was sensitive to compression temperature and the LTHR magnitude was sensitive to available time for completion of LTHR reactions. Combustion timing (CA50) was advanced on a time basis, but it was retarded on a crank angle basis.
For a range of diesel-like fuels including commercial ultra low sulphur diesel and n-heptane, two methods of combustion timing control, namely intake heating and intake pressure boosting, were examined over the engine speed range. It was found that significant variations in intake heating or pressure boosting were required to control the combustion timing. While cetane number was not correlated with heating requirements in this preliminary study, the higher cetane number fuels required less intake pressure boosting for combustion timing control.