Influence of Fuel Autoignition Reactivity on the High-Load Limits of HCCI Engines

This work explores the high-load limits of HCCI for naturally aspirated operation. This is done for three fuels with various autoignition reactivity: iso-octane, PRF80, and PRF60. The experiments were conducted in a single-cylinder HCCI research engine (0.98 liter displacement), mostly with a CR = 14 piston installed, but with some tests at CR = 18. Five load-limiting factors were identified: 1) NO_x-induced combustion-phasing run-away, 2) wall-heating-induced run-away, 3) EGR-induced oxygen deprivation, 4) wandering unsteady combustion, and 5) excessive exhaust NO_x.

These experiments at 1200 rpm show that the actual load-limiting factor is dependent on the autoignition reactivity of the fuel, the selected CA50, and in some cases, the tolerable level of NO_x emissions. For iso-octane, which has the highest resistance to autoignition of the fuels tested, the NO_x emissions become unacceptable at IMEP_g = 473 kPa. This happens before wandering and unsteady combustion becomes an issue for IMEP_g > 486 kPa. The NO_x is caused by high peak-combustion temperatures resulting from the high intake temperature required for this low-reactivity fuel. Iso-octane operation with a CR = 18 piston reduces the intake-temperature requirement. Consequently, the exhaust NO_x issue vanishes while the IMEP_g can be increased to 520 kPa before wall-heating-induced run-away become an issue. For a very reactive fuel like PRF60, large amounts of EGR are required to control the combustion phasing. Therefore, the maximum IMEP_g becomes limited at 643 kPa by the available oxygen as the EGR gases displace air. A fuel of intermediate reactivity, PRF80, exhibits the highest IMEP_g for the conditions of this study - 651 kPa. For this fuel, the maximum IMEP_g becomes limited by NO_x-induced run-away. This happens because even small amounts of NO_x recycled via residuals enhance the autoignition sufficiently to advance the ignition point. This leads to higher peak-combustion temperatures and more NO_x formation, thus making a very rapid run-away situation inevitable.