Influence of EGR Quality and Unmixedness on the High-Load Limits of HCCI Engines

This work explores how the high-load limits of HCCI are affected by fuel autoignition reactivity, EGR quality/composition, and EGR unmixedness for naturally aspirated conditions. This is done for PRF80 and PRF60. The experiments were conducted in a single-cylinder HCCI research engine (0.98 liters) with a CR = 14 piston installed. By operating at successively higher engine loads, five load-limiting factors were identified for these fuels: 1) Residual-NO_x-induced run-away advancement of the combustion phasing, 2) EGR-NO_x-induced run-away, 3) EGR-NO_x/wall-heating induced run-away 4) EGR-induced oxygen deprivation, and 5) excessive partial-burn occurrence due to EGR unmixedness.

The actual load-limiting factor is dependent on the autoignition reactivity of the fuel, the EGR quality level (where high quality refers to the absence of trace species like NO, HC and CO, i.e. simulated EGR), the level of EGR unmixedness, and the selected pressure-rise rate (PRR). For a reactive fuel like PRF60, large amounts of EGR are required to control the combustion phasing. Therefore, for operation with simulated EGR, the maximum IMEP becomes limited by the available oxygen. When real EGR (with trace species) is used instead of the simulated EGR, the maximum IMEP becomes limited by EGR-NO_x/wall-heating induced run-away. For the moderately reactive PRF80 operated with simulated EGR, the maximum IMEP becomes limited by residual-NO_x-induced run-away. Furthermore, operation with real EGR lowers the maximum steady IMEP because of EGR-NO_x-induced run-away. This is similar to PRF60.

Finally, the data show that EGR/fresh-gas unmixedness can lead to a substantial reduction of the maximum stable IMEP for operation with a low PRR. This happens because the EGR unmixedness causes occasional partial-burn cycles due to excessive combustion-phasing retard for cycles that induct substantially higher-than-average level of EGR gases.