Numerical Investigation of the Effects of Equivalence Ratio and EGR on Combustion Characteristics in an n-Heptane Fueled HCCI Engine

Homogeneous Charge Compression Ignition (HCCI) combustion offers low nitrogen oxides and soot emissions with high thermal efficiency, but direct control of combustion phasing is lacking. In this study, the effects of fuel-air equivalence ratio and exhaust gas recirculation (EGR) on the combustion characteristics of an n-heptane fueled HCCI engine were investigated using a single-zone chemical kinetic model implemented in Chemkin. The model employed the Seiser et al. n-heptane mechanism and was validated against in-cylinder pressure data from an Antor 6LD400 single cylinder engine converted to HCCI operation (maximum pressure error 0.99%, RMSE 0.70 bar, R² 0.998). The equivalence ratio was varied from 0.01 to 0.50, and the EGR rate from 0% to 35% at a fixed equivalence ratio of 0.4. A distinct transition from misfire to stable ignition occurred at equivalence ratios of approximately 0.34–0.36. Stable ignition at ϕ≥0.36 was predicted by all three mechanisms (Seiser et al., LLNL v3.1, and CRECK/POLIMI), whereas the response at ϕ=0.34 was mechanism-dependent. Above the threshold, the maximum in-cylinder temperature reached 1447 to 1919 °C and thermal efficiency 50% to 53%. Increasing the EGR rate from 0% to 35% reduced the maximum temperature from 1674 °C to 1449 °C and IMEP from 7.69 bar to 3.80 bar; at 35% EGR, the lower fuel energy supplied per cycle accounted for 87–92% of this IMEP reduction across the three mechanisms, and the remainder was attributable to a decrease in indicated thermal efficiency. All three mechanisms predicted an advance of combustion phasing with EGR, whereas the low-EGR response was mechanism-dependent. Cycle-end CO did not increase substantially because the predicted temperature remained above the CO oxidation threshold. Equivalence ratio therefore primarily governs the transition between misfire and stable ignition, whereas EGR modifies combustion phasing, temperature, and pressure within the stable-ignition condition.