Isolating the Effects of EGR on HCCI Heat-Release Rates and NO
High-load HCCI operation is typically limited by rapid pressure-rise rates (PRR) and engine knock caused by an overly rapid heat-release rate (HRR). Exhaust gas recirculation (EGR) is commonly used in HCCI engines, and it is often stated in the literature that charge dilution with EGR (or high levels of retained residuals) is beneficial for reducing the PRR to allow higher loads without knock. However, EGR/retained-residuals affect other operating parameters such as combustion phasing, which can in turn influence the PRR independently from any effect of the EGR gases themselves. Because of the multiple effects of EGR, its direct benefit for reducing the PRR is not well understood.
In this work, the effects of EGR on the PRR were isolated by controlling the combustion phasing independently from the EGR addition by adjusting the intake temperature. The experiments were conducted using gasoline as the fuel at a 1200 rpm operating condition. EGR levels were increased from zero up to the amount required to replace all excess air with combustion products (i.e. stoichiometric intake) at a relatively high-load for HCCI (corresponding to ϕ = 0.42 for no EGR). As the amount of EGR was increased, a systematic increase in fueling rate was required to maintain a constant IMEPg. This occurred because the thermal efficiency decreased with EGR addition due to the lower specific-heat ratio (γ) of the EGR gases. Despite the increased fueling, a small reduction in maximum PRR was observed. This allowed the IMEPg to be increased from 450 to 475 kPa for the same maximum PRR, albeit with a ~2% loss in thermal efficiency. The effectiveness of EGR for extending the high-load limit was also examined. Additionally, contrary to intuition, peak combustion temperatures actually increased with EGR because of the increased fueling required to maintain IMEPg. This resulted in a significant increase in NOX emissions, although NOX was still very low compared to spark-ignition or diesel engines.