In the quest for high efficiency IC engine operation, spark assisted compression ignition (SACI) can fill the gap between homogeneous charge compression ignition (HCCI) operation at low load and spark ignited (SI) operation at high load. SACI combustion utilizes a combination of flame propagation and auto-ignition to achieve ignition when unburned temperatures are too low for reliable auto-ignition and the mixture is too dilute for flame propagation with sufficient speed. Stoichiometric SACI combustion with cooled external exhaust gas recirculation (EGR) offers improved thermal efficiency compared to stoichiometric SI operation. It also reduces combustion temperatures and therefore NOx emissions, while still allowing for the use of a three-way catalyst (TWC).
This study investigates NOx spikes that can occur during transitions between different SACI operating points as a result of system time lags or mixture deviation from stoichiometry. Load transitions at various stoichiometric SACI operating points are investigated and NOx emissions before and after the TWC are reported. Significant engine-out NOx spikes are observed. A 1200 ppm NOx spike occurs during a load increase from 3 to 6 bar BMEP at 1800 rpm in 2 cycles (0.13 seconds), which is representative of a faster load change in the FTP-75 drive cycle. Observed NOx spikes are attributed to a time lag in external EGR during the transitions. NOx emissions after the TWC are reduced to below 50 ppm, indicating that NOx emissions during these transients can be handled effectively by a TWC.