Detailed Investigation into the Effect of Ozone Addition on Spark Assisted Compression Ignition Engine Performance and Emissions Characteristics 2019-01-0966
The aggressive move of modern spark-ignited (SI) gasoline engines towards challenging lean/dilute combustion regimes necessitate new and robust combustion strategies. Intake addition of the oxidizing chemical agent, ozone (O3), has previously been found to stabilize engine combustion and reduce intake heating requirements for advanced compression ignition strategies. For the present study, the impact of O3 addition on performance and emissions characteristics for spark assisted compression ignition (SACI) were explored in a single-cylinder, optically accessible, research engine for a range of engine loads (1.5 – 5.5 bar indicated mean effective pressure) and speeds (800 – 1600 revolutions per minute). Intake pressure and temperature were fixed at 1.0 bar and 42° C respectively, while intake O3 concentrations were varied from 0 – 40 ppm. Each cycle featured an early main with a fixed -230° CA start of injection (SOI), and a much smaller second injection just after the time of spark. For each condition a) O3 concentration, b) spark timing, c) second injection SOI, d) fuel split ratio between the main and second injection were optimized to maximize engine performance while maintaining nitrogen oxide emissions (NOx) below 5 g/kg-fuel. Ozone addition was found to decrease specific fuel consumption by up to 9% and across the board improvement in combustion stability relative to similar conditions without O3, with the most substantial improvement observed for the lowest loads. Moreover, because a higher fraction of the fuel burned was due to end-gas auto-ignition, specific NOx emissions likewise decreased by 20%. From complementary measurements of in-cylinder O3 decomposition acquired via an ultraviolet light absorption diagnostic, rapid decomposition of O3 into molecular and atomic oxygen coincide with the onset of end-gas auto-ignition. End-gas temperatures at the onset of auto-ignition are estimated to be around 680 K; roughly 400 K cooler than previous studies that use intake heating or retained residuals from negative valve overlap.