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Spark Assisted Compression Ignition Engine with Stratified Charge Combustion and Ozone Addition
Abstract:
Performance and emissions characteristics for stratified charge spark assisted compression ignition (SACI) with 30 ppm of added ozone (O3) were explored in a single-cylinder, optically accessible, spray-guided, research engine. For the present study, intake pressure and temperature were fixed at 1.0 bar and 42°C respectively, with a range of engine loads (1.5 – 5.5 bar indicated mean effective pressure) and speeds (800 – 1600 revolutions per minute) explored. Fuel stratification achieved by a late-cycle injection of ~ 10–25% of the total fuel was used to maintain stable operation at lower engine loads. For each condition spark timing, second injection SOI, and 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%, with across the board improvement in combustion stability relative to similar conditions without O3. The effect of O3 addition was most substantial 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 up to 30%. From complementary measurements of in-cylinder O3 decomposition acquired via an ultraviolet light absorption diagnostic, it was observed that rapid decomposition of O3 into molecular and atomic oxygen coincided with the onset of end-gas auto-ignition. The burst of resultant atomic oxygen was thought to accelerate low-temperature heat release (LTHR) reactions in the end gas. Optimal end-gas auto-ignition started between 20 and 30 crank angles before top dead center with temperatures at LTHR onset estimated to be between 575 and 700 K. An included analysis indicates that the spark deflagration was needed to add between 10 and 40 J of additional thermal energy to the end gas to achieve optimal auto-ignition.