Detailed Investigation into the Effect of Ozone Addition on Spark Assisted Compression Ignition Engine Performance and Emissions Characteristics

The impact of 50 ppm intake seeding of ozone (O₃) on performance and emissions characteristics was explored in a single-cylinder research engine operated under lean spark assisted compression ignition (SACI) conditions. Optical access into the engine enabled complementary crank angle resolved measurements of in-cylinder O₃ concentration via ultraviolet (UV) light absorption. Experiments were performed at moderate loads (4 - 5 bar indicated mean effective pressure) and low-to-moderate engine speeds (800 - 1400 revolutions per minute). Each operating condition featured a single early main injection and maximum brake torque spark timing. Intake pressure was fixed at 1.0 bar, while intake temperatures were varied between 42 - 80 °C. Moderate amounts of internal residuals (12 - 20%) were retained through the use of positive valve overlap.

Ozone addition was to found stabilize combustion relative to similar conditions without O₃ addition by promoting end gas auto-ignition. Ozone addition was most beneficial for the lowest engine speeds due to the longer available time per cycle for chemically controlled cool flame behavior to occur. Moreover, the homogeneous mixtures and low flame temperatures led to specific NOx emissions of less than 1 g/kg-fuel. From complementary measurements of in-cylinder O₃ decomposition acquired via UV light absorption, rapid decomposition of O₃ into molecular and atomic oxygen coincided with the onset of low-temperature heat release (LTHR). For a given intake temperature and engine speed, the appearance of LTHR was relatively invariant to spark timing and instead was more sensitive to the time at which O₃ decomposition occurred. End gas temperatures at the onset of high-temperature heat release were between 840 and 900 K, which are roughly 200 K cooler than those found in previous studies where intake heating or extensive retained residuals were used to pre-heat the charge. These results demonstrate that O₃ addition increased the charge reactivity of gasoline, and thereby enabled SACI operation for a broader range of conditions.