Ozone Added Spark Assisted Compression Ignition

Abstract

The mixed-mode engine combustion strategy where some combination of spark-assisted compression ignition (SACI) and pure advanced compression ignition (ACI) are used at part-load operation with exclusive spark-ignited (SI) combustion used for high power-density conditions has the potential to increase efficiency and decrease pollutant emissions. However, controlling combustion and switching between different modes of mixed-mode operation is inherently challenging. This chapter proposes to use ozone (O3)—a powerful oxidizing chemical agent—to maintain stable and knock-free combustion across the load-speed map. The impact of 0–50 ppm intake seeded O3 on performance, and emissions characteristics was explored in a single-cylinder, optically accessible, research engine operated under lean SACI conditions with two different in-cylinder conditions, (1) partially stratified (double injection—early and late injection) and (2) homogeneous (single early injection). O3 addition promotes end gas auto-ignition by enhancing the gasoline reactivity, which thereby enabled stable auto-ignition with less initial charge heating. Hence O3 addition could stabilize engine combustion relative to similar conditions without O3. The addition of ozone has been found to reduce specific fuel consumption by up to 9%, with an overall improvement in the combustion stability compared to similar conditions without O3. For the lowest loads, the effect of adding O3 was most substantial. Specific NOx emissions also dropped by up to 30% because a higher fraction of the fuel burned was due to auto-ignition of the end gas. Measurement of in-cylinder O3 concentrations using UV light absorption technique showed that rapid decomposition of O3 into molecular (O2) and atomic oxygen (O) concurred with the onset of low-temperature heat release (LTHR). The newly formed O from O3 decomposition initiated fuel hydrogen abstraction reactions responsible for early onset of LTHR. At the beginning of high-temperature heat release (HTHR), end gas temperatures ranged from 840 to 900 K, which is about 200 K cooler than those found in previous studies where intake charge heating or extensive retained residuals were used to preheat the charge. An included analysis indicates that in order to achieve optimal auto-ignition in our engine, the spark deflagration was needed to add 10–40 J of additional thermal energy to the end gas. We have leveraged these results to broaden our understanding of O3 addition to different load-speed conditions that we believe can facilitate multiple modes (SI, ACI, SACI, etc.) of combustion.