Study on Combustion Control in Natural-Gas PCCI Engines with Ozone Addition into Intake Gas

The Premixed Charge Compression Ignition (PCCI) natural-gas engine has been investigated extensively as a power source for stationary applications due to its potential for high thermal efficiency and very low NOx emissions. However, methane, which is a major component of natural gas, has a high auto-ignition temperature. Stable ignition of natural gas in PCCI engines can be achieved by high compression ratio, intake air heating, internal EGR and various other techniques. Although each of the above-mentioned methods shows positive effects, to some extent, on engine performance and emissions, the literature indicates that stable operation of the PCCI natural gas engine would require a combination of various techniques, which reveals the need for further investigation. The goal of the present study is to control the PCCI natural gas ignition and combustion by ozone addition into the intake air. Ozone, which has a strong oxidation characteristic, can be produced onboard using barrier discharge plasma. In this study, the effects of ozone intake addition on performance and emissions of a naturally aspirated single-cylinder PCCI natural gas engine were investigated under different equivalence ratios and amounts of ozone addition. The results indicate that ozone effectively enhances the ignition of natural gas and improves the thermal efficiency and the emissions of THC and CO, especially under low engine loads. Rough estimation indicates that addition of 100ppm of ozone to the intake gas corresponds to a 40°C increase in the intake air temperature. Detailed chemical kinetic study indicates that reaction of ozone with natural gas mixture at the early stage of the compression stroke produces species that play important roles in the natural gas ignition through the promotion of OH formation.