Need for the studies on ozone formation has been raised because the ozone formation, which is harmful to the environment by forming the photochemical smong, has been highly increased. Ozone is one of the photochemical oxidants made by the reaction of HC and NOx emissions under sunlight. Recently, many researches on the non-regulated emissions including ozone have been carried out widely as well as regulated emissions (HC, NOx, CO, PM) from the internal combustion engines.
To analyze the characteristics of ozone formation, individual concentrations of HC emission were measured under various engine design and operating conditions, using the natural gal and LPG fuel in a 2-liter 4-cylinder engine. The main fuel species of natural gas was methane while one of LPG was propane. The emission were characterized as a function of compression ratio(8.6 and 10.6) and intake swirl intensity as the engine design parameters, excess air ratio(λ=1.00~1.6), bemp(250~800kPa), ignition timing(BYDC 1-~55) and EGR ratio(0~7%) as the operating parameters. Fuel conversion efficiencies and cycle to cycle variation were also investigated together with emissions to study the of engine on the combustion performance in gas engines especially under the lean burn conditions. In addition, relationships between HC, NOx emissions and ozone formation were investigated under these various conditions.
Hc emissions from LPG engine consisted primarily of propane (greater than 60%), ethylene and propylene, while those from natural gas engine were methane (greater than 60%), ethane, ethylene and propane. Higher compression ratio, leaner mixture condition and advanced ignition timing resulted in the increase of the ozone formation by the increase of NMHC emissions, though it decreased the SR values by higher fuel components. The change of swirl intensity leads to no serious effects on ozone formation.
BSRs showed almost same values under high bmep, over 500kPa for both fuels. This means that the increase of NMHC emissions and the decrease of SRs with higher bmep affect each other simultaneously. The induction of EGR showed the increase of ozone formation by the rise of NMHC emissions and SRs though the amount of variation was very small. Natural gas was shown to have less effect on the ozone formation than LPG. This was accomplished by reducing the emissions of propylene ($C_3H_6$), which has relatively high MIR factor, and propane ($C_3H_8$) that has large portion of LPG. In addition, natural gas shows a benefit in the other emission (i.e. NMHC, NOx and $CO_2$), SR and BSR values except fuel conversion efficiency. It was also found that $CO_2$ emission decreased with smaller C value of fuel, leaner mixture strength, higher compression ratio, higher bmep and the ignition near the MBT spark timing. And the ozone formation was most affected by the variation of excess air ratio, among engine operating and design parameters.
