Effect of boosting on a performance and emissions in a port fuel injection natural gas engine with variable intake and exhaust valve timing

Recently, many studies have been conducted on the conversion of conventional fossil fuels to alternative fuels. Natural gas is one of the best alternative fuels because it is a low-carbon fuel. In natural gas engines, port fuel injection is generally adopted rather than direct injection because of the compatibility of the injection system in a positive ignition engine. However, methane, of which the global warming potential is 28 times than that of carbon dioxide, is highly likely to emit during the valve overlap in port fuel injection engines. The methane slip phenomenon could be reduced by changing the intake and exhaust valve timing. However, few studies have discussed engine performance and emission characteristics through valve timing changes in natural gas engines. Therefore, in this study, experiments were conducted under the valve timing variations with/without boosting in a natural gas engine. Through experiments, it was possible to investigate the trend of engine performance and methane emission according to the change in the intake and exhaust valve timing. As a result, under the boosted condition, the intake valve timing could only be advanced up to 45 CAD bTDC and the exhaust valve timing could only be retarded by 27.4 CAD bBDC. This is because the methane slip phenomenon increased and it resulted in a decrease of engine torque when the intake valve timing was excessively advanced or the exhaust valve timing was inordinately retarded under the boosted condition. In addition, considering a level of methane emission equivalent to EURO-6, the valve timings were optimized for each engine speed point of 1000–2500 rpm under boosted high load conditions. It was found that the trend of intake valve timing of the natural gas engine was unlike that of the conventional gasoline engine under the low engine speed of less than 2500 rpm