Exploration the effect of natural gas composition on exhaust emissions of a CNG SI (spark-ignition) engine by extreme N2 substitution

Abstract

Considering the increasingly tougher emission regulatory, applications of compressed natural gas (CNG) fuel in internal combustion engine must be more and more careful because composition of CNG fuel changes by regions and seasons. Composition of CNG fuel has important impact on engine exhaust emissions, many works in literature have been done and a big knowledge base has been built. But most previous studies directly changed the CNG composition during investigation. In this work, we only used CH4 and N2 to simulate the extreme CNG composition range (only the heat value of fuel was changed). Then the gaseous and particle emissions of a CNG engine were comparatively investigated between pure CNG and 13.6% N2 substitution CNG (denoted as N2-CNG) under World Harmonized Transient Cycle (WHTC). Results showed that the corrected fuel consumption (corrected by N2 substitution proportion) of N2-CNG was 258.4 g/kW·h which was slightly lower than that of Pure-CNG (263.8 g/kW·h), under N2-CNG operation, per unit weight fuel generated higher work. So, HC and CO emissions of N2-CNG were lower relative to Pure-CNG. CH4 and CO emission factors were reduced by 32% and 3.1%, respectively, by extreme N2 replacement, while N2-CNG emitted higher NOx relative to Pure-CNG; with three way catalyst (TWC), CH4 was decreased by 88.5%, and other species were reduced by about 99%, the difference of pollutant conversion between two fuels was ignorable. Although particle number emitted by N2-CNG was higher, the weight of PM emission of N2-CNG was lower than that of Pure-CNG, illustrating that the mean diameter of PM emitted from N2-CNG was smaller relative to Pure-CNG. And NOx and PM emissions of CNG engine were lower than by 2-3 orders of magnitude than diesel engine. Then, the transient emissions were comparatively analyzed by combining evolutions of engine condition, exhaust temperature and fuel rate. This study is anticipated to provide a good demonstration for better usage of NG fuels in traditional engines.