The increased availability of natural gas (NG) in the United States (US) and its relatively low cost compared to diesel fuel has heightened interest in the conversion of medium duty (MD) and heavy duty (HD) engines to NG fueled combustion systems. The aim for development for these NG engines is to realize fuel cost savings and reduce harmful emissions while maintaining durability. Transforming part of the vehicle fleet to NG is a path to reduce dependence on crude oil.
Traditionally, port-fuel injection (PFI) or premixed NG spark-ignited (SI) combustion systems have been used for MD and HD engines with widespread use in the US and Europe. But this technology exhibits poor cycle efficiency and is load limited due to knock phenomenon. Direct Injection of NG during the compression stroke promises to deliver improved thermal efficiency by avoiding excessive premixing and extending the lean limits which helps to extend the knock limit.
In this work, a single cylinder engine with 14:1 compression ratio (CR) was used to investigate spark ignited direct injection (SIDI) NG combustion using an integrated spark injector-igniter. Two parameters controlling dilution were investigated (i) increased air charge resulting in an increased air-fuel ratio (AFR) and (ii) exhaust gas recirculation (EGR). The impact of these parameters on fuel consumption, combustion stability, phasing, knock, and emissions of NOx, HC, CO is investigated and reported in this work. Partially stratified combustion (PSC) SOI and SPK timings from previous work will be used as a starting point to investigate the effects. Overall, both AFR and EGR were effective in controlling knock and NOx emissions; however, EGR required lower levels to obtain the desired effect. The use of charge dilution in the PSC range showed promise of trading off knock and combustion stability to within acceptable limits, thus demonstrating potential for increased load operation.