Improvements of Combustion and Emissions in a Natural Gas Fueled Engine with Hydrogen Enrichment and Optimized Injection Timings of the Diesel Fuel

In a natural gas fueled engine ignited by diesel fuel, the addition of hydrogen to the engine could be a possible way to improve thermal efficiency and reduce unburned methane which has a warming potential many times that of carbon dioxide as it promotes a more rapid and complete combustion. This study carried out engine experiments using a single cylinder engine with natural gas and hydrogen delivered separately into the intake pipe, and with pilot-injection of diesel fuel. The percentages of hydrogen in the natural gas-hydrogen mixtures were varied from 0% to 50% of the heat value. The results showed that the hydrogen addition has an insignificant effect on the ignition delay of the diesel fuel and that it shortens the combustion duration. The increase in the hydrogen ratio decreased the unburned hydrocarbon emissions more than the reduction of the amount of natural gas that was replaced by the hydrogen. Further, the direct injection timing of the diesel fuel was varied from early in the compression stroke to near top dead center to determine the optimum combinations of the hydrogen ratio and direct injection timing. The results showed that an early direct injection timing and higher hydrogen ratio are advantageous to achieve low NOx emissions and high thermal efficiency under low and medium load conditions as the early injection enhances lean mixture formation, and the higher hydrogen ratio reduces unburned emissions and improves the degree of constant volume heat release.