Numerical Study on Controllability of Natural Gas and Diesel Dual Fuel Combustion in a Heavy-Duty Engine 2017-01-0756
Natural gas is a promising alternative fuel for internal combustion engines due to its rich reserves and low price, as well as good physical and chemical properties. Its low carbon structure and high octane number are beneficial for CO2 reduction and knock mitigation, respectively. Diesel and natural gas dual fuel combustion is a viable pathway to utilize natural gas in diesel engines. To achieve high efficiency and low emission combustion in a practical diesel engine over a wide range of operating conditions, understanding the performance responses to engine system parameter variations is needed. The controllability of two combustion strategies, diesel pilot ignition (DPI) and single injection reactivity controlled compression ignition (RCCI), were evaluated using the multi-dimension CFD simulation in this paper. DPI is a strategy that premixed natural gas is ignited by a direct injection of diesel fuel near the compression top dead center, and RCCI employs earlier diesel injection to customize a desired in-cylinder fuel reactivity gradient to achieve a sequential, staged combustion. This study indicates RCCI generally has higher fuel efficiency and lower HC, CO and NOx emissions relative to DPI combustion. Its combustion phasing and fuel efficiency can be significantly influenced by the fuel and air system parameters including injection timing, PES, temperature and pressure at IVC and EGR. The highly sensitive responses to these parameters also suggest that RCCI should be carefully controlled to avoid misfire or undesired operation. While these parameters have a smaller effect on the combustion phasing and fuel efficiency of DPI except injection timing when compared to RCCI. For real applications, injection timing and PES should be considered the better parameters to control combustion on a cycle to cycle basic for both RCCI and DPI. A major challenge for DPI combustion is balancing the trade-off between NOx and HC.