Combustion Simulation of Dual Fuel CNG Engine Using Direct Injection of Natural Gas and Diesel

The increased availability of natural gas (NG) in the U.S. has renewed interest in the application to heavy-duty (HD) diesel engines in order to realize fuel cost savings and reduce pollutant emissions, while increasing fuel economy. Reactivity controlled compression ignition (RCCI) combustion employs two fuels with a large difference in auto-ignition properties to generate a spatial gradient of fuel-air mixtures and reactivity. Typically, a high octane fuel is premixed by means of port-injection, followed by direct injection of a high cetane fuel late in the compression stroke. Previous work by the authors has shown that NG and diesel RCCI offers improved fuel efficiency and lower oxides of nitrogen (NOx) and soot emissions when compared to conventional diesel diffusion combustion. The work concluded that NG and diesel RCCI engines are load limited by high rates of pressure rise (RoPR) (>15 bar/deg) and high peak cylinder pressure (PCP) (>200 bar). A high degree of premixing has been found by several researchers to cause excessively high rates of pressure rise thus limiting load. The dual fuel engine proposed in this work employed direct injection of natural gas (DI-NG) (modeled as methane), as the main fuel, during the compression stroke in addition to early and late injections of small quantities of diesel fuel (modeled as n-heptane) to provide the ignition source. The DI-NG concept creates enhanced stratification of the NG fuel portion and avoids excessive premixing, which tempers the RoPR, thus enabling higher load operation. A computational study was performed to examine the trade-offs of fuel consumption, PCP, and peak RoPR, with engine emissions. Several parameters were studied including: relative azimuthal angle between NG and diesel fuel nozzles, diesel pilot injection timing and quantity splits as well as injection timing sweeps. The results from the study indicated that DI-NG was successful in controlling the RoPR to below 10 bar/ deg and PCP to less than 180 bar, while improving the NOx, HC and soot emissions to meet engine out targets for engines equipped with modern aftertreatment systems.