Effect of Pilot Diesel Multiple Injections on the Performance and the Emissions of a Diesel/Natural Gas Dual Fuel Heavy-Duty Engine

For diesel/natural gas dual fuel engines, the combustion of pilot diesel injection plays an important role to subsequent mixture combustion process. To better understand the effects of multiple injections, a detailed study was conducted on a 6-cylinder turbocharged intercooler diesel/natural gas dual fuel heavy-duty engine at low loads. Multiple variables were tested, including the single injection timings, the multiple injections timings and the mass ratios. The investigated results showed that the multiple pilot diesel injections have an obvious effect on not only pilot diesel combustion process but also natural gas mixture combustion process. Early injection leads to a pilot-diesel-ignition-mode and it is a two-stage auto ignition mode. This mode differs from the compression ignition mode of traditional diesel engine in regard to its random occurrence location within the spray. Engine combustion and emissions characteristics, including cylinder pressure, heat release rate, start of combustion (SOC), ignition delay, crank angle of 50% heat release (CA50), nitrogen oxides (NOx) and total hydrocarbon (THC) are examined in this study with completely different variation trends of the multiple injections mode being shown. Overall, a higher thermal efficiency and a lower emission level can be achieved with the multiple injections when compared to the conventional single injection mode.