Controlling combustion with negative valve overlap in a gasoline–diesel dual-fuel compression ignition engine

Premixed charge compression ignition technology for diesel engines has been attracting increasing interest in recent years because of its ability to reduce nitrogen oxide emissions. At the same time, homogeneous charge compression ignition technology for gasoline engines also has been expected for high thermal efficiency and reduction in nitrogen oxide emissions. However, both methods have common challenges, such as limited part-load operation and ignition timing control. Additionally, under very low load conditions, thermal efficiency decreases due to the difficulty of providing steady combustion. In this study, a dual-fuel method has been applied to control the ignition timing and combustion duration. As a high ignitability fuel, diesel fuel was supplied to the direct injector, and as an inhibitor, gasoline was supplied to the port injector. Under very low load conditions, negative valve overlap and multiple fuel injection were applied. Diesel fuel injected during the negative valve overlap duration oxidised partially in the high-temperature environment with oxygen-contained exhaust gas of the previous cycle. The oxidation reaction of fuel during the main compression stroke was inhibited by the products from these reactions that occurred during negative valve overlap. Thus, it was possible to control the timing of the heat release immediately after top dead centre. As a consequence, high controllability of combustion and high thermal efficiency at low loads were achieved through independent control of the ratio of diesel fuel injected at negative valve overlap and the fractions of the two fuels.