Wissink, M. and Reitz, R., "Exploring the Role of Reactivity Gradients in Direct Dual Fuel Stratification," SAE Int. J. Engines 9(2):1036-1048, 2016, doi:10.4271/2016-01-0774.
Low-temperature combustion (LTC) strategies have been an active area of research due to their ability to achieve high thermal efficiency while avoiding the formation of NOx and particulate matter. One of the largest challenges with LTC is the relative lack of authority over the heat release rate profile, which, depending on the particular injection strategy, either limits the maximum attainable load, or creates a tradeoff between noise and efficiency at high load conditions. We have shown previously that control over heat release can be dramatically improved through a combination of reactivity stratification in the premixed charge and a diffusion-limited injection that occurs after the conclusion of the low-temperature heat release, in a strategy called direct dual fuel stratification (DDFS). This paper will focus on the role the of the reactivity gradients in the premixed charge, which are achieved by the relatively early injection of gasoline and the relatively late injection of diesel. Three regimes were identified for the diesel injection timing: premixed, reactivity-controlled, and diffusion-limited, with the reactivity-controlled regime being observed to offer superior control of combustion phasing and noise, while also having the 30% lower unburned hydrocarbons, 40% lower CO, 35% lower soot, and 4% higher gross efficiency than the premixed regime without any increase in NOx. It was also observed that increasing the energy fraction of the diesel fuel while in the reactivity-controlled regime resulted in further improvements to efficiency and hydrocarbon and CO emissions, while at the same time decreasing peak heat release rate and noise. This was achieved by simultaneously advancing combustion phasing and increasing combustion duration, which is a method of control unique to the DDFS strategy.