Effect of Mixture Stratification and Fuel Reactivity on Dual-Fuel Compression Ignition Combustion Process for SI-Based Engine 2016-01-2304

Compression ignition combustion with a lean mixture has high potential in terms of high theoretical thermal efficiency and low NOx emission characteristics due to low combustion temperatures. In particular, a Dual-Fuel concept is proposed to achieve high ignition timing controllability and an extended operation range. This concept controls ignition timing by adjusting the fraction of two fuels with different ignition characteristics. However, a rapid combustion process after initial ignition cannot be avoided due to the homogenous nature of the fuel mixture, because the combustion process depends entirely on the high reaction rate of thermal ignition.

In this study, the effect of mixture stratification in the cylinder on the combustion process after ignition based on the Dual-Fuel concept was investigated. Port injection of one fuel creates the homogeneous mixture, while direct injection of the other fuel prepares a stratified mixture in the cylinder at the compression stroke. The difference in equivalence ratio of the stratified mixture and fuel properties of each specific fuel results in sequential ignition. Thus, the heat release rate is lower; in other words, the combustion duration is extended. Combinations of n-heptane, iso-octane and ethanol were applied for the engine experiment. Applying ethanol to direct injection and applying n-heptane to port injection is the optimum combination among these fuels from the perspective of combustion duration prolongation. The stratified charge of ethanol extended the operation range under a higher load with high combustion efficiency.

According to the simulation results, at the timing of immediately before ignition, stratification of these fuel properties forms an uneven temperature distribution. Namely, low temperature oxidation by n-heptane forms a higher temperature region, while the inhibiting effect of ethanol forms a lower temperature region in the cylinder. Therefore, ignition occurs from the area of high temperature to the area of low temperature. In summary, the combustion duration as well as ignition timing is controlled under broad operating load conditions by adjusting the fuel fraction and injection timing of directly injected fuel.