Use of Detailed Kinetics and Advanced Chemistry-Solution Techniques in CFD to Investigate Dual-Fuel Engine Concepts

A multi-component fuel model is used to represent gasoline in computational fluid dynamics (CFD) simulations of a dual-fuel engine that combines premixed gasoline injection with diesel direct injection. The simulations employ detailed-kinetics mechanisms for both the gasoline and diesel surrogate fuels, through use of an advanced and efficient chemistry solver. The objective of this work is to elucidate kinetics effects of dual-fuel usage in Reactivity Controlled Compression Ignition (RCCI) combustion. The model is applied to simulate recent experiments on highly efficient RCCI engines. These engine experiments used a dual-fuel RCCI strategy with port-fuel-injection of gasoline and early-cycle, multiple injections of diesel fuel with a conventional diesel injector. The experiments showed that the US 2010 heavy-duty NO

and soot emissions regulations were easily met without aftertreatment, while achieving greater than 50% net indicated thermal efficiency. However, as with other low-temperature combustion strategies, CO and unburnt hydrocarbon emissions must be controlled. Homogeneous charge compression ignition (HCCI) engine experiments were also earlier performed and simulated using a less detailed primary-reference-fuel (PRF) mechanism and single-component surrogates. The present work introduces a more accurate multi-component representation of the gasoline and a more detailed kinetics mechanism for both the gasoline and diesel surrogates. The simulation results show accurate representation of combustion phasing and better predictions of unburned hydrocarbons and CO emissions as an outcome of using the detailed kinetics. The model demonstrates that the most-reactive surrogate component, n-heptane (component in both diesel and gasoline) ignites first, and the other, slower gasoline-surrogate components follow. The model also shows that the slowest gasoline surrogate component, toluene, is found disproportionately in the unburned hydrocarbon (UHC) emissions. Overall, the predicted UHC emissions can be broadly classified into three groups: surrogate-fuel species make up 37 wt%, large hydrocarbon intermediates that form from the surrogate fuels form 42 wt%, and small C₁-C₄ hydrocarbons form 21 wt%. Of the surrogate fuels present, toluene has the largest concentration. Analysis of uncertainties in IVC temperature input on emissions predictions has also been performed. As the combustion phasing is retarded, the UHC and CO emissions increase for the base case and the uncertainties in temperature have a more dominant effect on emissions.