Mixing-Controlled Compression Ignition With Exhaust Rebreathe on a Heavy-Duty Engine – A CFD Modelling Investigation Comparing Diesel Fuel and Ethanol

Abstract

Abstract As the demand for transportation energy increases, particularly in the commercial and heavy-duty sectors, the pollutant emissions requirements in this energy sector become further regulated. Battery electric powertrains have become a desirable replacement for the internal combustion engines (ICE) with its zero tailpipe emissions but poses practical problems when considering high-power density, long operating period applications like Class-8 trucks and heavy-duty off-road vehicles. A promising approach to reduce emissions for these applications is through alternative, low carbon renewable fuels. Ethanol is an attractive low carbon renewable fuel, especially in spark ignition (SI) engine applications due to its high-octane number. However, heavy-duty vehicles have operating cycles that are challenging for SI engines and thus mixing-controlled compression ignition (MCCI) operation is the preferred approach. To use a low carbon renewable fuel, like ethanol, in MCCI engines, an ignition assistance source must be implemented. This work investigates the use of an exhaust rebreathe strategy as an ignition assistance source to ignite ethanol in a MCCI engine by trapping hot exhaust residuals. In this concept, the exhaust valve reopens during the intake stroke to recirculate exhaust products from the previous cycle into the cylinder to heat the intake charge. The elevated trapped cylinder gas temperature aids in the auto-ignitability of low reactive fuels like ethanol. The amount of internal exhaust gas recirculation (i-EGR) directly affects the amount of heat induced into the cylinder during the intake stroke. To evaluate the ignition assistance potential of this concept, single cylinder computational fluid dynamics (CFD) multiple-cycle simulations were performed on a heavy-duty diesel engine. The baseline engine is a Caterpillar C9.3B and the simulations were performed using CONVERGE v3.0. Various exhaust pressures were simulated to quantify the amount of i-EGR needed to ignite ethanol and achieve similar combustion characteristics to diesel fuel, at a light load operating condition where ignition assistance is needed the most. Furthermore, multiple-cycle CFD simulations of the stock Caterpillar C9.3B camshaft without exhaust rebreathe were simulated with various intake air temperatures, as a different method to achieve ignition assistance. Comparisons are made regarding combustion characteristics and performance of diesel and ethanol fuels for the exhaust rebreathe strategy and the elevated intake temperature strategy. The simulation results demonstrate that at a highspeed, low-load condition an intake air temperature of 150°C, resulting in an IVC temperature (TIVC) of ∼440 K, was required to achieve ignition with pure ethanol fuel (E100). An intake air temperature of 225°C resulting in an TIVC of ∼480 K was required to achieve the same ignition delay as diesel fuel. Using exhaust rebreathe with an intake air temperature of 35°C, ignition with E100 was achieved with ∼30% i-EGR, resulting in an IVC temperature of ∼450K. The ignition delay however was ∼5 crank angle degrees later than with diesel fuel.