Divided Exhaust Period Implementation in a Light-Duty Turbocharged Dual-Fuel RCCI Engine for Improved Fuel Economy and Aftertreatment Thermal Management: A Simulation Study 2018-01-0256

Although turbocharging can extend the high load limit of low temperature combustion (LTC) strategies such as reactivity controlled compression ignition (RCCI), the low exhaust enthalpy prevalent in these strategies necessitates the use of high exhaust pressures for improving turbocharger efficiency, causing high pumping losses and poor fuel economy. To mitigate these pumping losses, the divided exhaust period (DEP) concept is proposed. In this concept, the exhaust gas is directed to two separate manifolds: the blowdown manifold which is connected to the turbocharger and the scavenging manifold that bypasses the turbocharger. By separately actuating the exhaust valves using variable valve actuation, the exhaust flow is split between two manifolds, thereby reducing the overall engine backpressure and lowering pumping losses. In this paper, results from zero-dimensional and one-dimensional simulations of a multicylinder RCCI light-duty engine equipped with DEP are presented. It is shown that while DEP helped reduce pumping penalty at medium and high loads, the pumping benefit was negated by crankshaft power consumption from a mechanical supercharger which made up for the boost deficit as the low exhaust enthalpy could not be efficiently utilized by a fixed geometry turbocharger (FGT). However, by replacing the FGT with a variable geometry turbocharger (VGT), a 1% improvement in brake-specific fuel consumption (BSFC) over the stock engine configuration was observed at high load, as the VGT allowed more efficient exhaust energy utilization through aspect ratio adjustment. In addition, by closing the blowdown valve at low load, higher exhaust gas temperatures were obtained by bypassing the turbocharger and thereby eliminating exhaust heat losses, which would be useful for aftertreatment thermal management.