Investigating Potential Efficiency Improvement for Light-Duty Transportation Applications Through Simulation of an Organic Rankine Cycle for Waste-Heat Recovery

Modern diesel engines used in light-duty transportation applications have peak brake thermal efficiencies in the range of 40–42% for high-load operation with substantially lower efficiencies at realistic road-load conditions. Thermodynamic energy and exergy analysis reveals that the largest losses from these engines are due to heat loss and combustion irreversibility. Substantial improvement in overall engine efficiency requires reducing or recovering these losses. Unfortunately, much of the heat transfer either occurs at relatively low temperatures resulting in large entropy generation (such as in the air-charge cooler), is transferred to low-exergy flow streams (such as the oil and engine coolant), or is radiated or convected directly to the environment. While there are significant opportunities for recovery from the exhaust and EGR cooler for heavy-duty applications, the potential benefits of such a strategy for light-duty diesel applications are unknown due to transient operation, the low thermal quality of exhaust gases at typical driving conditions, and the added mass of the system. Waste-heat recovery efforts will directly compete with NO_x aftertreatment systems for the limited thermal energy in the exhaust during low-load operation. We have developed an organic Rankine cycle model using GT-Suite^® to investigate the potential for efficiency improvement through waste-heat recovery from the exhaust and EGR cooler of a light-duty diesel engine. Results from steady-state and drive-cycle simulations are presented, and we discuss the operational difficulties associated with transient drive cycles and competition between waste-heat recovery systems, turbochargers, aftertreatment devices, and other systems for the limited thermal resources at typical driving conditions.