Optimization and Evaluation of a Low Temperature Waste Heat Recovery System for a Heavy Duty Engine over a Transient Cycle

Powertrain efficiency is a critical factor in lowering fuel consumption and reducing the emission of greenhouse gases for an internal combustion engine. One method to increase the powertrain efficiency is to recover some of the wasted heat from the engine using a waste heat recovery system e.g. an organic Rankine cycle. Most waste heat recovery systems in use today for combustion engines use the waste heat from the exhaust gases due to the high temperatures and hence, high energy quality. However, the coolant represents a major source of waste heat in the engine that is mostly overlooked due to its lower temperature. This paper studies the potential of using elevated coolant temperatures in internal combustion engines to improve the viability of low temperature waste heat recovery. The paper first uses engine experiments and multi-linear regression analysis to model the indicated efficiency and recoverable power for a Scania D13 heavy duty engine across a range of engine loads, speeds and coolant temperatures. The recoverable power is obtained from simulations of a dual loop waste heat recovery system using ten working fluids as potential candidates for recovering heat from the exhaust gases and the coolant. The paper then investigates the maximum potential fuel consumption benefit by using elevated coolant temperatures for the Scania engine running on the World Harmonized Transient Cycle. From the simulation results, it was seen that cyclopentane and methanol were the best performing working fluids for the coolant and exhaust gas heat sources respectively. From the analysis on the World Harmonized Transient cycle, when using the best performing working fluids and elevated coolant temperatures, a potential net reduction in fuel consumption of 9% could be obtained.