In order to comply with demanding Greenhous Gas (GHG) standards, future automotive engines employ advanced engine technologies including waste heat recovery (WHR) systems. A waste heat recovery system converts part of engine wasted exergies to useful work which can be fed back to the engine. Utilizing this additional output power leads to lower specific fuel consumption and CO2 emission when the total output power equals the original engine output power. Engine calibration strategies for reductions in specific fuel consumption typically results in a natural increase of NOx emissions. The utilization of waste heat recovery systems provides a pathway which gives both reduction in emissions and reduction in specific fuel consumption.
According to DOE (Department of Energy), US heavy-duty truck engines’ technology need to be upgraded towards higher brake thermal efficiencies (BTE). DOE target is BTE>55% for Class-8 heavy-duty vehicles in the United States. On the other side, the emissions legislation is currently under review in California aiming at around 80% reduction in NOx emission to improve air quality according to California Air Resources Board (CARB). The heavy-duty vehicles are the primary emitters of NOx. Reduction of NOx emission to such stringent proposed target demands a very high NOx catalyst efficiency and more investment in exhaust aftertreatment systems. The waste heat recovery system, however, reduces the fuel consumption as well as the engine out NOx emission at the original engine output power. The reason for that is the engine produces the same power with lower fuel energy which affects the engine operating points in engine fuel maps.
This paper will discuss a feasible waste heat recovery system for on-road heavy-duty diesel engine application under relevant boundary conditions. With the help of thermodynamic calculations the incremental power from waste heat recovery system as well as the fuel economy benefit will be calculated and discussed. As main topic, potentials for reduction of NOx emission and the other pollutants by using waste heat recovery system will be presented for a representative engine.