Impact of Engine Design on Vehicle Heating System Performance Gary D. Mandrusiak and Alex C. Alkidas General Motors Corp. COPYRIGHT 1 79 7 SOCIETY OF AUTOMOTiVE ENGINEERS, INC. ABSTRACT A global thermal model of a vehicle powertrain is used to quantify how different engine design and powertrain calibration strategies influence the performance of a vehicle heating system. Each strategy is evaluated on its ability to improve the warm-up and heat rejection characteristics of a small-displacement, spark-ignition engine while minimizing any adverse effect on fuel consumption or emissions. An energy audit analysis shows that the two strategies having the greatest impact on heating system performance are advancing the spark and forcing the transmission to operate in a lower gear. Changes in head mass, exhaust port diameter, and coolant flow rate influence the coolant warm-up rate but have relatively little effect on steady state heat transfer at the heater core. INTRODUCTION The performance of a vehicle's heating system is influenced by the cold weather warm-up and heat rejection characteristics of the engine. Under steady- state, moderate-load, conditions, an engine typically rejects about 30% of its input fuel energy to the cooling system. The vehicle heating system uses some of this energy to provide for thermal comfort in the passenger compartment in cold weather. During powertrain warm- up, however, most of the energy destined for the cooling system is absorbed by the engine structure. Any heat transferred to coolant usually goes into increasing its bulk temperature, leaving little to service the needs of the heating system. The rate at which excess energy becomes available for passenger compartment heating is largely determined by the geometry and operating characteristics of the engine. The connection between engine thermal performance and the effectiveness of a vehicle heating system is bounded by two extremes. Small engines installed in sub-compact vehicles, for example, usually warm-up quickly but don't reject much heat, especially under light load. In these cases, heating system performance is constrained by the amount of thermal energy transferred from the engine to the coolant. At the other extreme, larger engines typical of mid- and full-sized vehicles generate more heat but often take longer to warm up because of their higher thermal capacitances. Heating system performance in these vehicles is controlled by the time required for the coolant to attain the correct operating temperature. Design changes introduced to improve heating system performance at either extreme may hurt other powertrain attributes like emissions and fuel consumption. Engineers must understand the factors that influence engine thermal performance to design heating systems that work within the constraints set by the rest of the powertrain. Interest in learning more about how geometry and calibration influence engine thermal performance has prompted researchers to develop models of engine warm-up and heat rejection [I -91. Most of these models have focused on spark-ignition engine and exhaust system warm-up [I-51 and diesel engine thermal control [6-81, while only a few have examined powertrain thermal performance from a total systems perspective [9]. Each of these studies has provided insight into the complex flow and heat transfer phenomena that occur in a vehicle powertrain. The present study uses a thermal model similar in character to those discussed in [I-91 to examine how heating system performance is influenced by engine design and powertrain calibration. The analysis focuses specifically on the small-engine extreme of heating system performance in which the engine heat rejection rate, and not its warm-up rate, limits its effectiveness as a heat source. Each engine design is evaluated on its ability to increase heat transfer at the heater core Downloaded from SAE I