Improvement of bottoming cycle efficiency and heat rejection for HD truck applications by utilization of EGR and CAC heat

Abstract

Considering continuously rising fuel prices and the global warming problem it is significantly important to reduce fuel consumption of engines used in various applications. Of specific importance is the HD diesel engine used in large haul trucks because these vehicles have an extensive operating schedule, their engines have a high power output in the range of 200–400 kW and their number is significantly high. Considering current achievements, it appears that HDDI diesel engine bsfc cannot be significantly reduced in the future unless new ideas or techniques are employed. Under this framework the utilization of exhaust heat becomes inevitable because approximately 30–40% of fuel energy is rejected to the environment. A promising technique for the recovery of energy from the exhaust gas is the use of a Rankine bottoming cycle. This technical solution has been examined in the past with very positive indications and a strong potential for significant improvement. However various technical challenges have to be solved among which most important are packaging and rejection of excess heat from the engine cooling system. For this reason in the present work a simulation model which has been developed to describe the operation of a Rankine bottoming cycle is utilized to estimate the potential efficiency gain from its application on a heavy duty truck powered by a diesel engine. Using the simulation special attention is given to the utilization of EGR cooler and CA cooler (Charge Air) heat to increase the Rankine expander power output and thus improve bsfc reduction potential. Furthermore the utilization of both EGR and CAC heat amounts is used to minimize the negative impact of the Rankine cycle on the engine cooling system, the capacity of which is exceeded at high load. The last results to installation difficulties (larger engine radiator, etc.) and in some cases a significant amount of generated power is consumed to drive the cooling fan which obviously has a strong negative impact on the bsfc reduction potential. For this reason several scenarios are proposed and examined in the present work to avoid or minimize this problem. Results are produced for both organic and steam working media that reveal a very good potential for the application of Rankine bottoming cycles in HD engine applications. Furthermore it is revealed that the utilization of both EGR cooler and CA cooler heat beyond its positive effect on bsfc reduction potential is also beneficial for overall system packaging allowing the serious reduction of primary heat exchanger dimensions.