A modeling framework for second law analysis of low-temperature combustion engines

Abstract

Low-temperature combustion has shown the potential to provide solutions for future clean and efficient powertrain systems. Traditional approaches using the first law of thermodynamics have been established for describing energy flows within engine systems and comparing losses between low-temperature and traditional combustion modes. An augmented approach, using the second law of thermodynamics, can be utilized to gain insight into the exergy flows within the system and thus identify areas of irreversibilities and inefficiencies. The present article aims at introducing the required framework for the second law analysis of low-temperature combustion concepts and demonstrating its application to boosted homogeneous charge compression ignition engines. The framework consists of a combination of the first law and second law expressions combined with the University of Michigan homogeneous charge compression ignition combustion model and was applied on a modeled light-duty four-cylinder boosted homogeneous charge compression ignition engine. It was found that combustion irreversibilities in homogeneous charge compression ignition were ∼25% more than traditional spark ignition or diesel engines and increased with dilution. On the other hand, exergy transfer to the walls was reduced with low-temperature combustion. It was also found that the combination of negative valve overlap and turbocharging is not beneficial for high-load operation. Exergy analysis of the exhaust system revealed that turbine useful work was lower than 50% of exergy at the exhaust ports and indicated that boosting performance may be improved by manipulating exergy transfer in the exhaust manifold. Results from the present study are focused on boosted homogeneous charge compression ignition, but the conclusions reached are applicable to other low-temperature combustion concepts as well.