Combustion and Emission Characteristics of an LNG Engine for Heat Pumps

A gas-engine heat pump (GHP) is driven by an internal combustion engine fueled by natural gas. The objective of this research is to model the combustion and emission characteristics of a GHP engine and experimentally validate the model. In addition, the exhaust gas recirculation (EGR) and intake boost systems were numerically investigated to find an NO and NO2 (NOx) reduction strategy. The ignition and combustion processes were modeled using the level set method and the DPIK model, and experimental data was used for validation. The models predicted engine performance and emissions for various equivalence ratios and spark timing. The study results show that late spark timing leads to reductions of the indicated specific fuel consumption (ISFC) and NOx emission, and the ISFC and NOx emissions increase with the equivalence ratio until the equivalence ratio reaches 0.9. The indicated mean effective pressure (IMEP) increases with boost pressure and decreases with increase of EGR rate. Reductions of NOx emission and high IMEP can be achieved by employing a boost system and operating at the optimal operating point, from the performance and emission map.