Browse Publications Technical Papers 2019-24-0119
2019-09-09

Oxy-Fuel HCCI Combustion in a CFR Engine with Carbon Dioxide as a Thermal Buffer 2019-24-0119

A basic formula inspired by the Otto engine cycle shows that the thermodynamic efficiency of the engine increases as the mixture specific heat ratio and compression ratio increase. Homogenous charge compression ignition (HCCI) engines allow the combustion of a lean mixture at relatively higher compression ratios increasing the thermodynamic efficiency. At the same time, it is also a low temperature combustion and this means lower NOx emissions. One way to increase the thermodynamic efficiency of the engine is to increase the specific heat ratio by replacing the nitrogen in the oxidizer by a monoatomic gas that has the highest possible specific heat ratio. However, higher specific heat ratio results in elevated cylinder temperature and pressure leading to engine knock. The compression ratio is thus decreased to avoid this phenomenon. Also, elevated engine temperature will oxidize any nitrogen (no matter how small) forming undesired NOx emissions. A previous study by the authors of this paper showed that the elevated temperature oxidized the small nitrogen impurities found in the oxygen and argon cylinders. This paper aims to study the effect of replacing nitrogen in the oxidizer by carbon dioxide in terms of the engine performance and exhaust emissions. Carbon dioxide is a triatomic molecule and this means that it has more degrees of freedom and lower specific heat ratio which is 1.28. It should be noted that the ideal thermodynamic efficiency neglects heat transfer losses and this means that decreasing the specific heat ratio does not always deteriorate the thermodynamic efficiency. In fact, the lower specific heat ratio is expected to decrease the in-cylinder temperature and this means lower heat transfer losses and also eliminating any NOx coming from the nitrogen impurities. The experiments are performed on a standard Waukesha variable compression ratio CFR engine modified to run in HCCI mode. Emissions are measured by an AVL SESAM-i60 FTIR spectrometer.

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