Quantitative and qualitative analysis of thermodynamic process of a bi-fuel compressed natural gas spark ignition engine

A comparative energy and exergy analysis was experimentally performed using a bi-fuel compressed natural gas (CNG) spark ignition engine. The experiments were conducted for gasoline and CNG fuel at 1700 rpm and different operating loads from 5 to 30 Nm. The experiments were performed under stoichiometric air-fuel ratio and maximum brake torque ignition timing. Quantitative and qualitative analysis were conducted using the first and second law of thermodynamics, respectively. The effect of engine operating load on various energy and exergy parameters was compared for both the fuels. Output energy with respect to engine load was found higher for CNG compared to gasoline. Engine wall heat transfer was also higher in the CNG engine case due to its high combustion chamber temperature and lower burning velocity. The difference in heat transfer energy fraction between gasoline and CNG gradually increased with engine load. The exhaust energy was found maximum in the case of CNG under low operating load and reduced to a minimum at higher engine operating load. The unaccounted energy fraction was found lower for the CNG engine and reduced with respect to engine load. CNG exhibits the highest exergy efficiency (26.80%) compared to gasoline (25.50%) at 30 Nm load. On average, a 2% higher exergy efficiency was observed with the CNG engine at all operating load conditions. This indicates CNG has a higher potential to convert chemical energy present in the fuel into useful work output. CNG shows lower exergy destruction at all operating loads compared to gasoline, and it reduces significantly at higher engine loads. In all operating loads, exergy transfer due to exhaust gas and heat transfer to the wall was higher in the case of CNG.