Exploring the application of oxy-fuel combustion to methanol spark ignition engines

Methanol is a rational candidate for long-term energy storage in renewable energy systems, and efficient energy conversion as well as carbon cycling are the keys to its successful application. The use of oxy-fuel combustion in methanol engine is promising to achieve high thermal efficiency and carbon recovery. Large proportion of carbon dioxide (CO2) dilution is usually used in oxy-fuel combustion to accommodate the thermal load, but it also brings challenges in thermal efficiency improvement. In this paper, the effects of CO2 dilution on the performance of methanol spark ignition (SI) engines are investigated using one-dimensional thermodynamics simulations and chemical kinetics calculations. It is demonstrated that the net indicated thermal efficiency (ITEnet) decreases from 40.5% to 31% under 0.16 oxygen fraction compared to the traditional air operation. Increasing the oxygen fraction can alleviate the temperature drop in the cylinder due to the high specific heat capacity of CO2 meanwhile it also reduces the exhaust energy. Based on thermodynamics process analysis of internal combustion engine(ICE), the energy taken away by the exhaust gas accounts for about 55% and the recovery of exhaust gas energy is crucial to improving the engine thermal efficiency. Therefore, reforming methanol with waste heat and high compression ratio (CR) are adopted. Under the constraint of 150 bar peak combustion pressure, the maximum net indicated thermal efficiency with reforming (ITEnet,r) of the oxy-fuel methanol engine is 38.6%, which is only 1.9% lower than that of the baseline condition but allows cost-effective CO2 capture since the exhaust only consists of CO2 and water (H2O).