A simplified analysis of the processes occurring in a spark-ignition internal combustion engine is provided by the air-standard Otto cycle, as presented in Chapter 8. The purpose of this is to develop a more realistic model which considers combustion and heat transfer processes.
a) Calculate and plot the thermal efficiency and mean effective pressure for an air standard Otto cycle for compression ratios between 6 and 12 with a maximum cycle temperature of 3,000 K. Assume that the state of the air before compression is 25◦C, 1 atm.
b) The working fluid entering the engine is not air, but rather a mixture of propane (C3H8) and stoichiometric air. The maximum cycle temperature is the temperature that results during the adiabatic constant volume combustion process. Compute the maximum cycle temperature and plot the engine thermal efficiency (based on the lower heating value of the fuel) and the mean effective pressure as a function of the compression ratio for compression ratios between 6 and 12 using the same assumptions that are employed in the air-standard Otto cycle, i.e., isentropic compression and expansion with combustion occurring at constant volume.
c) Some of the energy released in the combustion process is transferred to the ‘cold’ engine walls which are maintained at Twall = 105◦C. Assume that the amount of heat transfer to the engine walls during the combustion process is given by:
where K = 628 kJ/K-kmol propane and T¯ is the average temperature occurring during the combustion process, e.g, (T2+T3)/2. Calculate and plot the thermal efficiency and mean effective pressure as a function of the compression ratio including this heat transfer consideration.