Throttling a Spark Ignition Engine A modern naturally-aspirated spark ignition engine has an indicated fuel conversion efficiency of ni = 38% and a volumetric efficiency of η-100% over its operating range. At 2000RPM, the mechanical friction MEPmf is 90kPa and the accessory and friction MEPaf is 20kPa. Both can be assumed to be independent of load. The engine operates at stoichiometric, A/F = 14.7, at all intake manifold pressures. The exhaust pressure is pe-1 atm and the intake temperature is Ti 300K, but the intake pressure, pi, depends on throttle position a) Write down and expression for the brake fuel conversion efficiency as a function of MEPmf, MEPaf, Pe. Pi, the fuel heating value, OHr47MJ/kg, the fuel air ratio F/A and the air density upstream and downstream of the throttle
b) What is the brake thermal efficiency of the engine for intake manifold pressures of latm and 0.75atm?
c) Assuming the mechanical and accessory friction values remain the same at idle, determine the degree of throttling that is required to run the engine properly. In other words, what is the necessary intake manifold pressure at idle?