Internal Combustion Engines, Final Examination Solution

2.61 Internal Combustion Engine Final examination solution, May 2017 Problem 1: ethanol cooling effect Problem 2 soluiton i 10(fi, i J. Q( fl j ~ ~f:tJ5;,.O(H . Note that since for the same torque output, if the engine efficiency does not change much, the amount of fuel used is approximately the same. Then the losses (a), (b) and (c) go down with the engine size B. Problem 3 solution Piston crevice knock (i) (ii) Detonation is more severe in the piston crevice gas is because the gas there is denser (due to the lower crevice gas temperature) than the combustion chamber gas. Hence the energy density is significantly higher (by the temperature ratio). The detonation of the crevice gas is fast; hence, the release of energy may be considered as instantaneous at constant volume. For ideal gas, applying the first law Problem 4: Split cycle engine Advantages: i. The splitting of the intake/compression and expansion/exhaust strokes to be performed by two independent cylinders enables one to have a higher expansion ratio  than compression ratio. The high expansion ratio gives higher fuel conversion efficiency. Note that the bore of the expansion cylinder could be larger than that of the compression one; hence larger  can be made large without going to longer stroke. This feature makes the packaging of the engine easier (do not have to have a tall engine). ii. Lower compression ratio (at the same expansion ratio) allows lower NOx emissions. iii. Lower compression ratio also prevents knocking. iv. The transfer of the charge into the expansion cylinder is just before ignition. Hence the turbulence generated by the fluid motion has little time to decay. The high turbulence promotes combustion speed. Disadvantages v. Substantial pumping loss in the valves and crossover passage in the charge transfer process. The loss is especially severe at high speed and high load. vi. Ditto for heat transfer loss. vii. The air intake is determined by the sweep volume of the intake/compression cylinder. Have to turbo-charge to increase power. viii. The valve stem seal for the two transfer valves must sustain the compression pressure and temperature. The tradeoff between leakage and friction is difficult. ix. Because ignition is in the expansion cylinder, ignition timing is essentially at or after TDC. Since the heat release is not instantaneous, the later combustion partly negates the effect of (ii) and efficiency suffers. x. If the fuel is injected in the crossover passage, there is not sufficient time for evaporation and mixing. So the charge will be inhomogeneous. There will be soot formation in the locally fuel rich region.