Effects of Cetane Number on Jet Fuel Combustion in a Heavy-Duty Compression Ignition Engine at High Load 2011-01-0335
The effects of jet fuel properties on compression ignition engine operation were investigated under high-load conditions for jet fuels with varying cetane number. A single-cylinder oil-test engine (SCOTE) with 2.44 L displacement was used to test a baseline #2 diesel fuel with a cetane number of 43, a Jet-A fuel with a cetane number of 47, and two mixtures of Jet-A and a Fishcer-Tropsch JP-8 with cetane numbers of 36 and 42, respectively. The engine was operated under high-load conditions corresponding to traditional diesel combustion, using a single injection of fuel near TDC. The fuels were tested using two different intake camshafts with closing times of -143 and -85 CAD BTDC. Injection timing sweeps were performed over a range of injection timings near TDC for each camshaft. The apparent net heat release rate (AHRR) data showed an increase in the premixed burn magnitude as cetane number decreased in agreement with previous work. The results indicated very little change in the mixing-controlled section of the AHRR curve. Ignition delay for the fuels showed small differences which correlate well with the increased premixed burn. Pressure rise rate data also showed a corresponding increase with decreasing fuel cetane number. Engine-out emissions of NOx, CO, HCs, and soot were comparable for all of the fuels with the exception of soot for the baseline #2 diesel sample. The diesel fuel showed higher soot emissions relative to the jet fuels likely due to its lower volatility and higher aromatic content. However, the other diesel emissions were comparable to those for the jet fuel samples. Overall the data show that, for the range of high-load diesel conditions tested, fuel volatility does not have a substantial influence on engine performance in heavy-duty compression ignition engines. Instead the data suggest that the amount of air entrainment into the fuel jet up to the point of ignition controls the amount of energy converted in the premixed burn and the resulting pressure rise rate. Therefore, for the conditions tested, for fuels with different volatility but with the same air entrainment and cetane number the ignition delay, the fraction of energy converted in the premixed burn, and the pressure rise rate should be similar.