Experimental investigations on controlled auto-ignition combustion in a four-stroke gasoline engine
The effects of air and exhaust gas dilution on the CAI combustion of a range of fuels including three gasoline compositions, four primary reference fuels, and two alcohols are experimentally investigated using a single cylinder research engine. Two of the three gasolines tested are manufactured from standard gasoline during engine operation by a novel fuel system, designed to improve the performance of both controlled autoignition and spark ignition engines. A series of experimental tests are performed to establish the satisfactory combined air and exhaust gas dilution regions for each fuel. Detailed in-cylinder pressure and exhaust gas speciation measurements are taken, and the fuels are compared and contrasted for their performance in terms of power output, fuel consumption, and harmful exhaust emissions. Results show that alcohol fuels are superior to hydrocarbon fuels for controlled autoignition combustion because their autoignition characteristics are less affected by the presence of exhaust gas species. Furthermore, the timing of autoignition is shown to be of minor importance for achieving efficient and stable controlled autoignition combustion, contrary to widely held beliefs. In addition, the novel fuel system is developed and commissioned for use on a single cylinder research engine operating with a spark ignition system. The two gasoline fuels produced by the system are evaluated individually for their knocking combustion characteristics over a range of compression ratios and spark advances. Results from these tests indicate that the fuel system used in conjunction with a specially modified production engine may allow the normal compression ratio of that engine to be increased by up to 1.0, increasing its efficiency.