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Stratified operation of a gasoline engine is one of the most efficient technologies for fuel economy improvement. This operation requires detailed knowledge and governance of component tolerances (fuel injector and spark plug) in order to ensure robust and smooth engine operation without unacceptable torque fluctuations. The coefficient of variation (COV) is a metric in engine development and calibration for fluctuation of indicated mean effective pressure (IMEP), resp. torque (critical to quality), which means for the customer that the engine is running smoothly (critical to satisfaction). It denotes the relation of standard deviation of IMEP over 300 combustion cycles to the average IMEP over these cycles. COV performance must be below the specified levels, as a function of operating point, which can be translated into limit states at chosen engine speeds.

CAI-combustion was achieved in a 4-cylinder four-stroke gasoline DI engine, with all cylinders running in CAI-mode. Standard components were used, with the exception of the camshafts which had been modified in order to restrict the gas exchange process. Results shown in the paper are between a load of 1.45 - 2.65 bar, an engine speed of 1500rpm and at a lambda value of 1.2. As is typical with this type of combustion, reductions in emissions of NOx were recorded as well as a slight decrease in HC emissions, also there was a reduction in the brake specific fuel consumption. The effect that injection timing on factors such as start of combustion, combustion duration and heat release rate are also investigated.

Regarding concepts for naturally aspirated engines, the high potential for fuel economy of Gasoline Direct Injection can only partially be utilized within the constraints of current or future emission legislation like EURO III / IV or LEV/ULEV. Instead of an expected improvement of 20 - 25 % currently only 10 - 15% can be obtained by the engine alone without vehicle optimizations considering all limitations of high volume production. A detailed analysis reveals concrete measures for further improvement. The application of DI gasoline technology clearly favors the combination with other fuel efficient technologies like downsizing by turbocharging and the application of a variable effective compression ratio by intake valve timing variation. Using the flexibility of direct gasoline injection some deficiencies of these technologies can be eliminated.