11 Normal vs Abnormal Combustion in SI engine

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Normal vs Abnormal Combustion in SI engine In a conventional SI engine, the fuel is completely vaporized and homogeneously premixed with air and residual gas, when combustion occurs. Under normal operating conditions, the mixture is ignited by the spark plug in a optimized moment of the cycle (usually 20-40° before TDC). The combustion is normal when 2 conditions are respected: 1) the ignition of combustible mixture is controlled by the spark plug, with a present timing 2) after the ignition, the flame propagates regularly to the whole mixture, without any sudden increase of velocity. Usually 3 combustion phases are distinguished: 1) Initial flame development, during which the small volume of mixture, ignited by the spark plug, is gradually transformed into a developed front of turbulent flame. This phase start with the spark ignition and ends when the pressure inside the cylinder clearly increase because of combustion. 2) Turbulent flame propagation The turbulent flame rapidly spreads over the main part of the combustion chamber, whose volume remains almost constant, since in this phase the piston moves slowly around TDC. This is the main phase characterized by a rapid burning. It begins when 4-8% of the mixture volume has been ignited and it ends when the flame front comes near the walls and the max pressure in the cylinder is reached. 3) Burnout, during which the mixture completes its oxidation processes behind the flame front, when the expansion stroke is occurring. Engine speed The engine speed varies but the engine is regulated such a way the peak of pressure is almost independent from n and happen between TDC and 20° ATDC. This is the optimal synchronization of pressure evolution inside the cylinder in order to obtain a max torque. The In-cylinder turbulence…