LN06 - Internal Combustion Engines ME422 COMBUSTION in SI...

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1 Internal Combustion Engines – ME422 COMBUSTION in SI ENGINES Prof.Dr. Cem Soru ş bay Internal Combustion Engines Combustion in SI Engines ¾ Introduction ¾ Classification of the combustion process ¾ Normal combustion : flame speed, turbulence ¾ Parameters influencing combustion process ¾ Ignition system ¾ Abnormal combustion : knock, surface ignition ¾ Parameters influencing knock ¾ Cyclic variations in combustion
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2 Introduction In a conventional SI engine, fuel and air are mixed together in the intake system , inducted through the intake valve into the cylinder where mixing with residual gas takes place, and then compressed during the compression stroke. Under normal operating conditions, combustion is initiated towards the end of compression stroke at the spark plug by an electric discharge. Following inflammation, a turbulent flame develops, propagates through the premixed air-fuel mixture (and burned gas mixture from the previous cycle) until it reaches combustion chamber walls, then it extinguishes. Cylinder Pressure
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3 Cylinder Pressure Combustion Combustion event must be properly located relative to the TDC to obtain max power or torque. Combined duration of the flame development and propagation process is typically between 30 and 90 CA degrees. If the start of combustion process is progressively advanced before TDC, compression stroke work transfer (from piston to cylinder gases) increases. If the end of combustion process is progressively delayed by retarding the spark timing, peak cylinder pressure occurs later in the expansion stroke and is reduced in magnitude. These changes reduce the expansion stroke work transfer from cylinder gases to the piston. The optimum timing which gives maximum brake torque (called maximum brake torque or MBT timing) occurs when magnitude of these two opposing trends just offset each other.
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4 Combustion Timing which is advanced or retarded from this optimum MBT timing gives lower torque. Optimum spark setting will depend on the rate of flame development and propagation, length of flame travel path across the combustion chamber, and details of the flame termination process after it reaches the wall - these depend on engine design, operating conditions and properties of the fuel-air and burned gas mixture. With optimum spark setting, max pressure occurs at about 15 degrees CA after TDC (10 - 15), half the charge is burned at about 10 degrees CA after TDC. In practice spark is retarded to give a 1 or 2 % reduction in brake torque from max value, to permit a more precise definition of the timing relative to the optimum. Combustion Normal combustion spark-ignited flame moves steadily across the combustion chamber until the charge is fully consumed. Abnormal combustion
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This note was uploaded on 10/04/2011 for the course ME 422 taught by Professor Cemsoruşbay during the Spring '11 term at Yeditepe Üniversitesi.

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LN06 - Internal Combustion Engines ME422 COMBUSTION in SI...

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