04 Combustion in spark ignition Otto engine

Page 1

Turbulent Combustion The motion of the charge in the engine cylinder is always turbulent, when it is reached by the flame front. The charge motion is usually composed by large vortexes, whose length scales are limited in size by system boundaries, that distort the flame front, and by small eddies, whose scale length is smaller than the thickness of the flame front, that intensify the processes of energy and mass transfer in the reaction zone, enlarging the front thickness. When the flame front approaches the relatively cold walls, quenching occurs, that leaves a thin (<0,1 mm) layer of unburned mixture. Sometimes the flame could even be extinguished due to a very low combustion rate, leaving a finite volume of charge only partially burned. During the expansion stroke the cylinder pressure decreases and the elements of unburned mixture come out of the small volumes and are desorbed from deposits and oil film. Then, during the exhaust stroke, the piston scrapes the quenching layer from the cylinder walls and mixes it up with the cylinder charge. Part of the hydrocarbons which escaped the primary combustion process are then oxidised, but only if the gas temperature is sufficiently high and in presence of enough oxygen. Normal vs Abnormal Combustion in SI engine SI Combustion In SI engines, combustion is started, close to the end of the compression stroke, by the electrical discharge produced by the spark-plug electrodes of the ignition system. 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 at an optimized instant of the cycle, around the end of the compression stroke (usually 20-40° BTDC). The combustion is…