Spark Ignition Engines

Abstract

First we need to describe the characteristics of spark ignition engine combustion, an then we can remind ourselves of what the optimum ignition timing is The flame kernel is created as a result of energy from the spark initiating chain reactions that lead to self-sustaining combustion. The initial size of the flame kernel will be little larger than the spark plug gap, so it will be of the order 1 mm in diameter. The spark will have created radicals that are important in the propagation of combustion, and it will have raised the temperature in the spark channel to 1000s of Kelvin. This initial flame kernel (say lmm in diameter) is very small compared to the turbulent length scales (which might be typically an order of magnitude larger). Thus the initial growth of the flame front will be determined by the laminar burning velocity. The transition to fully turbulent combustion will not be complete until about 10% of the fuel has been burnt; this period is also known as the early burn period or (less correctly) the ignition delay period since there will be negligible pressure rise due to combustion. The main part of combustion (say when 10–90% of the fuel is burnt), is dominated by turbulence so is often called the turbulent combustion phase. The final burn phase (90–100% burn) is much slower, since by now the flame front area will be progressively reducing because of contact with the combustion chamber surfaces. Combustion in the final burn period is also affected by the desorption of fuel from the oil films and crevices.