COMBUSTION IN SPARK-IGNITION ENGINES

COMBUSTION IN SPARK-IGNITION ENGINES
In a conventional spark-ignition engine, the fuel and air are homogeneously mixed
together in the intake system, inducted through the intake valve into the cylinder where it mixes
with residual gases and is then compressed. Under normal operating conditions, combustion is
initiated towards the end of the compression stroke at the spark plug by an electric discharge. A
turbulent flame develops following the ignition and propagates through this premixed charge of
fuel and air, and also the residual gas in the clearance volume until it reaches the combustion
chamber walls. Combustion in the SI engine may be broadly divided into two general types, viz.,
normal combustion and abnormal combustion.
STAGES OF COMBUSTION IN SI ENGINES
A typical theoretical pressure-crank angle diagram, during the process of compression
(a→b), combustion (b→c) and expansion (c→d) in an ideal four-stroke spark-ignition engine is
shown in Fig.1. In an ideal engine, as can be seen from the diagram, the entire pressure rised
uring combustion takes place at constant volume i.e., at TDC. However, in an actual engine this
does not happen. The detailed process of combustion in an actual SI engine is described below.
The combustion process in an SI engine consists of three stages.

The pressure variation due to combustion in a practical engine is shown in Fig. 2. In this
figure, A is the point of passage of spark (say 20°bTDC), B is the point at which the beginning of
pressure rise can be detected (say 8° bTDC) and C the attainment of peak pressure. Thus AB
represents the first stage and BC the second stage and CD the third stage.