The combustion process inside an engine is a chemical reaction of hydrocarbon based fuel (Carbon and Hydrogen) and Oxygen (the active oxidising component of atmospheric air).|
The Hydrogen and Carbon element of the fuel burns with the Oxygen to produce Carbon-Dioxide and Water. A phenomenal amount of heat is released during this process : the heat causing the gases to pressurise and expand inside the cylinder forcing the piston down on its power-stroke to produce useful work.
Combustion is initiated by a spark, with a flame front then radiating out from the spark location at a rate of between 20 and 40 m/s (Metres per Second). For most engines the combustion process takes 1mS to 2mS duration - this is a physical constant.
In their literature, Bosch indicate that the point of Maximum Heat Release (and therefore maximum cylinder pressure) should occur shortly after Piston Top Dead Centre (TDC) - usually between 5 and 10 degrees After Top Dead Centre (ATDC). Some papers indicate 15 degrees is optimal : the exact point will depend very much on the design geometry of combustion chambers / cylinder heads so this will naturally vary from engine design to engine design.
The ignition combustion process can be divided into four discernable stages :
A quoted figure is that when 80% of the VOLUME has burnt only 50% of the MASS has been burnt : the remaining 50% being squeezed into the final 20% of the VOLUME.
- Initiation of the flame kernal by the Spark. (2% of Mass Burnt)
- Growth of the flame kernal into a stable flame front. (10% of Mass Burnt)(20% of burn TIME)
- Propogation of the flame through the main charge. (90% of Mass Burnt)(80% of burn TIME)
(We may assume this correponds approx. with the point of maximum heat release)
- Slow burn of final remaining reactants (100% of Mass Burnt)(100% of Burn TIME)
The Constant nature of the flame front propogation rate means that the timing of the spark ignition point has to be varied to compensate for engine speed such that the point of Maximum Heat Release is maintained at the optimum point. This is referred to as an Ignition Advance Curve.
An interesting quoted figure is that 20% of the heat released from the burning fuel is lost to the surrounded metallic structures. Heat lost is pressure lost. Pressure lost is useful Work / Power lost from the engine.
Obviously, this figure is not exact as it will vary with geometry etc. It does demonstrate quite well the magnitude of the heat transfers that take place between the combusting gases and the combustion chamber walls.