A simplified description of the combustion process within the cylinder of a spark ignition engine is as follows. A single high intensity spark of high temperature passes between the electrodes of the spark plug leaving behind it a thin thread of flame.
Range and rate of burning
The range and rate of burning can be summarized by reference to the following graphs. Whole the second stage (B to C) is roughly dependent upon the degree of the turbulence (and on the engine speed), the initial delay necessitates ignition advance as the engine speed increases.
The detonation phenomenon is the limiting factor on the output and efficiency of the spark ignition engine. The mechanism of detonation is the setting up within the engine cylinder of a pressure wave travelling at such velocity as, by its impact against the cylinder walls, to set them in vibration, and thus produce a high pitched ‘ping’.
When the spark ignites a combustible mixture of the fuel and air, a small nucleus of flame builds up, slowly at first but accelerating rapidly. As the flame front advances it compresses the remaining unburned mixture ahead of it.
The temperature of the unburned mixture is raised by compression and radiation from the advancing flame until the remaining charge ignites spontaneously. The detonation pressure wave passes through the burning mixture at a very high velocity and the cylinder walls emit the ringing knock.
Detonation is seldom dangerous in small engines since it is usually avoided at the first warning by easing the load, but at higher speeds, where the noise level is high, the characteristic noise can and often does go undetected. It can be extremely dangerous, prompting pre-ignition and possibly the complete destruction of the engine.
Combustion chamber design
To avoid the onset of detonation and pre-ignition, a careful layout of the valves and spark plugs is essential. Smaller engines, for automotive use, are firmly tied to the poppet valve. This, together with the restriction of space involved with high compression ratios, presents the designer with interesting problems.
Stratification of cylinder charge
A very weak mixture is difficult to ignite but has great potential for reducing emissions and improving economy. One technique to get around the problem of igniting weak mixtures is stratification.
It is found that if the mixture strength is increased near the plug and weakened in the main combustion chamber an overall reduction in mixture strength results, but with a corresponding increase in thermal efficiency.
Compression ignition engines
The process of combustion in the compression ignition engine differs from that in a spark ignition engine. In this case the fuel is injected in a liquid state, into a highly compressed, high-temperature air supply in the engine cylinder.
Each minute droplet is quickly surrounded by an envelope of its own vapor as it enters the highly heated air. This vapor, after a certain time, becomes inflamed on the surface. A cross-section of any one droplet would reveal a central core of liquid, a thin surrounding film of vapor, with an outer layer of flame. This sequence of vaporization and burning persists as long as combustion continues.
Pre-combustion chambers, whether of the air cell or ‘combustion swirl’ type have the general disadvantage of being prone to metallurgical failure or at least are under some stress since, as they are required to produce a ‘hot spot’ to assist combustion, the temperature stresses in this region are extremely high. There is no unique solution and the resulting combustion chamber is always a compromise.