Spark ignition engine combustion process
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. From this thin thread combustion spreads to the envelope of mixture immediately surrounding it at a rate that depends mainly on the flame front temperature, but also, to a lesser degree, on the temperature and density of the surrounding envelope.
In this way, a bubble of flame is built up that spreads radially outwards until the whole mass of mixture is burning. The bubble contains the highly heated products of combustion, while ahead of it, and being compressed by it, lies the still unburnt mixture.
Range and rate of burning
The range and rate of burning can be summarized by reference to the following graphs. Increase in firing pressure (or maximum cylinder pressure) generally accompanied by a reduction in exhaust temperature. The effect of increasing the range of the mixture strength speeds the whole process up and thus increases the tendency to detonate.
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.
Evidence of the presence of pre-ignition is not as apparent at the onset as detonation, but the results are far more serious. There is no characteristic ‘ping’. In fact, if audible at all, it appears as a dull thud. Since it is not immediately noticeable, its effects are often allowed to take a serious toll on the engine? The process of combustion is not affected to any extent, but a serious factor is that control of ignition timing can be lost.
Pre-ignition can occur at the time of the spark with no visible effect. More seriously, the ‘auto ignition’ may creep earlier in the cycle. The danger of pre-ignition lies not so much in development of high pressures but in the very great increase in heat flow to the piston and cylinder walls. The maximum pressure does not, in fact, increase appreciably although it may occur a little early.
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. To achieve this, petrol injection is used – stratification being very difficult with a conventional carburation system. A novel approach to this technique is direct mixture injection, which, it is claimed, can allow a petrol engine to run with air-to-fuel ratios in the region of 150: 1. This is discussed in a later section.
Combustion chamber design – diesel engine
The combustion chamber must be designed to:
- Give the necessary compression ratio.
- Provide the necessary turbulence.
- Position for correct and optimum operation of the valves and injector.
These criteria have effects that are interrelated. Turbulence is normally obtained at the expense of volumetric efficiency. Masked inlet valves (which are mechanically undesirable) or ‘tangent’ directional ports restrict the air flow and therefore are restrictive to high-speed engines.
A hemispherical combustion chamber assists with the area available for valves, at the expense of using an offset injector. 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.