Combustion problems caused by
erratic burning and/or detonation of fuel rather than controlled and
even burning shorten engine life and also cause losses in economy and/or
power. A number of terms are used to describe them, including
pinking, pinging, detonation. Audi corteco oil seals.There
are a number of causes, with low octane fuel and distributors not
performing to spec, especially wear in the centrifugal advance
mechanism, being the leading culprits.
While there are several types of combustion problem, they fall into two general categories of phenomena which are loosely described as detonation which are:
spontaneous combustion which occurs independently of the ignition spark (diesel action), and
pre-ignition, brought about by the spark occurring too soon.
Technically they are quite different phenomena, but sometimes they are linked. Some of the causes are common to both.Audi corteco oil seals.
For simplicity, the following doesn’t differentiate between the two, as to the untrained ear it can be hard to tell the difference.
Note: Run-on (engine continues to run – somewhat erratically – when the key is turned off) is a combustion problem which is covered in a separate page: Run on.
Combustion – how it should be:
The fuel/air mix in the combustion chamber is supposed to burn evenly rather than explode all at once, and should spread out from the spark plug at a steady rate across the chamber. A certain amount of swirl or turbulence in the fuel/air mix is needed to achieve this. If there is too much turbulence, instead of spreading evenly across the combustion chamber after the spark ignites the fuel, the last bit of fuel/air in the chamber can explode spontaneously … which we hear as pinking or detonation.Audi corteco oil seals.
Peak useful combustive effort is achieved when the spreading flame front reaches its peak pressure at 12 degrees after top dead centre. The point at which the spark appears is timed to achieve this. That the spark is timed to appear anything up to 30 degrees before TDC imdicates that it takes quite a bit of time (relatively speaking!) for the flame front to take in most of the fuel compared with it all detonating at once.Audi corteco oil seals.
For a given amount of turbulence, the speed of the flame front depends mainly on the pressure in the cylinder – the less pressure, the slower it burns. Hence at low pressure (high vacuum) the spark needs to be even earlier – and so you have a vacuum advance.Audi corteco oil seals.
Because the speed of the front is independent on engine speed, it is necessary to make the spark appear sooner (ie advance its timing) as the engine speed increases. At higher speeds, the cylinder doesn’t fill completely so the reduced pressure tends to offset the increase in engine speed. Because of this counterbalancing effect, the amount of advance needed doesn’t keep increasing, but instead reaches a maximum figure after which engine speed increase is fully offset by reduced filling.Audi corteco oil seals.
If the peak pressure is too late, then engery is wasted, mostly as heat going to the cooling system rather than as mechanical energy in the piston. If it is achieved too early, then most of the energy is pumped back into the moving piston, increasing its temperature and hammering it against the bore … once again, we hear this as pinking/detonation.Audi corteco oil seals.
While there are several types of combustion problem, they fall into two general categories of phenomena which are loosely described as detonation which are:
spontaneous combustion which occurs independently of the ignition spark (diesel action), and
pre-ignition, brought about by the spark occurring too soon.
Technically they are quite different phenomena, but sometimes they are linked. Some of the causes are common to both.Audi corteco oil seals.
For simplicity, the following doesn’t differentiate between the two, as to the untrained ear it can be hard to tell the difference.
Note: Run-on (engine continues to run – somewhat erratically – when the key is turned off) is a combustion problem which is covered in a separate page: Run on.
Combustion – how it should be:
The fuel/air mix in the combustion chamber is supposed to burn evenly rather than explode all at once, and should spread out from the spark plug at a steady rate across the chamber. A certain amount of swirl or turbulence in the fuel/air mix is needed to achieve this. If there is too much turbulence, instead of spreading evenly across the combustion chamber after the spark ignites the fuel, the last bit of fuel/air in the chamber can explode spontaneously … which we hear as pinking or detonation.Audi corteco oil seals.
Peak useful combustive effort is achieved when the spreading flame front reaches its peak pressure at 12 degrees after top dead centre. The point at which the spark appears is timed to achieve this. That the spark is timed to appear anything up to 30 degrees before TDC imdicates that it takes quite a bit of time (relatively speaking!) for the flame front to take in most of the fuel compared with it all detonating at once.Audi corteco oil seals.
For a given amount of turbulence, the speed of the flame front depends mainly on the pressure in the cylinder – the less pressure, the slower it burns. Hence at low pressure (high vacuum) the spark needs to be even earlier – and so you have a vacuum advance.Audi corteco oil seals.
Because the speed of the front is independent on engine speed, it is necessary to make the spark appear sooner (ie advance its timing) as the engine speed increases. At higher speeds, the cylinder doesn’t fill completely so the reduced pressure tends to offset the increase in engine speed. Because of this counterbalancing effect, the amount of advance needed doesn’t keep increasing, but instead reaches a maximum figure after which engine speed increase is fully offset by reduced filling.Audi corteco oil seals.
If the peak pressure is too late, then engery is wasted, mostly as heat going to the cooling system rather than as mechanical energy in the piston. If it is achieved too early, then most of the energy is pumped back into the moving piston, increasing its temperature and hammering it against the bore … once again, we hear this as pinking/detonation.Audi corteco oil seals.
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