PISTON DAMAGE DUE TO COMBUSTION DISTURBANCES
Normal combustion of the air-fuel mixture takes place in the cylinder with a precisely predetermined sequence of events. It is initiated slightly before top dead center by a spark at the sparking plug. The flame spreads out from the plug in a circle and travels through the combustion chamber with steadily increasing speed from 5 - 30 m/sec. The combustion chamber pressure thus rises steeply, reaching its maximum level briefly after top dead center. The rate of pressure rise per degree crank angle should not exceed 3 - 5 bar to ensure long life of the power unit components. However, normal combustion can be disturbed by the most varied effects, of which the following three, which are quite different from each other, are the most important:
In the case of pre-ignition, combustion is initiated before the spark by a glowing area in the combustion chamber. Possible sources of pre-ignition are the valves (usually the exhaust valve), the sparking plug, the gasket, deposits on these parts, and the surfaces surrounding the combustion chamber. The pre-ignited flame them operates with uncontrolled timing on the components causing a sharp rise in piston crown temperature. Continued pre-ignition causes the piston material to reach melting point after only a few seconds. In engines with substantially hemi-spherical combustion chambers this results in holes through the piston crown, which usually occur on an extension of the sparking plug axis.
On engines with larger squish areas, scorching and melting of the top land occurs at the most highly loaded zone adjacent to the squish surfaces. Melting often continues down to the oil ring and onwards towards the engine interior.
CAUSES OF PRE-IGNITION
-- Sparking plug with incorrect heat range.
-- Damaged, leaking valves or too little valve clearance.
-- Glowing deposits on the piston crown, cylinder head, valves or sparking plugs.
-- Soft carbon formed in the combustion chambers of high performance engines with extended city driving.
-- Unsuitable fuel.
-- Diesel fuel in petrol.
-- Oil in the combustion chamber due to leakage past the piston rings or valve guides. -- High engine or high inlet temperature.
Pre-ignition can also be caused by the severe heating of individual combustion chamber components due to detonation, which always leads to high surface temperatures.
In the case of detonation, combustion is initiated by the sparking plug spark. The flame front generates pressure waves as it spreads out from the sparking plug, thereby cauisng critical reactions in the unburned mixture. This results in self-ignition simultaneously in many areas of the remaining mixture, giving a 10 - 15 fold increase in the flame speed. The rate of pressure rise per degree crank angle and the peak pressure then become substantially increased. High frequency presure oscillations then occur during the firing stroke.
In addition the surfaces surrounding the combustion chamber are overheated. When combustion chamber walls are burned clean of deposits, it is a sure sign that detonation has occured.
Light, interrupted detonation can be withstood by most engines over a long period without damage. More severe, maintained detonation results in erosion-like removal of piston material from the top land and piston crown. The cylinder head and head gasket can also be damaged in a similar way. Components in the combustion chamber (e.g. the sparking plug) can then be so severely heated up that pre-ignition and the associated severe thermal overload and melting of the piston occur.
Heavy, sustained detonation results in land and skirt fractures within short periods. These are not usually accompanied by melting or scuffing.
CAUSES OF DETONATION
-- Use of a fuel with insufficient detonation resistance (too low octane).
-- Diesel fuel in petrol.
-- Oil in the combustion chamber due to leakage past the piston rings or valve guides diminishes the detonation resistance of the fuel.
-- All engine related factors which cause high temperatures at the end of combustion also lead to detonation. These are:
-- High compression ratio, possibly due to deposits on the piston crown and cylinder head or to excessive grinding of the cylinder block and head surfaces.
-- Ignition timing too advanced.
-- Inlet temperature too high due to insufficient underbonnett ventilation or high exhaust back pressure. Failure to switch the inlet air flap to summer operation or a defective automatic inlet temperature control system also result in a significant increase in the inlet air temperature.
Fuel flooding can be caused by a too rich mixture, loss of compression pressure or combustion irregularities which lead to incomplete combustion. The lubrication of piston, piston ring and cylinder bearing surfaces becomes increasingly ineffective. Mixed friction with wear, oil consumption and scuffing are the consequences.