Don’t Ignore ‘Fuel Knock’ in Petrol Engines

If the fuel ignites on its own before the spark plug fires, because of engine pressure or engine heat, it explodes, making a knocking or pinging sound from the engine. The sound produced is termed as an “engine knock”.

There are a couple of types of knock as referred to in this article Knock from combustion being ill-timed or from fuel pre-igniting, and knock from worn out engine parts wandering and banging into each other. Engine knock from the wrong fuel can often be remedied by using a higher octane fuel. Sometimes, however, it may require ignition timing to be adjusted as well.

Knocking from worn out parts can only be remedied by repairing the engine with new or reconditioned parts. A qualified mechanic can determine the type of knock and perform the remedy as recommended in the vehicle workshop manual.

The most typically used engine management systems found in automobiles today have a knock sensor that monitors if knock is being produced by the fuel being used. In modern computer controlled engines, the ignition timing will be automatically altered by the engine management system to reduce the knock (detonation) to an acceptable level.

There are a number of possible causes of knocking or pinging sounds in vehicle engines.

Sometimes knocking sounds can be caused by using the wrong type of fuel for a particular vehicle, for instance, using regular unleaded if a vehicle requires higher octane fuel.

Mechanics also say at times the sounds could be caused by a malfunction with the spark point. Issues that fall into this category include spark plugs that need to be replaced, ignition timing issues, or any number of other possibilities according to the vehicle ignition system.

There are also tales on the Internet from users who say switching to synthetic oil solved their engine knock and others say the switch to synthetic caused their cars to start knocking.

However, many well trained mechanics concur that type of oil has nothing to do with traditional engine knock, detonation or pinging one way or the other. The only oil-related problem that might ever cause the sound in an engine is a lack of oil.

If engine knock due to fuel is allowed to persist under extreme conditions or over many engine cycles, engine parts can be damaged or destroyed. The simplest deleterious effects are typically particle wear caused by moderate knocking, which may further ensue through the engine’s oil system and cause wear on other parts before being trapped by the oil filter.

Severe knocking can lead to catastrophic failure in the form of physical holes punched through the piston or cylinder head that is rupture of the combustion chamber. Either of these depressurises the affected cylinder and introduces large metal fragments, fuel, and combustion products into the oil system.

Detonation can be prevented by any or all of the following techniques:

The use of a fuel with high octane rating, which increases the combustion temperature of the fuel and reduces the proclivity to detonate

Reducing the load on the engine.

Knock detection:

Due to the large variation in fuel quality, a large number of engines now contain mechanisms to detect knocking and adjust timing or boost pressure accordingly. This offers improved performance on high octane fuels while reducing the risk of engine damage caused by knock when the engine is running on low octane fuels.

Various monitoring devices are commonly utilised by tuners as a method of seeing and listening to the engine in order to ascertain if a tuned vehicle is safe under load.

Source : The Independent

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