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Torque Shop: Knocking in cars

I understand my car’s engine has a knock sensor. Does this mean I do not have to use high-octane petrol since the engine will adjust accordingly?

The term “knocking” in internal combustion engines refers to a phenomenon of spontaneous combustion even before the spark plug has fired. The result is a series of shock waves that are usually audible, sounding like rattling or metal rods slapping against one another.

This happens when the ignition or spark timing is outside of the programmed range. It can be caused by a faulty ignition system, but the most common cause is the use of incorrect fuel grade.

Knocking could lead to poor performance, higher fuel consumption, increased emissions and, worst of all, damage to the pistons, valves and combustion chamber.

Under normal circumstances, whenever the onset of knock is detected by the knock sensor – such as when accelerating hard at low speeds or when moving off aggressively from standstill – the car’s electronic control unit (ECU) is able to retard the ignition timing so as to avoid knock.

But if knocking occurs as a result of low octane fuel, the ECU cannot retard ignition continuously or it might eventually result in the engine shutting down altogether.

With the way petrol prices are rising, there is good reason to consider cheaper and lower octane fuels. There are three octane ratings in Singapore – 92, 95 and 98. Which grade your car requires is stated in the owner’s manual and usually also on the fuel filler flap.

Most car engines today are designed to run on 95 octane, although there are also many which are able to run on a minimum 92 octane.

High-compression engines usually require 98-octane fuel for optimum performance, as do many high-performance, direct-injection and high-output turbocharged engines.

There is no harm in using the higher-than-recommended octane fuel. But there are no advantages, such as better performance or fuel economy, either.

As a rule, stick to what the manufacturer recommends.

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