Using lower-octane petrol may damage engine if it does not meet carmaker specifications

With rising petrol pump prices, can I downgrade from the usual 95- and 98-octane fuel to 92-octane to cut cost?

With the way petrol prices are spiking, there is good reason to consider cheaper, lower-octane fuels, but this is not necessarily a good course of action.

In general, there are three grades of petrol sold in Singapore. The classification is based on what the industry refers to as octane ratings, being 92, 95 and 98. Usually, the higher the number, the higher the price.

The grade of petrol your car requires is usually stated in the owner’s manual, as well as on the back of the fuel filler flap. Most modern car engines are designed to run on 95-octane petrol, although there are some specified to run on minimum 92-octane petrol.

Technically, there is no harm in using fuel of a higher octane than recommended. However, you may not experience any advantage, either in terms of performance or fuel economy.

On the other hand, using fuel with lower octane than the minimum specified in the owner’s manual might save you some money, but this will cause engine “knock” and, eventually, serious engine damage.

The term “knocking” in the context of internal combustion engines is when there is spontaneous combustion occurring before the spark plug has fired. The result is a series of shock waves that is usually audible and sounds like a rattle or metal rods slapping one another furiously.

This happens when the ignition or spark timing is out of sync. Although it could be due to a faulty ignition system, the most common cause is incorrect fuel grade or an octane rating that is too low.

When this happens, fuel consumption and emissions increase because fuel is not completely burned. Worst of all, engine components like the piston, valves and combustion chamber will be damaged.

The knock sensor in engines with electronic control units (ECU) constantly monitors and detects knocking. Even the slightest knock detected is relayed to the ECU, which instantly adjusts ignition timing to restore smooth combustion and prevent further knocking.

However, due to combustion chamber design and, more specifically, compression ratio, the controller is not programmed to accommodate lower than design-specified minimum fuel octane rating. So, the ECU cannot handle 92-octane fuel if the engine is designed for minimum 95-octane fuel.

In older carburetted cars, it is necessary to manually tweak the ignition timing at the specified setting suited for the octane rating of the fuel, since there is no knock sensor or ECU.

As a rule, stick to what the manufacturer recommends or higher, but never use a lower-octane fuel because the resulting engine damage will cost you far more than what you save on fuel.