Most of the big oil companies offer a high-octane petrol in addition to their regular unleaded products, promising better performance or improved fuel efficiency. Will using it benefit your car?

While filling your car you may have come across names such as BP Ultimate , Mobil Synergy 8000 or Shell Optimax. High-octane fuels claiming a number of benefits, such as:

High octane rating (RON=98) promising more engine power and less engine knock.

Higher density for lower fuel consumption.
Added detergents that keep engine parts clean, providing smoother engine performance and lower fuel consumption.

What is Engine Knocking?
Knocking is the sound caused if fuel isnít burned smoothly in a carís engine. In ideal conditions, the spark plug creates a flame that travels across the cylinder, burning the fuel/air mix and steadily building up pressure. If part of the fuel/air mix instantaneously self-ignites before the flame reaches it, the pressure rises rapidly, causing a knocking or pinging noise. Heavy knocking can cause engine damage.
Knocking is influenced by several factors ó for example, the build-up of deposits, hard acceleration in a high gear, high-load driving (uphill or towing) and an out-of-tune engine (wrong spark timing or fuel/air ratio) can lead to increased knocking.

Octane numbers (Rating)
A fuelís octane number is a measure of its ability to resist self-ignition when burnt in an engine ó or its anti-knock quality.
The octane number is measured under standardised test conditions against reference fuels, one of which is iso-octane (hence the name Ďoctane numberí). There are actually several octane numbers, depending on the test conditions. Car fuels are described by two numbers: research octane (RON) and motor octane number (MON), with the RON being higher than the MON.

Typical RON values are:

Regular unleaded petrol: 91
Premium unleaded petrol: 95
Lead replacement petrol : 96
High-octane petrol: 98
Each engine requires a certain octane number to prevent knocking ó for example, your car may need premium unleaded rather than regular unleaded petrol. This requirement is measured under worst-case conditions, and takes into account a slight increase over time ó for example, due to the build-up of deposits. In less severe conditions, the engine can do with petrol of a lower octane number.

Apart from being less likely to develop knocking, an engine wonít benefit from using a petrol with a higher octane number (for example, it wonít provide more power).

Density and detergents

The oil companies claim high-octane fuels are about 3 to 5% denser than regular unleaded petrol, potentially decreasing your carís fuel consumption by up to that amount.
As your car gets older, and particularly if youíve been using petrol without an added detergent, deposits may build up in its fuel system, injector nozzles, valves and combustion chamber. This can lead to increased engine knock , less responsive running and lower fuel efficiency.

A cleaner fuel system leads to better engine response and fuel efficiency. All the companies offering a high-octane fuel add detergents to their regular unleaded products, however they claim the detergents in high-octane products are:

either of better quality, cleaning engine parts not cleaned by regular detergents; and/or
added in a higher concentration, not only preventing build-up, but also cleaning up existing deposits after a few tankfuls.

And if deposit build-up is a problem for you, using a detergent additive available from your petrol station may also be an alternative (such as Redex)

By how much, if at all, a high-octane fuel can improve your carís fuel efficiency depends on a number of factors ó for example, on what type of petrol youíve been using before, or on the degree of deposit built-up in your engine. And as everyday driving conditions (and therefore fuel consumption) can vary considerably, it may actually be quite difficult to reliably identify any improvement.
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