How petrol is different

Every time we pull up at a petrol pump, we see the octane ratings of the different kinds of petrol available. On the highveld it's 93 or 95 for unleaded and 93 for lead-replacement petrol.

Every time we pull up at a petrol pump, we see the octane ratings of the different kinds of petrol available. On the highveld it's 93 or 95 for unleaded and 93 for lead-replacement petrol.

What do these numbers mean, is the petrol good enough for all vehicles, even high-powered sports cars, and why do we see different octane ratings on pumps in coastal areas?

The octane rating of petrol is a measure of its resistance to detonation, which causes engine knocking. Knocking occurs when the air/fuel mixture in the parts of a combustion chamber farthest away from the spark plug ignite explosively before the flame front of regular combustion. This is caused by a sudden rise in temperature, due to the compression of the air/fuel mixture during the compression stroke, and the heat radiating from the advancing flame front immediately after the spark has jumped. Knocking is a very undesirable thing in an engine. Not only does it rob the engine of power, but the explosions of these pockets of end gas produce sharp spikes of pressure inside the combustion chambers which place serious strain on the pistons and the head gasket.

The higher the pressure inside the combustion chambers at the top of the compression stroke, the greater the tendency for detonation. And the pressure inside the combustion chambers depends on the engine's compression ratio as well as on how completely the cylinders were filled with air/fuel mixture during the intake stroke.

At wide open throttle, there's no obstruction to the in-rushing air, and the cylinders are well filled, while at small throttle openings the air struggles to get past the throttle valve and you end up with poorly filled cylinders. At sea level you will get more air/fuel molecules into the cylinders than at Gauteng altitude in similar conditions.

The other factor, besides pressure, which influences the tendency for detonation is the spark timing. By retarding the spark slightly, that is waiting until the piston is closer to the top of its travel before the spark is allowed to jump, you can diminish the chances of knocking.

To prevent knocking, even when conditions are most favourable, the mix of hydrocarbons used in petrol must be chosen so as to have sufficient resistance to detonation. Petrol companies use a scale of octane ratings to indicate the degree of resistance to knocking.

The octane rating of a fuel is determined by running the fuel in a test engine with a variable compression ratio.

With the engine running at low speed, the compression ratio is slowly increased until where knocking first occurs.

Octane rating does not relate to the energy content of the fuel; it's a measure of its tendency to burn rather than explode.

There are fuels with octane ratings greater than 100. But for ordinary vehicles, the octane ratings available are adequate.

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