There appears to be some confusion about when to use 95 unleaded petrol as opposed to 93 unleaded petrol. In these days of high fuel prices, it would be a sheer waste of money if you filled your car with the more expensive 95 unleaded petrol, when your car only requires 93 unleaded petrol.
WHAT DOES THE ‘93’ AND ‘95’ MEAN WHEN BUYING FUEL?
The numbers ‘93’ and ‘95’ refer to the octane numbers or octane rating of petrol. The performance of an engine is measured by using octane rating. The greater the compression fuel can experience before detonating, the higher the octane rating. Generally, high performance petrol engines must have high compression ratios. On the other hand, fuels with low octane ratings, such as diesel, do not compress fuel but compress air instead. Fuel is then injected into the hot, compressed air.
WHAT IS A COMPRESSION RATIO?
A compression ratio is the ratio of the volume of the cylinder and the combustion chamber when the piston is in the bottom position compared to when the piston is at the top position. Engines, such as those using diesel, have high compression ratios, which greatly improves fuel efficiency.2 Diesel engines are called ‘compression-ignition’ engines.
95 PETROL DOES NOT HAVE MORE ENERGY THAN 93 PETROL
We tend to think that ’95 petrol’ has more energy than ’93 petrol,’ but that is not true. The octane rating refers to the fuel’s resistance to auto-ignition, or otherwise known as ‘knocking.’ Thus the higher the rating, the greater the resistance.
WHAT IS ‘KNOCKING’?
‘Knocking’ or auto-ignition refers to a fast combustion process which is similar to an explosion. This is when some of the air-fuel mixture burns due to specific conditions of temperature and pressure inside the combustion chamber. This type of combustion process can destroy an engine.
HOW ARE PETROL ENGINES MEANT TO WORK?
In petrol engines, a spark from the spark plug initiates the combustion process of an air-fuel mixture in the combustion chamber, at the right moment. The latter is called spark timing. The normal combustion process in petrol engines is not an explosion, but a controlled release of energy while the air-fuel mixture burns.
WHY DOES THE OCTANE RATING CHANGE WHEN AT THE COAST?
According to the AA, cars should use 93 octane petrol, unless they drive a supercharged or turbo engine. Driving in Johannesburg requires the use of 93 octane petrol, but this changes to 95 octane petrol when at the coast.4 This is due to atmospheric pressure, which is lower in Johannesburg compared to Durban. You won’t obtain any improved engine performance when using the more expensive 95 octane petrol in Johannesburg. Always check your car manual for the correct octane fuel to be used.
WHAT SHOULD YOU DO WHEN REFUELLING?
This may sound like a silly question, but it’s not really. Respectfully, many drivers sit in their cars, while the garage attendant fills the car with either petrol or diesel. This can be a dangerous practice, because the attendant may not have heard you correctly, or may have been distracted by someone. Instead of putting in 95 octane petrol, 93 octane petrol was used. It is much better to get out of the car, and make sure that the attendant is filling your car with the correct octane fuel.
WHAT IF YOUR TANK WAS FILLED WITH 93 OCTANE FUEL INSTEAD OF 95 OCTANE FUEL?
Sometimes, if you happen to be in the middle of nowhere, the garage may only have 93 octane fuel available, and yet your car requires 95 octane. So what do you do if the wrong octane rating fuel is in your tank? First, there is no need to panic,it just means your engine’s performance on the lower octane fuel will be less, while fuel consumption will be higher.
DON’T MAKE A HABIT OF USING THE INCORRECT FUEL RATING
However, if you keep on running your car on 93 octane petrol instead of the required 95 octane petrol, your engine could start to knock. This, in turn, could cause serious engine damage.
WHAT ABOUT USING 95 OCTANE PETROL WHEN MY CAR SHOULD RUN ON 93 OCTANE?
Conversely, if you are filling up every time at the garage with 95 octane fuel instead of the required 93 octane fuel, you are just wasting your money and energy. The 95 octane fuel won’t make any difference to the performance of your engine, which only needs 93 octane fuel, according to the car’s manual.
When your car is running smoothly on the correct octane fuel, remember that you should protect it by buying the right kind of car insurance.
Disclaimer:
This article was prepared by Eric Sandmann in his personal capacity. The views and opinions expressed in this article are the author’s own and do not reflect the views and opinions of Prime Meridian Direct (Pty) Ltd, FSP41040. The views and opinions in the article should not be attributed to anyone but the author unless expressly stated. Nothing in this article should be relied upon as advice, this publication is presented for informational purposes only. No person should act or refrain from acting in reliance on any information found in this article, without first obtaining proper financial advice from the appropriate professional. The author makes no claims, promises or guarantees about the accuracy, or completeness, of any information linked from, referred to, or contained in this article. The author reserves the right, to edit and change the content of this article.