Engineers’ Angle | 87 or 90 octane - which fuel should I use?

Undoubtedly, the continuing increase in the price of gasolene is of great concern to the motoring public.

Nowadays, the weekly price change serves as a harsh reminder to motorists to check their pockets.

Some motorists consider which octane fuel to use as they glance at the prices at the pump. Should I use 87 or 90? Some persons will anecdotally swear that with 90 octane fuel the car runs better, the fuel lasts longer, the vehicle has more power and runs smoother. Actually, the octane fuel you chose is all about the vehicle you take to the pump.

First, the designation of 87 and 90 octane is not just to distinguish the cost of the two grades of petrol at the gas station. These numbers represent more than what meets the eyes, they have everything to do with the characteristics of the fuel.

The octane number, or rating, is the measure of a fuel's ability to resist detonation (ignition) when compressed in an internal combustion engine. The higher the octane number, the higher the pressure the fuel can withstand before self-ignition. Therefore, the 90 octane fuel can handle greater compression before it spontaneously combusts compared to the 87 octane rated fuel.

It is important to know the octane rating of the fuel because in an internal combustion engine, the fuel is compressed just before combustion. The combustion is initiated by a spark produced by a spark plug.

This process is precisely timed to ensure that you get maximum power from your engine. The issue with fuels is that they can combust spontaneously when compressed without any spark, a phenomenon which is exploited in the diesel engine.

REASON FOR NOISY ENGINE

The problem is that the gasolene engine was not designed to combust the fuel by compression but rather a controlled ignition using a spark after the fuel is compressed. If the fuel in the gasolene engine should prematurely combust during compression, it produces an audible 'knocking' or 'pinging' sound in the engine. This is bad for your engine.

There is the likelihood of getting a shorter distance per litre, and potentially paying for engine repairs if a lower than manufacturer's recommended fuel is used. In such a case, cheaper gasolene wouldn't be saving money at all.

The requirement for higher octane fuel is based on the compression ratio of your car engine. Higher octane rated fuels are typically required in cars with high compression ratios because the fuel is compressed to a high pressure before ignition.

High compression ratio vehicles typically include high-performance cars such as sports cars and racing cars. In these cars with high compression ratios, a lower octane rated fuel might detonate before the compression process is completed and will, therefore, result in knocking, reduced engine performance and a risk of overheating.

But what happens when you use a higher octane fuel in a car which does not require it? An article on the website www.fueleconomy.gov, which identifies itself as the official United States Government source for fuel economy information, states that, "for most vehicles, higher octane fuel may improve performance and gas mileage and reduce carbon dioxide emissions by a few per cent during severe duty operation, such as towing a trailer or carrying heavy loads, especially in hot weather. However, under normal driving conditions, you may get little to no benefit."

The US Federal Trade Commission on its website notes, "In most cases, using a higher octane gasolene than your owner's manual recommends offers absolutely no benefit. It won't make your car perform better, go faster, get better mileage or run cleaner."

If your car was designed to use 87 octane fuel and you use 90 octane, it is likely that the engine will not perform better. What is certain to increase is the amount you spend at the pump.

So why pay the higher price for higher-octane-rated fuels? Simply put, higher-octane fuels cost more to produce. The promise of better performance of your vehicle is likely due to additives such as detergents to remove deposits and clean your engine. However, you can purchase a fuel injection cleaning additive and add it to your regular gasolene when you fill up at the pump.

So the question remains, which octane fuel should you use in your vehicle? Simply stated, you should use the octane rating stated in the vehicle's manufacturer manual. Whenever in doubt, open the vehicle manual and search.

Additionally, these days, Internet search engines like Google can provide easy access to answers whether from manufacturers or car enthusiasts. In any case, most vehicles are designed to run on 87 octane fuel.

- Kirkland Rowe is a senior lecturer in the Mechanical Engineering Department

at the University of Technology and a certified energy manager. Send questions and comments to, editorial@gleanerjm.com or jie@cwjamaica.com. You may also leave your comments for the JIE's Technical Committee at our Facebook page: Jamaica Institution of Engineers - JIE.