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Regular, Mid, Premium

As the price of fuel continues to rise many of you using premium fuel are tempted by the lower cost of regular or mid grade fuel. What fuel do you need? What is the consequence of not using the premium fuel you are currently using?

Initially your owner’s manual should be your guide for what kind of fuel your vehicle needs. It is all about octane rating. Our premium fuel is 91 AKI ((RON+MON)/2). What the heck does that mean? AKI means anti knock index and this is the number you will see under or near the button you press on the pump. This is the Canadian and US method of displaying octane rating. Note that this method is an average of two other methods; RON (Research Octane Number) and (Motor Octane Number). The RON method is the one used in Europe and Australia and it is generally 8 to 10 points higher than the MON method and thus 4 to 5 points higher than our AKI rating. Thus a 91 octane Canadian fuel will suffice for a European vehicle that requires 95 RON fuel and our 87 will work in a European vehicle that requires 91 RON fuel. Clear as mud?

Octane rating is all about resistance to knock. These terms may all be Greek to you. I will try to clarify. When your engine knocks severely it is on the path to destruction. Knock is defined as uncontrolled combustion. The term knock is used as the resultant sound of this uncontrolled combustion is similar to hitting a hammer on a solid piece of steel. For the trained ear the onset of knock is easily identified but many vehicle owners will not recognize the sound.

The spark ignition internal combustion engine produces power by exploding a mixture of air and fuel with a spark. The timing of the spark so that the resulting explosion applies the maximum force or best emissions or best fuel economy is critical. The timing of that spark is called spark advance. There is a situation where too much spark advance will result in spark knock. This is known as the knock limit. The knock limit is to some extent determined by the octane rating of the fuel. The higher the octane rating the more spark advance allowable. The more spark advance, the more power and better fuel economy. Higher octane fuel is therefore a win win situation.

When designing a spark ignited internal combustion engine for a particular vehicle the engineer will be faced with many trade offs. Cost of the design will always be one but the other is a concern for the cost to the future owner. One of the significant costs to the new owner is the cost of fuel.

The engine design is generally optimized for regular (87) or premium (91) fuel. Prior to the eighties using regular fuel in an engine designed for premium would very likely result in engine destruction. Enter electronic spark control ignition and the knock sensor. Knock sensor technology allows the engineer to tune engines very close to the knock limit. When low octane fuel is used in a high octane design the knock sensor will force the ignition system to reduce spark advance at the onset of knock. Reduced spark advance will lower both the power produced and the fuel economy but it will save the engine.

Knock sensor technology has therefore allowed engine designs that will make the most of high octane fuel or tolerate lower octane fuel. If an engine is designed for the lower or 87 octane fuel there is virtually no benefit to the use of the higher octane fuel.

For your mechanic the knock sensor has added a new dimension to tuning up your vehicle. A good scan tool will allow your mechanic to check how optimally tuned your vehicle is. Many owners will not recognize the few reduced degrees of advance resulting from a carboned up engine. They also may not recognize the lack of knock limit timing control when the knock sensor is not functioning at all. Only a computer scan will determine that.

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