Though 2015 saw some of the lowest gasoline price in years for Ontario drivers, no one wants to spend more than necessary, even at the 80-cents per litre mark. So is the extra price for premium gas necessary? High-performance and luxury cars are often directed to the expensive stuff by the manufacturer. Is it just because the owners of those cars can afford it or are there real reasons behind the added cost? Some background into how car engines work is helpful.
The Combustion Cycle
Cars gain motion from the activity in the engine. Fuel and air are mixed, ignited and pushed out of the engine in a little self-contained flurry of activity. Power comes from the explosion and expansion of the fuel and air mixture, which pushes the piston away. With several cylinders and pistons working in coordinated effort, enough power to propel a car occurs.
The spark plug lights a bit of fuel-air which in turn starts a flame which burns until all the fuel-air is consumed. This takes about one-third of the combustion cycle. The fuel-air mixture furthest from the spark is subject to increasing heat and pressure from the advancing flame.
If it’s heated and squeezed enough, it can explode before the flame reaches it and, as you can imagine, this extra explosion means trouble.
The Formulation of Gasoline
Gasoline is a combination of about 200 different compounds called hydrocarbons. Each has a different property. Some are very resistant to this premature explosion phenomenon, while others are very susceptible to it.
Premium gasoline packs more of the resistant compounds into its mixture. It may not have any more potential energy content than lower grades, but it takes more heat and pressure for it to spontaneously explode.
Octane and Engines
The susceptibility to explosion is rated as an octane number. The higher the number, the less likely early combustion will happen.
An engine with a high compression ratio is more likely to start that early explosion, called a knock because, like a novice drummer playing off the beat, it creates noise outside the regular rhythms of engine operation.
High compression engines extract more power from the fuel-air mixture and these types of engines are found, you guessed it, in high-performance and luxury models. It’s one reason why a Porsche 911 accelerates differently than a Prius.
Other Uses for Premium Gas
As a car ages, it may become susceptible to knocking. This is particularly true in older cars with mechanical distributors, but even cars without distributors can build up carbon in cylinders which result in hotspots. Environmental factors may combine, such as hot ambient temperatures and low humidity, to create knock. If your older car is starting to run rough, despite tune-ups and maintenance, moving to mid-grade or premium fuel may be an answer to the knocking issue, but it won’t turn your beater into a BMW.