myth busting
I don't know if it may apply to you, but when I was young, my dad used to say that sometimes, filling your gas tank with supreme gas is good. I was fueling my car yesterday and I was thinking about it. Here in Quebec, ordinary fuel is 1.08.9 CAN dollars per litre. At this time, it means around a US dollar. What ever the price, it is always too expensive! The supreme gas is always 5 to 8 cents more expensive than the ordinary one. I was asking my self, what's so different in the supreme gas to be more expensive and is it good to pay a bit more and have supreme gas in my little car?
After doing some research in my old notes of chemistry and aviation propulsion, I finally found some interesting facts. Fuel is constituted from hydrocarbons between 5 and 12 carbon-long molecules. Diesel is more between 15 and 18 carbon-long molecules. There no explosion in the engine, but rather a rapid combustion of a mix of gas and air (usually ratio 15:1 air/fuel).
The octane rating is calculated for it's tendancy to resist to knocking. Knocking is a phenomenon when fuel burns before its time and causes the engine to knock. There are two mains way to "achieve" the knocking: the detonation and pre-ignition. The first one occurs when "when the remaining unburned mixture is heated and pressurized by the advancing flame front for a certain length of time". The pre-ignition is caused by another ignition source than the spark plug, like hot spots (remainings in the cylinder).
More there are carbons in the chain, more it is likely to ignite. Heptane (7 carbons) and octane (8 carbons) are totally different in their behavior. In the octane rating, heptane has the value of 0, and octane -20. But why do we say octane-87, 91 or 94? Actually, in the gas we put in our car is the 2,2,4-trimethypentane:
CH3-C(CH3)2-CH2-CH2-CH3
who has the octane ratio 100 on the scale. As told before, this one is 5 carbon-long chain, so much likely to burn before it is supposed to comparely to octane. A few years ago, lead was added to delay the combustion (until it was proven that it was very nocive fot the environment, although it is still used in aviation). For each car, the manufacturer tells us in the car guide which gas is the good one for our piece of metal. The engine is synchronized to ignite at a certain point in time, not before neither after. Which means that you need to fuel with the good one. If you put supreme fuel in your car, then the combustion will be delayed in time and the combustion won't be as good as it is supposed to be. So don't be fooled with ads who tells you that it is good sometimes to put high-octane rating fuel when fueling. And… in the same time, you'll save money!
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