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What's that knocking sound?Apr 18 '00 Write an essay on this topic.Myths surrounding premium and regular gasoline are amazing! Let me give you a few examples that I've heard in my travels: "Premium is only for turbocharged engines, because it burns colder," or, "premium gives you way more power, because it's thinner and it mixes with the air better." These people obviously have no idea what they're talking about, so I'm here to dispel the myths and help make your decision at the pumps a little easier. Gasoline is rated by its octane content, the higher the octane the higher the grade (and the higher the price). The price is higher to help pay for the further refining processes and additives, and to provide a tidy profit for the fuel companies. Octane only serves to make the fuel burn slower, not colder or thinner or whatever else you might have heard. When the fuel burns slower, it can be used under higher compression and higher volumetric efficiency without detonating. Detonation occurs when the air/fuel mixture is compressed and heated to the point of combustion before the piston reaches top dead-center. This makes an audible knock as the force of the explosion encounters the moving piston, the two forces attempting to cancel each other out. The result of this exchange is that the wrist pin and connecting rod take a nasty shock and the piston slaps against the cylinder wall. The small clearances between wrist pin and bushing or connecting rod and crank journal are filled with oil under normal conditions, but during extreme detonation the oil can be forced out and allow the metal parts to touch. High octane fuel prevents detonation because it burns slower, allowing the piston to complete the compression stroke and insuring a more complete burn. So what does all this mean? It means check your manual or ask the dealer what type of gas to use, and stick with it. In newer cars with high-compression, lean-running engines, 87 octane is usually fine. Modern engine management systems have knock sensors which sense detonation and retard the ignition timing accordingly, so fuel grade doesn't make that much difference. In older cars with throttle-body injection or carbeuration, changing fuel grade is the easiest way to control detonation. Another way to control knock is to retard your ignition timing a few degrees, which makes the spark plug fire later in the combustion process. The same principles apply to turbocharged or supercharged engines as well. There is also the question of fuel quality. My mechanics teacher told us a story about his dad the trucker, who used to haul fuel on a regular basis. He would meet his dad on a dark farm road at around 3:00 in the morning with a 45 gallon drum, at which point they filled the drum and replaced the volume in the truck with 45 gallons of water. His dad worked with a driver who never had a fuel bill for the whole year, because he was hauling diesel and just filled his truck from the trailer, replacing the lost fuel with an equal volume of water. This means that you are probably getting a lot less than you think when you buy gas, and it's a good idea to keep this in mind when wondering whether to spring for the premium or not. I hope I have provided some insight into this dilemma, it is certainly a prickly issue and I'm sure it won't be resolved any time soon. Until then, don't listen to all those idiots out there, just buy the gas you need and don't waste your hard-earned money on something that's not all it's cracked up to be.  |
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