Date: Mon, 01 Mar 1999 04:07:15 -0800
From: Justin VanAbrahams <>
Subject: Re: Can someone explain OCTANE?

GK wrote: > > Can someone please explain exactly what octane is and how it effects an > engine? Let me first establish I am not a scientist. I might not know what I'm talking about. If that's the case, anyone out there is free to correct me... :) Octane is a chemical present in fuel which resists ignition. In gasoline, it's primary function is the prevent the mixture from burning abnormally, which can happen when the gas reaches a temperature (or a pressure) when it actually ignites itself. There are two types of these abnormal burns... one is caused preignition, which results when the mixture reaches this pressure or temperature before the plug fires. The second is caused detonation, and it happens after the plug fire. Preignition can result in all sorts of problems - but the most common is burnt or damaged pistons. Usually what happens is engine temps reach a critical point and the fuel will start igniting without the aid of the spark plug. Detonation usually will result in damage to the piston or the rod bearings. In this case, the spark plug will ignite the fuel on one side of the cylinder, and the ambient temperature will ignite the fuel on the other side of the cylinder. Rather than a controlled burn of fuel from one side to the other, you end up with two 'flame fronts' travelling opposite one another. Since the mixture burns more quickly than it should (due to 'double combustion') the force may try to force the piston back down before it's had a chance to come all the way up - this damages the rod bearing, but can also damage the crank itself. > Why do some cars require hig octane and others recommend lower octane fuels? An engine's power is derivative of its compression ratio - how much force with which the pistons compress the mixture before it ignites and pushes the piston back away (essentially). Thus, basically, the higher the compression ratio, the more power an engine makes. More power means more heat, and that means that the fuel needs to be more resistant to abnormal ignition in order to be sure it burns at the right time, and only the right time. In order to ensure this, you use a higher octane fuel. The fuel octane requirements of an engine are based almost exclusively on compression ratio - the higher the compression ratio the more resistant to abnormal ignition the fuel needs to be and thus the higher its octane rating needs to be. > Many motorcycles recommend that a lower octane fuel will minimize backfire > and automobiles suggest that higher octane will reduce engine ping. This gonna be *really basic* but you'll get the idea: Without going into a big explanation, compression causes torque. RPM causes horsepower. Motorcycles generally use low compression, low-torque engines that rev really high and make lots of horsepower. Since they don't have a lot of mass to move, their torque is more or less insignificant. Thus, they use low octane fuels. Automobiles, on the other hand, rely on torque to get their big mass moving. Putting a motorcycle engine in a car would cause it to accelerate VERY slowly. So you need torque in a car, thus higher compression, thus higher octane. > What is engine ping? I know the sound, I have heard it, but what > is making the noise and why does a high octane fuel reduce it? Pinging is the sound you get when you have preignition in an engine. Most (if not all) modern engines have facilities built in to deal with pinging - usually by adjusting the timing (when the spark plug fires) of the engine to ignite the fuel before it would preignite. With that in mind, it's not totally uncommon to hear pinging from time to time in an engine. If it persists, you should first try switching gasoline grades (or stations!). If that doesn't solve it, there may be something wrong with the fuel system or timing system in the car. High octane fuel reduces the tendency to ping for obvious reasons - more octane means more resistance to combustion and thus less chance for preignition. > And finally, is there a reason to avoid the highest octance fuels? 76 sells > "racing fuel" 100 octane gas for $3.99US a gallon. Yes, and precisely for the same reason that you should avoid it in a motorcycle (for the most part). Using a high octane fuel in a low-compression engine may result in unburned fuel. If the engine cannot make enough pressure to overcome the octane present in a fuel, an incomplete burn may result. You may end up with lowered performance or fuel economy as the engine struggles (and fails) to take full advantage of the fuel present in the cylinders. In the case of a motorcycle, and any vehicle for that matter, some of this unburnt fuel may escape the cylinder and end up in the hot exhaust system, where the temperature alone can ignite it and cause backfires. Hope this helps... I made some generalizations but I believe it gets the point across... :) -Justin

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