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Old October 10th, 2001, 01:09   #3
mickey
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Default Re: Diesel #1 vs Diesel #2

Basically (very basically), hydrocarbon fuels come in "grades" that have to do with the length and complexity of the hydrocarbon chains. They range from propane at the "top" to asphalt at the "bottom". Diesel #1 is "higher" than Diesel #2...it's "thinner." The hydrocarbon chains are generally shorter.

In practice, it's a bit more complicated than that. Motor fuels like diesel fuel and gasoline are kind of a "soup" of several different hydrocarbons, but the bottom line is that the thinner, more volatile fuels have more of the "shorter" hydrocarbons.

Each grade of hydrocarbon fuel has a flash point AND an "auto-ignition point." The flash point is the temperature at which the stuff will burst into flames WHEN EXPOSED TO AN IGNITION SOURCE, like a spark or a flame. The auto-ignition point is the temperature at which it will burst into flames when exposed to nothing but heat. (Most importantly, the "heat of compression." When a gas is compressed, its temperature rises since the same heat energy is concentrated in a smaller volume.)

A fuel's resistance to auto-ignition is called its "octane rating." Octane is a type of hydrocarbon which is used as a reference for resistance to auto-ignition.

A fuel's "desire" to auto-ignite is called its "cetane rating." Cetane is a hydrocarbon that is used as a reference for a fuel's tendency to WANT to self-ignite.

Oddly enough, as you go up the hydrocarbon "ladder" from heaviest to lightest you find that flash point drops (it's easier to set fire to propane with a Bic lighter than a puddle of cold asphalt), but auto-ignition temperature FALLS. (It's EASIER to get a spray of hot asphalt to ignite by simply compressing it than to ignite a same-termperature spray of propane.) A heavy fuel oil resists "flashing", as from a spark or a flame. It's relatively difficult to set fire to a puddle of fuel oil, compared to gasoline. But if the fuel oil is atomized in an oxygen atmosphere it is EASIER to ignite by compressing the mixture than it would be to ignite, say, gasoline or propane.

Octane and Cetane are "opposites", in a way. The higher the octane number, the lower the cetane number...and vice-versa.

Gasoline has high octane but low cetane. It RESISTS auto-ignition, which is why it's used in gasoline engines. Those engines compress an air-fuel mixture, and it's important that the fuel resist auto-ignition until the spark plug fires.

Diesel fuel has low octane but high CETANE. It LIKES to auto-ignite under compression. Diesel engines compress only air; then the fuel is sprayed into the hot combustion chamber where it instantly begins to burn. Gasoline in a diesel engine resists ignition at first, but then explodes violently and burns way too hot. That's how gasoline damages diesel engines. It hammers them to pieces, and burns holes in the pistons...precisely because it RESISTS igniting in the absence of a spark.

#2 Diesel has a higher energy content and a higher cetane number than #1 Diesel. But it also has a higher gel point, so it's mixed with (or replaced by) #1 Diesel in the winter to prevent fuel gelling. This is why your engine is generally noisier in the winter, even when warm, and your fuel economy and power will drop a bit. It is helpful to add some cetane boost to your winterized fuel. The car will start easier and run quieter, and you'll regain SOME of the lost power and economy because the fuel will burn more efficiently. But cetane booster cannot change the fact that the lighter grade fuel oil has less chemical energy content in the first place. You can't make something out of nothing...you can only make the most of the "something."

-mickey

[ October 10, 2001: Message edited by: mickey ]
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