Thread: Basics Of Fuel
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      05-16-2012, 07:55 AM   #1
EFI Logics
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Drives: '08 BMW 135i, 08 535Xi
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Basics Of Fuel

I want to take a second to talk to you guys about fuel in order to help give you a better understanding of where the actual energy in fuel comes from and the pros and cons of different fuel types. Fuel is very complex subject matter that extends beyond what I have posted here, but there seems to be a lot of interest here on the forums regarding different fuels so I figured this would be helpful to outline some of the basics.

Engine calibration is used in order to create precisely controlled conditions in which we can extract energy from the hydrocarbons in fuel in order to produce power. Air comprised mainly of oxygen and nitrogen is pumped through the engine, compressed by the piston with the fuel and is then ignited by a controlled spark ultimately yielding acceleration as the cylinder pressure builds and pushes the piston down. The energy from fuel comes from the heat that the hydrocarbons give off after being mixed with air and ignited in the cylinder. It is the breaking of the molecular bonds in the fuel under heat and the reforming of new ones that generate the heat energy that builds cylinder pressure and drives the piston. Octane is only a measure of knock resistance, not energy or power output. Fuels with higher octane ratings have more stable molecules that require more heat to break and are thus more resistant to detonation. Fuels that can make good power will have high chemical energy or “heating value” so that more energy can be extracted given the same fuel mass. That does not mean you cannot make good power from a fuel with lower heating value though. Other things to consider when thinking about fuel are the burn rate and heat of vaporization. The burn rate is important to keep in mind because high compression, high revving engines need fuel that burns at a faster rate in order to ensure the fuel burns quickly enough to keep up with the engine speed. The fuel density can give you a hint at the burn rate. Usually denser fuels burn slower while lighter fuels burn more quickly. A fuel’s density is usually shown by a measure of Specific Gravity, or density as a it relates to water being 1.0. Heat of vaporization is the ability of the fuel to cool the intake charge making it more dense packing more energy per volume in the engine.

Here is a cool chart that nicely lays out the energy value in different types of fuel all in one place.

For those of you interested in E85 as your fuel source, it’s interesting to look at the heating value and stoichiometric ratio in comparison to other fuel types. Although it has a lower heating value or thermal energy in comparison to other fuels it also has a high resistance to knock. E85’s knock resistance allows you to extract more power by advancing the ignition timing and increasing the boost for a more complete, powerful burn. Ethanol has a high heat of vaporization that helps it cool down the intake mixture. The downside is many people experience worse gas mileage with ethanol because ethanol has less energy than gasoline and you have to run rich mixtures in order to offset the lower energy per unit volume. 100 octane has a higher resistance to knock than regular 93-octane pump gas and a higher heating value, but as you know it also costs more an availability is an issue. Diesel is also interesting to look at because of its low octane rating and high heating value in comparison to other fuel types. Diesel engines use the heat from compression in order to create ignition. They are very thermally efficient.

An important thing to note is you do not want to use leaded race gas in your BMW! Not only is it illegal for road use, but it is also bad for oxygen sensors and catalytic converters. Fuel manufacturers like VP and Sunoco offer unleaded, oxygenated fuels that will not harm you oxygen sensors or catalytic converters. Also note that simply filling your tank with race fuel will not do much other than waste your money. In order to properly leverage the benefits of different fuel types you should get calibrated to run the specific fuel mixture.

Here are a few helpful links to help you learn about/locate different fuels that are available to us.


Last edited by EFI Logics; 05-16-2012 at 08:01 AM.