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Fuel FUEL Energy in an engine ultimately comes from the energy stored within the chemical bonds of the mol- ecules used as fuel. Historically, these have been hydrocarbons (usually liquids) because they have a high energy content and are easily transported and combusted. One gallon of gasoline produces 1.323108 J, the same amount of energy as from eating 110 ham- burgers! One aspect of creating efficient engines is to match engine design with fuel design. Fuel design? Yes. The size and shape of the fuel molecules play a role in how an engine operates. Combustion engines work by combining a fuel with oxygen, generally from the atmosphere. In a piston engine, this occurs near the end of the compression stroke when the spark plug fires. It is important for the fuel to burn at the cor- rect rate. If it burns too slowly, not all the fuel will burn, producing hydrocarbon pollution in the exhaust and carbon deposits that leave hot spots in the cylinder. If it burns too quickly the gas-air mixture can detonate and cause severe and sudden pressure and tempera- ture spikes that will damage the engine. The noise from this is called “knocking.” Straight alkane molecules like hexane and hep- tane knock severely while a branched molecule like trimethylpentane is very resistant to knock. The reason for this is a bit complex, but simply stated, oxygen can get to the carbons in the straight chain all at once, and so the molecule reacts very quickly, causing knocking. The branched hydrocarbon has carbon atoms that are surrounded by other carbons and can’t react with oxy- gen until after the outside carbons react, thus making it burn slower and be more resistant to knocking. emissions of harmful lead into the environment led to this being banned in the 1980s. The octane rating is a way to measure how smoothly a fuel burns. Simply stated, heptane, which knocks severely, is burned in a one-cylinder engine and the noise is recorded. This noise level is assigned an octane rating of “0.” Trimethylpentane, which knocks much less, is burned and the sound is assigned a rating of “100.” If your car uses “87” octane gasoline, this mixture makes the same amount of knocking as a mixture of 87 percent trimethyl pen- tane and 13 percent heptane. It is possible to have a mixture of hydrocarbons with octane ratings above 100. Aviation gas at the end of World War II was rated at 115 octane. Jet fuel, unlike piston engine fuel, does not need to be timing dependent since combustion in a turbine engine is continuous. Most commercial jet aircraft today burn one of two kinds of jet fuel, either JET A–1 (JET A), which is a kerosene-based fuel, or JET B–1 (JET B), which is a naphtha-based fuel. The major dif- ference is that Jet B has a lighter composition, higher volatility, and thus is easier to ignite, which makes it better for cold weather operation but more dangerous to handle. It is Jet A that is used for most commercial jet operation. Military fuel in this country is designated by a jet propellant (JP) number. JP–8 is most similar to Jet A and again is most commonly used since its flashpoint is around 38 °C, making it safer to handle. JP–4 is more like Jet B and has cold weather applications. The U.S. Air Force stopped using JP–4 in 1996 because it is more hazardous. An estimated 60 billion gallons of JP–8 are used each year worldwide. Other specialty fuels are made by blending hydro- carbon distillates to meet specific needs. These include JP–5, which is used on carrier-based aircraft because it has a high flashpoint (60 °C) that reduces the risk of fire onboard ship. Another is JP–7, which was used in the SR–71 Blackbird. Since the SR–71 operated at a wide range of temperatures, from the extreme cold of H3C CH3 Pb H3C CH3 Tetraethyl lead H3C CH3 H3C H3C n-Heptane H3C CH3 Trimethyl-pentane CH3 Previously, tetraethyl lead was used in fuel mix- tures that contained a high percentage of straight hydrocarbons to slow down the rate of burning. The Pushing the Envelope: A NASA Guide to Engines 35 CHEMISTRYPDF Image | NASA Guide to Engines
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