Fuel Chemistry


LAST MONTH, WE looked briefly at how crude oil is refined—separated into different chemical groups by molecular weight, from dark, sludgy bitumen (for asphalt and tar paper) to fine gases like butane (for pocket lighters and portable stoves). We saw that gasoline is not a single chemical, but a mixture of light chemicals which typically have between 5 and 12 carbon atoms per molecule. To understand why we need a fairly even distribution of molecular weights, just remember this rule: the lighter the distillate (the lower its molecular weight), the more easily it will vaporize, and—importantly—the less heat that will be created by its combustion. Therefore, we need some of the heavier elements for heat and strong power, and we need the lighter ones so our fuel will resist detonation, allow easy starting and give good throttle response. Next, let’s look at some of the characteristics that are important in gasoline.


We measure a fuel’s energy density as a ratio of its weight compared to water at 60° F—which is called its specific gravity. As noted, relatively heavy fuels will contain more heat energy, but lighter, higher octane fuels will vaporize more easily and burn faster. Because gasoline is lighter than water, its specific gravity is less than 1.0 and typically measures between 0.720–0.770 for pump gas. Remember that air-to-fuel mixture ratios are determined by weight, not volume. We say a ratio of 14.7 parts of air to 1 part fuel is “perfect” (or stoichiometric) because, if it burns correctly, all the oxygen atoms in the air will combine with all the hydrocarbon atoms in the fuel, leaving just CO2 and water vapor as residuals—in theory. In reality, if we make sure all the fuel is consumed by an excess of oxygen in a lean mixture, such as 16:1, we can get lower emissions and better fuel mileage, but will also have to deal with more heat. On the other hand, if we make sure all the oxygen is burned by a slightly richer mixture, such as 13.5:1, we will achieve better power and also cool the combustion chambers to prevent overheating. Computerized engine management systems usually deliver lean mixtures during cruise, and richer mixtures during wide-open throttle. REID VAPOR PRESSURE RVP measures volatility, or the ability of the fuel to create a flammable vapor— important for easy starting and good throttle response. Defined as the absolute vapor pressure exerted by a liquid at 100° F, a high RVP (say 12–14 psi) is highly volatile, while a low RVP (like 5–7 psi) doesn’t vaporize as easily. To reduce evaporative emissions that contribute to ground level ozone, the EPA regulates RVP levels in pump gas by latitudes, elevations and prevailing temperatures, requiring different fuel blends during summer and winter in different parts of the country. In southwestern summertime locations, pump fuel RVP will be low, while in northern wintertime locations, it will be high. However, high RVP fuels in warm weather are vulnerable to vaporizing in the fuel lines, creating vapor lock, and low RVP fuels in cooler weather can make engines hard to start. Leaded gasolines, still widely available for racing use, are not particularly sensitive to RVP. Some unleaded racing gasolines can have such high RVP measures that, once opened, cans of race gas can lose their high-octane fractions in mere days in hot weather, regardless of how tightly they are recapped.


As we’ve previously discussed, octane is a measure of the fuel’s resistance to detonation or auto-ignition, not burning speed. Higher octane allows higher compression ratios, which are thermally more efficient, giving better power and performance. For decades, high-octane gas relied on poisonous tetraethyl lead for knock resistance, but when it was phased out, a stew of nasty aromatic ingredients took its place, like toxic Benzene, Toluene, Ethylbenzene and Xylene. Government efforts to clean the air have almost eliminated lead and greatly reduced BTEX, but gave us oxygenated fuels as an alternative, primarily mixed with MTBE, a suspected carcinogen, which lived up to its evil reputation and turned out to badly pollute groundwater. When it, too, was phased out by the EPA, the ethanol mandate gave us corn alcohol instead. And while ethanol does boost octane, we’re also well-acquainted with its drawbacks: aluminum corrosion, plastic embrittlement, and a strong affi nity for atmospheric moisture resulting in rapid degradation and phase separation. Racing fuels on the other hand, for the most part, don’t use oxygenates to supply octane. Some use lead, other blends use various aromatic distillates to give high octane without either lead or alcohol. Race fuel suppliers also create special blends that are very stable, just for rarely used vehicles like classics and customs, which are only slightly more expensive than pump gas.


Fuel burning speed is especially important because many motorcycles use very high-rpm engines, which allow little time for the fuel to burn. For instance, at 12,000 rpm, each cylinder will be fi ring at 100 times per second, and fuel that isn’t consumed in the fi rst 20 percent of the power stroke won’t be used effi ciently. Remember, just after the piston reaches top dead center (TDC), it begins to fall, increasing the volume of the combustion chamber as it moves, which reduces the effective compression ratio and thus pressure on the piston. Large displacement V-twins used in drag racing have similar demands, as the fl ame must travel very quickly across their much larger combustion chambers to deliver optimum pressure on the pistons. Racing fuel suppliers can customize burning speeds to optimize the heat/ pressure curve over the course of the combustion event, for instance, to avoid exceeding the piston strength of certain models. Race fuels can also be designed to burn so rapidly that they require less ignition advance, which also aids power. Consider that if a sparkplug fi res at 30 degrees before top dead center (a common fi gure), the early pressure rise created by the burning fuel is actually trying to stop the piston, not drive it downward—less ignition advance equals less resistance. Once upon a time, we were warned not to run race gas in our street motorcycles without changing the jetting, otherwise they would run lean and risk overheating damage. That’s no longer true. And while highly modified engines are perfect candidates for race fuel, there are also race fuels available that require no changes whatsoever. Just buy a five-gallon can for your next track day and enjoy the extra power. Horsepower gains achieved from using race fuels can range from as little as 2 percent, which could still be a winning advantage, to as much as 15 percent—an edge no competitor could dare ignore. Depending on the type and quantity purchased, race fuel can range from $6 per gallon to more than $40. There are reasons for buying race fuel, besides the added horsepower. When racers need every advantage, the sheer consistency of race gas can be equally important. Remember, pump gas is a global commodity, created at the lowest possible price, and all the refineries share the same pipelines. When pipelines switch brands and octane ratings, the contents inevitably mix, to some degree. Was your last tankful of Texaco premium diluted with Exxon regular, or was your Exxon regular mixed with low-sulfur diesel? Was the hose at that last gas station previously dispensing regular, and how much of it remained in the hose to dilute your purchase of premium? Have you ever tried to save money at an out-of-way station that doesn’t turn over the gas in its tanks very often? The volatile high octane portion of the fuel might have had time to evaporate before you paid to put it in your tank. Hopefully, this provides some fuel for thought.


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