Different Types of Fuel

by Richard Rowe

If it burns, sparks or gets hot, somebody's tried to run a car on it. Since the earliest internal combustion engines, folks have tried to run engines on hydrogen, acetylene, gunpowder, moonshine, potatoes and almost anything else with a hydrogen and oxygen atom. Yes, that even includes seawater -- slightly modified, of course. The auto industry might have spent the last century standardizing on liquefied dinosaurs, but the way things are going it's only a matter of time before liquid fossil fuels go all but extinct. Again.

Oil-Derived Fuels

Oil storage tank.

Since J.D. Rockefeller started underwriting auto development in the 19th century, the world has been largely fixated on finding new ways to separate liquefied dead stuff into different kinds of fuel, according to the "weight" of the different parts. "Heavy" fuels have long, complex chains of hydrogen and carbon atoms, and "lighter" fuels typically have shorter chains with fewer carbon atoms. Refining oil means essentially splitting the heavier parts of the oil from the lighter parts. About half a barrel of oil is gasoline-grade hydrocarbon fuel, and roughly a quarter of it is heavier diesel. About 10 percent of the barrel is heavier jet fuel, and 4 percent or so is liquefied petroleum gas composed primarily of propane and butane. LPG is often confused with "natural gas," which is primarily methane and ethane, and isn't necessarily or often a "fossil fuel."

Biofuels

Corn field that will be used for ethanol fuel.

The primary difference between fossil fuels and "biofuels" is that fossil fuels are hydrocarbon fuels made from stuff that died a long time ago, and biofuels are made from stuff that died more recently. But the standard definition of biofuels today includes two primary types: "light" fuels made from different types of alcohols -- primarily ethanol -- and "heavy" types made from organisms that naturally contain or produce a lot of carbon-rich oils. Ethanol can be made from almost any plant, including dry grass and wood clippings if they go through a separate process to become "cellulosic ethanol." But sugar- and starch-rich plants like corn, sugar beets and potatoes are a bit easier and cheaper to turn into alcohol. Seed oils -- including grapeseed, soybean and sunflower oils -- can be easily processed into high-grade bio-diesels, and so can the algae that grows on sewage treatment ponds. Natural gas might also be considered a biofuel, since methane and ethane can be extracted from decaying organic and waste matter -- like cow dung.

Electrical Vector Fuels

Electric car receiving special fuel.

Electric cars run on power from the plug, which comes directly from the power station; so in effect, they run on whatever the power station runs on. This has lead many to criticize electric cars for being not so much "zero emissions" as "remote emissions" vehicles, because odds are good that the powerplant that charges them produces emissions, too. But maybe it doesn't; if the powerplant is solar, wind or hydroelectric, then the car's effectively powered by the sun or gravity. Electricity can serve as a "vector" or "medium" for all kinds of energy exchanges, and that goes both ways. Fuel-cell vehicles use hydrogen, which is made by passing massive amounts of electricity through water. So thermodynamically speaking, the hydrogen is just a vector, a kind of "liquid form" of the electrical energy it took to make it. The same is true of cars that run on "synthetic" hydrocarbons like the Navy's new line of infinitely renewable jet fuels made from seawater. These things are just vectors -- storage devices -- for the electrical or fuel energy it took to convert the seawater to jet fuel.

Hydrocarbon Comparison

Woman filling gas tank at gas station.

Ideally, the human race would never have to burn another drop of dead dinosaurs. Every ounce oil manufacturers pull from the ground winds up as more carbon going into the atmosphere than should be present for the biomass currently on the planet. There just aren't enough plants on Earth to eat all of the carbon dioxide fossil fuels are putting into the air, on top of the greenhouse gasses produced by other sources like volcanoes and livestock -- which is a surprisingly significant amount. Switching to biofuels made from stuff that just died puts our planet a bit closer to a state of equilibrium, something known to biofuel advocates as the "net zero" state. In this state, the plants used to make the fuels eat the carbon in the atmosphere, die, get burned and turned into CO2, and that CO2 gets eaten by the next generation of biofuel plants. If autos used nothing but biofuels, mankind could theoretically burn them forever without destroying the planet. Performance-wise, biofuels are almost always superior to fossil fuels, though it often takes a bit more fuel -- particularly in the case of ethanol and natural gas -- because they're not as energy-dense as their fossil-fuel counterparts. In terms of cost, biofuels are rapidly catching up to fossil fuels, and they're often incorporated with fossil fuels in ethanol and biodiesel blends.

Electrical Vector Comparison

Electric charging sign for electric vehicles.

Electric cars are eventually going to all but take over the marketplace -- that's a simple projection resulting from purely practical considerations, so it may be wildly erroneous. With emerging battery technologies, replaceable battery pack systems like those demonstrated by Tesla, and a degree of inherent efficiency no internal combustion engine can hope to match, there's just no way electric cars shouldn't come to dominate the auto industry in the next 25 years or so -- that is, if technology and common sense prevail. The only question will be how to best get the electrons from the point of production to the car's motor. It's unlikely anything is ever going to exceed the efficiency of simply moving those electrons along a wire and into a battery, provided it's an efficient battery. So the electrical-vector fuel sources listed earlier may or may not be able to compete with simple plug-in battery electrics over the long term. As it stands in 2014, hydrogen fuel cells are looking like a dead-end road as energy vectors go. A very good fuel cell while dead cold can, at present, theoretically run at about 83 percent efficiency. But in the real world, that often plummets to around 40 percent once the cell heats up. And that problem is compounded by the fact that the plants that make the hydrogen lose energy in the process of doing so. Ultimately, perhaps 30 to 50 percent of the electrical energy that went into making the hydrogen gets to the vehicle's wheels -- and that's no better than any diesel.

About the Author

Richard Rowe has been writing professionally since 2007, specializing in automotive topics. He has worked as a tractor-trailer driver and mechanic, a rigger at a fire engine factory and as a race-car driver and builder. Rowe studied engineering, philosophy and American literature at Central Florida Community College.

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