In automotive, there are a handful of common combustion fuels used in passenger vehicles. Most people are familiar with gasoline and diesel, but there are also others as well as alternative fuel options to many common fuels. Here, we’ll describe the most common fuels for combustion engines in use in automotive today and some of the alternative options to them.
One of the most-used fuels in transportation globally, diesel fuel is also one of the simplest of the combustion fuels available. It’s most telling attributes are its safety, as it cannot be combusted by flame, and its ease of manufacture. Diesel is combusted through pressure rather than spark, making the core of a diesel engine relatively simple by nature. Also important is the basic nature of the diesel engine, which was originally designed to burn a variety of oil-based liquids, vegetable oil being chief among them.
The downside to diesel is its emissions-filled exhaust, which includes high amounts of particulate matter (PM) and, depending on engine design, oxides of nitrous (NOx). Most automotive diesel engines today have after-treatment scrubbers and capture systems to reduce exhaust emissions.
Alternatives to diesel are plentiful, including biologically-sourced “biodiesel,” which can come from a variety of plants. Many diesel engines can easily be converted to run on second-hand kitchen grease (“greasel“), pure vegetable oil, etc.
Ethanol is a gasoline alternative that produces less energy per gallon than does standard gasoline, but does so with a lower overall pollutant footprint. Ethanol can be sourced from a variety of plants, as it is derived from plant sugars. Globally, ethanol is often made from sugar cane, rapeseed, soy oil, and more. In the United States, it is most often made from corn (maize).
Gasoline is the most common passenger vehicle motor fuel in North America. It is derived from petroleum, usually alongside diesel fuel refinement, and burns at a relatively high energy level. The most efficient gasoline engines vaporize the liquid gasoline into the combustion chamber, maximizing burn potential.
Alternatives to gasoline are relatively few, with ethanol (above) being chief among them.
Although not often used as a combustion fuel, hydrogen (H2) is highly combustible and thus has been used as a fuel or fuel additive for some time. Hydrogen is the most compact source of usable energy known to man, but has some downsides. Chief among those is storage. Hydrogen is one of the smallest of the atoms in the periodic table, making it difficult to contain as it can “pass through” most materials. Especially when stored under pressure, which is more efficient in terms of fuel per volume.
Today, most hydrogen used as fuel is used as an additive rather than the “main course” for the engine. Hydrogen is also being explored as an alternative to chemical batteries in hydrogen fuel-cell vehicles. Hydrogen is most often produced as a byproduct of natural gas “burnoff,” in which the carbon of NG is released, leaving behind the H2 molecule. It can also be produced by splitting water atoms (H2O) with electricity. New methods of chemically splitting water are also being explored.
Natural gas (methane) is delivered as fuel in several forms. The most common are liquid petroleum (LP), compressed natural gas (CNG), and liquefied petroleum gas (LPG, “propane”). There are only slight differences between them in terms of how they’re stored, but they otherwise operate largely the same in a vehicle’s combustion engine.
Natural gas is very combustible and can be highly volatile, as the majority of its makeup is carbon and hydrogen (above). This makes it both energetic (and efficient) as a propulsion fuel as well as dangerous. In piston-using combustion engines such as those commonly used in cars, however, natural gas is generally less efficient than is gasoline. This is more to do with the mechanics of the engine than with the efficiency of the fuel.
There are few downsides to natural gas outside of the inefficiency of the mechanical engines most often used to burn it. In some engines, such as turbines, NG can be very efficient. It is also readily sourced, regionally, throughout much of the world, including North America.
This is not meant to be a comprehensive list, nor are the explanations meant to be complete. Most fuels have both pros and cons to their use. There are often many reasons that one may be chosen over another.