Transportation Fuel in Internal Combustion Engines 101

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There are many forms of transportation and also many sources of energy that enable these forms of transportation to get where they are going. Most vehicles, whether on land, in the air or at sea, require some type of fuel for their engines.

Fuels are materials that store potential energy in a form that can be released and used as heat energy. In most fuels, this potential energy is stored in the bonds of molecules that make up the fuel. This is known as chemical potential energy or chemical energy. To release this stored energy, most fuels undergo a chemical reaction called a combustion reaction (we call this burning). In this type of reaction, a fuel (often a hydrocarbon or mix of hydrocarbons) reacts with oxygen (typically from the air) to produce carbon dioxide and water. Combustion reactions also release a lot of heat (reactions which produce heat are known as exothermic reaction), which make them very useful. In most vehicles, the heat energy released by the fuel is converted into mechanical energy in the engine, which is transferred to other moving parts such as wheels and propellers as kinetic energy (energy of movement).

Fuels may be solid (wood, coal, etc.), liquid (gasoline, diesel, ethanol, etc.) or gas (propane, natural gas, hydrogen, etc.). Most modern vehicles use liquid and gas fuels, but in the past, coal and wood were used to heat water to create steam, which allowed the steam engines found in cars, trains and ships to turn the wheels and propellers.

Most of the fuels we use in vehicles today can be sorted into two categories –petroleum-based (fossil fuels) and biofuels (such as ethanol and biodiesel). The word “petroleum” refers to both naturally occurring unprocessed crude oil as well as to the products that are made up of refined crude oil. Liquid fuels used in vehicles that are in the petroleum group include gasoline, petrodiesel, aviation gasoline, aviation jet fuel and marine fuel oil. Gaseous fuels used in vehicles in the petroleum group include natural gas and propane (a gas which is stored as a liquid).

FOSSIL FUELS

Compressed Natural Gas (CNG)

CNG is made up mostly of methane which is a hydrocarbon with one carbon atom.
CNG Bus in Hamilton Ontario
Figure 1: CNG Bus in Hamilton, Ontario. Image source: Wikimedia Commons

Natural gas is used as a hydrocarbon-based substitute for gasoline, diesel fuel and propane primarily in buses (see Figure 1). Natural gas is a naturally occurring mixture of hydrocarbon gases (mostly methane – CH4) found in underground deposits called natural gas fields or near crude oil deposits (oil fields). Compressed natural gas (CNG) is made by compressing natural gas to less than one percent of the volume it occupies at standard atmospheric pressure. The compressed gas can then be stored in cylinders and used in vehicles in much the same way as gasoline is used. Biogas (gas produced through anaerobic digestion of organic matter) can be used as an alternative fuel in CNG vehicles.

Propane (Liquified Natural Gas, Auto Propane)

PROPANE is a short hydrocarbon chain with three carbon atoms.

Although propane may be more familiar as a fuel in BBQs, it is also a type of fuel used in fleet vehicles such as police cars and taxis. Some vehicles can run solely on propane, whereas others have a dual system which can use both propane and gasoline. Propane is a short gaseous hydrocarbon (C3H8) which is created as a by-product of natural gas processing and crude oil refining. When stored under pressure inside a tank, propane turns into a colourless, odorless liquid (an additive is put in so that we can smell it for safety reasons). When the pressure is released, liquid propane vaporizes and turns into gas that is used for combustion. Like gasoline engines, propane engines also use spark-ignition. Most vehicles on the road which use propane have been converted from vehicles which originally were designed to use gasoline.

Aviation Gasoline (Avgas)

AVGAS is made up of hydrocarbon chains that have 4-12 carbon atoms.

Aviation gasoline is a special type of gasoline used in piston-engined aircraft (aircraft with spark-ignition). This type of gasoline is mainly used in small private and vintage aircraft. Like automotive gasoline, avgas is a blend of liquid hydrocarbons. However, unlike automotive gasoline, avgas contains tetraethyl lead (TEL), a toxic substance used to prevent engine knocking (improper igniting of fuel which can interfere with the engines cycles). Many types of avgas have dyes added to them so that they are easily visible if spilled.

Automotive Gasoline (Mogas)

MOGAS is made up of hydrocarbon chains that have 7-11 carbon atoms.

Probably the most familiar fuel used in vehicles is gasoline (called “petrolin other Commonwealth countries such as the United Kingdom and Australia). People use gasoline as fuel in cars, pickup trucks, vans, SUVs, power boats, snowmobiles, scooters and motorcycles. Gasoline is a clear flammable fluid that ignites easily and is made up of a blend of hydrocarbons refined from crude oil plus additives such as ethanol (see below). In internal combustion gasoline engines, the fuel is mixed with air and then ignited using spark plugs (spark-ignition). Today, automotive gasoline does not contain lead as it did in the past (unleaded).

Aviation Jet Fuel (Aviation Turbine Fuel)

AVIATION JET FUEL is made up of hydrocarbon chains that have 12-16 carbon atoms.

Aviation jet fuel is a type of liquid fuel similar to diesel fuel that can be used in either compression ignition engines or turbine engines (internal combustion engine which turns a turbine). Aircraft which use this type of fuel include turboprop aircraft, jets and helicopters. There are two types of aviation jet fuels - unleaded kerosene (Jet A, JP5, JP8) and blended naphtha-kerosene (Jet B, JP4 – only used in very cold temperatures). Both types of fuels contain short hydrocarbons (5-15 carbon atoms) refined from crude oil. A number of organizations are now in the process of developing jet biofuels from plant sources such as algae and camelina.

Petroleum Diesel (Diesel, Petrodiesel)

DIESEL is made up of hydrocarbon chains that have 15-18 carbon atoms.

The next most common type of fuel after gasoline is diesel fuel. Diesel fuel is used in cars, pickup trucks, vans, SUVs, school buses, city buses, trains, power boats and ferry boats. Like gasoline, petroleum-based diesel fuel is a liquid hydrocarbon refined from crude oil. As new types of diesel fuels are being developed, diesel fuel which is petroleum-based is becoming more commonly known as petrodiesel to differentiate it from vegetable oil- or animal fat-based diesel fuel, which is known as biodiesel (see below). Diesel fuel is used in Diesel engines that are named after the German inventor Rudolf Diesel. In Diesel engines, unlike in gasoline engines, it is the heat from compressed air, rather than a spark, that ignites the fuel.

Heavy Fuel Oil (Bunker Fuel)

A sample of bunker fuel (residual fuel).
Figure 2: A sample of bunker fuel (residual fuel). Image source: Wikimedia Commons

Heavy fuel oil is a thick, dark liquid made up of long hydrocarbon chains. It is made up of distillate fuels (hydrocarbons that boil and condense during the process of fractional distillation) as well as residual fuels (the hydrocarbons that are too heavy for distillation and end up as residue). As the residual fuel is very thick, the addition of the distillates allows the fuel to flow (lowers its viscosity). Heavy fuel oil can only be used by very large ships because it must first be heated before it can be combusted using specialized equipment which would take up too much space on smaller vehicles. Heavy fuel oil is also known as bunker fuel (see Figure 2) – a name which comes from the containers in which the oil is stored on ships and in ports.

BIOFUELS

Ethanol

ETHANOL is an alcohol with two carbon atoms (C2H5OH).

Ethanol, a biofuel, is a type of alcohol which is produced from agricultural feedstocks (raw plant material). In Canada, ethanol is primarily made using corn and wheat. Extensive research is also occurring involving the potential use of cellulose from waste plant material to make ethanol. Ethanol and other alcohols are often added to gasoline because the combustion of alcohols produces less carbon monoxide and soot than the combustion of gasoline. Gasoline containing 5% ethanol is called E5 while gasoline containing 10% ethanol is called E10. There are also Flex Fuel Vehicles (FFVs) that can run on any combination of gasoline and ethanol up to 85% ethanol (this is called E85). All gasoline vehicles manufactured since the early 1980s can use gasoline that contains ethanol, and all major vehicle manufacturers approve the use of up to 10% ethanol. In 2010, the government of Canada introduced regulations requiring an average of 5% renewable content (which is typically in the form of ethanol) in gasoline across Canada.

Biodiesel

BIODIESEL is made up of esters with 8-20 carbon atoms.
Figure 3: B100 made from soybeans.
Figure 3: B100 made from soybeans. Image source: Wikimedia Commons

Biodiesel, also a biofuel, is a type of diesel fuel made by chemically reacting fat (vegetable oil or animal fat) and alcohol. Most biodiesel comes from canola, but it can also be produced from plants such as soybeans, jatropha, algae and waste vegetable oil as well as from waste animal products such as beef and chicken fat. All diesel engines can run on 100% biodiesel (see Figure 3), though it is typically sold in lower-level blends of 2%, 5%, or 20%. Most diesel engine warranties allow the engine to run on blends from B5 (5% biodiesel) to B20 (20% biodiesel). Biodiesel is currently used in some cars, trucks, buses and trains and some companies are testing biodiesel in aircraft. In 2011, the government of Canada introduced regulations requiring an average of 2% renewable content (biodiesel) in diesel fuel across Canada.



References

Thanks to Nick MacCallum, P.Eng for helping in the review of this backgrounder.

Kim Taylor

Kim Taylor is an Education Specialist at Let’s Talk Science. A true ‘nature-girl’, she is happiest outdoors, whether it be in a beautiful provincial park or her community vegetable garden. She is interested in science and technology of all kinds and enjoys sharing cool stuff about the world with people of all ages.



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