MEC 2050 Fuel & Exhaust
Hydrocarbons
The gasoline that powers your vehicle is produced from crude oil or petroleum.
Hydrocarbons are organic chemical compounds made up of hydrogen and carbon molecules. Petroleum consists of hydrocarbon chains of all sorts of different lengths. The overall length of a particular hydrocarbon chain depends upon the number of carbon molecules present.
Crude oil is made up of a mixture of hydrocarbons that can be turned into a wide variety of products. For example, very short chains of one to four carbons form the petroleum gases known as methane, ethane, propane, and butane.
Hydrocarbon chains of five to seven carbons form solvents, including dry cleaning fluids and paint solvents. The gasoline we use for automobile fuel is made up mainly of hydrocarbonchains with seven to eleven carbons.
The hydrocarbon chains that are longer than twelve carbons are used for heavier fuel oils, including kerosene, diesel fuel, and heating oil; lubricating oils, such as engine oil; and solid forms of petroleum, including paraffin wax, tar, and asphalt.
Normal Ignition of Gasoline Engine
Normal combustion happens when the piston reaches top dead center (TDC) after the compression stroke.
The spark plug begins the ignition of the air/fuel mixture.
Spontaneous Ignition
Spontaneous igntion is something that happens to gasoline when unique temperatures and pressures are reached that allows for various hydrocarbons to ignite randomly in relation to the piston & valve timing cycle. This is either before or after the piston reaches top dead centre.
In gasoline engines, spontaneous Ignition causes Engine Knock, which can cause harm to engine components especially if the spontaneous combustion happens in a stroke other than the combustion stroke.
Preignition
Preignition is when gasoline and air mixtue in the cylinder ignites before the spark plug fires. Pre-ignition happens because an ignition source other than the spark, such as hot spots in the combustion chamber, a spark plug that runs too hot for the application, or carbon deposits in the combustion chamber heated to incandescence by previous engine combustion events.
In order for an engine to operate at high speeds while mantaining the timing for the ignition, we need a fuel that will combust consistently and predicatbly. Because of the high degree of variability of the hydrocarbons in gasoline, we rate our fuels based on one of the most predictable eight carbon hydrocarbons we know of. Hence Octane (octa means 8):
Of the 18 isomers of normal octane (C8H18), octane gets its name from the 2,2,4-trimethylpentane compound, which is highly resistant to auto-ignition. This iso-octane has been assigned the reference value of 100 for testing purposes. The extremely unstable normal heptane (C7H16) molecule is the 0 octane reference fuel.
Because higher octane gas burns slower, it is more resistant to knock when subjected to higher RPM and cylinder pressures. Compression ratios also factor into cylinder pressures. Higher ratios cause higher cylinder pressures and therefore cause the engine to be more susceptible to pre-detonation or knock.
Gasoline is tested for it's "resistance to auto-ignition" and is assigned an equivalent octane rating.
This is NOT necessarily the amount of 2,2,4-trimethylpentane in the gasoline.
Additives
The use of nitroxide compounds alone or in combination with aromatic amines, such as substituted phenylenediamines, or phenolic antioxidants provides an effective way to prevent oxidative degradation and gum formation in gasoline.
These additives, however, must be dealt with by the exhaust system to limit air pollution.
Ethanol is a common additive to Canadian gasoline fuels. Ethanol serves to solvate water in the fuel to help stabilize it in winter months (when the moisture in the air will condense due to cold). The water can then move through the fuel system, rather than accumulating in the gas tank!
