Petroleum or crude oil is taken out of the ground as a mix of impurities and highly flammable compounds of hydrogen and carbon, called hydrocarbons.
It is processed into many products, including petrol/gasoline and diesel fuel.
Gasoline is used in different forms in different engines.
It is very volatile, mixing easily with air to form gas or vapor. The more effectively liquid gasoline is changed into vapor, the more efficiently it burns in the engine, so high volatility is desirable.
And if liquid gasoline is heated, it is even more volatile.
However, if it vaporizes in the fuel line, bubbles of vapor can block the flow of fuel and stop the engine. This is called vapor lock.
Chemical analysis and production
Gasoline is produced in oil refineries. These days, material that is simply separated from crude oil via distillation, called natural gasoline, will not meet the required specifications (in particular octane rating; see below) for modern engines, but these streams will form part of the blend.
The bulk of a typical gasoline consists of hydrocarbons with between 5 and 12 carbon atoms per molecule.
The various refinery streams that are blended together to make gasoline all have different characteristics. Some important streams are:
- Reformate, produced in a catalytic reformer with a high octane and high aromatics content, and very low olefins (alkenes).
- Cat Cracked Gasoline or Cat Cracked Naphtha, produced from a catalytic cracker, with a moderate octane, high olefins (alkene) content, and moderate aromatics level. Here, "cat" is short for "catalyst".
- Hydrocrackate (Heavy, Mid, and Light), produced from a hydrocracker, with medium to low octane and moderate aromatic levels.
- Natural Gasoline (has very many names), directly from crude oil with low octane, low aromatics (depending on the crude oil), some naphthenes (cycloalkanes) and zero olefins (alkenes).
- Alkylate, produced in an Alkylation unit, with a high octane and which is pure paraffin (alkane), mainly branched chains.
- Isomerate (various names) which is made by isomerising Natural Gasoline to increase its octane and is very low in aromatics and benzene content.
(The terms used here are not always the correct chemical terms. Typically they are old fashioned, but they are the terms normally used in the oil industry. The exact terminology for these streams varies by oil company and by country.)
Overall a typical gasoline is predominantly a mixture of paraffins (alkanes), naphthenes (cycloalkanes), aromatics and olefins (alkenes). The exact ratios can depend on
- the oil refinery that makes the gasoline, as not all refineries have the same set of processing units.
- the crude oil used by the refinery on a particular day.
- the grade of gasoline, in particular the octane.
These days, gasoline in many countries has tight limits on aromatics in general, benzene in particular, and olefins (alkene) content. This is increasing the demand for high octane pure paraffin (alkane) components, such as Alkylate, and is forcing refineries to add processing units to reduce the benzene content.
Gasoline can also contain some other organic compounds: such as organic ethers, (deliberately added) plus small levels of contaminants, in particular sulfur compounds such as disulfides and thiophenes. Some contaminants, in particular mercaptans and hydrogen sulfide must be removed because they cause corrosion in engines.
Gasoline fuel characteristics
The most important characteristic of gasoline is its Research Octane Number (RON) or octane rating, which is a measure of how resistant gasoline is to premature detonation (knocking). It is measured relative to a mixture of 2,2,4-trimethylpentane (an octane) and n-heptane. So an 87-octane gasoline has the same knock resistance as a mixture of 87% isooctane and 13% n-heptane.
There is another type of Octane, called "Motor Octane Number" (MON), which is a better measure of how the fuel behaves when under load. Its definition is also based on the mixture of isooctane and n-heptane that has the same performance. Depending on the composition of the fuel, the MON of a modern gasoline will be about 10 points lower than the RON. Normally fuel specifications require both a minimum RON and a minimum MON.
In most countries (including all of Europe and Australia) the 'headline' octane that would be shown on the pump is the RON: but in the United States and some other countries the headline number is the average of the RON and the MON, sometimes called the "roaD Octane Number" or DON, or (R+M)/2. Because of the 10 point difference noted above this means that the octane in the United States will be about 5 points lower than the same fuel elsewhere: 87 octane fuel, the "normal" gasoline in the US and Canada, would be 92 in Europe.
Romania is a supplier of "light-sweet" crude oil, which, when distilled, resulted in a gasoline with an 87 rating (DON).
It is possible for a fuel to have a RON greater than 100, because isooctane is not the most knock-resistant substance available. Racing fuels, Avgas and LPG typically have octane ratings of 110 or significantly higher.
It might seem odd that fuels with higher octane ratings burn less easily, yet are popularly thought of as more powerful. Using a fuel with a higher octane lets an engine be run at a higher compression ratio without having problems with knock. Compression is directly related to power, so engines that require higher octane usually deliver more power. Some high-performance engines are designed to operate with a compression ratio associated with high octane numbers, and thus demand high-octane gasoline. It should be noted that the power output of an engine also depends on the energy content of its fuel, which bears no simple relationship to the octane rating. Some people believe that adding a higher octane fuel to their engine will increase its performance or lessen its fuel consumption; this is false - engines perform best when using fuel with the octane rating they were designed for.
The octane rating was developed by the chemist Russell Marker. The selection of n-heptane as the zero point of the scale was due to the availability of very high purity n-heptane, not mixed with other isomers of heptane or octane, distilled from the resin of Jeffrey Pine. Other sources of heptane produced from crude oil contain a mixture of different isomers with greatly differing ratings, which would not give a precise zero point.
Controlling fuel burn
How gasoline burns must be controlled. If it burns too quickly, it can cause detonation, sometimes called ping or knock.
So special additives are used to slow down the rate of combustion.
Octane rating is a measure of how well a gasoline resists detonation. The higher the rating, the less likely it is that detonation will occur.
Higher operating pressures in the combustion chamber can also cause abnormal combustion. A lead-based, anti-knock additive used in leaded gasoline can allow these higher pressures, without loss of performance.
Lead additives are also lubricants. In high compression engines, lead compounds coat valve faces and seats and help prevent wear.
Leaded gasoline is used in older cars that have few or no emission controls. It should never be used in engines designed for unleaded fuel.
Governments now limit levels of lead in gasoline and unleaded gasoline is replacing leaded gasoline. So-called “unleaded gasoline” may contain small amounts of lead but maximum levels are tightly controlled.
For gasoline to burn properly, it must be mixed with the right amount of air.
For a gasoline engine, the air-fuel ratio by mass is about 15 to 1.
By volume, it is about 11,000 to 1. Not much gasoline, but lots of air.
A lean air-fuel mixture has more air in proportion to the amount of fuel.
A slightly lean mixture gives good fuel economy and low exhaust emissions - suitable for cruising conditions.
A mixture that’s too lean can make an engine run roughly and overheat.
A rich air-fuel mixture has less air in proportion to the amount of fuel.
A slightly rich mixture can produce more power at lower temperatures, but the extra fuel it uses steps up fuel consumption and emissions.
A too-rich mixture fouls spark plugs and causes incomplete burning, and that reduces power.
In normal combustion, the spark plug ignites the mixture, and a small ball of flame forms around the tip of the plug. The piston compresses the mixture. The flame spreads faster and moves evenly to halfway through the mixture. The piston reaches top dead centre. The flame picks up more speed, then shoots out to consume the rest of the mixture. Combustion ends with the piston a short way down the cylinder.
Detonation is a violent collision of flame fronts in the cylinder, caused by uncontrolled combustion. It occurs after the spark plug has fired. The sudden rise in pressure can cause a knocking sound. Sustained detonations can lift temperatures enough to cause pre-ignition, also called auto-ignition.
Pre-ignition occurs before normal combustion, when something in the combustion chamber heats up enough to ignite the mixture before the spark plug fires.
A mixture may also ignite simply because it is unstable under the higher heat and pressure.
Detonation and pre-ignition can cause severe damage and must be avoided.
Engine management systems use knock sensors to detect detonation and they retard ignition to stop it.
An engine that keeps running after it is switched off is said to be running-on or dieseling.
That’s because, as in a diesel engine, the fuel is igniting just from heat and pressure, with no spark from the plug.
It may even cause an engine to run backwards for a short time.
Dieseling can be caused by a high idling speed, an overheated engine, too many carbon deposits in the chamber, or by using a gasoline with an octane rating that is too low.
It can be prevented by a special valve in the carburetor idle circuit that cuts off fuel when the ignition is turned off. Or a solenoid can be used to return the throttle to a below-idle position.
Stoichiometric ratio
The term stoichiometric ratio describes the chemically correct air-fuel ratio necessary to achieve complete combustion of the fuel.
In an internal combustion engine the fuel is a mixture of different hydrocarbons, and for gasoline/petrol the average Hydrocarbon Ratio,is approximately 1.78:1 This corresponds to a mass Air to Fuel Ratio (AFR) of between 14.6 and 14:7:1. This is known as the "stoichiometric ratio".
In operation the difference between the engine's actual "air fuel ratio"(AFR) to the "stoichiometric air fuel ratio" (SAFR) is known (and is represented by the Greek letter), lambda.
If the actual mixture fed to the engine contains more air (a leaner ratio), then some oxygen is left in the exhaust. On the other hand, if the fuel is in excess, (a rich mixture ratio), then complete combustion achieved. In reality this produces a mixture of CO and CO2 in the exhaust.
To combat this modern petrol engines are fitted with catalytic converters in the exhaust that remove this CO along with any hydrocarbons left over due to poor combustion, and in addition any other polutants such as nitrogen oxides (NOx). These devices, however, only work correctly when the mixture is very close to the stoichiometric ratio. In practice the system must regulated so the average value of "lambda" is be between about 0.996 and 1.003 at all times.
Lambda is normally given a value of "1" and the stoichiometric air fuel ratio is sometimes refered to as "lambda" for simplicity where in reality lambda is the ratio between AFR and SAFR.
For simple and practical purposes it 's worked out in the following way:
For a gasoline engine the optimal operational value of the fuel /air ratio is 14.7 parts of air, to 1 part of fuel. By mass, that’s 14.7 kilograms of air to each kilogram of fuel.
So if "lambda" is given a value that equals "1", and the the air-fuel mixture is at the stoichiometric ratio, of 14.7 to 1 then "lambda" equals 14.7:1 the same as the stoichiometric ratio.
If a gasoline air-fuel mixture has a higher figure, say, 1.03, there is more air in proportion to the fuel than 14.7 to 1, and the mixture is slightly lean. A mixture with a lower lambda value has less air, proportionately, than fuel, and the mixture is slightly rich.
Engines are often run slightly on one side or the other of this perfect mix for a variety of reasons.
Other common fuels include:
- Methanol, 6.4
- Ethanol, 9.0
- MTBE, 11.7
- ETBE, TAME, 12.
The exhaust gas oxygen sensor is also called the lambda sensor, since it can be used to maintain the air-fuel ratio at lambda equal to 1, within very close limits. It can be installed in the exhaust manifold, where it measures the percentage of oxygen in the exhaust gases.
A high percentage of oxygen may mean too little fuel is entering the engine, the mixture is too lean, and lambda is greater than 1. The sensor delivers this information to the ECU, which adjusts the mixture accordingly.
Similarly, a low percentage of oxygen may indicate too much fuel is entering the engine, the mixture is too rich, and lambda is less than one.
Etymology
Stoichiometry (/stoi-kE-'a-m&-trE/) (from Greek
stoicheion meaning element or principle, and
metron measure). The
Stoichiometria of Nicephorus gave line counts of the canonical books of the New Testament and some of the Apocrypha.
Air density
The density of air is its mass per unit volume. This means a volume of air at high density has a higher mass than the same volume at low density. And if there is a larger mass of air, it will contain proportionally more oxygen.
The density of air in the atmosphere changes at different temperatures, and altitudes. That means the air that enters an engine at different locations could have very different amounts of oxygen.
The amount of oxygen in air directly affects how well it supports combustion, so it can be important in determining an air-fuel ratio for an engine.
Fuel supply system
The purpose of any fuel supply system is to deliver fuel to the engine in a form in which it can mix with air to form a combustible mixture.
The fuel must be finely atomised to mix with the air. This is why injection systems deliver the fuel under pressure.
The EFI system is a circulation system. Fuel is drawn from the tank by a fuel pump, and delivered to solenoid-operated injection valves. Fuel pressure at the valves is maintained by a fuel pressure regulator, and excess fuel flows back to the tank through a return line.
The fuel filter is directional. It is fitted between the pump and injection valves, to stop contaminants reaching other parts of the system.
The fuel pump can be mounted externally on the chassis frame, or submerged in the fuel in the tank. It is electrically operated, and electronically controlled.
It is driven by a permanent-magnet electric motor, a sealed unit integral with the pump. Fuel flows through the pump and around the electric motor when it is running. There is never an ignitable mixture inside the pump housing so there is no danger of explosion.
The pump delivers more fuel than the maximum requirement of the engine, so pressure in the fuel circulation system is maintained at all times. A fuel pressure regulator maintains fuel pressure between the pump and the injectors.
Pressure & vacuum
“Pressure” and “vacuum” are terms in everyday use in the automotive industry. Manufacturers recommend a pressure to which tires need to be inflated. And manifold vacuum is used to operate a brake booster.
Gases exert pressure on all bodies they make contact with. This applies also to the air in earth’s atmosphere. Air has mass, and as a result it exerts pressure, called atmospheric pressure, not only on the earth’s surface, but also on all objects on the earth’s surface.
Atmospheric pressure varies with altitude, but at sea level, it is calculated as 101.3 kilo-Newtons per square meter, or, 101.3 kilo-Pascals; or 14.7 pounds per square inch or PSI.
But if this is so, why does a pressure gauge read zero when its not in use? This is because the gauge indicates pressure above atmospheric pressure only. This reading is called gauge pressure.
If absolute pressure was needed, an Absolute Pressure Gauge would be required, and it would read 101.3 kilo-Pascals or 14.7 PSI when not in use.
Absolute pressure equals gauge pressure plus atmospheric pressure.
Readings on an oil pressure gauge, or a tire gauge, should really have 101.3 Kilo-Pascals or 14.7 PSI added to them. It is normal practice however, for the gauge reading alone to be taken as the accepted value.
If pressure being measured is below atmospheric, a pressure gauge with its zero reading is of no value. This is why, for pressures below atmospheric, as in a manifold vacuum or depression, a vacuum gauge is normally used.
In a gasoline engine, the position of the throttle-plate controls the volume of air, or air-fuel mixture, entering the manifold. At idle speed, the pistons draw air away from the manifold at a faster rate than it can pass the throttle-plate into the manifold. A high vacuum, or a depression, exists.
At wide-open throttle, depending on load, the vacuum is much less, and pressure in the manifold rises, closer to atmospheric.
A vacuum gauge can be calibrated in millimeters of mercury, and the scale reads from zero to 760, or inches of mercury in a scale reading from '0' to '30'.
The scale is derived from the fact that atmospheric pressure supports a column of mercury 760 millimeters or 30 inches high. This glass tube, closed at one end, is filled with mercury, then inverted in a bowl of mercury. The column of mercury in the tube falls to about 760 millimeters or 30 inches. The space above the mercury is a vacuum, and atmospheric pressure on the exposed surface of the mercury supports the column.
At sea level, atmospheric pressure supports a 30 inches or 760 millimeter column of mercury but at higher altitude, its height falls, which indicates that atmospheric pressure at that altitude is less.
A scale of zero to 760 millimeters or 0 to 30 inches can thus be used to indicate the degree of vacuum, or depression that exists, below atmospheric pressure.
At idle speeds, a vacuum gauge connected to the intake manifold will indicate a reading of approximately 450 to 500 millimeters or 17.5 to 19.5 inches of mercury, depending on altitude.
Some engine management systems signal changes in atmospheric pressure by using a barometric pressure sensor in the ECU. This is because, above sea level, air pressure is reduced. So to maintain correct air-fuel ratio, a vehicle has to reduce the amount of fuel delivered to the engine.
