- Hydrocarbons
- Hydrocarbons in exhaust gases
- Oxides of nitrogen
- Particulates
- Carbon monoxide
- Carbon dioxide
- Sulfur content in fuels
Hydrocarbons
Gasoline, diesel, LP and natural gas are all hydrocarbon compounds.
Hydrocarbon emissions react with other compounds in the atmosphere to produce photo-chemical smog. Hydrocarbons are a major source of motor vehicle emissions.
Gasoline needs to evaporate easily to burn properly in an internal combustion engine. But this property also means it evaporates easily into the atmosphere at ordinary temperatures and pressures.
When a vehicle is being refuelled, hydrocarbon vapors can escape from the filler neck into the atmosphere. When the vehicle is left in the sun, its temperature increases, and fuel evaporates from the tank and, if there is a carburetter, from it as well.
An evaporation control system is fitted to modern vehicles to collect, and store the hydrocarbon vapors from the tank and the carburetter. Then, when conditions are suitable, these vapors are then drawn into the intake manifold, and burned, as part of the combustion process.
In a gasoline-engined vehicle, during an engine cycle, some of the gases escape past the piston rings into the crankcase. This is called blow-by. Some of these blow-by gases mix with heated, vaporized oil and can usually escape out into the atmosphere.
Modern vehicles direct these gases and vapors back through the inlet system to be burned during normal combustion.
Hydrocarbons in exhaust gases
Hydrocarbons can also be part of the exhaust gases.
In a 4-stroke gasoline engine, during valve overlap at top dead centre, some intake charge is drawn out of the combustion chamber into the exhaust port. Raw fuel, a mixture of hydrocarbons and air, is released into the atmosphere.
When combustion occurs in the cylinder, the walls, piston and piston rings are slightly cooler than points closer to the burning mixture. Some of the air and fuel molecules come in contact with these cooler parts, and they cool down, until their temperature becomes too low for combustion to occur. They are left unburned, and when the exhaust port opens, they leave the cylinder.
Misfiring of the ignition can result in unburned fuel leaving the cylinder when the exhaust port opens.
If an excessively rich air-fuel mixture is used, there is too much fuel for the quantity of air. Combustion will be incomplete, and any unburned fuel will leave the cylinder through the exhaust port.
If an excessively lean mixture is used, then combustion takes longer, and the flame may extinguish before it is complete. When the exhaust port opens, unburned hydrocarbons will be exhausted from the cylinder.
Oxides of nitrogen
Air that’s drawn from the atmosphere into an engine contains almost 80% Nitrogen. Under the high temperatures and pressure of combustion, this nitrogen combines with oxygen to produce oxides of nitrogen. Almost all internal combustion engine exhaust gases contains these chemicals. They are more likely to be produced when high peak temperatures occur during combustion.
If a lean mixture is used, formation of hydrocarbons and carbon monoxide is reduced, but for oxides of nitrogen, it is increased. This is due to the high temperature, and the increase in available oxygen.
Oxides of nitrogen are claimed to be major contributors to photo-chemical smog.
Compression-ignition engines can produce high levels of oxides of nitrogen.
Oxides of nitrogen irritate the eyes, nose and throat. In extreme cases, coughing and lung damage can occur
articulates
Particulates from modern engines are usually carbon-based.
Older vehicles may produce lead-based particulates. This is caused by lead compounds used in the fuel to raise its octane rating.
In spark ignition engines, particulates are caused by incomplete combustion of rich air-fuel mixtures.
In compression-ignition engines, they are caused by a lack of turbulence and lack of oxygen.
Carbon monoxide
Carbon monoxide is an extremely poisonous gas. Inhaling it in a confined space can be lethal, and since it is has no odour or colour, it is very dangerous.
It is produced during combustion when there are not enough oxygen molecules around the hydrocarbon molecules. This can be caused by an incorrect air-fuel ratio.
In modern vehicles, carbon monoxide emissions have been reduced by better engine designs, and by chemically treating the exhaust gas.
Carbon monoxide, is a colourless, odourless, flammable and highly toxic gas. It is a major product of the incomplete combustion of carbon and carbon-containing compounds.
Sources
Carbon monoxide has many common sources. The exhaust of the internal combustion engine, when burning a carbon-based fuel (i.e. almost any fuel except pure hydrogen) contains carbon monoxide, especially when the temperature is too low to effect complete oxidation of the hydrocarbons in the fuel to water and CO
2, because the time (i.e., the residence time) available in the combustion chamber is too short, or because there is insufficient oxygen present. Usually, it is more difficult to design and operate a combustor for very low CO than for very low unburned hydrocarbons. Carbon monoxide is also present in small but significant concentrations in cigarette smoke.
Toxicity
CO binds very strongly to the iron atoms in hemoglobin, the principal oxygen-carrying compound in blood. The affinity between CO and hemoglobin is 200 times stronger than the affinity between hemoglobin and oxygen. As CO binds to the hemoglobin, it cannot be released nearly as readily as oxygen would be. Thus, the body's hemoglobin becomes saturated with CO and is rendered incapable of carrying oxygen to the body. A sufficient exposure to carbon monoxide can reduce the amount of oxygen taken up by the brain to the point that the victim becomes unconscious, and can suffer brain damage or even death from anoxia. Hemoglobin acquires a bright red colour when bound to carbon monoxide, so a casualty of CO poisoning can actually look abnormally pink-cheeked and healthy.
First aid for carbon monoxide poisoning is to immediately remove the victim from the exposure without endangering oneself, call for help, apply CPR and if possible apply oxygen first aid. Hyperbaric oxygen therapy is a treatment for carbon monoxide poisoning.
Carbon dioxide
Carbon dioxide is produced, with water, when complete combustion of air and fuel occurs.
It isn’t poisonous, but many scientists consider it a serious contributor to global warming.
Catalytic converters in gasoline-engined vehicles convert carbon monoxide to carbon dioxide.
Carbon dioxide is also produced by diesel and LPG-fuelled vehicles.
Sulfur content in fuels
Gasoline and diesel fuels contain sulfur as part of their chemical makeup. Sulfuric acid is produced when sulfur combines with water vapor formed during the combustion process, and some of this corrosive compound is emitted into the atmosphere through the exhaust.
Sulfuric acid is a major environmental pollutant, coming back to earth in contaminated rainwater. This 'acid rain' has been responsible for destroying or degrading vast areas of arable land.
As a result, the removal of sulfur from motor fuels has become a major part of most countries vehicle emission control programs.
High sulfur levels in fuel, when combined with water vapor, can also cause corrosive wear on valve guides and cylinder liners, which can lead to premature engine failure. The use of proper lubricants and correct oil drain intervals helps combat this effect and reduces the degree of corrosive damage.
Sulfur reduces catalyst efficiency in modern vehicles, and vehicles operating with higher sulfur gasoline have higher emissions than vehicles operating on lower sulfur gasoline.
There is evidence that in some instances, sulfur in gasoline may degrade the performance of oxygen sensors, which may also result in high emissions. High sulfur levels in gasoline may also impair the performance of OBD II systems on some vehicles.
Although regulations have reduced the permissible levels of sulfur in fuel, there are some side effects from using low sulfur diesel fuel.
The refining process used to reduce the sulfur level can reduce the natural lubricating properties of the diesel fuel, which is essential for the lubrication and operation of fuel system components such as fuel pumps and injectors.
Also, to reduce the sulfur content, oil companies change the overall chemical composition of the fuel and this can affect fuel pump seals, engine seals, and 'O'rings, some of which react to changes in fuel composition by swelling or shrinking.
This problem can be fixed by regular servicing and by replacing the seals with ones made from newer, less susceptible, materials.
