|MadSci Network: Biochemistry|
The reaction that works the engine of an automobile is simply a combustion (burning) of petrol (gasoline), diesel oil, or LPG (propane). If the reaction goes efficiently, and to completion, the fuel reacts with oxygen from the air, and the products are carbon dioxide and water vapour, in roughly equal quantities. But because of the way the motor is designed and tuned, the actual composition of exhaust fumes is rather more complicated than that.
Air is about 78% nitrogen and 21% oxygen. Only the oxygen is used in the burning process, so 60-70% of the exhaust is the unused nitrogen from the air. It is less than 78% because more gases are produced than are used up.
So ideally the composition might be something like 70% nitrogen, 15% carbon dioxide, 15% water vapour.
However, not all of the fuel burns completely. So the exhaust stream may contain carbon monoxide (very poisonous), soot, and unburnt petrol. This is especially so if the engine is 'running rich' or 'burning oil'. Modern cars have catalytic convertors in the exhaust system to finish off the combustion process and change these substances to carbon dioxide and water as well. If a car does not have an efficient catalytic convertor, the exhaust stream may contain several per cent of these materials.
Again, the exhaust stream may contain several per cent of unused oxygen especially if the engine is 'running lean.'
The other significant material that is present in car exhaust is nitric oxide. An engine works more efficiently the higher the combustion temperature is. But whenever air is heated above about 2000 deg. C (about 3650 deg F), some of the nitrogen and oxygen react together to produce nitric oxide. When this nitric oxide cools, it can react further with the air to produce nitrogen dioxide. Nitrogen dioxide is a poisonous and corrosive brown gas. It is the substance that reacts in sunlight to start off the very complicated series of reactions that produce photochemical smog (Los Angeles type smog). In cool damp conditions, it can alternatively react with water droplets to produce nitric acid, and acid rain.
Nitric oxide may be present in exhaust fumes at levels up to 1%, but typically rather less. Ironically, more of it is produced when the engine is tuned more efficiently (higher operating temperature), or particularly when LPG fuel, which is generally cleaner burning but also hotter burning, is used instead of petrol. You might have noticed a strange smell from the exhaust when running behind a LPG car in traffic -- that is the smell of nitric oxide (or nitrogen dioxide).
Until recently, tetraethyl lead was added to petrol to help motor engines to run more smoothly. Leaded petrol is still used in many places (including here in Australia). When petrol burns, the tetraethyl lead produces lead oxide as a very fine dust. This is a poisoning hazard, both in terms of direct inhalation, and in terms of helping maintain a high content of lead in the street dust along busy roads. It is a matter of significant controversy whether or not the quantities are sufficient to make long-term lead poisoning as a result of vehicle exhaust emissions a real problem. The main factor which lead to the withdrawal of leaded fuel was not so much the human health problem as the fact that the lead compounds stopped the catalytic convertors from working properly.
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