My name is Isaac and I am in year six. I have a project at school about car exhaust pollution and I am try to find out the main gases that come out the car exhaust and how they affect the environment. I was wondering if you could help me find out what they are, and if these questions are too hard could you please put me in contact who knows more about car exhaust gases.
Happy to help you out, mate. I’m an engineer. I studied combustion at university. This is pretty simple. (I'll let you know if it gets too hard...) Let's get to it:
WHAT'S IN AIR?
Dry air is 78 per cent nitrogen gas, 21 per cent oxygen gas and one per cent everything else. (‘Everything else’ being argon, carbon dioxide, neon, helium, methane, krypton and hydrogen gasses.)
There is also up to about 5% water vapour in air, in the form of humidity.
The basic theory is: ‘Everything else’ above is pretty insignificant by volume, if not effect - it’s either totally inert (like argon, krypton) and/or in such tiny amounts it does not really affect the combustion process (like hydrogen gas, which burns but is only 0.000055% of what you are breathing now).
More on the composition of air >>
This means, in terms of internal combustion, air is overwhelmingly just nitrogen gas and oxygen gas. In a properly running engine, the nitrogen doesn’t really do anything chemical at all - it just passes through and gets warmed up, and is then blown out the exhaust pipe. It’s the oxygen that allows the fuel to burn. Nitrogen is just along for the ride.
Petrol is (broadly) a chemical named octane (eight carbon atoms and a8 hydrogens).
Petrol is actually a cocktail of a bunch of chemicals similar to octane, which collectively burn like octane..
Petrol is really cool stuff, but potentially dangerous. It is a very condensed form of stored energy. Every kilogram of petrol (about 1.4 litres) contains 47 million joules of chemical energy. That’s enough to lift a Toyota Corolla about 3.8 kilometres into the air. (Imagine having to lift it just one centimetre into the air yourself.)
WHAT HAPPENS IN THE ENGINE?
Engines seem very complex, but really all that happens in them is a lot of burning. Very controlled burning. In a six-cylinder engine at 3000rpm, there are 150 different combustion events every second.
Blink your eyes normally. (About one-tenth of a second.) That's 15 combustion events.
It happens really fast.
WHAT GOES IN AND WHAT COMES OUT?
Petrol and air go into an engine. When you burn petrol you get water and carbon dioxide (and a lot of energy). Water and carbon dioxide, and a bunch of really warn nitrogen gas along for the ride, come out the exhaust. Mathematically, those three things are the main three components in car exhaust. Some other toxic stuff is in there as well, in much smaller amounts. We'll get to that.
Anyway, every kilo of petrol needs about 3.5 kilos of oxygen gas to burn efficiently in the engine. This produces 3.1 kilos of carbon dioxide and 1.4 kilos of water in the form of steam.
Carbon dioxide is a major greenhouse gas, which contributes to global warming.
HOW MUCH DOES AUSTRALIA BURN?
We burn about 20 billion litres of petrol in Australia every year. That’s about 14 billion kilos (14 million tonnes). It releases about 49 million tonnes of carbon dioxide into the atmosphere every year.
It’s very difficult to reduce this amount, without making engines more efficient or reducing our mobility (driving less).
This is the fundamental problem with reducing carbon dioxide emissions: the amount of fundamental change to the way we would need to live live is unacceptable to most people living in the first world.
Because combustion is not totally efficient, and because fuel and air contains impurities, exhaust gas contains other pollutants and poisons in small but significant concentrations.
Apart from carbon dioxide (which is also a deadly poison in addition to being a greenhouse gas) inefficient combustion also produces carbon monoxide (an even more deadly poison).
If there is insufficient fuel in the engine and excessive air (called a ‘lean’ mixture) this can form oxides of nitrogen (also called ‘NOx’) which forms into acid rain. It’s also a poison that irritates the respiratory system and aggravates heart disease. NOx is treated in modern engines by catalytic converters in the exhaust.
Volkswagen is very much in trouble at the moment as a result of governments around the world, including Australia, being made aware the company was cheating on the NOx emissions of its 2.0-litre diesel engines, which emitted up to 40 times the amount of NOx permitted by emissions laws. It's costing the company billions in legal fees and compensation payouts. See How Volkswagen Betrayed the World >>
Car exhaust can also contain small amounts of formaldehyde (a poison, used to preserve medical specimens) as well as unburned hydrocarbons, which are especially common while engines are warming up (because they are running too rich for efficient combustion) as well as volatile organic compounds (which smell like paint thinner, because that’s basically what they are) which form ground-level ozone - a cause of smog.
Car exhaust can contain benzene, which is a poison that damages bone marrow and can cause cancer. Diesel engine exhaust contains fine particles (Called PM10 and PM2.5) which can cause asthma, cardiovascular disease and lung cancer. Many modern diesel engines have diesel particle filters designed to trap these particles and burn them off into less harmful products during highway cruising. This process is known in the automotive industry as ‘regeneration’.
Vehicle exhaust used to contain lead, which is a neurotoxin that lowers the human IQ and causes antisocial behaviour. Tetraethyl lead >> was used as an anti-knock agent and an upper cylinder head lubricant in petrol, but was completely phased out in Australia by 2002. This is why we have ‘unleaded’ petrol today, like most of the rest of the world.
Exhaust gas, in excruciating detail >>
Exhaust is poisonous - and for this reason you should always avoid breathing it, and never run a car or any other internal combustion engine in an enclosed space.
The biggest long-term problem with exhaust is carbon dioxide, because of its link to global warming. This is also the hardest problem to solve, because the formation of carbon dioxide in exhaust is fundamental to the release of energy in the engine, from which we derive our mobility.
Clever engineering of the combustion process cannot subvert the formation of carbon dioxide in the way it can remove particles or treat NOx. This is a significant scientific challenge. Which is why young blokes like you need to learn as much about it as possible, and grow up as smart and technically trained as possible - because the future of humanity might depend on exactly this.
Please let me know how we went in our assignment.