What's really in car exhaust?
You’re sucking on it every day. You might as well know what it is
Here’s a pleasant thought: Car exhaust will kill you in at least four different ways. Three are quick and one is pretty slow.
One: There’s no oxygen in exhaust - so that’s generally bad, unless you want to suffocate. And even if you put the oxygen back, there are two deadly poisons in it that will kill you regardless: Two: Carbon dioxide - people forget the number one greenhouse gas is also a deadly poison, which is directly toxic to humans in reasonably low concentrations, no matter how much oxygen is present in the air. (Did someone just remember Apollo 13?)
Three: carbon monoxide - an even deadlier deadly poison. And then there are...
Four: the carcinogens… And you’re breathing some car exhaust every minute of every day. We all are. You’re sleeping in it, eating dinner in it, exercising in it. In the immortal words of Agent Smith from the Matrix, you’re breathing it when you pay your taxes, and when you help your land-lady carry out her garbage.
The other day I had this kid, about 12 years old, named Isaac, track me down here on the website and ask something almost no strident greenie, no government minister for the environment, and no car enthusiast is capable of answering, off the bat:
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.
My challenge to you:
Are you any smarter than that minister for the environment? Than that car-hating greenie? Than the other car nuts you know? Can you name the top three gasses (in order of volumetric appearance) spewing from the exhaust pipe of your car?
Write them down. I dare you. Then keep reading - let's see how you go.
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 by effect - it’s either totally inert (like argon, krypton, and the other so-called 'noble' gasses) 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 the air you are breathing right 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 18 hydrogens). It's actually a hydrocarbon cocktail; 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 stored 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.) That's what makes it such a good fuel.
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 in that engine we just discussed.
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.
So, unless you said:
- Carbon dioxide
- Water (as steam)
In answer to the question I challenged you with above, you were kinda wrong...
Every kilo of petrol (a.k.a. gasoline) you burn converts into about 3.1 kilos of CO2 and 1.4 kilos of water as steam.
(It’s easier to think about the fuel - and in fact everything else in chemistry - in kilos, because the density changes with temperature but the mass is constant. A kilo is about 1.4 litres.)
And before you tell me that this doesn’t add up, the faux ‘gotcha!’ moment where you reveal 1 kilo of fuel can’t actually turn into 4.5 kilos of products, you need to add about 3.5 kilos of oxygen in the air to burn it. It’s why engines have air intakes. Mass in equals mass out for the reaction.
Air’s mostly nitrogen - about four-to-one nitrogen-to-oxygen - so every time you burn a kilo of petrol, about 12.3 kilos of nitrogen gas surfs on through for the ride. So: petrol, oxygen and nitrogen go in - nitrogen, CO2 and water come out. That’s now an engine rolls. In theory. The answer to the question is: The big three tailpipe gas emissions are, in order: really warm, but otherwise unaltered nitrogen gas, carbon dioxide, and water (in the form of steam).
Unfortunately, combustion’s not perfect, and some impurities are also produced. Twisted freaks of the combustion world. See below.
Thanks to the intense focus on the interplay between global warming and CO2, people forget that CO2 is directly toxic to humans. Death is such a cheery topic. CO2 will kill you at concentrations above about six per cent - regardless of how much oxygen there is in the air. That’s about 400 times the latent level of atmospheric CO2. Carbon monoxide is much more deadly: just one third of one per cent will kill you in about half an hour, and four times that amount will knock you unconscious in 2-3 breaths, and kill you in under three minutes. Which is why gassing yourself with car exhaust - intentionally, or not - is so damn effective - carbon monoxide will kill you quicker than, and with about as much remorse, as an ex-wife who meets your new girlfriend the same day her new hormone meds get horribly mixed up at the pharmacy.
Just to be clear on this, however: CO2 is philosophically different to all these other exhaust emissions: It’s a direct consequence. A consequence of burning petrol (or diesel, or coal, wood, ethanol, LPG - whatever). You cannot engineer CO2 out. If you want that energy, you must break those chemical bonds. CO2 is the consequence. The only solution for CO2 is to do more with less fuel, by making the combustion process more efficient, and/or by making the machine itself more efficient (by cutting rolling resistance, or aerodynamic drag, etc) and/or by reducing the mass of the vehicle.
Preferably, by doing all of those things.
The point being there’s no mad science that can abrogate the production of CO2. You burn; you get CO2 in strict mathematical proportion. End of story.
IS CO2 REALLY TOXIC?
CO2 is definitely toxic, but a lot of scientifically illiterate people argue the toss with me on this. They cite the CO2 in the air (currently about 400 parts per million) and the fact that we're not presently all dropping dead from that, or the fizz in carbonated beverages, like Coke, beer and Champagne (the fizz is CO2) and the fact that we're all not keeling over from that, similarly.
The fact is, the toxicity of everything is dosage related. CO2 is directly toxic to humans above 5-6% concentration in the air you breathe. At concentrations above that it prevents the haemoglobin in your blood from carrying oxygen to your brain and other organs. Result: You lose consciousness and die - no matter how much available oxygen gas is present in the air you're breathing.
This is why NASA set the emergency limit for CO2 in the Apollo spacecraft at 4 per cent. This was a real problem, you might recall, for Apollo 13. Mission Commander Jim Lovell said:
"We would have died of the exhaust from our own lungs if Mission Control hadn't come up with a marvellous fix."
He was talking about the famous 'square peg/round hole' lithium hydroxide canister hack to scrub CO2 from the air they were breathing in the LM. (Apollo 13: world's most ingenious engineering hack, ever.)
The US National Library of Medicine says 7-10 per cent CO2 in air causes unconsciousness in “a few minutes” and concentrations greater than 10 per cent may cause (quote) “convulsions, coma and death”.
Arizona Energy says:
“At levels over five per cent concentration, CO2 is directly toxic”.
PhD Chemist Blair King says people don’t commonly think CO2 is toxic because it can also suffocate you. He says:
“This is a common misconception based on a misunderstanding of the difference between the two terms since a chemical can be both toxic and an asphyxiant. As an asphyxiant, it will displace oxygen, and at high enough concentrations it will kill.”
The world-renowned US Centers for Disease Control and Prevention also says CO2 is toxic.
On balance: Pretty safe to label it 'toxic'.
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 vehicles more efficient or reducing our mobility (ie - 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. This is something I call the Petrochemical Paradox >>
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 Isaac need to learn as much about it as possible, and grow up as smart and technically literate as possible - because the future of humanity might depend on exactly this.