Emissions: Hydrocarbons
by David Finlay (20 March 2000)
The term hydrocarbon (HC) describes any substance whose molecules consist of atoms of hydrogen, carbon and nothing else. The simplest of them all is methane, which has one carbon atom and four hydrogen atoms.
Petrol is substantially more complex. As well as various additives, it's made up of a cocktail of several dozen HCs. Some of these are aromatic hydrocarbons, which means they contain the same number of carbon and hydrogen atoms in each molecule. This is a very unpleasant characteristic, because it means they can cause cancer if you are exposed to them in sufficient quantities.
For that reason it's not a good plan to have too many aromatics coming out of your car's exhaust. An even bigger problem is that HCs create smoke - in fact they are smoke - which in turn leads to smog in the lower atmosphere.
The same smoke causes yet another problem when it works its way up to the ozone layer. Ozone is unstable, and when it comes into contact with smoke it reacts by turning into oxygen. Ultra-violet rays from the sun can't pass through ozone but they go straight through oxygen, so the more smoke you send skywards the more you increase the risk of global warming.
This is worth remembering the next time you pass a power station and watch the rubbish coming out of the chimneys.
It's theoretically possible to build an engine which emits no hydrocarbons at all, and produces nothing but water and carbon dioxide as waste products, but as with most theoretical possibilities this one is a practical non-starter. The main problems are as follows:
(1) over-rich mixture, caused by running a carburettor car with the choke on or a fuel-injected car with poor fuel mapping. So much fuel gets hurled in that it can't possibly all be burned, so it goes straight through the exhaust and vastly increases the total amount of HCs coming out of the tailpipe. The same problem arises if the engine is misfiring, when the fuel does not ignite.
(2) over-lean mixture. If there isn't enough fuel it won't burn properly, causing the same problems noted above.
(3) low operating temperature. Fuel in a cold engine does not atomise properly and remains partially liquid. Liquid fuel does not burn, so it's important for the engine to warm up as quickly as possible. The cylinder walls are also much cooler than the ignited fuel, so modern combustion chambers and pistons are designed to direct the fuel as close to the spark plug as possible.
(4) oil burning. Piston rings remove oil from the cylinder walls on downstrokes, but they can't do this with 100% efficiency; so small amounts of oil are left in the part of the cylinder where the fuel ignites. Also, the area between the top of the piston and the first piston ring always contains some leftover oil. Oil burns in a different manner from fuel and results in more - and different - HCs being produced. The blue smoke you see being produced by a worn engine is pure hydrocarbon. Two-strokes, in which the oil is actually mixed with the petrol, are a disaster area when it comes to HC emissions.
Modern design, construction quality and fuel control have resulted in a tremendous improvement in HC production. Twenty years ago a typical engine would use a carburettor - so you simply threw the fuel in and hoped that most of it burned properly - and would produce perhaps 1000 parts per million of the stuff. Today, depending on the actual car, the figure is down to about 500ppm.
However, the amount of HC which actually reaches the atmosphere can be as low as 5ppm. This is because catalytic convertors do a fabulous job of converting hydrocarbons into the aforementioned water and carbon dioxide, by the simple method of burning them.
A cold catalytic convertor isn't much use in this respect, but once it reaches 250 degrees C the heat starts breaking down the HCs. This process in turn creates more heat, so from now on the catalyst is self-heating, up to around 600 degrees C, at which no hydrocarbon will last long.
Although the major improvement in HC emissions reduction has undoubtedly come from catalytic convertors, the improvements in engine design and construction have still been worthwhile. For a start, they have contributed to the enormous improvement in emissions mentioned elsewhere in this series of articles.
More relevant to this particular discussion, though, is the fact that you can't just slap a catalyst on a car and expect it to deal with 1000ppm of hydrocarbons. The operating temperature would skyrocket and you would burn the thing out very quickly. So fitting a cat to your cherished classic car and hoping the exhaust becomes magically cleaner is not, unfortunately, a realistic option.
