Emissions: Carbon Monoxide
by David Finlay (30 May 2000)
Along with plugging yourself into the mains and strolling off the top of Beachy Head, one of the most popular forms of committing suicide used to be to gulp down a few lungfuls of car exhaust gas. The reason this worked so well was that the exhaust contained large quantities of carbon monoxide (CO).
CO is more easily absorbed into the bloodstream than oxygen, so it goes straight into your system and effectively suffocates you. You can get away with breathing about 50 parts per million (ppm) of CO for eight hours, or up to 400 for half an hour, before poor circulation and shortness of breath set in.
As recently as the early 1980s, it was legal for 3% of the emissions from an idling engine to consist of CO, which doesn't sound too much until you realise that it equates to 30,000ppm - easily enough to send you to your eternal rest within a few seconds.
One of the aims of this series of articles is to demonstrate the astonishing improvement in exhaust emissions over the past few years, and the results in terms of CO are as impressive as they are in any other area. Within half a minute of starting up, a modern engine may be producing only 2ppm at idle, rising to perhaps 5ppm under acceleration.
As ever, the two reasons for this advance are the better engine control and the use of catalytic convertors. An engine will produce lots of CO if the air-fuel ratio is biased too heavily in favour of fuel - if the engine is running rich, in other words - so this is kept under control by an exhaust oxygen sensor consisting of a piece of beryllium.
One of the properties of beryllium is that, when warm, it produces a voltage if it comes into contact with oxygen. The sensor therefore works as a switch: if it does not detect any oxygen, it produces zero volts, but if it does detect oxygen it produces a one-volt signal which is sent to the engine management system.
When the management system receives this signal it increases the amount of fuel in the mixture going into the cylinders. But left to its own devices the system will always feed in as little fuel as possible, so when the oxygen level goes back down to a point at which the beryllium sensor no longer reacts, the air-fuel mixture will be leaned off again. Eventually oxygen starts appearing in the exhaust again and the sensor is triggered, so the whole process starts once more.
This all happens incredibly quickly. The sensor can send up to 10,000 signals per second, so the fuel mixture is being constantly adjusted and the engine is in effect always running as lean and efficiently as possible.
On its own, this efficiency reduces the amount of CO in the exhaust to about 500ppm, which is pretty good but not all that spectacular compared with the drop to 2ppm. The catalytic convertor looks after that bit. The heavy metals inside the catalyst are rich in oxygen, and at operating temperature they add this oxygen to the CO.
Carbon monoxide plus oxygen equals carbon dioxide, which is actually a greenhouse gas; but since, like every other animal on the planet, we exhale the same stuff ourselves several times a minute, we can't complain too forcibly about that. It's also far, far less dangerous than carbon monoxide.
Although the catalyst does the main work in reducing CO emissions, it can't do it all on its own. Without the fuel control, engines would produce far more CO than today's catalysts could ever cope with, and they would have to be replaced with much bigger, more expensive and less practical units.
This extraordinary improvement in CO levels is good news for those of us who like to breathe clean air, but not so good for the suicide contingent. The most damage you can do to yourself by sitting next to an exhaust pipe these days is to give yourself a slight headache from the noise.
