Tell me about… exhaust gas
By Simon Hargreaves
There are two things worth knowing about exhaust gas: 1) what’s in it, and 2) how it behaves.
What’s in exhaust gas?
For the most part, nothing toxic. Most of it (70%) is nitrogen (which has gone in one end and come out the other unchanged), and water vapour (12%). The trouble starts with carbon dioxide (14%), which is also non-toxic but which contributes to Britain’s most successful export, talking about the weather (or Global Warming/Climate Change, as it’s called).
The connection between ‘man-made’ CO2 and global warming is, depending on who you believe, incontrovertible, tenuous or a fallacy. Either way it’s worth taking seriously, in a sort of green Pascal’s wager, because we’ve nothing to lose by accepting it and a whole planet to lose if we don’t.
But if CO2 is a long-term nasty, there’s plenty of other badness. Some of it comes from incomplete or inefficient combustion (an inherent characteristic of internal combustion engines) and some from photochemical reactions between the exhaust gas and sunlight in the atmosphere.
Carbon monoxide (1.5%) comes from partially oxidised carbon in the fuel and reduces oxygen in the blood causing a loss of consciousness and, soon after, death. It’s colourless, tasteless and has no smell, which is why it was once a popular method of dispatching oneself. Hydrocarbons (0.25% or less) come from unburned or part-burned fuel react in sunlight to form, among other things, ozone. Ozone is good high in the stratosphere, where it soaks up UV radiation but at ground level it causes severe breathing problems. Most breeds of exhaust hydrocarbons, like benzene, are carcinogenic. Then there are nitrogen oxides (0.25% or less) made from nitrogen combining with oxygen under high pressure and temperatures. NOx combines with hydrocarbons to form ozone, and also forms acid rain. There are also significant traces of peroxyacyl nitrates (lung and eye irritants), aldehydes (carcinogens).
It sound fairly unpleasant bunch of stuff, and it is. You wouldn’t want to be stuck in a lift with that lot, which is why emissions regulations exist.
How does exhaust gas behave?
This is much more interesting because it’s about how well an engine breathes, and how well an engine breathes is a measure of how efficient it is; how good it is at making torque and power.
You’d think, once you’d done the useful combustion bit of a four-stroke cycle, the exhaust stroke is all about getting rid of the spent mixture as quickly as possible. And it is, which is why the shape and dimensions of exhaust headers and pipes is crucial to aiding that gas flow.
But the engine is effectively a pump, and does not process gas at a constant velocity; piston motion and valve events set up pulsating pressure waves in the gas movement. By managing the timing of these waves – which reflect up and down both intake and exhaust – they can be used to help the engine work more efficiently (or, if you get it wrong, less efficiently). For example, if an exhaust header is ‘tuned’ dimensionally to the right length and shape, it can deliver a negative pressure wave at the exhaust port when the exhaust valve opens, which helps draw out the spent mixture from the cylinder faster than it would otherwise move. If the inlet valve is open – during overlap – the same wave can help draw fresh charge in. This is called cylinder scavenging.
One problem is the ‘helpful’ negative pressure waves can only be timed to occur at specific engine revs – which is why manufacturers developed various types of exhaust valve to help broaden the spread of revs at which useful exhaust waves will be reflected. In BMW’s S1000RR and the new GSX-R1000R, the headers feature linking pipes with valves solenoid-driven valves to make use of these waves.
One of the bummers about exhaust gas is it’s wasted energy. Internal combustion engines only convert around 20 - 25 per cent of the fuel’s energy into work (or torque, and then that’s not counting how much is subsequently lost to friction, noise, aero drag etc). Of the remaining 70 – 75 per cent, around half is lost as heat through cylinder walls into the cooling system, and the other half goes out the exhaust. Which is why, on the face of it, turbos are such a good idea – using knietic exhaust gas energy to stuff more air into the engine. Some automotive manufacturers also use exhaust heat to generate more power – BMW’s system is called the Turbosteamer (those Germans, eh?) and it uses exhaust heat to drive a steam-powered turbine to augment engine torque and increase efficiency.
Not sure how you’d fit that on a bike, where space is already at a premium. But it would be fun trying.