Exhaust pipes (4 strokes)

Bike builders tend not to put parts onto a bike unless they have a purpose. So what is the purpose of the exhaust pipe. Why is is it shaped like that, and what's inside it?

You could theoretically run an engine without an exhaust pipe. It would be incredibly noisy - the exhaust valve is opened pretty quickly after there's just been a big explosion inside the cylinder. There's probably still some burning gases, and the whole mixture is going to still be extremely hot and would burn your legs. So, you need some way of getting these hot gases away.

The exhaust pipe serves three purposes. It takes the noxious gases which come out of the engine and moves them a bit further from the rider. We've already seen how carbon dioxide, carbon monoxide and water are formed during the burning of hydrocarbons. Sulphur dioxide is formed when the sulphur in the fuel is burned. Additionally, we find oxides of nitrogen in the exhaust gases. Normally nitrogen is pretty unreactive (it makes up 80% of the air we breathe) but the extremely high temperatures generated by the spark plug can cause it to react with oxygen.

Secondly, the exhaust pipe cools down the gases. It does this by allowing them to expand. The exhaust pipe is quite narrow when it leaves the engine, and then it gets wider. Allowing a gas to expand makes it cooler. Remember how compressing the fuel/air mixture with the piston made it hotter? This is the opposite effect. Once the gases have passed through the expansion chamber they are much cooler. That's why your exhaust pipe dries out fastest near the engine when it's been raining.

Finally, the exhaust pipe reduces the noise of the engine. Every time the exhaust valve opens, there's a pressure wave (shockwave) travels along the exhaust pipe. If the exhaust pipe was just a straight tube, this wave would escape to the outside world and you'd have a noisy bike. To stop this, the exhaust pipe also acts as a silencer. It contains various plates with holes in them (baffles), which force the exhaust gases to take a convoluted route to the outside world. This tends to breaks up and cancel out much of the pressure waves, which results in a quieter bike. If you get a hole in your exhaust before the silencer then the exhaust gases will escape there and everything will get much noiser.