Ah the humble exhaust system. Lying beneath your car getting covered in dirt and soaked in the rain, it does a hard and unappreciated job directing exhaust gases out to the rear of your car. Have you ever spared a thought about the evolution of what used to be the simplest part of the car?
It started off as a pipe exiting above the passengers heads, then to the side of the of the passenger compartment. Its sole purpose was to keep the soot and smell out of the passengers way. They still had to be collected from the engine first, so the manifold was invented joining all the cylinders together. Then one day someone worked out that by slowing the gasses as they travelled through the pipes you could make them quieter. That then progressed to allowing them to slow and expand in large chambers which were imaginatively called silencers.
Hold on, if we can control the sounds with the silencer surely we can control the booming or droning noise at certain revs? So now we have a manifold, silencer and a resonator all connected by some tubing. Simple. The engine creates the gases which pass through the simple mild steel system and exits to the rear of the car. Sometimes the simplest ideas are the best.
Hold on, surely we can do better than that though?
We can control the valve timing to assist in the gasses being pushed out of the cylinder, join opposed cylinders to create a scavenging system that sucks the gasses out. We can then also add a sensor to measure the oxygen content that will control the fuel injection. If we have fuel injection we can add a catalyst to convert the harmful gasses to less harmful gasses. We can then add a second chamber after that to store all the soot, let’s call it a particulate filter.
If we have a particulate filter we’ll have to measure the pressure before it so we can tell if it’s blocked, and another one to measure the temperature. We’ll also need to measure the gasses after the catalyst to confirm it works and an extra pressure sensor to help measure the pressure across the particulate filter.
Well if we can control the gasses with the catalyst lets add another to help control the NOx so that’s another sensor added too.
Remember how simple the system was? We now have two pressure sensors, a temperature sensor, a NOx sensor, two expensive metals catalysts with a resonator and two silencers. And it’s now usually made out of stainless steel.
Is that it, I hear you ask?
No. Someone found out that injecting pig pee into the exhaust gasses reduces harmful emissions, so we now have a pee injector added to the system which in turn needs a sonar sensor to measure the amount of urine in the separate tank.
What happens to all the particulate matter that is trapped by the filter, I hear you ask? Simple. It stores it as you drive around town and releases it when you are on the motorway or in the countryside. How does it does that? It injects fuel on the exhaust stroke to assist the burning off and the gas velocity literally blows it out of the back. Ah, but how does it know you are on a motorway or the countryside?
Good question! That too is a simple answer. It looks for a constant speed above 50/60mph over a measured distance, usually 5+ miles. That way all the people in the cities can breathe fresh clean air whilst the cows and rabbits in the countryside get all the harmful, freshly burnt gasses.
Like I said, keep it simple, eh?
