What is the point of the exhaust?

Question

I was thinking today, what is the actual point of an exhaust?

Why can't we just have a pipe going out the front of a car instead of taking all the gases to the back of the car?

I can only see disadvantages of the current design:

• The expense of manufacturing a long metal pipe, which has to be the length of the car. It would be a lot cheaper to make it come out at the front or the side

• More to repair. Loads of old cars have leaking exhausts, and are always a nightmare to find and are expensive to fix. It would be easier to move the exhaust to the side or front and shorten it

• Added weight means more fuel consumption

• Repairing damage from going over speed humps at speed, loads of them round here have scrapes from exhausts on.

As everyone always asks what things are, here is a speed hump (shamelessly stolen from Wikipedia):

What are the points of exhausts?

Why can't we just let the gases out into the engine compartment, or out the front or side of the engine?

If we can't let them into the engine compartment, why can't we just let it out at the side of the car, instead of at the back?

Answer 1

Safety, comfort, noise and space are the things that come to mind.

Safety

Exhaust gasses are hot as hell. Hot enough that we put heat shields all over the exhaust line. In engines, we are actively trying to remove heat. Adding more from the exhaust goes completely against that and would increase overheating. Also, would you want a hot exhaust pipe near your door, nicely exposed for you to brush a leg against when getting out? I don't.

Comfort

If the exhaust came out the front or the side, you would be far more likely to breathe in your own exhaust, either through an open window or the ventilation system. No thanks.

Noise

We need to put a muffler somewhere. The space near the engine is already being used pretty heavily. So you would either have to design something smaller or live without one. Since there are laws some places regarding having a muffler and creating excess noise, car manufacturers are going to put them on there so they can sell cars.

Space

As alluded to above, there are large components (muffler, catalytic converter) that need to go somewhere. And they don't fit up by the engine.

Answer 2

Well, we can have a pipe going out the front of a car as in this design here: or its heavier predecessor: Arguably, the aerodynamic properties are not optimal in both, but here is a similar approach where aerodynamics have been considered for sure: other designs (like trucks) do account for aerodynamics by putting the exhaust tubing behind the relevant frontal surfaces and letting the exhaust stream exit sideways or upwards. You might even see this in a passenger car, although I'd doubt its usefulness:

What's common to all of these cases is that designers had plenty of room to work with to direct the gases away from the drivers'/pilots' noses. Specific properties of the respective vessels were serving the same purpose, too.

You might not have this in most standard factory model passenger car designs, so the problem had to be solved differently. As an exacerbation from the engineer's point of view, you will have to accomodate exhaust gas treatment facilites and silencers somewhere and something like a DPF/SCR system (Diesel exhaust cleaning) will take up plenty of space on its own:

Answer 3

Several things;

1. Exhaust coming out the front is dangerous. If the fumes are ingested into the cab the driver may be harmed. Normally a car is driven forward. If the exhaust is coming out the back this gives the least chance of the fumes making it inside the cab.
2. Mufflers are large components. Often there is not enough room inside the engine compartment to house them.
3. Exhaust comes out screaming hot. The long trip to the back allows it to cool off some. No one would want a car that if you walk by you run the rick of being burned.

Answer 4

The exhaust also creates a pressure differential, which can help scavenge exhaust gases from the chamber. So a properly tuned exhaust will therefore increase the fuel efficiency of the engine. Whilst an engine without an exhaust would make more power, it would also consume more fuel.

Answer 5

Adding to the excellent answers already given.

• An important point to note is that we need to have a catalytic converter somewhere in the exhaust a short exhaust will not have room to fit it.

Then there are a host of other things car manufacturers determine while designing an exhaust system.

For an engine to have optimum efficiency, the intake , combustion and exhaust systems have to be tuned together as a single entity. The exhaust is more than a stupid long steel pipe at the end of the exhaust manifold. It CONTRIBUTES to the performance of the engine.

Within the exhaust itself there are various parameters like the length, the width or the diameter etc which determine a host of factors.

For example, if you want a setup with low end power and good efficiency , you will have to go for a traditional long exhaust, if you want a race setup a short exhaust is preferred.

So for any car you drive , the exhaust pipe characteristics are carefully adjusted and tuned according to the purpose of the car , now in 90% of the commuter cars , the performance factor takes a back seat and economy and comfort come in first , thus the long exhaust till the tail.

Now , one can argue that if length of the exhaust affects the performance then why do super cars have long exhausts? Reason? to stick to Sound and emissions standards.

Answer 6

Take that airplane with the short, up-front, direct exhaust pipes. Start it up in your driveway early on a Saturday morning. See how popular you are with your neighbors.

Answer 7

Because, as has been pointed out, the infernal combustion engine has dangerous byproducts - heat and poisonous gases, as well as noise - which need to be carried away from the persons using the vehicle. Society made a bad choice a century ago, and we live with the consequences to this day. Get an electric car and you won't have to replace your exhaust/tailpipe, ever. You'll still have to watch the speed-bumps, however.