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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):

enter image description here

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?

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    "What is the point of the exhaust?" ... It's the bit at the back, that the smoke comes out of. (sorry, couldn't resist!) – PeteCon Jun 15 '16 at 20:09
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    Adding to the excellent answers already provided: have you EVER heard a car run with nothing but an exhaust manifold? At idle it's like a barrage of extra-loud shotguns. – MonkeyZeus Jun 16 '16 at 3:10
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    @MonkeyZeus: Not to mention that if you run with an open manifold, you run the risk of sucking cold air in and possibly warping an exhaust valve. – TMN Jun 16 '16 at 12:13
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    What is the point of the speed hump picture? – Nayuki Jun 17 '16 at 5:24
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    @Nayuki To force people to slow down. – Luaan Jun 17 '16 at 7:42
52

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.

  • 3
    With a couple of these, you could think of the old aircooled VWs. I know I burned myself on the tail pipes once when I was younger. Decently loud for such a small engine due to the small muffler and short pipes. – rpmerf Jun 15 '16 at 19:49
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    Great answer, especially since I half expected all your answers to just say 'Becuzz'. – MooseLucifer Jun 15 '16 at 21:37
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    Great answer. The fumes should be in the safety category, in my opinion. The hazard of Internal Combustion fumes range from deadly poisonous in the worst case (carbon monoxide), to irritating in the midrange (NOx, oxides of nitrogen and HC, unburned gasoline), and merely oxygen-displacing in the best case (carbon dioxide). – SteveRacer Jun 15 '16 at 22:00
  • Adding to the noise comment, most noise on a car comes from the exhaust. If this is rearwards of the occupants ears, pointing backwards the noise will effectively be behind the car. If it's infront of the occupants, the noise will be significantly louder. Drive through a tunnel with the windows open and stamp on the pedal. That cacophony of noise you hear would be the constant reality if the exhaust was infront of the occupants ears. – Steve Matthews Jun 16 '16 at 8:37
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    Back in the late 60s and early 70s "side pipes" were a popular modification. This ran the exhaust out through pipes running under the rocker panels underneath the doors. I never heard of anyone getting burned (although I imagine it did happen), but I did hear about cars losing traction due to oil getting blown out the pipes and coating the rear wheel. – TMN Jun 16 '16 at 12:09
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Well, we can have a pipe going out the front of a car as in this design here: tractor or its heavier predecessor: enter image description here Arguably, the aerodynamic properties are not optimal in both, but here is a similar approach where aerodynamics have been considered for sure: piston engine plane 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: Mazda 3 tuned with truck exhausts

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: SCR catalyst

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    LRO - the steam engine led to the steam tractor and then to the tractors we know today. But even if it didn't, your comment is really not constructive. – Rory Alsop Jun 16 '16 at 14:07
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    @LightnessRacesinOrbit: As Rory mentioned, train engines were LITERALLY the predecessor to the predecessor of modern tractors (in other words, the predecessor to early tractors): historylink101.com/lessons/farm-city/early_farm_tractors.htm – slebetman Jun 16 '16 at 20:37
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    @RoryAlsop: What is this "not constructive" fad going around lately? What's "not constructive" about providing critique about a statement made in an answer? That's literally what the comments section is for. No need to get so defensive. – Lightness Races in Orbit Jun 16 '16 at 20:47
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    @LightnessRacesinOrbit I probably should have used this picture as not to overstretch the imagination of the audience. At the time of writing, I figured it would be kind of obvious to anyone familiar with the industrial history. Also, the train engine pic had black smoke in it - I love black smoke. – the-wabbit Jun 17 '16 at 7:43
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    @LightnessRacesinOrbit I agree that your comment was constructive enough, but it would have been more so if you had provided reasoning as to why you didn't agree. Either way, most of us appreciate all contributions so please keep it up c: – MooseLucifer Jun 20 '16 at 23:26
14

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.
9

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.

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    Indeed. Gas leaving the engine has sufficient momentum to cause a slight dip in pressure at just the right time to coincide with the next exhaust cycle - helping to draw out exhaust gases from subsequent exhaust strokes. A well tuned exhaust system can significantly improve engine efficiency and power output. – Wossname Jun 16 '16 at 14:04
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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.

2

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.

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    Welcome to the site! I'm thinking since OP asked the question in the first place, they don't understand how loud straight pipes can be, thus they won't understand what you're alluding to with your answer. If you could elaborate on why you won't be too popular, this would be a good answer. Thanks for contributing! – MooseLucifer Jun 16 '16 at 18:48
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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.

  • Welcome to the site. We do appreciate you being here. While you may not have to replace the exhaust/tailpipe, the battery will need to be replaced in about the same amount of time in an electric car. There are a lot larger consequences environmentally than just replacing an exhaust system. While this answer is appreciated, it surely seems more like a rant than something which will help the OP or answer the question. – Pᴀᴜʟsᴛᴇʀ2 Jun 18 '16 at 16:03

protected by Pᴀᴜʟsᴛᴇʀ2 Jan 21 '17 at 13:34

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