It's a strange dilemma, but if the power of an engine is determined by the difference in pressure between the combustion chamber and the exhaust, it should follow that turbocharging should be equally effective pumping air out the exhaust, than in the inlets, right? So, if this is the case, why are turbochargers exclusively pumping air in, and none are pumping air out?

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    You need to understand how a turbocharger works - it is an enthalpy change...
    – Solar Mike
    Commented Jun 19, 2020 at 14:29
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    See my answer to a somewhat related question, with a worked out example
    – Zaid
    Commented Jun 19, 2020 at 20:37
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    Ultimately you want more air to burn more fuel to make more power. Commented Jun 20, 2020 at 4:36
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    If it sucked air out, it would be a turbodischarger. :D Commented Jun 20, 2020 at 4:41
  • Why not both - a turbo and a freeflow extractor manifold/pipe, which helps scavenge during the exhaust cycle
    – Criggie
    Commented Jun 21, 2020 at 21:35

9 Answers 9


IMO this is not a stupid idea, however it doesn't actually make sense for multiple reasons:

  1. A naturally-aspirated Otto or Diesel engine by itself doesn't expand the gas even to atmospheric pressure. When opening the exhaust valve, there's an overpressure escaping – thus wasting energy – before the exhaus stroke itself starts. (This is the concrete reason why the Atkinson cycle is more efficient: it reduces the pressure at that point, by having less air in the cylinder in the first place.)
    Therefore, the turbo's backpressure actually makes sense to have – although in the exhaust stroke that requires the piston to put in some extra work to get the burnt gas out of the cylinder, the energy lost in this is less than the energy the turbo can scavenge.
    (With a non-turbo supercharger – a “compressor” – you don't get this benefit.)
  2. For efficiency, you also want a high total compression ratio (as that increases the maximum Carnot efficiency). Therefore, it would be counterproductive if the turbo were used to lower pressure in the engine. Instead, a turbo is a very practical way to increase the compression ratio.
  3. A supercharger simply puts more air into the engine. Correspondingly you can also put in more fuel, and thus get a higher power in the same displacement. But the amount of air is not something you can change after the fact at the exhaust, it needs to be done by pumping extra air in the intake – which is of course exactly what any supercharger does.

So in summary, a turbo-supercharger increases both power and effiency.
Your proposed sucker-turbo would reduce efficiency, the suction during the exhaust stroke would only provide insignificant extra power, whilst that vacuum would require a separate power source. Energy isn't for free.

  • The (swept volume + clearance volume)/clearance volume does not change whether there is a turbo or not.
    – Solar Mike
    Commented Jun 21, 2020 at 12:39
  • @SolarMike of course not, why would that change if there's a turbo? And why do you bring it up here? Commented Jun 21, 2020 at 15:52
  • Because in your point 2 you say turbo charging increases the compression ratio...
    – Solar Mike
    Commented Jun 21, 2020 at 15:54
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    @SolarMike yes it does. Compression ratio consists of the turbo boost on top of the engine's own compression. Commented Jun 21, 2020 at 17:21

The power of an engine is not determined by the difference in pressure between the combustion chamber and the exhaust. Power is determined by how much energy one can put into the combustion chamber and the efficiency of how that energy is applied.

When one is compressing the intake air, additional oxygen is being included in the "mix" allowing for more power and cleaner burning, if the engine is designed properly. This applies to both turbochargers and superchargers.

  • "The power of an engine is not determined by the difference in pressure between the combustion chamber and the exhaust" If this were true, we could use car engines all the way up into the inonosphere, no?
    – tuskiomi
    Commented Jun 19, 2020 at 14:19
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    @tuskiomi So up there where would you get enough air?
    – Solar Mike
    Commented Jun 19, 2020 at 14:25
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    @SolarMike well, if the above quote is to be believed, it's because they don't have turbochargers.
    – tuskiomi
    Commented Jun 19, 2020 at 14:35
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    @tuskiomi you can't reduce the pressure on the exhaust to less than 0 psi. So the maximum "suck" you can get is 1 bar, but the boost pressure on high performance engines can be 2 to 3 bar. Commented Jun 19, 2020 at 15:01
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    Take a look at the P51 Mustang or Supermarine Spitfire. You'll note that not only are these among the fastest piston-powered aircraft of WWII but that they're quite happily supercharged, precisely because there's so little oxygen at their service ceilings of FL410 (P51) and FL344 - FL392 (Spitfire). (Links to their supercharged engines.)
    – FreeMan
    Commented Jun 20, 2020 at 13:53

OP's question states:

It should follow that turbocharging should be equally effective pumping air out the exhaust, than in the inlet.

No, it is not as effective.

You can't reduce the pressure to less than 0 psi. So the maximum "suck" you can get is 1 bar.

The boost pressure on high performance engines can be 2 to 3 bar. But working from the exhaust side cannot achieve that. The best that could happen is better scavenging.

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    @DavidSupportsMonica The fact that compressing the intake puts more air and fuel in the cylinder, and vacuuming the exhaust does not, is not more important? Intuition says it should be Commented Jun 19, 2020 at 22:47
  • @user253751 I think you're stating the same thing, using different words. Commented Jun 19, 2020 at 22:55
  • @DavidSupportsMonica Is a 1 bar intake boost equally effective to a -1 bar exhaust boost? Commented Jun 19, 2020 at 22:56
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    Ok so if this is the correct answer then my follow up would be, "why not both"?
    – Michael
    Commented Jun 20, 2020 at 2:03
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    Where would you get the power to drive a “suck” turbo from? A normal turbo uses the enthalpy change in the exhaust gas turbine to drive it. Drive it from the crankshaft like a supercharger - then you end up with less as the pressure difference is limited.
    – Solar Mike
    Commented Jun 20, 2020 at 6:43

There is no direct path from the intake to the exhaust, at least one set of valves will be closed at all times. The exhaust valves open and the piston pushes the exhaust out of the cylinder, then the exhaust valves close and the intake valves open to allow fresh fuel-air mix in. Low pressure on the exhaust may pull the exhaust out a bit quicker but it won't increase the cylinder pressure one bit, which is what a turbo or supercharger does. Increasing exhaust flow isn't a bad thing and you may get a bit of benefit from it, but not nearly as much as compressing the intake.

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    On many engines there is an overlap when the exhaust valves are still open and the inlet valves are opening to help push the last of the combustion gases out of the combustion chamber - it’s called scavenging well, at least it was when we were doing valve timing and combustion theory...
    – Solar Mike
    Commented Jun 19, 2020 at 16:33
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    One thing to add here is that an engine's ability to fill a cylinder full of fresh air is limited by the intake side - be it through valves or otherwise - and not the exhaust. If you were to pull a full vacuum whenever the cylinder exhausts the restrictions would still be how much the intake can flow. This may improve the intake stroke slightly but in even the best case you cannot achieve more than atmospheric pressure. This ability to fill the cylinder is an engine's volumetric efficiency.
    – Techlord
    Commented Jun 19, 2020 at 16:42
  • Last thing, a NA engine can get a high percentage here - say mid 90s - but the only engines to achieve 100% or more is one that has forced induction.
    – Techlord
    Commented Jun 19, 2020 at 16:47
  • @Techlord the term you are looking for is volumetric efficiency.
    – Solar Mike
    Commented Jun 19, 2020 at 17:55
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    Is that possibly simply because they are going fast and the air piles up in the air inlet? Commented Jun 19, 2020 at 22:46

Pumping air into an engine requires work, but each joule spent pumping in pre-combustion air will increase the amount of work that is produced downstream by more than a joule. The effective power increase is the difference between the added input work and the increase in output work.

If one were to pump out the exhaust, one could reduce the amount of work being done by the non-supercharger part of the engine, but the amount of reduction would be less than the amount of extra work done by the supercharger. If one had a high speed partial-vacuum blower which was powered with free energy, and one had limited places where one could put it, placing it on an engine's exhaust would let one harvest some of that free energy as useful work, but if e.g. the supercharger would be powered by electricity, it would be more efficient to simply feed that electricity into a motor that's assisting the engine directly by driving the shaft.


Since the question doesn't specify any type of engine...

Funnily enough, there are engines which pull a vacuum on the exhaust.

They are steam engines, so the vacuum is created by simply adding cold water in a condensor, without wasting power (*) to create the vacuum. This extracts energy from the steam down to about 30C instead of the 100C boiling point at atmospheric pressure.

(*) OK, a tiny amount of power is required to pull the condensed water out. But this is crucial : if you had to create that vacuum by pumping steam or exhaust gas, that would eat the benefits.

While the very first "Newcomen atmospheric engines" worked this way, they were very inefficient because the inlet steam was at atmospheric pressure, and the condesing process was done in the cylinder itself. (This allowed low pressure boilers, avoiding boiler explosions, which was safer until materials improved)

Later, compound and triple-expansion engines used high pressure steam in 2, then 3 cylinders of increasing diameter as the pressure decreased. Here's one producing 2,000 hp at nearly 60 rpm, with the low pressure cylinder working from about 2 psia (just above atmospheric) down to a pretty good (< 1 psi) vacuum.

This lives on in turbine systems in thermal power stations.

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    Great addition. The crucial thing is of course that the steam engine really emits almost only water vapour, and that water has a boiling point above ambient temperature. This way, the condensor actually emits further energy whilst creating the vacuum (in modern power stations, to provide district heating). — Car exhaust does also contain water vapour, but mixed with too much CO₂ and N₂ for for condensation-vacuum to be usable. Commented Jun 20, 2020 at 15:07
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    @leftaroundabout in other words, condensors suck. Commented Jun 20, 2020 at 15:08

A very simple way to look at this is how much can you pump. You can only drop pressure down to zero. You can increase pressure as much as the material can take. A pump to push water uphill would only raise it about 4 feet with vaccuum, not even counting the problems with the water boiling, but by compressing at the bottom the water can be raised as much as needed.

Same thing here. It may be more complicated because there is pressure in the engine, but the best way to increase compression ratio is to start with more pressure because you cannot go below zero.


Firstly, vacuum comes with a limitation: once you create a perfect vacuum, you cannot create a further pressure drop. In contrast, generating pressure has virtually no upper bound. For instance, with respect to regular atmospheric pressure, the best vacuum we can generate is about -15 PSI. At that point, we have sucked all the air out of the container; a greater pressure difference is not attainable. If we compress air, we can attain ever-greater pressures, limited only by the ferocity of the equipment.

Secondly, the point of supercharging is to drive more fuel-air mixture into the combustion chamber: to raise the pressure there. That cannot happen if we are sucking instead of pushing. You cannot pressurize a container by sucking gas out of it; at best you will will only de-pressurize it. Even if another valve is open, only enough gas will come inside to replace what was sucked out.

Lastly, it would never make sense to be actively pulling exhaust out of the combustion chamber other than during the exhaust cycle. During the compression stroke, the exhaust valve is closed. If the exhaust valve were open during the compression stroke, the stroke would turn into an exhaust stroke: the unburned fuel-air mixture would pass right through the chamber and be ejected into the exhaust manifold.


The power in an internal combustion engine is created by the difference in pressure between the cylinder/combustion chamber and the crankcase. The higher pressure in the cylinder causes the combustion chamber to expand against the piston, which moves away from the combustion chamber toward the crankcase. This linear movement is turned into rotation by the piston/crankcase. The pressure in the combustion chamber is increased by allowing a chemical reaction between gasoline and oxygen to occur. Before the reaction, you have hydrogen bound to carbon, which is compact. After the reaction, the hydrogen and carbon that were together in gasoline are now each bound to oxygen instead. The resulting chemicals are water and carbon dioxide (mostly). The water and CO2 take up more space than the original Oxygen O2 and gasoline C5H12, and the result has less energy stored in the molecule. So the pressure, and therefore the power are increased by having more O2 molecules reacting with more C5H12 molecules. It’s easy to squirt more gasoline into the combustion chamber, but at a certain point there is no more oxygen left to react with that gasoline, and you end up wasting gasoline for no more power. To get more power, you need more oxygen inside the combustion chamber and you do that by forcing more air in there with a supercharger or turbocharger. So the point is having more chemical reactants in the chamber and not about increasing pressure for the sake of the pressure itself.

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