Why is it that when supercharging, it's preferred to compress the input, rather than pull a vacuum on the exhaust?

Question

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?

Answer 1

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.

Answer 2

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.

Answer 3

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.

Answer 4

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.

Answer 5

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.

Answer 6

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.

Answer 7

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.

Answer 8

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.

Answer 9

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.