Naked Science Forum
Non Life Sciences => Technology => Topic started by: Wmkethi on 20/11/2011 12:30:01
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Wmkethi asked the Naked Scientists:
How does a turbocharger work?
What do you think?
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A turbo is essentially a pump.
It takes exhaust gas flow to pump air into the engine. The more exhaust, the more air pumped into the intake.
There is essentially a pressure bypass with the waste-gate, so if too much intake pressure is built up, exhaust is bypassed.
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A turbocharger is (usually) a way of getting more power out of an engine without increasing the displacement of the engine. This might allow a 1.5l engine to behave more like a 2.0l engine when extra power is required. You can think of it as a sort of temporary engine enlarger.
As Clifford said, it's basically a pump that uses kinetic energy from the hot exhaust gas when the engine is working hard to increase the pressure of the air going into the engine. Because the air is above atmospheric pressure, it contains more oxygen than the same volume of air at atmospheric pressure. This means that more fuel can be mixed with the air to produce more power, and that results in even more hot exhaust gas to drive the turbine.
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A lot of people often think that a turbocharger raises the pressure in an engine, but actually a turbocharger is an air pump (powered by the exhaust) which raises the air pressure, but only to push air into the combustion chamber faster.
The reason it's not there to simply raise the pressure in the chamber is because air/fuel mixtures ignite prematurely if you compress them too much, for efficiency reasons the pressure in the combustion chamber has to be about as high as it can be anyway, just from the action of the piston, without the turbocharger.
However, at high engine speeds, the air can't physically get into the chamber quickly enough and normally you wouldn't get enough air to give good power, a turbocharger pushes the air in past the inlet valves, and this permits much the same amount of air and combustion energy per revolution to be made at high speeds as low, and this greatly increases power.
The big problem with turbochargers is that when it compresses the air, the air can get too hot, so high performance cars often use intercoolers which cool the air after it has been compressed.
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A lot of people often think that a turbocharger raises the pressure in an engine, but actually a turbocharger is an air pump (powered by the exhaust) which raises the air pressure, but only to push air into the combustion chamber faster.
Why do you think they put superchargers on piston engine aircraft?
It's because the oxygen per volume of air increases so that the engine is effectively operating at a lower altitude. A turbocharger on a car engine effectively increases the density of the intake air so that a smaller volume of intake air can oxidize a greater mass of fuel. Because of this, the engine can have a greater effective displacement than a normally aspirated engine with the same displacement.
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A lot of people often think that a turbocharger raises the pressure in an engine, but actually a turbocharger is an air pump (powered by the exhaust) which raises the air pressure, but only to push air into the combustion chamber faster.
The reason it's not there to simply raise the pressure in the chamber is because air/fuel mixtures ignite prematurely if you compress them too much, for efficiency reasons the pressure in the combustion chamber has to be about as high as it can be anyway, just from the action of the piston, without the turbocharger.
That may be true for a carbureted engine.
Diesel engines don't have a problem with pre-ignition because they inject the fuel at near the maximum pressure of the compression cycle. So, more air in the engine means more air being compressed and higher compression. Although, this may still be a compensation for engine speed.
There has been discussion about not wanting to add turbos to engines not designed for higher compression pressures due to damaging the heads and head bolts.
One additional thing about Diesels is that they regulate power based on changing the amount of fuel being injected into the cylinder. And, thus have relatively little control over the fuel/air mixture. The turbocharger helps extend control over the fuel/air mixture. So, adding more fuel can be coupled with adding more air, and thus a cleaner burning engine.
Superchargers are different from turbochargers in that the superchargers run off of crank power and the turbochargers run off of exhaust power.
There is at least one brand of 2-stroke Diesel supercharged engine that uses the supercharger to blow the exhaust gases out of the cylinder and refill the cylinder with clean air in a single stroke.
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The reason it's not there to simply raise the pressure in the chamber is because air/fuel mixtures ignite prematurely if you compress them too much, for efficiency reasons the pressure in the combustion chamber has to be about as high as it can be anyway, just from the action of the piston, without the turbocharger.
You are correct in saying it's not there to raise the pressure in the chamber. In fact, turbocharged engines sometimes operate with lower compression ratios than the normally aspirated version of the same engine.
It's not about compression; it's about volume. The object of a turbocharger, or a supercharger, is to increase the density of the air going into the engine so that a smaller (by volume) engine can consume more fuel to produce as much maximum power as a much larger normally aspirated engine.
If standard atmospheric pressure was 150kPa instead of 100kPa, internal combustion engines would have much smaller displacement volumes, even though their thermal efficiency would not be much different. They would weigh a bit less because they would have smaller dimensions and that would help to reduce fuel consumption.
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This may be a silly question, but can someone explain to me WHY a turbocharger works, using Newtonian principles?
I never really found someone that could explain it to me satisfactorily.
As far as I understand how a turbo works, simply said:
the exhaust fumes push one side of a fan, and the other side pushes in the fresh air faster because of this, which makes it possible for the engine to work harder and produce more power. Correct?
I know about action = reaction, conservation of energy, and such.
To me a turbo seems to contradict some laws of physics. I know I am making a mental mistake here(obviously), but I don't know what I am missing/overlooking.
A turbo feels a bit like blowing into your own sails, or pushing your steering wheel to make your car go faster.
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The fact that you burn fuel in the middle changes things.
For example a jet engine is basically just a turbocharger with a flame in the middle.
The air gets compressed on the way in, and decompressed on the way out, so you would think that there's no power. And indeed there isn't, if the flame is off.
But the trick is that the flame heats the air in the middle, so you get more air coming out than went in (by volume); so more power and energy (force x distance, each volume of air leaves the engine fsster than it went in, because it's bigger).
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It does seem a little counter-intuitive that some people invest in jetting the exhaust ports to get better flow of gasses out of the engine. Then they add a restriction in the exhaust line. It certainly isn't "free energy" as the engine is expending power to pump the exhaust gases out of the engine.
I agree with wolfekeeper that it is not just exhaust out/air in. But, rather increasing the amount of air pumped into the engine allows one to increase the amount of fuel, and thus an overall increase in the power produced by the engine.
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It's true that the expander part of the turbocharger increases the back pressure in the engine's exhaust, so that means the engine has to do a bit more work to drive the turbine, so it's not exactly "free" work. However, it's more complicated than that (isn't it ever!)
The expansion ratio of the IC engine is (usually) the same as the compression ratio, and the compression ratio is pretty much determined by the type of fuel, so it's not all that negotiable. But, if the expansion ratio could be made a bit greater than the compression ratio (some engines do that btw), there is still enough energy in the products of combustion to do a bit more work.
The turbocharger takes advantage of that by further expanding the exhaust in the expander section to produce work to drive the turbine. It could, instead, do work on a generator to charge the battery in a hybrid vehicle and I believe the additional expansion would improve the overall fuel consumption, but probably not enough to justify the added complexity and cost.