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Power requirement of Turbocharger vs Supercharger #4970436 01/05/19 12:05 PM
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NICAT Offline OP
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Hi.
How is it that true that turbocharger needs less power than supercharger?
The only theory is that, Turbocharger is driven by "Exhaust" gas, while the supercharger is driven by crankshaft.
Exhaust gas doesn't have its pushing power alone. Does it ? it is pushed by pistons, so, by the crankshaft at all.
Please correct me if i am wrong.

Last edited by NICAT; 01/05/19 12:07 PM.
Re: Power requirement of Turbocharger vs Supercharger [Re: NICAT] #4970442 01/05/19 12:11 PM
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A turbocharger is far more efficient as far as “free” power. It’s driven by the natural flow of exhaust gases. There is drag produced by the supercharger belt. The belt also puts some side loading on the front of the crankshaft but to what extent is debatable. The engine has to work harder to turn the blower vs spinning a turbine on a turbocharger. The movement of the pistons upward on the exhaust stroke moves the air from the cylinders into the exhaust

Last edited by mattd; 01/05/19 12:12 PM.

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Re: Power requirement of Turbocharger vs Supercharger [Re: mattd] #4970447 01/05/19 12:19 PM
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Originally Posted by mattd
A turbocharger is far more efficient as far as “free” power. It’s driven by the natural flow of exhaust gases. There is drag produced by the supercharger belt. The belt also puts some side loading on the front of the crankshaft but to what extent is debatable. The engine has to work harder to turn the blower vs spinning a turbine on a turbocharger. The movement of the pistons upward on the exhaust stroke moves the air from the cylinders into the exhaust


Sir, i still miss the point,
Why exhaust gas naturally wants to flow out ? Isn't it pushed by pistons which means it is not actually natural flow ?
Does Exhaust gas has any energy (Except the heat energy) on it, which will drive turbocharger ?



Last edited by NICAT; 01/05/19 12:19 PM.
Re: Power requirement of Turbocharger vs Supercharger [Re: NICAT] #4970449 01/05/19 12:22 PM
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mattd Offline
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There is no load on the crankshaft when the exhaust valve is open allowing the air out of the cylinder.


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Re: Power requirement of Turbocharger vs Supercharger [Re: NICAT] #4970450 01/05/19 12:25 PM
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The energy in the exhaust is mostly wasted on a non-turbo engine, fighting drag in the muffler, etc. This is your "free" energy.


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Re: Power requirement of Turbocharger vs Supercharger [Re: mattd] #4970451 01/05/19 12:27 PM
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Originally Posted by mattd
There is no load on the crankshaft when the exhaust valve is open allowing the air out of the cylinder.



If the exhaust is not connected to a turbocharger, Yes, there is almost no load on the crankshaft.
But if we close the exhaust hose complately, will there by any load on the crankshaft (at least before engine fails)? Closing the exhaust is complate restriction, while connecting exhaust to a turbocharger is partially restriction. isn't it ?




Last edited by NICAT; 01/05/19 12:28 PM.
Re: Power requirement of Turbocharger vs Supercharger [Re: NICAT] #4970457 01/05/19 12:32 PM
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Exhaust coming out of the engine is pushed out by the rising piston. It's also hot--it might be fully combusted but it's still expanding. It wants to expand, and thus has some pressure. Put a turbo in the way and it'll push against the wheel.

I'm sure it increases the work the piston has to do push the exhaust upwards, but it's still a net gain for little cost.


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Re: Power requirement of Turbocharger vs Supercharger [Re: NICAT] #4970458 01/05/19 12:32 PM
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mattd Offline
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If you close the exhaust completely the engine won’t run because it can’t breathe. The crankshaft still has momentum from the power stroke, as well as the continued power strokes from the other cylinders. You are completely over thinking this. The restriction of a turbo is negligible from what the engine gains in power. Why do you think turbo vehicles are more fuel efficient than a supercharged one?

Last edited by mattd; 01/05/19 12:33 PM.

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Re: Power requirement of Turbocharger vs Supercharger [Re: supton] #4970462 01/05/19 12:39 PM
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Originally Posted by supton
I'm sure it increases the work the piston has to do push the exhaust upwards, but it's still a net gain for little cost.


Of course, the net gain is significant compared to energy loss by the turbocharger.
But here i want to complare energy used by the turbocharger vs supercharger.

Re: Power requirement of Turbocharger vs Supercharger [Re: NICAT] #4970463 01/05/19 12:41 PM
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It doesn’t take any extra fuel to spin a turbine. It takes extra fuel to turn an additional accessory drive component.

Last edited by mattd; 01/05/19 12:44 PM.

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Re: Power requirement of Turbocharger vs Supercharger [Re: NICAT] #4970465 01/05/19 12:44 PM
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100% of the energy to drive a supercharger comes directly off the crankshaft.
In a turbocharger, part of the drive energy is subtracted from the crank in the form of lost torque due to pumping work pushing the exhaust out against turbine pressure.
But most of the drive energy comes from expanding the exhaust through the turbine, which causes a drop in temperature and pressure of the gases.
In round numbers, about 30% of fuel energy goes out the exhaust. Putting a turbine in the exhaust allows some of that energy to be recovered.
Another advantage that turbochargers have is the centrifugal compressor is more efficient than Roots or Screw compressors. Mechanically driven centrifugal superchargers, such as Vortech or ProChargers have efficiency on a par with a turbo compressor, but they take all of their power from the crankshaft.


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Re: Power requirement of Turbocharger vs Supercharger [Re: mattd] #4970468 01/05/19 12:45 PM
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Originally Posted by mattd
If you close the exhaust completely the engine won’t run because it can’t breathe. The crankshaft still has momentum from the power stroke, as well as the continued power strokes from the other cylinders. You are completely over thinking this. The restriction of a turbo is negligible from what the engine gains in power. Why do you think turbo vehicles are more fuel efficient than a supercharged one?


Again, i am not saying that , turbocharger consumes energy, so it does not worth having it. smile
If you are consuming 1Kw energy, and gaining 50Kw, it is great.

Just wanted to understand how energy required to drive the turbocharger is less than a supercharger.

Re: Power requirement of Turbocharger vs Supercharger [Re: NICAT] #4970471 01/05/19 12:51 PM
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In case you missed my other post, plain and simple, it takes no extra fuel to drive a turbocharger turbine, however there is an increased demand due to another accesoery drive the crankshaft has to spin when using a supercharger.

Last edited by mattd; 01/05/19 12:52 PM.

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Re: Power requirement of Turbocharger vs Supercharger [Re: NICAT] #4970478 01/05/19 01:01 PM
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As stated above, the energy to drive a supercharger (typically a Roots-type blower) comes directly from the crankshaft. The energy to drive the turbocharger comes from the exhaust. Some of this is basically "free" because when the exhaust valve opens the cylinder pressure is typically much higher than ambient. This is called the blowdown energy; depressurizing the cylinder to manifold pressure. The remainder of the energy is subtracted from the crankshaft due to the increased cylinder pressure during the exhaust stroke.
The turbocharger is also more efficient because the compressor stage runs about 70% peak efficiency versus 60% at best for a lobed (positive displacement) blower.
On some applications it's possible to have the intake manifold pressure be higher than the exhaust manifold pressure and extract extra power from the difference. This condition usually requires the engine be operating near maximum output (rpm/boost) with a properly sized free running (no wastegate) turbocharger(s).


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Re: Power requirement of Turbocharger vs Supercharger [Re: NICAT] #4970495 01/05/19 01:26 PM
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One of the reason a diesel (without emissions) is so much more efficient than gas is the is expanded and work extracted 18 times as opposed to 12 times for gas.

The exhaust is still hot and has heat energy. So the turbo extracts this heat for work to compress air. A perfect turbo would take in gas at say 1000 degrees and 1 PSI and extract say 500 degrees of heat energy and have that available for work, and exhaust at the same 1 PSI. Modern turbos are getting quite close to perfect. There is still a pressure drop it is now slight.

There exist turbo compounded engines where after using some of the heat energy to compress air, an additional turbo is hooked thru gears to the crankshaft, They can gain several percent of efficiency from this. We do not see if often because so much heat can be extracted there is not enough left to light the catalytic converter and regenerate the particulate filter so they have to inject extra fuel, reducing the gain. Sometimes the turbo is after the emissions controls, and it can extract much of the heat left and that increases costs and complexity a lot from the location.. I have seen this on demo engines but I do not know if this has made it into the field yet. When 5 dollar diesel comes . back as it always does, there may be enough efficiency gain to pay for it. I have also seen the compounding turbo on the exhaust and the intake air turbo located after the exhaust treatment here there will be massive turbo lag so do not know if any in the field. There is much extra expense from 2 turbos and lots of plumbing.

Another thing is common turbo used to make a jet engine. Youtube is your friend here.

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