Turbocharger power requirements

[WOT]

And I have never disagreed that a turbo doesn't create more power than it uses. I want to calculate how much power it is using! That is the purpose of the entire thread. HOW DO YOU CALCULATE THE LOSS (energy required to drive the turbo).

I am taking this all in good spirit.

The power required to drive the compressor on a belt driven centrifugal SC and a turbo is essentially the same.

So, based on a given mass airflow rate and boost pressure desired and factoring in the efficiency of the compressor itself, you can calculate fairly accurately what the power would be required to drive the compressor. This forum does not make it easy to paste complex expressions and formulas inline with the text, so if you really want the derivation of how to calculate the power required, I guess I could try to include the formulas as a jpg image or something.

Anyway, as an example, a 350ci engine turning 6000rpm with a desired boost of 15 psi with a compressor efficiency of 65% would require around 53 hp to drive the compressor. So, if you were to power the compressor side with an electric motor, it would take around a 53 hp motor to drive it. If you were to then bolt the compressor onto the engine itself and power the compressor with a belt (such as on a belt driven centrifugal SC), the engine would make about 53 hp less since it is now having to power the SC.

If you were to then mount the compressor onto a turbine in the exhaust stream (such as a turbocharger), you could have it set up so that you gain most or all of the 53 hp back at the same 15 psi boost pressure.

The difference is that the power to drive it on the turbo can be set up so that there is no net cost to the motor. Since the backpressure differential in the exhaust can be less than the boost differential in the intake.

But, you are correct in that the example with the electric motor would create more power than the example with the turbo, and yes it would be due to back pressure, which makes my previous example several pages back a bit inaccurate, but it was for illustrative purposes only.

The power required to drive the compressor is given by W=MCp(T2-T1), where M = mass air flow rate, Cp = specific heat of air, and T2 and T1 are the temperatures after and before compression repectively. If you can measure these, you can calculate power required to drive the compressor. But, that still won't equal the power cost to the engine if the compressor is connected to a turbine rather than to the crank through a belt.

 

[WOT]

Well I have made a few phone calls. I have been told that no one has calculated it or measured it. I guess if I ever want to find out I am going to need an engine dyno and $$$$$$$$$$.

Engineering labs all of the country have measured and calculated this, but they typically don't share their data.

I would try to talk to an engineer at Garrett Air Reasearch if you can get one to talk to you.

 

[pokeytemplar]

The power required to drive the compressor on a belt driven centrifugal SC and a turbo is essentially the same.

So, based on a given mass airflow rate and boost pressure desired and factoring in the efficiency of the compressor itself, you can calculate fairly accurately what the power would be required to drive the compressor. This forum does not make it easy to paste complex expressions and formulas inline with the text, so if you really want the derivation of how to calculate the power required, I guess I could try to include the formulas as a jpg image or something.

Anyway, as an example, a 350ci engine turning 6000rpm with a desired boost of 15 psi with a compressor efficiency of 65% would require around 53 hp to drive the compressor. So, if you were to power the compressor side with an electric motor, it would take around a 53 hp motor to drive it. If you were to then bolt the compressor onto the engine itself and power the compressor with a belt (such as on a belt driven centrifugal SC), the engine would make about 53 hp less since it is now having to power the SC.

If you were to then mount the compressor onto a turbine in the exhaust stream (such as a turbocharger), you could have it set up so that you gain most or all of the 53 hp back at the same 15 psi boost pressure.

The difference is that the power to drive it on the turbo can be set up so that there is no net cost to the motor. Since the backpressure differential in the exhaust can be less than the boost differential in the intake.

But, you are correct in that the example with the electric motor would create more power than the example with the turbo, and yes it would be due to back pressure, which makes my previous example several pages back a bit inaccurate, but it was for illustrative purposes only.

The power required to drive the compressor is given by W=MCp(T2-T1), where M = mass air flow rate, Cp = specific heat of air, and T2 and T1 are the temperatures after and before compression repectively. If you can measure these, you can calculate power required to drive the compressor. But, that still won't equal the power cost to the engine if the compressor is connected to a turbine rather than to the crank through a belt.

So that is why the turbo is about 15% more efficient than belt driven systems then (on average). Of that 53HP required to drive the compressor on a belt driven system (100% driven by the crank) only 45HP(avg) would be needed on the turbo system (85% driven by the crank) and the other 8HP would be from utilizing the wasted energy stored in the exhaust. This percentage would change based on load, accel or decel, etc... so it could be worse in some conditions or considerably better than 15% in other conditions.

 

[pokeytemplar]

Engineering labs all of the country have measured and calculated this, but they typically don't share their data.

I would try to talk to an engineer at Garrett Air Reasearch if you can get one to talk to you.

I couldn't find any contact info for Garrett.:dontknow:

 

[DevilDawg3097]

I couldn't find any contact info for Garrett.:dontknow:

If your just looking for general info Holset engineers are pretty cool to talk to.

 

[WOT]

Jay Kavanaugh from Garret used to be active on several forums. I don't know if he still is.

I think the best way to summarize all of this discussion from my point of view is the following:

The supercharger is driven directly off of the crank. Therefore, it gets it's power required to compress the air by robbing some horsepower from the engine, which means that it is getting it's power directly from the combustion of fuel and air.

The turbo gets it power from the exhaust energy (both temperature and pressure). Since it creates backpressure, the engine will have a reduced output over say an open pipe. But it is worth mentioning that there are 2 elements at work here. First, increasing the backpressure causes the motor to have to work to pump out the exhaust, so that's where your "cost" comes into play. Second, increasing the backpressure will have the upstream effect of reducing the amount of intake charge the motor consumes at a given boost level (if the exhaust can't get out, the intake can't get in). But, since there is less air going in for a given boost level, there is less fuel required. Meaning, the motor is consuming less air and fuel at a given boost level on a turbo motor vs a SC motor. And, since turbo motors typically make more power than SC motors at a given boost level, that means less fuel burned for a given horsepower output.

So, not only does a turbo motor make more horsepower at a given boost level (due to the belt drive consuming more power than the pumping loss due to backpressure), but due to the backpressure it is also actually consuming less air (and fuel) in doing so. Hence, that's why it is hard to provide a direct answer to your question: "What are the costs to drive a turbo?" as the backpressure costs horsepower due to the pumping loss as well as the throttling effect. But the throttling effect portion (which is likely the larger of the two components) does not cost any fuel, it is mearly the equivalent not opening the throttle as far.

As a summary:
A turbo motor makes more power with less boost but requires more boost to have the same airflow through the motor (which in turn would make even more power). So to keep the same airflow rate between a turbo motor and an SC motor, a turbo would have to generate say 2-3 psi more boost, but on a large motor it would probably make 50-75 more hp than the equavalent SC motor in doing so.

This all translates to turbo motors having a lower brake specific fuel consumption (BSFC) number for a given output, meaning they use less fuel to make the same power as a SC motor.