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Re: Power requirement of Turbocharger vs Supercharger [Re: NICAT] #4970519 01/05/19 01:42 PM
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BMWTurboDzl Offline
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Friction and resistance
Originally Posted by NICAT
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.


Turbochargers operate passively because it's the movement of exhaust gas that moves the turbine.
Superchargers have relatively more friction because they are mechanically connected to the crankshaft.


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Re: Power requirement of Turbocharger vs Supercharger [Re: A_Harman] #4970558 01/05/19 02:46 PM
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I think A_Harmann has a pretty good answer here.

Originally Posted by A_Harman
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.

Originally Posted by A_Harman
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.

Re: Power requirement of Turbocharger vs Supercharger [Re: NICAT] #4970600 01/05/19 03:39 PM
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It's the heat of combustion that forces the "natural" flow. Energy goes from higher state to lower by nature. The turbo just takes some of that energy and uses it.

But, the blower will do things a turbo won't like instant torque just off idle, zero throttle lag, etc.

There is a lot of parasitic friction in blower system. Belts, bearings, rotors, seals, etc. Much less parasitic friction in a turbo.

If you ever try to turn one over by hand you know immediately what is what ...


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Re: Power requirement of Turbocharger vs Supercharger [Re: NICAT] #4970674 01/05/19 04:49 PM
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The best answers here are the ones that refer to taking the energy from the hot high pressure gas and converting it to kinetic energy to spin the turbine and compress fresh air into the cylinder. Because there is so much energy in that exhaust gas that is about to be released into the atmosphere, some of this energy is recovered to turn the turbine. Because of this, process is more efficient than the supercharger. laugh

Last edited by Snagglefoot; 01/05/19 04:49 PM.

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Re: Power requirement of Turbocharger vs Supercharger [Re: NICAT] #4970676 01/05/19 04:49 PM
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Originally Posted by NICAT
... Just wanted to understand how energy required to drive the turbocharger is less than a supercharger.
Maybe (at least to simplify things) it is the same amount of energy. However, the supercharger obtains its input energy from a more costly source, compared to the nearly free energy available from exhaust to drive a turbine. Does that clarify?


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Re: Power requirement of Turbocharger vs Supercharger [Re: NICAT] #4970917 01/05/19 10:22 PM
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To help illustrate the point, air conditioning systems on large aircraft use turbocharger-like devices to cool the air (often called air cycle machines) . By extracting heat energy, they provide a source of extremely cold air (typically colder than -30f) for use in the cabin. For obvious reasons, ice forms instantly and must be melted to avoid trouble. It's good to recognize that a pressure drop across the turbine is normal.

I’ll avoid a discussion on all the principals involved with the air cycle machine. However, the turbine side works the same way as a turbocharger’s turbine and extracts heat from the air source (engine or APU) by expanding it through a turbine to perform work.

As most know, anytime a gas is compressed it heats up. And anytime a gas is expanded it cools.

Under heavy load, (when the turbine is doing it's work) (there can be other times it's really hot, but not doing much work) the temperature drop becomes obvious in pictures:

[Linked Image]

[Linked Image]


Last edited by Cujet; 01/05/19 10:34 PM.

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Re: Power requirement of Turbocharger vs Supercharger [Re: NICAT] #4973528 01/08/19 07:44 PM
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a turbo is ONLY on sometimes depending on your right foot + ECU tuning where a crank driven supercharger is on to some extent ALWAYS so power is generally better right off the get go but todays DI allows quicker boost no matter whats under the hood. for that reason supercharging uses more power. thats the beauty of forced induction as power can be easily increased as long as everything else is up to it. my traded stock lowly 150 hp 162 tq 2001 jetta 1.8T but a chip tune + better intake + exhaust netted 240 hp + 262 TQ safely until i traded @ 200,000 miles!! the pics of the really HOT exhausts ONLY happen when the engine is REALLY pushed + not even a bit of spirited driving will do that unless your really lean on fuel + prolly melting pistons!! with todays DI increases of 100 hp is possible safely by a good chip tuner, look at goapr for some amazing increases on VW's 2.0T. VW tunes VERY conservatively for longevity + no warranty claims as my port injected 2001 1.8T only boosted to 7 lb stock, but 25 lb after tuning with no issues!!

Last edited by benjy; 01/08/19 07:46 PM.
Re: Power requirement of Turbocharger vs Supercharger [Re: mahansm] #4973817 01/09/19 02:04 AM
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Originally Posted by mahansm
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).



Nice detail. Make sure you get to the Naval Museum or somewhere where they display the incredibly complicated engines used in many helicopters with compound turbos geared directly back to the crank. Amazingly complex!

Everyone glosses over the extreme care that must be used to keep back pressures in check with a turbo installation, and I like that you noted the dual nature of the exhaust, as it gets pushed out the open exhaust valve by expansion AND directly via the piston displacing it.


Modern turbo setups like the 3.5 Ford I rented for a month have virtually no lag. That particular engine behaves like a much larger displacement naturally aspirated one. I also recently drove a 2018 Cadillac ATS with the 2.0 turbo, which, as long as it was cool out, had no lag and instant boost right off idle. Common these days, but not always in the past...


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Re: Power requirement of Turbocharger vs Supercharger [Re: NICAT] #4974547 01/09/19 08:15 PM
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There is some extra energy that turbochargers harness that would otherwise go wasted:
Exhaust valves typically start opening before BDC so they can be open wider during the main part of the exhaust stroke.
Cylinder pressure is still pretty high at that point; and exhaust flow builds quickly.
That's why an exhaust manifold leak can make a ticking sound.
The energy of that quick "blow down" puff of gas would be muffled and wasted in a NA setup, but it will give a turbine an extra kick.


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Re: Power requirement of Turbocharger vs Supercharger [Re: NICAT] #4975384 01/10/19 06:41 PM
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DGXR Offline
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Turbos are more efficient not only because they don't take power directly off the crankshaft, but also because exhaust gases can be compressed. When the exhaust gases reach the turbine, the resistance in exhaust flow is not directly transferred back to the pistons. In other words, some of that exhaust resistance force (energy) is being converted into heat energy during exhaust compression, rather than working against the pistons.

Another turbo efficiency was mentioned above, because turbos are not always making boost. They spin fast enough to make boost only when the exhaust gases are flowing above a certain threshold, typically when the throttle is open more than 30-40% approx.

Last edited by DGXR; 01/10/19 06:44 PM.

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Re: Power requirement of Turbocharger vs Supercharger [Re: NICAT] #4976192 01/11/19 03:09 PM
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Buy what you need for each repsir .

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Re: Power requirement of Turbocharger vs Supercharger [Re: NICAT] #4976198 01/11/19 03:14 PM
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Originally Posted by NICAT
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.


When the exhaust valve first opens , the exhaust gas " excapes " very hot & under considerable pressure / velocity " .

The turbo extracts dome of that energy which otherwise would have been wasted .


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Re: Power requirement of Turbocharger vs Supercharger [Re: NICAT] #4976321 01/11/19 05:48 PM
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Can't argue with the theory of a turbo being more efficient however in practice I haven't seen much difference. I have a supercharged car that was superseded by a turbo on the same engine. The turbo version at least in it's first incarnation was no more economical than the supercharged one it replaced, in fact the official figures were slightly worse.
I suspect the reason is that due to the supercharger being electronically controlled it's not being driven much of the time, certainly not in cruise. If a comparison was made with both engines flat out the whole time then the turbo would probably win out easily.

I prefer a supercharger for the instant response.


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Re: Power requirement of Turbocharger vs Supercharger [Re: barryh] #4976480 01/11/19 08:44 PM
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Only roots style blowers have instant response. Centrifugal superchargers require higher rpm for peak efficiency.


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