Power requirement of Turbocharger vs Supercharger

[Cujet]

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:

 

[benjy]

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!!

 

[SteveSRT8]

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...

 

[circuitsmith]

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.

 

[DGXR]

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.

 

[WyrTwister]

Buy what you need for each repsir .

At my age , I but a fair amount from Harbor Freight , particurally , hand tools .

 

[WyrTwister]

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 .

 

[barryh]

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.

 

[mattd]

Only roots style blowers have instant response. Centrifugal superchargers require higher rpm for peak efficiency.