Author Topic: Turbo or mechanical supercharger?  (Read 12849 times)

Offline Frank N. O.

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Turbo or mechanical supercharger?
« on: March 14, 2006, 09:45:05 AM »
Now, orignally the term supercharger covers any form of compressor before the pistons in a piston-engine, or wankel rotary for that matter, but in modern days at least in automotive circles supercharger specifically means the mechanical type and turbo means the exhuast-turbine driven version, named turbonormalizer in aviation right? For the P-38 (the Tomcat of WW2 at least in terms of size and impressive looks in the air) was said to be supercharged, is that the mechanical form or the turbo, and in general, what are the pros and cons for mechanical vs exhuast-driven compressors for aviation piston engines?
For a car then a mechanical compressor, especially the twin-screw type is very good for low power-use and high low-end torque with no need for wastegates or modified pistons but a plane-engine doesn't need low-end acceleration ability like that so is that why in current times I've only heard of turbocharged engines?

I hope the question is good enough to be here otherwise feel free to remove it.
Frank
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Offline Gulfstream Driver

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Re: Turbo or mechanical supercharger?
« Reply #1 on: March 14, 2006, 02:50:49 PM »
I'm not an expert in engine design, so feel free to correct me if I'm wrong.  I know that superchargers are heavy and sap engine power because they're mechanically driven.  Turbochargers are lighter and therefore better for light planes.  I think superchargers are used on some larger airplanes, like the old warbirds, that need the extra power.

I'm not sure what the P-38 had...

Excellent question, btw. 
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Re: Turbo or mechanical supercharger?
« Reply #2 on: March 14, 2006, 04:08:15 PM »
Yes, it's a good question,

I'm not sure why but superchargers were used on most WWII aircraft including the P-38, I suspect that's because they were superior to exhaust turbine technology at that time.   Superchargers are still used for drag racing, for instance, because of the need for rapid throttle response as the turbocharger is slow to spool up.

Some very large engines like the 4350 compounds (used in the Super Constellations) had superchargers for manifold pressure  boost AND turbochargers that were geared  to the propellers to utilize waste gasses for thrust.  These were very complex, high maintenance engines and problems were common, especially fires.

Maybe some of our engineer brethren can cast more light on the subject.


Offline Roland

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Re: Turbo or mechanical supercharger?
« Reply #3 on: March 15, 2006, 08:22:16 AM »
Frank, slowly but surely you become a technical expert. I can watch it clearly. While posting your question, which is valid since introduction of turbochargers not only in aviation, you’ve answered it already.

“For a car then a mechanical compressor, especially the twin-screw type is very good for low power-use and high low-end torque with no need for waste-gates or modified pistons but a plane-engine doesn't need low-end acceleration ability …”

See your answer? In planes wee need nominal engine speeds to drive the propeller at its best aerodynamic rate. A propeller has to:
  • accelerate the aircraft from zero to take-off speed in the shortest possible time (keeps runways short) and
    • keep the aircraft in the air by pulling it through this element at, again, the best aerodynamic rate of the wing and the propeller.
[/list]
So you will find that there are only a few speed and power rates necessary, but this few rates you try to have at their best, most economical and so on. With all this in mind constructors come out with the exhaust gas driven turbocharger. And there is no gear-shifting, too!

There are a few engines around with geared turbo chargers like the Lycoming GTIO 540 or so. “G” stand for geared (turbocharger), “T” for turbocharged, “I” for injection, “O” for opposite piston arrangement. Again they are quite rare.
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Offline Sleek-Jet

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Re: Turbo or mechanical supercharger?
« Reply #4 on: March 15, 2006, 05:20:58 PM »
There are a few engines around with geared turbo chargers like the Lycoming GTIO 540 or so. “G” stand for geared (turbocharger), “T” for turbocharged, “I” for injection, “O” for opposite piston arrangement. Again they are quite rare.


To my knowlege, the "G" stands for a gear reduction propellar drive.  GTSIO-520 and GTIO-540 really arean't all that rare.  There are still quite a few 421's, 404's, Pressurized Navajo's and Aero Commanders running around that have these hanging off the firewalls.  Continental uses the "TS" to represent turbo-supercharged. Lycoming uses the simple "T".

Frank, a Turbonormalized engine is one that doesn't surpass a sea level manifold pressure.  The exhuast driven turbo charger is rigged to maintain no more than 31 inches of manifold pressure.  Though it can provide that manifold pressure at high altitudes.  This allows the engine to operate a sea level power ratings as altitude increases.

A Turbo-supercharged engine has a turbocharger rigged to "over boost" sea level pressure.  They will be set up to provide 40, 50, or even 60 inches of manifold pressure.  Some of the WWII vintage radials and V12's had  manifold pressure red lines upward of 120 inches or more of pressure.  Of course, on todays fuels, the engine would destroy itself with detonation, but it's intersting to know they were designed for this kind of power.
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Offline Frank N. O.

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Re: Turbo or mechanical supercharger?
« Reply #5 on: March 17, 2006, 07:50:45 PM »
I ask the questions and I answer them myself, lol that's also a quote from a good old danish comedymovie. Actually I've read a lot about these things from my life-long interest in cars, driving and technology, and electronics but I know there are differences due to the nature of flying.
I however now understand why turbocharging was, so I thought, called turbonormalizing and Cessna's I've seen had the exact same power output as the NA (naturally aspirated) versions. Are there then any actually turbocharged GA engines around or doesn't that work well with planes?

Since turbos are connected with a special exhaust then I'd like to ask about these performance exhuasts I've seen being specially made for some Cessna's etc. in Plane and Pilot I think it was, they claim better performance overall, is that true and if so then how come I haven't heard of updated exhuasts for planes as much as you hear it for cars? (even though I know that far too many exhuasts for cars are more for noise than increasing efficiency)

And does planes use catalytic converters? Or is the fuel cleaner than std. fuel (from what I heard then US car-fuel is very dirty compared to the European std. especially the new one this year from EU that demands a total removal of sulfur).

Thank you very much for the friendly and informative answers :)
Frank
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Offline Sleek-Jet

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Re: Turbo or mechanical supercharger?
« Reply #6 on: March 18, 2006, 04:10:08 PM »
Frank, there are lots of GA engines are turbo-supercharged.  Cessna Turbo 210 and 206, Turbo Saratogas, any pressurized light GA airplane, the list goes on.

Keep in mind that a normally aspirated engine is rated at sea level, as soon as you start climbing, power out put starts to diminish.  A turbo charged engine will maintain it's rated horse power as the airplane climbs, until it reaches what's called it's criticle altitude.  This is the point where the turbocharger can no longer compress enough air to maintain the maximum manifold pressure.  Above this alititude, the engine responds as a normally aspriated engine does, power output drops as altitude increases.  So even though an IO-520 and a TSIO-520 are both rated at 310 hp, the TSIO-520 can make that power at much higher altitude.

I think I might have confused you in the turbonomralized vs. turbo-supercharged explanation.  A normalized eingine's turbo system will only maintain sea level pressure and no more. 

A turbo-superchaged engine's turbo will produce more pressure than sea level.  Either way produces the same results.  Induction air is forced into the cylinders to maintain sea level power output.  Just be aware there is a difference in the two systems.

Typically, a turbo normalized engine is an add on turbo system.  The manufacturer has not designed to engine to withstand high boost pressures, so someone adds a trubo, but limits the boost to no more than sea level.  The Turbo Alley Bonanza modifications is the hot thing right now.  The Cessna Turbo 182's are a good example of an OEM doing the same thing from the factory.  The engine is an 0-540, but Cessna added a simple turbo system to it, limits the boost to no more than sea level pressure, and viola, you have a turbo charged airplane. 

As far as tuned exhaust... it all comes down to $$$$$$$$$$.  Yes, there are advantages to putting a tuned system on a normally aspirated engine (like the 0-320 and 0-360 series Lycomings).  However, the pilot operating handbook does not reflect any change in performance due to the installation of a different exhaust system.  So, even though your engine is putting out a few more ponnies, as far as the feds and the book are concerned, nothing has changed.  One other factor that seems pretty obvious wants you've seen it.  The Cessna 172 and Piper Cherokee series airplanes have lots of room available between the bottom of the enigine and the cowling.  Perfect for nice long tuned exhaust runners.  However, in higher performance airplanes, like a Bonanza or Cessa 210, space is a at a premium under the bonnet.  You don't have enough room to for the long runners.   

And finally... emissions.  No, airplanes do not run catialytic converters.  The amount of emissions from general aviatoin gasoline engines is minuscule compared to cars and trucks.  Also, since we pretty much have to squeez ever last drop of performance out of our birds, we are running pretty clean as it is.  Pilots have direct control of the mixture (at least most of us, FADEC not withstanding).  Most operators run 100LL aviation fuel in the US, not sure about the EU.  Yes, it still has lead in it.  No, I would not consider it a "dirty" fuel when burned.   

« Last Edit: March 28, 2006, 10:45:16 PM by Sleek-Jet »
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Offline Frank N. O.

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Re: Turbo or mechanical supercharger?
« Reply #7 on: March 18, 2006, 05:02:10 PM »
Thanks for the explanation, although I did understand your former post explanaining the normalization, I just didn't write it that well, and in fact I suspected that even when I wrote it. Ever since I saw a turbo plane-enigne named "-normalized" and not "-charged" then I thought there must be a reason to it, also since the peak power was the same as the NA-versions I saw, but until your former former post I never knew the details however I didn't explain it well enough that I now understood it.

In cars then I've read that a mechanical compressor can keep the peak-power up to some altitude in mountain-driving where-as a turbo-charged engine looses a bit and of course a NA engine looses a lot in comparisom but the plane-enigne appearently works differently.

I know that commuter traffic and trucks are the main points of polution, although factories also produce a fair bit, in Europe I believe the car-traffic is "only" 1/3 of the total polution, however then there are other factors such as polution concentration for people in the cities walking next to busy downtown roads, and diesel-engines without particle-filters are deadly there, especially chip-tuned engines (25% extra power = 300% increased emissions tested by a danish news-magazine for a late-model Peugeot HDI turbodiesel-engine, horrible!!!). Airplane emissions have however been said to be a problem with high-altitude jet-planes, a new theory is in fact that when jetliners form trails then it's poluting heavyli and it should fly lower, but that gives a whole lot more problems though across the board, heavy increase in fuel-consumption that go means more money, higher fuel-feel => higher production, higher demand => higher prices, more congestion in the traffic lanes etc. etc. etc.

I think the aviation fuel std. is seperate from the car-fuel std. In Europe and Scandinavia we use the RON octane-rating and 80% of all cars use 95 octane unleaded and some used to use 98 octane but that's rare now, and not many use 92/93 octane that we have as well. I believe that the regular 89 octane in USA is something like 93 Octane RON but I've been told that the fuel isn't as clean, maybe sulfur but also other particles that makes moderm direct-injection gasoline and diesel-engines from Europe work at less than design-efficiency.

You say that large positive boost pressures can damage the engines, but airplane engine are massive compared to a average passenger-car, isn't the block strong enough for it? A 4-cylinder Cessna-engine has a bigger displacement than all 4 cylinders on my mom's Pueoget 206 and that alone must require some block-strength with all that torque, even thouhg an airplane-engine revs not much more than a big truck-engine (real truck like a Scania or Kenworth, not a pick-up).

Speaking of exhuasts, isn't there some amount of aerodynamic gain by covering the exhuast pipes, like on a Lancair and such instead of just having a hole in the bottom with the pipe sticking out straight down into the wind? I know it would at most be a minor gain, but still.

Sorry if this is written confusingly but some things have been happening here at home and while waiting for something to do I'm trying to learn something about a favourite topic of mine.
Frank
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Offline Sleek-Jet

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Re: Turbo or mechanical supercharger?
« Reply #8 on: March 18, 2006, 05:25:45 PM »
It has more to do with compression ratio's than the strength of the block or the case.

Nomrally aspirated (and trubo normalized) aircraft engines produced today have a C/R of about 8.5:1 or so, sometimes a little higher.  If you add to much boost to that (i,e, more than sea level pressure) the fuel octane rating isn't enough to keep the engine from entering detonation (uncontrolled burning of the air fuel mixture) and the engine will shortly destroy itself.

Turbocharged engines run a compression ratio of 7.0:1 or perhaps even a little lower to enable more boost to be forced into the cylinders without risking detonation.

The compression ratio is usually achieved with a dome or dish cast into the top of the piston. 

That's a pretty simplified explanation, but I think you'll see what I'm getting at.
« Last Edit: March 18, 2006, 05:27:18 PM by Sleek-Jet »
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Offline Frank N. O.

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Re: Turbo or mechanical supercharger?
« Reply #9 on: March 22, 2006, 05:12:28 PM »
Ah yes, compression-ratios normally need to be lowered for a turbocharged engine, for car-engines then several sources claim you don't have to do that for lower-pressure mechanical superchargers, nor do they need intercoolers but that sounds weird since compression must create heat regardless if it's a turbo or mechanical compressor but perhaps it's because the turbo is right next to the exhuast-turbine that naturally gets really hot.

The lower compression for aviation engines is that due to the thinner air these engines might fly in? it's about 1.0 lower in general than similar car-enignes are (NA 9.5-10.5:1 and turbo-engines 7.5-8.5:1). The Fireball chamber used on the Jaguar V12 did however enabled an extre ratio of around 14:1 I think it was, that's wild, especially for the 80s!

Frank
"When once you have tasted flight, you will forever walk the earth with your eyes turned skyward, for there you have been, and there you will always long to return."
— Leonardo da Vinci

Offline Roland

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Re: Turbo or mechanical supercharger?
« Reply #10 on: March 28, 2006, 06:36:54 AM »
Did you guys here in this ‘coop realize what just happened in this thread? We became witnesses in a high skilled discussion about turbocharged piston engines. Except my stupid comment, for which I want to place my apology. :-\

If it comes to piston engines I will keep my mouth shut from now on and let the real expert talk. I will take this discussion to our trainees and let them translate and learn it. Further they have to do some home work about and with it. Honestly my deepest respect, Mr. Sleek-Jet, sir. (saluting smily placed here)
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Offline Frank N. O.

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Re: Turbo or mechanical supercharger?
« Reply #11 on: March 28, 2006, 10:30:16 PM »
Uhm, Roland what do you have to apologize for? I certainly don't think you've said anything dumb, it was just a conversation of a common interest and I certainly don't think you yourself are dumb, you clearly know a lot and I'm grafeful for you for sharing that as well as your good spirits.

Frank
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— Leonardo da Vinci

Offline Sleek-Jet

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Re: Turbo or mechanical supercharger?
« Reply #12 on: March 28, 2006, 10:41:32 PM »
Geez Roland... I'm by far no expert, I was just correcting what the "G" stands for in the prefix letters, at least on this side of the pond... anyone's input is welcome at anytime. That's the great thing about these forums.  Hopefully if I post something way off base, someone will call me on it.

Post away my friend (where's a beer toasting smilie when you need one??? ;D ;D )
« Last Edit: March 28, 2006, 11:35:37 PM by Sleek-Jet »
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Offline Mike

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Re: Turbo or mechanical supercharger?
« Reply #13 on: March 29, 2006, 12:40:55 AM »
Gee, I guess we'll have to put some of those in, huh?! Would be fun!
I pretty sure they'll have those smileys somewhere (*insert head scratching smiley*)
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Offline Stef

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Re: Turbo or mechanical supercharger?
« Reply #14 on: March 31, 2006, 10:47:32 AM »
Wish granted! At least a saluting smiley... Still have to work on the head scratching smiley.