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AFGMEL
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Piston Power And Turbines

Tue Sep 29, 2009 3:55 am

I am really ignorant about engineering and was hoping one of you could answer a question.

I am given to understand that regular propellers become very inefficient at about 400mph which is a virtual barrier. If this is correct, could an engine with turbine blades exceed that? I'm not talking about now with jet available, but in the prop days turbines had been around for decades and I wondered why nobody had tried it; as my non-scientific guess would be that it would have overcome this problem.
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tdscanuck
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RE: Piston Power And Turbines

Tue Sep 29, 2009 4:14 am



Quoting AFGMEL (Thread starter):
I am given to understand that regular propellers become very inefficient at about 400mph which is a virtual barrier.

Correct. In order to get useful thrust, the exhaust exit speed has to exceed the vehicle's airspeed. In an unducted prop, you're effectively limited to somewhat subsonic exhaust.

Turbofans, with the duct, can generate higher duct pressure and translate that into higher exahaust velocity...a modern turbofan can exhaust very close to Mach 1, which is how you can get cruise speeds up in the low Mach .9x. Past that, you need a convergent-divergent nozzle, like a fighter, to get supersonic exhaust.

Quoting AFGMEL (Thread starter):
If this is correct, could an engine with turbine blades exceed that? I'm not talking about now with jet available, but in the prop days turbines had been around for decades and I wondered why nobody had tried it;

I'm not really following you here...the speed limit on props comes from the aerodynamics of the propellor, not the engine driving it. A turboprop (turbine driven propellor) has no more or less inherent speed advantage than a piston driven prop.

Tom.
 
AFGMEL
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RE: Piston Power And Turbines

Tue Sep 29, 2009 4:22 am



Quoting Tdscanuck (Reply 1):
I'm not really following you here...the speed limit on props comes from the aerodynamics of the propellor, not the engine driving it. A turboprop (turbine driven propellor) has no more or less inherent speed advantage than a piston driven prop.

I probably didn't explain myself well. Take a piston engine, gear it to a turbine so the turbine blades are producing thrust. In effect you're replacing the propeller with the turbines. Err, do you see what I mean?
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MrChips
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RE: Piston Power And Turbines

Tue Sep 29, 2009 4:43 am

Quoting AFGMEL (Reply 2):
I probably didn't explain myself well. Take a piston engine, gear it to a turbine so the turbine blades are producing thrust. In effect you're replacing the propeller with the turbines. Err, do you see what I mean?

What you're describing is essentially a motorjet engine. This has been done before with this aircraft:



http://en.wikipedia.org/wiki/Caproni_Campini_N.1

As you can see from the article, it wasn't too successful. In a nutshell, this propulsion arrangement combines all the worst attributes of both the piston and jet engines into one slimy, ugly mess, with very few (if any) of the positives of either. A gas turbine needs to be able to generate a substantial increase in pressure for it to be efficient, but the piston engine simply cannot provide enough power to drive a turbine to the kinds of speeds where this is possible.

[Edited 2009-09-28 21:45:47]
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tdscanuck
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RE: Piston Power And Turbines

Tue Sep 29, 2009 4:57 am



Quoting AFGMEL (Reply 2):
I probably didn't explain myself well. Take a piston engine, gear it to a turbine so the turbine blades are producing thrust. In effect you're replacing the propeller with the turbines. Err, do you see what I mean?

OK, gotcha. Yes, that's the thing MrChips is talking about.

In that case, you're actually spinning a compressor, not a turbine. A turbine has a pressure reduction as you go through it...for propulsion, you want a pressure rise, which is what a compressor does. And a compressor is basically a ducted propellor.

At the end of the day, you're basically talking about a turbofan...use a turbine to spin a big set of ducted blades. This does work really well, but for it to work properly you need a very high power engine and, as MrChips very correctly notes, it's very hard to build a piston engine that can generate that kind of power.

Tom.
 
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Starlionblue
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RE: Piston Power And Turbines

Tue Sep 29, 2009 5:47 am

The Caproni Campini N.1 injected fuel into the air compressed by the compressor, producing thrust through it's combustion. So it's not really a ducted fan or turboprop analogue. It is like a turbojet sans turbine, the turbine's function (powering the compressor) being replaced by the piston engine. In any case an awesome looking plane for the time.
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
AFGMEL
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RE: Piston Power And Turbines

Tue Sep 29, 2009 5:57 am

Excellent stuff, thanks guys! Yes, piston powered turbofan. So, in theory if you could have a whopping great engine it could go fast enough?
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MrChips
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RE: Piston Power And Turbines

Tue Sep 29, 2009 6:36 am



Quoting AFGMEL (Reply 6):
So, in theory if you could have a whopping great engine it could go fast enough?

In theory yes. But realistically, a piston engine that large would be so big and heavy that it would be completely impractical. How impractical? Well, an engine the size of a CFM56 would need a piston engine that is capable of five digit horsepower levels. And while they do exist, they're literally the size and weight of small buildings.
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Starlionblue
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RE: Piston Power And Turbines

Tue Sep 29, 2009 10:41 am



Quoting MrChips (Reply 7):
Well, an engine the size of a CFM56 would need a piston engine that is capable of five digit horsepower levels. And while they do exist, they're literally the size and weight of small buildings.

I am assuming you are talking marine diesels and such. Very cool pieces of engineering but as you say not great in the power produced to weight ratio department.
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
ImperialEagle
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RE: Piston Power And Turbines

Wed Sep 30, 2009 1:26 am



Quoting AFGMEL (Thread starter):
regular propellers become very inefficient at about 400mph

Ah,well generally speaking I get what you are alluding to, but the old TU-114's regularly cruised well over 500MPH, and on some very long route segments. So efficiency is achieveable especially considering the age of the TU-114 technology.
"If everything seems under control, you're just not going fast enough!"
 
tdscanuck
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RE: Piston Power And Turbines

Wed Sep 30, 2009 2:41 am



Quoting ImperialEagle (Reply 9):
Ah,well generally speaking I get what you are alluding to, but the old TU-114's regularly cruised well over 500MPH, and on some very long route segments. So efficiency is achieveable especially considering the age of the TU-114 technology.

I'm not sure that they were that efficient...just hugely powered with giant fuel tanks. Russian engines, in general, aren't known for being efficient and there's just no way that the Bear was sipping gas charging ahead at those kinds of speeds. The B-52 also had enourmous range and nobody in their right mind would claim that was an efficient engine.

Tom.
 
speedracer1407
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RE: Piston Power And Turbines

Wed Sep 30, 2009 3:31 am



Quoting ImperialEagle (Reply 9):

Quoting AFGMEL (Thread starter):
regular propellers become very inefficient at about 400mph

Ah,well generally speaking I get what you are alluding to, but the old TU-114's regularly cruised well over 500MPH, and on some very long route segments. So efficiency is achieveable especially considering the age of the TU-114 technology.

Perhaps 4-500 MPH over the ground, but don't forget that any speed measurement that's relevant to the plane's aerodynamic properties (efficiency, max speed, etc) will be measured in KIAS (Knots indicated air speed) or mach number. Both of those units vary depending on local air density (and therefore altitude) or outside air temperature, so at high altitude, where the air is thin and cold, KIAS is sure to be far, far lower than 4-500 knots.
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Viscount724
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RE: Piston Power And Turbines

Wed Sep 30, 2009 3:58 am



Quoting Starlionblue (Reply 8):
Quoting MrChips (Reply 7):
Well, an engine the size of a CFM56 would need a piston engine that is capable of five digit horsepower levels. And while they do exist, they're literally the size and weight of small buildings.

I am assuming you are talking marine diesels and such. Very cool pieces of engineering but as you say not great in the power produced to weight ratio department.

Photos of some of the largest marine diesels here.
http://people.bath.ac.uk/ccsshb/12cyl/

Photos are of 10 and 12 cyl. models but the largest 14 cyl. version (data in table at top of above page) generates 109,000 hp (at 102 rpm) but the engine is 89 ft. long and weighs 2300 tons, roughly equal to 4 A380s at MTOW. It powers the Emma Maersk, the world's largest container ship when launched in 2006. It now has 7 sister ships the same size.



 
speedracer1407
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RE: Piston Power And Turbines

Wed Sep 30, 2009 4:51 am



Quoting Viscount724 (Reply 12):
Photos are of 10 and 12 cyl. models but the largest 14 cyl. version (data in table at top of above page) generates 109,000 hp (at 102 rpm) but the engine is 89 ft. long and weighs 2300 tons, roughly equal to 4 A380s at MTOW. It powers the Emma Maersk, the world's largest container ship when launched in 2006. It now has 7 sister ships the same size.

Wow. Most automotive piston engines develop a horsepower number that is roughly similar to it's peak torque. But check this out:

At 102 RPM, that engine twists out 5,612,431 lb/ft of torque. I have no idea how to compare that to something tangible.

But let's compare to some other remarkable engines:

The the ubiquitous EMD 645 16 cylinder locomotive Diesel that probably powers your local suburban commuter train displaces 169 Liters and produces 3000 HP and 17,507 lb/ft of torque at 900 RPM.

A Formula 1 engine (V8) displaces 2.4 liters and produces roughly 750 HP and 219 lb/ft of torque at 18,000 RPM.
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ImperialEagle
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RE: Piston Power And Turbines

Wed Sep 30, 2009 5:43 am



Quoting Tdscanuck (Reply 10):

I'm not sure that they were that efficient

Oh yeah, I'm laughing because I know what you mean. On April 9th, 1960 one flew a 3,107 mile circuit at an average speed of 545MPH. You know those Kuznetsovs were just sucking fuel like pigs!
Some of the performance criteria on the aircraft show a range with a 33,000lb payload as 5,560miles cruising at 540MPH at 39,300 feet.
Ssssssssmokin! Wow!
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Dalmd88
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RE: Piston Power And Turbines

Wed Sep 30, 2009 12:42 pm

The ship analogy is pretty good considering many of the new cruise ships are powered completely by aircraft turbine engines. The last one I was on used two GE CF6 cores to provide all electric power for the ship. That included the propulsion drive motors. They said that one was capable to do the job. The second was mostly for a back up.
 
planewasted
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RE: Piston Power And Turbines

Thu Oct 01, 2009 7:50 pm



Quoting Viscount724 (Reply 12):
Photos are of 10 and 12 cyl. models but the largest 14 cyl. version (data in table at top of above page) generates 109,000 hp (at 102 rpm) but the engine is 89 ft. long and weighs 2300 tons, roughly equal to 4 A380s at MTOW. It powers the Emma Maersk, the world's largest container ship when launched in 2006. It now has 7 sister ships the same size.

An interesting note: A GE90 is still more powerful.
 
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DocLightning
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RE: Piston Power And Turbines

Fri Oct 02, 2009 10:57 pm



Quoting PlaneWasted (Reply 16):

An interesting note: A GE90 is still more powerful.

Is it? What is the power output of a GE90-115? I know it can do 115Klbs, but I've never heard its power quoted in HP.

BTW, I believe could theoretically power a high-subsonic aircraft with piston power. You would have to gear the drive shaft to the fan to provide the RPM's, and in order to get the power output of a turbine, the piston engine would have to be about the size of one of those ship engines. 90 foot long, 2300 ton engines (and you need a minimum of two of them) are not terribly attractive to aerospace engineers, for some odd reason, though.

The ultimate reason is that a piston is only operating at peak temperature and pressure for a brief instant of its cycle. A turbine is always operating at peak temperature and pressure, and each part of the turbine can be optimized for the temperature and pressure in which it must operate.

But on a ship, you need to do things that turbines aren't designed to do, like reverse and sudden starting and stopping. They have been used on ships, but typically in high-speed military applications. And, on a huge container ship, weight just isn't that much of an issue.
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rwessel
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RE: Piston Power And Turbines

Sat Oct 03, 2009 3:10 am



Quoting DocLightning (Reply 17):
Is it? What is the power output of a GE90-115? I know it can do 115Klbs, but I've never heard its power quoted in HP.

It's hard to do an exact conversion, since thrust is force and horsepower is, *ahem*, power, but a GE-90-115 is generally assumed to be producing about 110,000hp.

Quoting DocLightning (Reply 17):

BTW, I believe could theoretically power a high-subsonic aircraft with piston power. You would have to gear the drive shaft to the fan to provide the RPM's, and in order to get the power output of a turbine, the piston engine would have to be about the size of one of those ship engines. 90 foot long, 2300 ton engines (and you need a minimum of two of them) are not terribly attractive to aerospace engineers, for some odd reason, though.

It's certainly possible to build piston engines with a far higher output per unit mass than the very slow diesels intended for use shipboard. The large piston engines of WWII were producing on the order of a horsepower per pound. Consider the Pratt R-4360-VDT (the Wasp Major), which weighed about 3900lbs, and produced 4300hp. 26 of those, to equal a GE-90, would only net about 50 tons. A GE-90 is only about a sixth of that. Of course you'd need to add a transmission if you actually ganged a bunch of engines, and the fan itself.

The GE-90 is also far more reliable - A flight with 52 Wasp Majors and no engine failures would be a rare thing (so we've already got 26 on each side, just toss in a couple extra as spares).

But just think, you'd need some 3136 spark plugs when you did a tuneup on your Wasp powered 777!
 
tdscanuck
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RE: Piston Power And Turbines

Sat Oct 03, 2009 3:56 am



Quoting DocLightning (Reply 17):
But on a ship, you need to do things that turbines aren't designed to do, like reverse and sudden starting and stopping. They have been used on ships, but typically in high-speed military applications.

That was true primarily before really good solid state electronics came into their own. Reversing and stop/start are only problems for turbines that are direct coupled to the prop. If you spin a generator with the turbine, and spin your props with electric motors, you get a very nice propulsion system. It's, by far, the favorite propulsion of cruise ships and drillships now.

Tom.
 
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jetmech
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RE: Piston Power And Turbines

Sat Oct 03, 2009 4:23 am



Quoting DocLightning (Reply 17):



Quoting PlaneWasted (Reply 16):

According to this link, http://geae.com/education/theatre/ge90/ the GE 90 puts out an "equivalent" of 145,000 hp.



Because the GE90 N1 and N2 spools spin at 2552RPM & 10850 RPM respectively,

http://i94.photobucket.com/albums/l118/Jet-Mech/ge90-1.jpg

the torque output would be proportionally less than the RTA96-C diesel. If 145,000 hp was delivered at 2552 RPM you would have about 298,409 ftlb of torque.

Regards, JetMech
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DocLightning
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RE: Piston Power And Turbines

Sat Oct 03, 2009 2:35 pm



Quoting Rwessel (Reply 18):
26 of those, to equal a GE-90, would only net about 50 tons

"only"  Wow!
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prebennorholm
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RE: Piston Power And Turbines

Mon Oct 05, 2009 12:38 am



Quoting Tdscanuck (Reply 19):
If you spin a generator with the turbine, and spin your props with electric motors, you get a very nice propulsion system.

Right, but you get also a very heavy weight penalty of the electric generators and electric motors in addition to the engine.

Quoting Viscount724 (Reply 12):
Photos of some of the largest marine diesels here.
http://people.bath.ac.uk/ccsshb/12cyl/

Photos are of 10 and 12 cyl. models but the largest 14 cyl. version (data in table at top of above page) generates 109,000 hp (at 102 rpm) but the engine is 89 ft. long and weighs 2300 tons, roughly equal to 4 A380s at MTOW. It powers the Emma Maersk, the world's largest container ship when launched in 2006.

Part of the reason for this very heavy engine is that it is an extremely complicated engine and likely the most fuel efficient engine in the world.

It it featured with several turbines. Some of them charge the engine inlet. Others produce electric power.

Also heat from cooling and the exhaust goes to heat steam boilers, in fact two step boilers, high pressure and low presure boilers. Pressurised steam is used to power other turbines which produce electric power.

And finally the electric power is used by an electric motor which is an integrated part of the propeller shaft.

That's what it takes to get the very last power out of every droplet of oil.

I have been told that the overall efficiency of the Emma Maersk engine is 51%. That compares to 25-30% on most modern gasoline car engines and 30-35% on ordinary diesel engines.
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tdscanuck
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RE: Piston Power And Turbines

Mon Oct 05, 2009 3:48 am



Quoting Prebennorholm (Reply 22):
Quoting Tdscanuck (Reply 19):
If you spin a generator with the turbine, and spin your props with electric motors, you get a very nice propulsion system.

Right, but you get also a very heavy weight penalty of the electric generators and electric motors in addition to the engine.

Not really. Compared to the prop shaft & transmission, the generator + motors solution is quite elegant. For ships that use the capability of having multiple azipods, you come out *way* ahead on weight with the electric solution.

Tom.
 
MrChips
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RE: Piston Power And Turbines

Mon Oct 05, 2009 8:15 pm



Quoting Tdscanuck (Reply 19):
That was true primarily before really good solid state electronics came into their own. Reversing and stop/start are only problems for turbines that are direct coupled to the prop. If you spin a generator with the turbine, and spin your props with electric motors, you get a very nice propulsion system. It's, by far, the favorite propulsion of cruise ships and drillships now.

From what I understand, pure turbine-powered ships have fallen out of favour in the last year or so because of high fuel prices. Even though they utilise the remaining energy in the exhaust to drive other systems, marine turbines still take an efficiency hit compared with the diesel engines.

Where the turbine engine loses in efficiency, however, it has one big advantage over a diesel - its compactness. Depending on the type of ship, the increase in revenue from the added floor space can offset the added fuel cost. And if the ship uses electric drive for the propellers, those gains can be increased again by placing the engine room in a location that minimises the amount of fresh air ducting needed for the engine.
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rwessel
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RE: Piston Power And Turbines

Tue Oct 06, 2009 1:44 am



Quoting MrChips (Reply 24):
From what I understand, pure turbine-powered ships have fallen out of favour in the last year or so because of high fuel prices. Even though they utilise the remaining energy in the exhaust to drive other systems, marine turbines still take an efficiency hit compared with the diesel engines.

Where the turbine engine loses in efficiency, however, it has one big advantage over a diesel - its compactness. Depending on the type of ship, the increase in revenue from the added floor space can offset the added fuel cost. And if the ship uses electric drive for the propellers, those gains can be increased again by placing the engine room in a location that minimises the amount of fresh air ducting needed for the engine.

Being Green is important for many cruise lines right now, so minimizing fuel consumption is a win there too. That being said, a number of recent designs, like the Carnival Spirits, use azipods but have stuck with diesel generators. The "all electric" design has advantages in that you don't need a single big engine room with everything positioned just-so relative to the shafts, and you don't need to try and balance the very large electrical loads on a modern cruise ship with your propulsion needs.
 
Oryx
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RE: Piston Power And Turbines

Wed Oct 07, 2009 7:13 pm

My work group designs turbocharger for engines like the one powering Emma Maersk so I can add some figures to this subject:
-The rating of Emma Maersk includes some 8(?) MW of electrical power from the use of the waste heat of the engine in a steam cycle.
-Engines of that size are two-stroke diesel engines.
-Smaller engines used for diesel-electric applications use a four-stroke cycle. These engines are rated up to twenty MW each. Cruise ships have about five of those - delivering either electrical power for the hotel load or for the electrical drive.
- The turbine of our largest turbocharger is rated 10 MW at a velocity in excess of Mach 1.5 at the outer diameter. Emma Maersk needs three of them.
 
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jetmech
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RE: Piston Power And Turbines

Thu Oct 08, 2009 6:17 am



Quoting Oryx (Reply 26):
The turbine of our largest turbocharger is rated 10 MW

Interesting stuff! Probably the same power as a CFM-56. IIRC, the particular design of diesel two stroke as used on large ships requires an external scavenging / charging pump to function. I take it that the turbochargers you design provide this function.

However, when the engine is starting, the exhaust gases produced may not be sufficient to drive a turbocharger. How does the Emma Maersk get around this? Can your turbochargers provide sufficient scavenging / charging during engine start, or is a separate mechanically driven pumping device required?

Regards, JetMech
JetMech split the back of his pants. He can feel the wind in his hair :shock: .
 
Oryx
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RE: Piston Power And Turbines

Thu Oct 08, 2009 7:33 am

Usually the engines are equipped with up to three turbochargers, several electrically driven auxiliary blowers and for start up with compressed air stored in large bottles. Before engine start the hot oil is circulated for several hours in the engine to warm it up. Then the aux blower are turned on. After that the compressed air is used to turn the engine and often to drive the turbocharger. At something like 30 percent of the rated power the turbocharger takes over and the aux blower are shut down. There are some similarities to the start of gas turbines by the use of air starter. Only the time is hours and minutes not minutes and seconds  .

On the comparison with the CFM65: The 10 MW is the power of the turbine olone. So one must not forget to substract the power consumption of the compressor which will be exactly the same as we do (usually) not produce net power but only warm air.

[Edited 2009-10-08 01:26:12]
 
tdscanuck
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RE: Piston Power And Turbines

Fri Oct 09, 2009 4:22 am



Quoting JetMech (Reply 27):
Quoting Oryx (Reply 26):
The turbine of our largest turbocharger is rated 10 MW

Interesting stuff! Probably the same power as a CFM-56

A CFM-56 is ~20 MW, depending on the variant and how you do the conversion.

Tom.

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