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darkhorizon11
02-03-2006, 10:52 AM
I posted this little thread as a response to Tagert and his K4 climbrates and the other thread with the how stuff works link. I figure I'll go a little more in depth and start from the basics a little better. This is also good practice for me since I'm working on my CFI and I'll have to teach this stuff soon anyway! http://forums.ubi.com/groupee_common/emoticons/icon_biggrin.gif

PART I

Okay first off all, I'm going to assume everyone understands the basic concepts of how a piston engine works since thats a whole nother thread in itself thats twice as long. It should also be pointed out that there are some major differences between car and aircraft chargers.

1. Terms: Work= force X distance, Power force x distance / time, or how much work is done over a given amount of time. Standard Atmospheric pressure is 29.92 inches of Mercury or 14.7 psi.
The basic concepts of all mechanical machines basically involves pressure differential, the idea that there is more force pushing one way than the other and there is a general equilibrium trend.

A more indepth explanation can be found in any basic physics book.


2. The ambient air in the atmosphere is 78% nitrogen 21% oxygen and 1% inert gases. When we suck the fuel air mixture into the engine (supercharged or not) to burn we look to combust it, the spark ignites the fuel and oxgyen which heat up to extremely high temps almost instantly. The piston however is not forced downward by the actual burning, but by the expansion of the Nitrogen as a result of the burning oxy and fuel. In a perfect (stoicometric) burn where the ratio of air:fuel is about 12:1 and there are no additives in the fuel the only thing coming out of the exhaust is carbon dioxide and water. Combustion where occur with a ratio of anything between about 8:1 to 18:1, but stoiciometric is where the most power is developed. Factors that also control power are the compression ratio of the engine, number of cylinders, surface area of the piston, number of strokes per cycle (2 stroke vs. 4), and the factor were talking about now... the density of the fuel air mixture.

3. As altitude increases density of the air decreases. An aircraft much travel faster at higher altitudes to remain aloft since the wings will not create as much lift, the same concept goes for an engine. Just like a person breathing the higher you go the harder it is for the engine to "breathe". This is because with the less dense air, there is less oxgen and fuel to combust (since the amount of fuel must be decreased to maintain stoiciometric) and therefore less nitrogen expansion and force pushing the cylinder down. So the concept is, what if we can maintain standard atmospheric pressure inside the engine itself, that way we can maintain the same amount of power from the ground up?

4. Before we go over the differences I'll go over the similarities, force air induction is what turbos and supers do. They both maintain standard atmospheric pressure up to altitude allowing for more power available and the both use a compressor to increase deck pressure going into the combustion chamber. The compressor is a series of spinning metal blades that is extremely efficient in moving air with almost no backflow in the opposite direction. The key is HOW the compressor is spun.

5. All superchargers including the WWII era ones use gearing from the crankshaft to spin the compressor. The problem is this gearing takes horsepower away from the engine in the first place. All turbochargers used the extremely hot and fast moving gases leaving the engine which are otherwise wasted to spin a turbine which is directly connected to the compressor.

darkhorizon11
02-03-2006, 11:46 AM
PART II

6. Components:

Throttle, everyone knows what it does, but not necessarily how it works. The throttle in an airplane only controls the airflow into the engine before it is mixed with fuel. It is nothing more than a valve, as the throttle or valve is opened more air is allowed to flow through the intake manifold (where air is seperated and mixed with fuel) to enter the engine. After being filtered, the air enters the turbo or supercharger where it is compressed THEN passes through the throttle valve and enters the intake manifold.

The other big thing I would like to hit on is the Wastegate, the key is ONLY the turbocharger has a wastegate. When the exhaust gases are exiting the engine they pass through the exhaust manifold. Then it is split in one of two directions, it can pass through the turbine thus being forced to spin it or pass through an alternate route and skip the exhaust turbine (for the turbo) completely and leave the aircraft. In this alternate route there is another valve, the wastegate. The wastegate (depending upon its setting) will allow a certain amount of exhaust gases past, the rest must pass through the turbine. It acts as a relief to the turbine since if all the exhaust gases flowed through the turbine an overboost situation can occur which the pilot could not control. Some waste gates are pilot controlled, but most are automatic and simple use a differential pressure controller and/or a density controller to maintain a constant boost pressure. I'm not going to get into those. The trick is when the waste gate is fully open, more exhaust gases avoid the turbine and less pressure is being developed in the manifold. As the wastegate closes, more gas is forced through the turbine. RPMS in the turbine and compresser increase, pressure increases in the manifold.

7. Whew, okay now that you have an idea of what they both do, lets look at the pros and cons: Superchargers take power away from the engine to add it. Its generally assumed that their better, but not necessarily. the 1" of MAP to 1/2" MAP rule is mostly used for cars and the size and efficiency of the charger is a major factor in this. During WWII superchargers were used, honestly I should look this up but I'm not sure when the first turbos were used. The reason being because not only were superchargers more efficient at the time, but the turbocharger turbines are exposed to EXTREMELY high temperatures and the alloys available at the time did not have the reliability as ones today. Metal fatigue and failure was much more of an issue back then and since there wasn't time to swap chargers every two or three flights superchargers were prefered. Superchargers are also much easier install on to an engine than a turbo and work pretty much autonomously. Also superchargers are connected directly to the crankshaft so as soon as the engine is throttled up the compressor RPMs increase and manifold pressure increases. In turbochargers since the pressure of the exhaust gases must first increase for the compressor RPMs to increase this is a 1 or 2 second lag. This is known as turbo lag.

8. Here are some of the types of engines in reference to turbo and superchargers:

a. Normally aspirated: ambient unaltered and unpressured air sucked into the engine and combusted. No turbo or supercharging at all.

b. Sea Level boost: the turbochargers maintains sea level pressure inside the intake manifold. Therefore the waste gate is almost entirely open on the ground since no boosting is necessary, exhaust gases are allowed to avoid the exhaust turbine and flow out. As the aircraft climbs to higher altitudes the waste gate slowly shuts, as altitude increases the RPMs of the turbine and compressor increase to maintain sea level pressure. I could be wrong but I've never heard of sea level boost supercharger.

c. Ground Boost: the most powerful of the three and what we see in WWII era aircraft looking for that extra boost. Here starting from the ground the supercharger or turbocharger spools up and to a higher than sea level pressure and maintains that pressure up to altitude.

9. Finally heres some other pertinent stuff to know with forced air induction. Every turbocharger and supercharger has a critical altitude. Critical altitude is the point where the compressor and/or turbines are spinning at max RPM and full boost can no longer be maintained. IF you look at a rate of climb vs. altitude chart like Tagert posts alot you can tell the critical altitude by looking at the line. The rate of climb will remain constant or increase to a certain altitude, then decrease rapidly. The point of the rapid decrease is the critical altitude. The the ROC climb will drop off proportionately to the boost pressure. The ambient pressure outside is so low that sea level or ground boost can't be maintained. There are three reasons why we are limited:

a. The altitude is so high there literally isn't enough air to compress.
b. The compressor for a turbo or a supercharger will max out around 100,000 and some up to 150,000 RPM (no that isn't a typo). Therefore the compressor can't spin any faster to maintain pressure.
c. Manifold pressure limitations. This isn't ussually why the charger is limited, but should be mentioned. There is a huge pressure differential across the manifold. Inside the air is boosted, yet outside at high altitude the air density is extremely low. I'll use an example to better explain this...

The new and debated P-38L late boasts a boost of 70" manifold air pressure or MAP (inside the intake manifold). The inches refer to inches of mercury btw, standard atmospheric pressure at sea level is 29.92" or for simplicity I'll round up to about 30". That means just sitting on the ground with full throttle the pressure inside the manifold is more than twice the standard atmospheric pressure for every square inch!

I don't know off the top of my head what the critical altitude is for this 1720hp beast but lets say for reference its about 20,000ft. (its probably actually higher than that). The standard ambient pressure for 20K is about 13.75" which means that the air inside the manifold is 5 times more compressed than the outside atmosphere!!!

What does all this mean? Well despite popular belief horsepower and boost are not proportionate to speed. A super/turbocharged aircraft will be faster than a normally aspirated one, but not by that much. Superchargers and turbochargers give us extra POWER. And excess POWER allows us to accelerate faster, and climb faster.

Quite a mechanical feat. I must say, I hope this gives everyone a better understanding of turbos and superchargers and how they work, specifically on aircraft. Actually I have some questions myself, for you guys to field.

1. American aircraft use MAP and German ones use ata. What does ata stand for?
2. What other measures of boost are there? What methods of boost did the Russians use? or the Japanese?
3. Also if there were any WWII era aircraft that did use turbochargers I'd be curious as to which ones...

Anyways thanks for reading, I hope I didn't put you to sleep! http://forums.ubi.com/images/smilies/25.gif

Viper2005_
02-03-2006, 11:58 AM
1) ata = atmospheres. Pretty simple really. 1.0 ata = 1 atmosphere = 29.92" Hg = 0 psi boost.

2) mostly superchargers I think

3) P-38, P-47, B-17, B-29...

Turbocharging has been around for a long time (since 1905) and is especially useful for relatively slow, high altitude aircraft. The wonderful thing about it is that the higher you go the lower the back pressure on the turbine, so the high the expansion ratio available.

For high speed fighter aircraft, mechanical supercharging is generally preferable since the exhaust energy is most efficiently used for the direct generation of thrust. At 375 mph, 1 lbf = 1 hp. However, because airscrew efficiencies are only around 80% or so, exhaust thrust is actually even more attractive than this relationship would suggest.

The Merlin can develop over 150 lbf of exhaust thrust at high speed and high altitude, which is worth over 200 bhp. In effect this "pays" for the supercharger.

StellarRat
02-03-2006, 01:14 PM
Originally posted by Viper2005_:
1) ata = atmospheres. Pretty simple really. 1.0 ata = 1 atmosphere = 29.92" Hg = 0 psi boost.

2) mostly superchargers I think

3) P-38, P-47, B-17, B-29... I know you know this, but for everyone elses benefit, I'd like to point out that the P-47 was supercharged and turbocharged. It used both systems. That's why it is such an awesome plane at very high altitudes.

FYI: Properly designed air intakes can also give you a slight boost at high speeds.

darkhorizon11
02-03-2006, 01:28 PM
Originally posted by Viper2005_:
1) ata = atmospheres. Pretty simple really. 1.0 ata = 1 atmosphere = 29.92" Hg = 0 psi boost.

2) mostly superchargers I think

3) P-38, P-47, B-17, B-29...

Turbocharging has been around for a long time (since 1905) and is especially useful for relatively slow, high altitude aircraft. The wonderful thing about it is that the higher you go the lower the back pressure on the turbine, so the high the expansion ratio available.

For high speed fighter aircraft, mechanical supercharging is generally preferable since the exhaust energy is most efficiently used for the direct generation of thrust. At 375 mph, 1 lbf = 1 hp. However, because airscrew efficiencies are only around 80% or so, exhaust thrust is actually even more attractive than this relationship would suggest.

The Merlin can develop over 150 lbf of exhaust thrust at high speed and high altitude, which is worth over 200 bhp. In effect this "pays" for the supercharger.

I guess that answers my questions. So it was turbocharging that was invented first then? I couldn't remember although, yes the invention of the turbo and super charger are considered to be a major step in the development of gas turbines since it meant the ability too mechanic compress the air.

Haha, and you learn something every day, I wasn't aware the P47 has a supercharger and a turbocharger, too bad we can't switch in game huh? http://forums.ubi.com/images/smilies/25.gif

darkhorizon11
02-03-2006, 01:38 PM
For the second question... I know almost everyone used superchargers but what I meant is what unit of measure? inches? cm? or ata also?

WTE_Ibis
02-03-2006, 01:54 PM
Thanks for the explanation although as an engine reconditioner I know the application in motor vehicles but height rarely matters, (well for most it rarely matters). http://forums.ubi.com/groupee_common/emoticons/icon_biggrin.gif

.

Viper2005_
02-03-2006, 02:27 PM
Ditto the P-38; those V-1710s have superchargers as well as turbochargers.

Most piston aeroengines of the period have mechanical superchargers, even quite small ones.

The turbocharger was a separate unit which effectively served to trick the engine into thinking it was at sea level, when in fact it was flying at high altitude. The supercharger then just did its thing.

Of course later the turbocharger was used to provide increased boost lower down too, but the original conception was based around the turbocharger simply supplying the supercharged aeroengine with sea level esque conditions at high altitudes in order to permit operations at higher altitudes.

The advantage of this approach is that with a good installation the engine hardly knows that the turbocharger is even there, which considerably simplfies development, at least in theory...

As for the units, they're all units of pressure, the measurement of which is taken in the induction manifold. All other things being equal, the higher the pressure in the induction manifold, the more power you'll get.

Dash_C.
02-03-2006, 02:29 PM
Good info all! You learn something new every day http://forums.ubi.com/images/smilies/16x16_smiley-wink.gif

Cajun76
02-03-2006, 03:03 PM
http://rwebs.net/avhistory/history/p-47.htm

Pics and info on the P-47 and design, including turbo-supercharging system.

Sergio_101
02-03-2006, 03:57 PM
ALL turbochargers are superchargers.
All superchargers are not necessarially turbochargers.

Turbocharger is the shortening of "Turbine driven compressor powered by
energy recovered from exhaust gasses".

Most WWII era turbocharger applications
act as a first stage in a two stage supercharger
and offer no boost beyond altitude compensation.
The turbocharger compresses air whicht passes
through an intercooler then through the carbuerettor
maintaing sea level air pressure at the carb
or injection control intake.
The engine driven compressor further increased the pressure
above sea level to "Boost pressure"

With the exception of light planes, no WWII era combat
planes flew without at least an engine driven supercharger.
Even turbocharged engines had the engine driven supercharger.

Most superchargers of the era were of the centrifugal type
including turbochargers.
Junkers Diesels used a roots type posative displacement
supercharger which is required as they were two stroke
double acting Diesels (two crankshafts pistons meeting at
the center line).
Some were turbocharged as well.

The variations in supercharger drives of the era are many
and are in some cases larger than the piston engine itself!

Two speed drives, constant speed drives three speed drives
and on and on.........

In theory the turbocharger offers better brake specific fuel consumption
at all altitues. In practice this is true, to a point. At low
power settings the difference is little.
At high boost and high altitudes the turbocharged engine
is more efficent.

Late in WWII experiments were done with turbo compounding.
This type of power recovery helps in a big way, but with
some negative side effects. High altitude boost is sacrificed
for greater power and fuel economy.

In a large Diesels the recovered power can be as high as 10%.
In a large aircraft supercharged engine the recovered power
can exceed 30%.

The only large scale use of TC engines was after WWII on the
CW R-3350TCW radial engines powering DC-7C, Super Constellations
Lockheed Neptunes and C-119 flying boxcars.
Rated power for the R-3350 was maxed out at 2,800 HP for non TC
engines in service, up to 3,700 HP for the TCW series (normally most were
rated at 3,450 HP).
P&W experimented with exhaust recovery and got around 4,400 HP from
the R-4360 VDT.

As to RPM, I believe the fastest RPM for WWII era turbo's was 22,000 RPM*.
Today some small automotive turbo's spin up to 200,000+ RPM.

Chief advantages of turbocharged applications are
improved fuel consumption and lower crankshaft
power loading for a specific power output.
Smooth boost pressure up to critical altitude, no stepping
of pressure as in multiple speed/stage mechanical drives.

Chief disadvantages are increased complexity,
weight and heat load on the engine.

Todays compact turbochargers are the same in theory but use
the turbocharger for boost (effectively a lowering of pressure altitude)
Also they are a lot more compact!

Sergio

Refrence, "Allied Aircraft Piston Engines" Ghram White, SAE press. Page 191,
second paragraph.

Unknown-Pilot
02-03-2006, 04:00 PM
The thing to remember is that all Chargers are Super, whether it has a turbo or not. (although the best were the '69 and the Daytona)

But a turbo Charger would be pretty sick. http://forums.ubi.com/images/smilies/icon_twisted.gif

effte
02-04-2006, 01:46 PM
Originally posted by WTE_Ibis:I know the application in motor vehicles but height rarely matters, (well for most it rarely matters)

But some people do find out where it matters, only they may not be aware of it.

Driving in mountains, one might discover the engine running bad. The natural thing to do is to pull over, pop the hood and have a look. Step two in any problem solving procedure for your typical motorist is to scratch head or beer gut, as applicable. Step three is to restart the engine, to verify the problem. And voila, it runs as smooth as ever again!

The problem is that all the electronics for the fuel injection needs to know the density of the ambient air. Many engines only sense this when the engine is being started.

ATLAS_DEATH
02-04-2006, 02:02 PM
Originally posted by effte:
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by WTE_Ibis:I know the application in motor vehicles but height rarely matters, (well for most it rarely matters)

But some people do find out where it matters, only they may not be aware of it.

Yes. Some require to to unplug the air density sensor or the ecm and then plug it back in.. it will then take some readings once you start the vehicle.
Driving in mountains, one might discover the engine running bad. The natural thing to do is to pull over, pop the hood and have a look. Step two in any problem solving procedure for your typical motorist is to scratch head or beer gut, as applicable. Step three is to restart the engine, to verify the problem. And voila, it runs as smooth as ever again!

The problem is that all the electronics for the fuel injection needs to know the density of the ambient air. Many engines only sense this when the engine is being started. </div></BLOCKQUOTE>

SnapdLikeAMutha
02-05-2006, 08:02 AM
The P-63 was also turbocharged, the P-39 was also supposed to be turbocharged originally (the prototype was TC'ed and had quite awesome performance)

Sergio_101
02-05-2006, 12:03 PM
Originally posted by SnapdLikeAMutha:
The P-63 was also turbocharged, the P-39 was also supposed to be turbocharged originally (the prototype was TC'ed and had quite awesome performance)

Sorry, the P-63 was never turbocharged.
What the P-63 had was an auxillary first stage
blower on a constant speed drive.

Only the XP-39 was turbocharged.

Sergio

SnapdLikeAMutha
02-05-2006, 12:25 PM
http://home.att.net/~jbaugher1/p63_3.html (http://home.att.net/%7Ejbaugher1/p63_3.html)

"One Allison V-1710-93 twelve-cylinder Vee liquid cooled engine with a single-stage supercharger and auxiliary hydraulic turbosupercharger"

Is a turbosupercharger not the same thing as a turbocharger then? Shoot, I've been scuppered by semantics again http://forums.ubi.com/groupee_common/emoticons/icon_frown.gif http://forums.ubi.com/images/smilies/cry.gif

Staga
02-05-2006, 01:26 PM
They are same thing but Baugher has some false info in his site.

Like Sergio said P-63 had Allison with single speed supercharger like P-39 had BUT it also had another supercharger behind the engine, driven by short axle, and with bit similar hydraulic coupling like Daimler-Benz engines had.

Sergio_101
02-05-2006, 02:07 PM
Yea, there's a lot of bad press out there.
like the un supercharged P-39 and P-40 myth.
Truth is NO Allison engine got into a WWII
fighter without a supercharger.

100% of all airworthy Allisons had at least
an engine mounted single stage supercharger.

The popular press frequently screws the pooch.

Sergio

SnapdLikeAMutha
02-05-2006, 02:23 PM
Originally posted by Sergio_101:


The popular press frequently screws the pooch.

Sergio

Isn't that the truth?!!

Unknown-Pilot
02-05-2006, 02:33 PM
There actually isn't a "Super Charger", technically.

I think the confusion comes from the fact that this -


http://www.cumminsrestoration.com/pics/web/superbird.jpg

Looks so much like the Charger, and did have "super" in the name, but it was actually the "Superbird".

Wherease this was the Charger -
http://www.fast-autos.net/dodge/daytona4.jpg



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