Turbo Horsepower

Here's a question I think may be fun to figure out:

How much Horsepower does the Turbocharger produce?

I don't mean "how much does it add to the engine"... but more like,"How much horsepower does the turbine producek to drive the compressor?"

Any engineers out there?

Oh, boy- I've just GOT to jump in here...

Think about this:

When your piston has reached the bottom of its power stroke, even in a diesel, what do you think the pressure in the cylinder is? Pressure developes as the burned gasses expand, and the majority of the mechanical power we create is used up by the time the power stroke is over, but not all of it. There is still considerable pressure in the cylinders when the exhaust valve opens, and those gasses immediately begin to rush out of the cylinder due to the difference in absolute pressures. There is a pressure differential in the cylinder and the turbo inlet, there is a differential in the turbo inlet and turbo outlet, and another between the turbo outlet and the tailpipe.
The difference between the pressures in the gasses in the manifold and the turbo are very small, as the turbine section of the turbo is not a positive displacement driven unit. If you held your finger in the blades (STUPID- DON'T EVER TRY THIS!!! IT IS FOR EXPLANATIVE PURPOSES ONLY!!! YOU WILL LOSE YOUR FINGER IF THE TURBO IS SPINNING WITH ANY KIND OF SPEED, AND YOU WON'T FEEL IT HAPPENING!!!- uh, so I've heard...) the gasses would still pass through both sides of the turbo with relative ease.
The point here is, due to the gasses still expanding as they leave the cylinder, the mechanical expansion that would normally be lost to the atmosphere is captured and used to increase our engine's volumetric efficiency. It's all well and good to play with the numbers, but that 18 horsepower that we say is being "lost" to drive the turbo is actually being captured instead of thrown away.
I'm an engineer too.

Orginally from xlr82v2:

The way I look at it is that with turbocharging, you get the 7 course meal for the price of a bag of chips, and maybe a Coke!

Click to expand...

<font size="2" face="Verdana, Helvetica, sans-serif">Very good.

The 10 or so HP it takes to drive the turbo is the price to pay for getting the approx. 20 hp net from turbocharging. A similar 1.9l naturally aspirated diesel would probably be around 70+/- HP as compared to our 90 HP TDI's.

The turbine in the exhaust causes greater backpressure that requires more power from the engine. The pistons have to push harder to get the exhaust out on the exhaust stroke.

Also, be careful about how you are interpretting a change in temperature of a working fluid. The change in temperature doesn't mean work is taking place, or there is a energy flux to infer a work process is taking place. Compressing air in a cylinder gets the temperature of the air up but work need not be done by that air, the temp just goes up.

The turbine blades act like sails on a sailboat. The sail catches the wind and pushes the boat. The turbine blades catch the exhaust and pushes the turbine wheel. The turbine section acts like a diffuser. As a pressurized gas passes through a diffuser, its temp is reduced due to the reduction in pressure although it could have been an isothermal process where no heat transfer took place. The pressure drops significantly across the turbine, therefore the temp does too.

The temperature is used to affect the pressure which affects mechanical work. The pressure is the link between temperature and mechanical work.

[ March 15, 2002, 14:05: Message edited by: Boundless ]

Oops... hit the wrong button. Sorry.

[ March 15, 2002, 14:04: Message edited by: Boundless ]

You guys brought back the memory from when I used to fuel aircraft and the conversations about holding a PT6 prop while it started up...at least you could hold it for a while (no I never tried it).

Just in case somebody gets the nerve to try this make sure your not grabbing a hold of a direct drive Garrett engine doing a series start


DB

[ March 15, 2002, 14:31: Message edited by: Drivbiwire ]

Originally posted by Drivbiwire:
You guys brought back the memory from when I used to fuel aircraft and the conversations about holding a PT6 prop while it started up...at least you could hold it for a while (no I never tried it).

Just in case somebody gets the nerve to try this make sure your not grabbing a hold of a direct drive Garrett engine doing a series start

DB

Click to expand...

<font size="2" face="Verdana, Helvetica, sans-serif">Drivbiwire,

I have done the PT6 thing . It doesn't pull very hard. But watch out when you let go.... you aren't going to stop it!!

The TPE331 will throw you over to the other side of the airplane before the blades are moving fast enough to hurt!!

Originally posted by Drivbiwire:
You guys brought back the memory from when I used to fuel aircraft and the conversations about holding a PT6 prop while it started up...at least you could hold it for a while (no I never tried it).

Just in case somebody gets the nerve to try this make sure your not grabbing a hold of a direct drive Garrett engine doing a series start

DB

Click to expand...

<font size="2" face="Verdana, Helvetica, sans-serif">why? I think it would be fun to be thrown 200 feet through the air! MMMM Hard concrete. Love it!

ok,, with all losses added in,, a turbo will net at least 20% increase in hp over a non-turbo diesel engine... that formula is safe to use for all turbo engines.. some add a little more.. but the rule is generally close.. generators, farm equip, otr trucks, catepillers,, waterpumps, you name it.. etc...

There was an article in the November 2001 issue of Diesel Progress about the Scania 470 diesel truck engine (now in production) which features "turbocompounding". Turbocompounding is a system which uses a second turbo downstream from the regular tubocharger. The second turbo is larger and has a turbine wheel but no compressor wheel. The turbine wheel is connected to the engine output shaft through a high-speed clutch, so residual energy in the exhaust is recovered by this turbine.

This engine is rated 50 hp more that it's predecessor (470 hp vs. 420 hp), but it has a new high-pressure fuel injection system in addition to the turbocompound system. The article doesn't say how much of the 50 hp can be attributed to the turbocompounding.

Not all of the power that it takes to spin the turbocharger are taken from crankshaft horsepower. Some of it comes from increased crankshaft power due to increased exhaust system back pressure, the rest comes from the expansion of the hot gases.

If we assume that exhaust manifold back-pressure is not too different from intake manifold pressure - which is a fairly good approximation - then if we assume 1 bar exhaust back-pressure, 0.0019 m3 x 0.5 x (3000/60) x 100000 = 4750 watts, about 6 horsepower of crankshaft power is consumed.

The calculation of 8.7 compressor horsepower above is not quite right; when dealing with significant compressible flow you can't just multiply volume flow rate by pressure difference (although it will be in that order of magnitude). On top of that, the compressor efficiency is in the 65% range which means the required power is bigger ...

I forgot all about turbocompounding! If you aviation buffs remember back, the Lockheed Super Constellation with the R-3350's used turbocompounding to increase each engine's output by some 450hp if I remember correctly (it's been a while). That is recovered energy that would otherwise have been dumped out the exhaust stacks. The viscous clutches they used to connect the turbines to the crankshaft gave some trouble, but it worked!

Boundless quoted:

"The 10 or so HP it takes to drive the turbo is the price to pay for getting the approx. 20 hp net from turbocharging. A similar 1.9l naturally aspirated diesel would probably be around 70+/- HP as compared to our 90 HP TDI's."

I believe you're looking at the engine/turbocharger system as a "fluid power" system (like a hydraulic pump powering a hydraulic motor) where 10hp from the engine (pump) is tapped off the total power output to drive the turbo (hyd. motor). That's not the way it works.

The engine does't have to produce 10 more hp in order to drive the turbo... the turbo extracts that 10hp from the potential heat energy already contained in the exhaust stream that would otherwise be dumped out the exhaust.

If what I think you're saying is correct, then the turbocompounding on the Super Connie and also the new Scania trucks could not possibly work, at least the way I'm seeing it. For example, using your theory, if the R-3350's get 450 more hp from turbocompounding (turbocharger connected directly to the crankshaft), then when you factor in efficiency losses, the engine would have to produce perhaps 525hp MORE horsepower just to drive the turbos and associated gearing just to obtain the 450hp from the Turbocompounding. That's a net LOSS of power, 75hp in this example, than if turbocompounding was not used.

Since the difference in turbocharging and turbocompounding is what the exhaust driven turbine powers (either a compressor to compress the intake charge air or the crankshaft itself in the case of turbocompounding), what you're saying can't be happening.

Turbocharging is about as close to free lunch as you can get!

Very interesting xlr82v2,

Explain how a TDI has more output power than a similar naturally aspirated engine of the same displacement.

In particular, how does pumping more air into the diesel engine, using the exhaust stream, make the engine more powerful?

Here ya go....

Let's look at what it takes to pump the inlet air charge at 3100 engine RPM.

Pumping Power = Pressure across pump (inlet-to-outlet) x Volumetric Flow Rate

Let's assume 1 bar boost pressure (pressure across the pump I/O) which also equals 100,000 Newtons/ Sq. Meter and a flow rate of 0.06492 cubic meters/sec. (3100 RPM flow rate from VW specs.)

The power required to achieve this flow is

Power = 100,000 N/m^2 x 0.06492 m^3/sec

= 6.5 kW

= 8.7 HP

TURBOCHARGING AIN'T FREE!!!!!!!

Now that's just what is required to move that inlet charge air. Apply efficiency factors and the LOAD on the engine is MUCH greater.

Anybody have an idea as to the efficiencies of the compressor, turbine, and turbo overall?

TURBOCHARGING AIN'T FREE!!!!!!!

TURBOCHARGING AIN'T FREE!!!!!!!

TURBOCHARGING AIN'T FREE!!!!!!!

Oh yeah, you can include the pressure drop effects due to the airfilter in the pressure term and see how little affect a high flow air filter has on a TURBOCHARGED engine. Just change the pressure by a couple millibars and note the difference in the answers. Heck, take the entire pressure drop out and see how meaningless it is. Per VW specs, the A3 filter box with stock panel has 18 mbar of pressure drop at 3100 RPM. That will result in a 0.16 HP differential in pumping power to drive that air charge. How much less than 18 mbar is a high flow air filter... BFD!!! That high flow air filter has to be some of the most expensive and highest maintence HP($).

Have a nice weekend!!!

Now if I can make a nice efficient APU out of a spare turbo that I have, I can have it turn my 5,000watt generator!

I wonder what my spare Holset turbo can make?

Hondo! Can I come over and play build a turbine again?

LMAO!
DB

[ March 15, 2002, 12:14: Message edited by: Drivbiwire ]

Hondo, keep pete away from fire.

Pete:

You should see how many more BTU's you can get from your propane heater by attaching a turbo to it. That should keep your garage and my feet warm.

Cool! That's about what I guessed using the SWAG method of engineering... I guessed about 10-11 horsepower.

But I don't think that ALL of that power is "robbed" from the engine, rather it is extracted from the energy already contained in the exhaust that would otherwise be routed out the tailpipe. Mostly in the form of heat.

For example, if you had a way to measure the TIT vs. TOT temps, the difference would probably amaze you. I think this is the majority of the engergy that drives the turbine, not so much the engine "forcing" the exhaust gases through the turbo, like a "fluid power" system. The heat in the exhaust is converted to mechanical energy by the turbine.

So, true, turbocharging is not a "free" lunch, but it doesn't cost nearly as much as you would think.

Another application of this principle is in the Air Cycle Machines on aircraft (the "air conditioners"). They take hot bleed air from the compressor sections of the engines (that air is heated to over 600ºF just from compression- it hasn't reached the combustor section of the engine yet) and with a series of heat exchangers that take the temperature of the air down to about 250-300ºF and then a "cooling turbine" (basicly a turbocharger where the compressor side drives the air through the heat exchangers) that air is reduced to below freezing temperatures with no freon or other refrigerants at all! The way the temperature is regulated is by mixing warm air back into the cold air from the cooling turbine...but enough of that.

The way I look at it is that with turbocharging, you get the 7 course meal for the price of a bag of chips, and maybe a Coke!

Or am I way off base here ?

Originally posted by Boundless:
Very interesting xlr82v2,

Explain how a TDI has more output power than a similar naturally aspirated engine of the same displacement.

In particular, how does pumping more air into the diesel engine, using the exhaust stream, make the engine more powerful?

Click to expand...

<font size="2" face="Verdana, Helvetica, sans-serif"> Huh?

OK, I'm sensing some sarcasm here, but here goes.

What we're debating, I think, is where the energy to drive the turbine comes from. I think you're saying it is tapped off the total power output of the engine. I'm saying that it is tapped out of the wasted energy that is being dumped out of the exhaust. But here's how a turbocharger makes the engine more powerful.

For simplicity's sake we will ignore heat losses to the cooling system, and just focus on the exhaust.

Let's say that we have a normally aspirated diesel engine that is....oh, let's say 25% efficient. That means that 25% of the heat energy contained in the fuel is converted to mechanical energy at the crankshaft. The remaining 75% of the heat energy in the fuel is wasted as hot exhaust, and dumped to the atmosphere.

Now, let's add a turbocharger. The turbocharger is placed in the exhaust stream which contains 75% of the heat energy from the burning fuel. The turbine extracts another let's say 30% of the total heat energy from the burning fuel, converts that to rotational energy and uses it to drive the compressor. The compressor compresses the intake air to roughly double the density of the natural atmosphere (for this example). Now we're using 55% of the heat energy from the burning fuel to ultimately produce power at the crankshaft. So now the intake valve opens, and air at double the density of the natural atmosphere fills the cylinder. Since it is double the density of the natural atmosphere, there is double the normal amount of oxygen available to burn more fuel than with the same size normally aspirated engine. So, if you burn more fuel, you get more energy (horsepower) at the crankshaft, and also more heat in the exhaust from the same displacement engine than if it were normally aspirated.

Eventually, you get to the point that the turbocharger is extracting the maximum amount of energy that it can from the exhaust and compressing the maximum amount of air that it can going into the engine, and the engine is burning the maximum amount of fuel that it can for the amount of oxygen in the cylinders, and the maximum power output of the engine is reached.

That's the basics of turbocharging, at least the way I understand it. It lets you burn the same amount of fuel in our 1.9L engine that would only be possible in a normally aspirated engine if it had a displacement of perhaps 2.5L or more.

And it gets the ability to do that from the otherwise wasted energy contained in the exhaust stream, not from the energy already converted to mechanical power by the engine itself. It's not the mechanical action of the pistons pushing the exhaust gases out of the cylinder that drives the turbine (like others have said, even if the turbine were held stationary, the exhaust flow restriction of the "locked up" turbocharger would be minimal) but rather the hot exhaust gases expanding and cooling inside the turbine housing that powers the turbine. If you routed the same flow volume of cold air through the turbine housing as what flows through it at 3100rpm on the TDI, it would not even come close to the 180,000+ rpm that it reaches in normal use. The turbine harnesses that wasted energy (from the hot exhaust expanding and cooling in the exhaust system) and uses it to cram 2.5 liters of air (or more)into a 1.9 liter space, and thus be able to burn that much more fuel.

I hope this makes it a little clearer than mud

Am I making any sense anyone?

[ March 16, 2002, 09:15: Message edited by: xlr82v2 ]

xlr8, you have given the clearest answer, and from other research I have done elsewhere, The correct one. Boundless's comments about the turbo being pushed by the upward exhaust stroke are not to be believed.

The insanely high pressures created by expanding hot gasses are what drives the turbo, and Yes, it IS a free lunch. I'm going to be installing dual turbos on my other car, a Lincoln Mark VII to add power, and I know I won't be "spending" power to get the bonus. Maybe I'll dyno the car first to see exactly how much power that V8 will get.

--Jim

Originally posted by xlr82v2:
</font><blockquote><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><hr /><font size="2" face="Verdana, Helvetica, sans-serif">Originally posted by Boundless:
Very interesting xlr82v2,

Explain how a TDI has more output power than a similar naturally aspirated engine of the same displacement.

In particular, how does pumping more air into the diesel engine, using the exhaust stream, make the engine more powerful?

Click to expand...

<font size="2" face="Verdana, Helvetica, sans-serif">

Huh?

OK, I'm sensing some sarcasm here, but here goes.

What we're debating, I think, is where the energy to drive the turbine comes from. I think you're saying it is tapped off the total power output of the engine. I'm saying that it is tapped out of the wasted energy that is being dumped out of the exhaust. But here's how a turbocharger makes the engine more powerful.

For simplicity's sake we will ignore heat losses to the cooling system, and just focus on the exhaust.

Let's say that we have a normally aspirated diesel engine that is....oh, let's say 25% efficient. That means that 25% of the heat energy contained in the fuel is converted to mechanical energy at the crankshaft. The remaining 75% of the heat energy in the fuel is wasted as hot exhaust, and dumped to the atmosphere.

Now, let's add a turbocharger. The turbocharger is placed in the exhaust stream which contains 75% of the heat energy from the burning fuel. The turbine extracts another let's say 30% of the total heat energy from the burning fuel, converts that to rotational energy and uses it to drive the compressor. The compressor compresses the intake air to roughly double the density of the natural atmosphere (for this example). Now we're using 55% of the heat energy from the burning fuel to ultimately produce power at the crankshaft. So now the intake valve opens, and air at double the density of the natural atmosphere fills the cylinder. Since it is double the density of the natural atmosphere, there is double the normal amount of oxygen available to burn more fuel than with the same size normally aspirated engine. So, if you burn more fuel, you get more energy (horsepower) at the crankshaft, and also more heat in the exhaust from the same displacement engine than if it were normally aspirated.

Eventually, you get to the point that the turbocharger is extracting the maximum amount of energy that it can from the exhaust and compressing the maximum amount of air that it can going into the engine, and the engine is burning the maximum amount of fuel that it can for the amount of oxygen in the cylinders, and the maximum power output of the engine is reached.

That's the basics of turbocharging, at least the way I understand it. It lets you burn the same amount of fuel in our 1.9L engine that would only be possible in a normally aspirated engine if it had a displacement of perhaps 2.5L or more.

And it gets the ability to do that from the otherwise wasted energy contained in the exhaust stream, not from the energy already converted to mechanical power by the engine itself. It's not the mechanical action of the pistons pushing the exhaust gases out of the cylinder that drives the turbine (like others have said, even if the turbine were held stationary, the exhaust flow restriction of the "locked up" turbocharger would be minimal) but rather the hot exhaust gases expanding and cooling inside the turbine housing that powers the turbine. If you routed the same flow volume of cold air through the turbine housing as what flows through it at 3100rpm on the TDI, it would not even come close to the 180,000+ rpm that it reaches in normal use. The turbine harnesses that wasted energy (from the hot exhaust expanding and cooling in the exhaust system) and uses it to cram 2.5 liters of air (or more)into a 1.9 liter space, and thus be able to burn that much more fuel.

I hope this makes it a little clearer than mud

Am I making any sense anyone?</font><hr /></blockquote><font size="2" face="Verdana, Helvetica, sans-serif">No sarcasm, I just wanna see where you're coming from.

First, couple things:

</font><ul type="square">[*]<font size="2" face="Verdana, Helvetica, sans-serif">An engine is a positive displacement air pump. </font>[*]<font size="2" face="Verdana, Helvetica, sans-serif">Exhaust isn't exhaust until it clears the turbine. Until then, the 'exhaust' is a working fluid. </font>[/list]<font size="2" face="Verdana, Helvetica, sans-serif">A diesel engine has four strokes in the cycle. Power is had on one out of the four strokes. The other three strokes cost power. The exhaust stroke costs power because it pumps the exhaust gasses out of the cylinder. The cylinder that is doing the exhaust pumping gets its energy from the other cylinders' power strokes and its own energy stored in the flywheel just for this purpose. Pumping the spent gasses out of the cylinder requires power tapped off the total power of the engine. And the expansion of the gasses is also contributing to the pushing against the pistons on the exhaust stroke, causing more power off the top.
From xlr82v2:

What we're debating, I think, is where the energy to drive the turbine comes from. I think you're saying it is tapped off the total power output of the engine. I'm saying that it is tapped out of the wasted energy that is being dumped out of the exhaust. But here's how a turbocharger makes the engine more powerful.

Click to expand...

<font size="2" face="Verdana, Helvetica, sans-serif">In a turbocharged engine, the exhaust is not exhaust until it clears the turbine. Until then, it is a working fluid. Nothing is waste until it is past the turbine.

From xlr82v2:

For simplicity's sake we will ignore heat losses to the cooling system, and just focus on the exhaust.

Let's say that we have a normally aspirated diesel engine that is....oh, let's say 25% efficient. That means that 25% of the heat energy contained in the fuel is converted to mechanical energy at the crankshaft. The remaining 75% of the heat energy in the fuel is wasted as hot exhaust, and dumped to the atmosphere.

Click to expand...

<font size="2" face="Verdana, Helvetica, sans-serif">xlr82v2, big mistake, big big mistake.... the simplification that is. Way too simple...

About 75% to 85% of your diesel fuel goes right to heat waste by:

</font><ul type="square">[*]<font size="2" face="Verdana, Helvetica, sans-serif">Cooling System </font>[*]<font size="2" face="Verdana, Helvetica, sans-serif">Oil System </font>[*]<font size="2" face="Verdana, Helvetica, sans-serif">Exhaust </font>[/list]<font size="2" face="Verdana, Helvetica, sans-serif">You've grossly over simplified the situation to the point where it is not representative or transferable to reality. Your simplified waste stream is much larger than reality.
From xlr82v2:

That's the basics of turbocharging, at least the way I understand it. It lets you burn the same amount of fuel in our 1.9L engine that would only be possible in a normally aspirated engine if it had a displacement of perhaps 2.5L or more.

Click to expand...

<font size="2" face="Verdana, Helvetica, sans-serif">BRAVO!!!!! Well put!!!! You understand that additional fuelling is possible with a turbo'd application and this additional fuel is where the additional power comes from.

From xlr82v2:

And it gets the ability to do that from the otherwise wasted energy contained in the exhaust stream, not from the energy already converted to mechanical power by the engine itself. It's not the mechanical action of the pistons pushing the exhaust gases out of the cylinder that drives the turbine (like others have said, even if the turbine were held stationary, the exhaust flow restriction of the "locked up" turbocharger would be minimal) but rather the hot exhaust gases expanding and cooling inside the turbine housing that powers the turbine. If you routed the same flow volume of cold air through the turbine housing as what flows through it at 3100rpm on the TDI, it would not even come close to the 180,000+ rpm that it reaches in normal use. The turbine harnesses that wasted energy (from the hot exhaust expanding and cooling in the exhaust system) and uses it to cram 2.5 liters of air (or more)into a 1.9 liter space, and thus be able to burn that much more fuel.

Click to expand...

<font size="2" face="Verdana, Helvetica, sans-serif">The engine is a positive displacement gas (as in phase) pump. It is the pumping action of the pistons that pushes the spent gasses out of the cylinder. It is called the exhaust stroke. This gas flux is a working fluid in a turbo'd application. It drives the turbine.

The turbine in the exhaust results in a backpressure that is much greater than a naturally aspirated engine. This pressure must be produced by and overcome by the engine. This pressure wouldn't be there if it weren't for the turbocharging and the load it presents to the engine. This is power off the total power of the engine.

If you routed the same volumetric flow of cold air through the turbine, the turbine would probably be destroyed. The max speed would most likely be exceeded. The gasses exert pressure on the turbine blades. Cold gasses have higher density than hot gasses, therefore for the same volumetric flow, much greater kinetic energy. The force a working fluid exerts on a turbine blade is represented by dM/dt, or the change in momentum with respect to time. A higher density working fluid yields a greater change in momentum. Here's an obvious example: aircraft require more power for take off on hot days or at high altitude airports because of the lower density of the air. The lower density air can not develop the lift (force) as well as the higher density air at sea level or the not-hot conditions air.

A turbine is a mechanical device that extracts work from a fluid and converts that into shaft work. This could be isothermal, no heat interactions. A turbine looks like a throttling device, such as an orifice. When a gaseous working fluid is pumped through a throttling device, the pressure is reduced. This results in a reduction of the temperature of the working fluid although heat was not added or lost. This is why the temp of the working fluid stream drops through the turbine. If it is otherwise, where is the heat going?

There are two ways to reduce the temp of a gas:

</font><ul type="square">[*]<font size="2" face="Verdana, Helvetica, sans-serif">Remove heat </font>[*]<font size="2" face="Verdana, Helvetica, sans-serif">Reduce the pressure </font>[/list]<font size="2" face="Verdana, Helvetica, sans-serif">If things are happening as you say, a heat flux is driving the turbine. Where is the heat flux coming from and going to? How does that heat flux drive the turbine?

[ March 16, 2002, 20:25: Message edited by: Boundless ]

Originally posted by Boundless:
Here's an obvious example: aircraft require more power for take off on hot days or at high altitude airports because of the lower density of the air. The lower density air can not develop the lift (force) as well as the higher density air at sea level or the not-hot conditions air.[/QB]

Click to expand...

<font size="2" face="Verdana, Helvetica, sans-serif">OK, I'll admit that you know more scientific formulas and jargon than I do, and I just can't argue with that. But I still don't believe that what you're saying is actually what is happening. So is turbo rpm directly proportional to engine rpm?

However, you have encroached into my area of expertise, and your obvious example of an aircraft needing more power at altitude to take off is obviously flawed.

An aircraft does NOT require MORE power to take off at higher altitudes as it does at lower altitudes. It requires the SAME power, regardless of altitude. It still takes 12,000 pounds of lift to raise 12,000 pounds aircraft, no matter what the altitude.

You are correct in stating that the wings cannot produce lift as effectively in high density altitude (thin air) conditions as they can in low density altitude (dense air). But you have forgotten, that the same density altitude conditions that decrease the lifting properties of the wing ALSO decrease the power output of the engines as well. So, in your high altitude takeoff scenario, what is REALLY happening is you are taking off at higher density altitudes (like Denver) with LESS power than what you have available at lower density altitudes (like here in St. Louis). And remember, that 12000 pound airplane weighs 12000 pounds at ANY altitude. So, in order for the wing to be able to lift 12K pounds at high altitudes, you have to attain a higher TRUE AIRSPEED (not groundspeed or indicated airspeed) to develop the same amount of lift, not have more power. More power will get you into the air faster, and allow you to climb faster, but More Power is not available. That is why airplanes take longer to get off the ground at Denver than they do here in St. Louis--they have less power available, so they do not accelerate as quickly, plus they also have to accelerate to a higher true airspeed, and thus take up considerably more runway up in Denver than they do down in St Louis.

Ain't this fun?

"Here's an obvious example: aircraft require more power for take off on hot days or at high altitude airports because of the lower density of the air. The lower density air can not develop the lift (force) as well as the higher density air at sea level or the not-hot conditions air.

In the case of a fan(jet) engine rather than a recip, the engine is still producing the same thrust rating except for the fact the fan section has to spin at a higher % of rated RPM to accomplish the desired thrust setting. Simply put the fan has to spin a bit faster to develope the same amount of lift on each fan blade due to the higher density altitude.

In certain situations thrust must be limited or even reduced to accomodate VMCG or VMCA considerations and the effects of density altitude on directional control. If your in Salt Lake City you may not be able to use the same thrust setting as you could in Orlando, reason being if you lose an engine on the Take-Off roll the nose gear or vertical stabilizer may not be able to keep you pointed in the direction you "intend to go" . If a thrust reduction is required then the take-off roll will be increased proportionally but not as a factor of lift on the wings but because of directional control.

Since all take-off speeds are predicated on IAS not TAS most of the factors taken into consideration relate to the most critical engine out scenerio.

I have a few runway analysis laying around the links are below. They make for some good denisty altitude conversations.

I posted the DC8 Data below the links are about .56mb so be patient on the downloads.

Below are links to Dayton (1009 MSL and SLC at 4227 MSL)

http://pics.tdiclub.com/members/Drivbiwire/KDAY%20Data.jpg

http://pics.tdiclub.com/members/Drivbiwire/SLC%20Data.jpg

DB

Originally posted by Drivbiwire:
"Here's an obvious example: aircraft require more power for take off on hot days or at high altitude airports because of the lower density of the air. The lower density air can not develop the lift (force) as well as the higher density air at sea level or the not-hot conditions air.

In the case of a fan(jet) engine rather than a recip, the engine is still producing the same thrust rating except for the fact the fan section has to spin at a higher % of rated RPM to accomplish the desired thrust setting. Simply put the fan has to spin a bit faster to develope the same amount of lift on each fan blade due to the higher density altitude.

DB

Click to expand...

<font size="2" face="Verdana, Helvetica, sans-serif">DB,

Your paragraph response above addresses what I was after. We need the same thrust, but if I remember correctly, it requires a higher 'power setting', aka: 'hot day' conditions.

BTW,

In this sentence,

Simply put the fan has to spin a bit faster to develope the same amount of lift on each fan blade due to the higher density altitude.

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<font size="2" face="Verdana, Helvetica, sans-serif">I think you want to delete the word "density", or say something like "low density air at high altitude"?

Just as the fan blades need to spin faster in lower density air to develop the equivalent thrust, the same principle holds true for a turbine blade or an aircraft wing. Less dense working fluid requires higher relative speeds between the airfoil and working fluid to achieve the desired power transfer, be it a compressor or turbine.

Think about hydro-turbines that spin massive generators for making electricity. Water from a dam is the working fluid.

In order to get the lift for that 12,000 lb aircraft at Denver, you need more airspeed and/or AOA to get the lift you get at sealevel under normal conditions. Power is not the same as lift. Higher AOA also increases drag (lift-to-drag ratio). Higher airspeed and the acceleration to achieve that airspeed requires more power.

From xlr82v2:

That is why airplanes take longer to get off the ground at Denver than they do here in St. Louis--they have less power available, so they do not accelerate as quickly, plus they also have to accelerate to a higher true airspeed, and thus take up considerably more runway up in Denver than they do down in St Louis.

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<font size="2" face="Verdana, Helvetica, sans-serif">My point about airspeed (true) exactly.

From xlr82v2:

So is turbo rpm directly proportional to engine rpm?

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<font size="2" face="Verdana, Helvetica, sans-serif">Good question. And the answer is yes, but not as direct as if direct shaft driven. Turbo RPM is a function of the volumetric flow rate of the spent gasses, which is a function of the compressor load (boost pressure & increased air volume), altitude, throttle, etc. Assume a regular fixed geometry turbine section. But, there could be a high load at a low RPM that would still result in very high volumes of spent gasses that would drive the turbo very fast. So the turbo speed is proportional to engine RPM and the load at that RPM.

I spent several years developing flight control computer systems (fly-by-wire) and aircraft ice protection systems working with NASA, FAA, airframers, engine houses, etc. I'm pretty familiar with what it takes to get and keep an aircraft airborne. Not as a pilot, but an engineer.