What is "power"?

SixPapaCharlie

SixPapaCharlie

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I don't quite understand the concept of power I guess.
I have kind of thought full throttle = full power.

Not the case and I don't quite understand what all the components are that determine power.
This question is based on cruising along at full throttle and the avidyne telling me 80% or watever the number was.
I don't recall but I expected it to be a lot closer to 100.

So what is power?
 
Power is what you didn't have!

engine speed, manifold pressure, fuel flow (to some extent, depends on LOP, Peak, ROP condition)
 
Power is the end result of the integral of pressure on a cylinder over an engine cycle.

But for what you need to know, power is determined by air, fuel, and ignition timing. Ignition timing is fixed on our planes (unless you get one of the recently STC'd electronic ignitions, or fly an experimental that is so equipped). Air is a component of your throttle position and also your altitude (or, if you have a turbo, it's pretty much just throttle position) as well as outside air temperature (in Texas it's usually hot). Then there's your fuel, as depending on your mixture setting you will be producing more power or less power. Think about it like this: At full rich, you make power. At idle cut off with zero fuel, you make no power. And somewhere in between is where power drops off because of fuel.
 
Power is the ability to do an amount of work in a set time.

With an engine that comes down to torque multiplied by RPM. That is basically the pressure of combustion on the piston multiplied by how many times a minute you get those combustions.

So you have two factors, throttle/mixture which controls how much pressure per combustion, and the prop which regulates how many times a minute. You can reduce either of the independently and reduce power.

Percent power is also altitude dependent.
 
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SixPapaCharlie said:
I don't quite understand the concept of power I guess.
I have kind of thought full throttle = full power.
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Full throttle only means the throttling valve is fully opened. There are plenty of reasons why that would result in less than full rated power, including being at a density altitude above sea level with a normally aspirated engine, or having a high drag load on the prop (like at climb speed with fixed pitch) so the RPM doesn't reach the full rated RPM, or the mixture is too rich/lean so combustion isn't at peak efficiency. They only time you're developing full power (i.e., the rated power stamped on the engine's ID plate) is when you're at sea level standard day (29.92" and 59F/15C) with the engine turning rated RPM and the mixture at optimum with the right load on the crankshaft (i.e., using a dynamometer).

This question is based on cruising along at full throttle and the avidyne telling me 80% or watever the number was.
I don't recall but I expected it to be a lot closer to 100.
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I'm guessing you were at about 6000 feet when you saw that, right? At that point, air density is only about 80% of SL density so the most a normally aspirated engine can develop is 80% power because it only has 80% of the air molecules to oxidize the available fuel.

So what is power?
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As noted -- work, measured in ft-lb/hr, or similar units (1 HP being 33,000 ft-lb/hr).
 
A question I'd often ask primary students, in a C150, let's say, was:

"What HP does this plane have?"

"100"

"Good. Up to what altitude do you think it can produce that power?"

I think the most common answer I'd get was around 5,000'.

In any case, your new Cirrus has 310 HP at SL on a standard day. That would be the 100% you were looking for. Up to what altitude do you think your plane can produce that power?
 
Power for this purpose is better understood as the rated power of the engine at sea level. So for a typical io-540 that might be something like 260 horsepower. That is at sea level it can generate 746 watts of power x 260.

As you rise in altitude the engine can do less work generate less of its max rated hp because (for simplification) whereas at sea level the ambient pressure pushed 100 molecules of o2 per stroke with the ten molecules of 100ll it now only pushes 70 molecules of O2 into the combustion chamber per stroke. To get max power for that ambient pressure you must scale the amount of 100ll that goes into the combustion chamber to 7 molecules of 100ll.

BUT because you are only getting 70% of sea level O2 and 70% of sea level 100ll you only get 70% of sea level rated HP. Or about 190 hp.
 
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I'll take "Things my wife has that I don't" for $200, Alex.
 
FastEddieB said:
In any case, your new Cirrus has 310 HP at SL on a standard day. That would be the 100% you were looking for. Up to what altitude do you think your plane can produce that power?
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I will use my go to answer when answering flying questions:
Well, that depends on a lot of factors.
 
FastEddieB said:
A question I'd often ask primary students, in a C150, let's say, was:

"What HP does this plane have?"

"100"

"Good. Up to what altitude do you think it can produce that power?"

I think the most common answer I'd get was around 5,000'.

In any case, your new Cirrus has 310 HP at SL on a standard day. That would be the 100% you were looking for. Up to what altitude do you think your plane can produce that power?
Click to expand...

What is the density altitude?
 
"Power" is that thing I never have enough of.
 
SixPapaCharlie said:
...This question is based on cruising along at full throttle and the avidyne telling me 80% or watever the number was.
I don't recall but I expected it to be a lot closer to 100...
Click to expand...

Your SR22 can only achieve 100% power at ideal standard conditions (sea level) Once you leave the ground you can no longer make 100%, at 10,000 feet you might be lucky to get 75%
 
Are cars the same way?
Is a 200 hp car actually producing less due to atmospheric conditions?
 
I'm gonna try flying at sea level more often.
 
SixPapaCharlie said:
Are cars the same way?
Is a 200 hp car actually producing less due to atmospheric conditions?
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Yes! And people back in the dark ages (when I was a young kid) had to get their carburetors re-jetted when they lived in places like the Rockies. But since you are rarely running your car at anything like 70%-100% power, you don't really notice.

John
 
SixPapaCharlie said:
Are cars the same way?
Is a 200 hp car actually producing less due to atmospheric conditions?
Click to expand...

Yes. It's not so noticeable in a car because a car engine only has to deliver a small fraction of its rated power to do the day-to-day things the driver demands of it.

Edit: Sheesh--John beat me by a minute. Gotta be quick around here.

Tim
 
timrb said:
Yes. It's not so noticeable in a car because a car engine only has to deliver a small fraction of its rated power to do the day-to-day things the driver demands of it.

Edit: Sheesh--John beat me by a minute. Gotta be quick around here.

Tim
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Fastest post in the west! :D
 
SixPapaCharlie said:
Are cars the same way?
Is a 200 hp car actually producing less due to atmospheric conditions?
Click to expand...
I think if you look carefully, you'll find your "200 HP" car engine is rated at redline RPM -- like 6500 or so. When you're cruising along at 55 mph, it's probably only turning around 2000 RPM, and thus is producing perhaps only one-third of rated power. That said, if you take it up to the top of the Sandia Peak outside Albuquerque, at that 10,000-foot elevation, even at rated RPM, it's probably only capable of producing about 130 HP due to the roughly 65% air density -- just like a "200 HP" aircraft engine.

Of course, if your car's engine is turbocharged and capable of producing the same intake air pressure at 10,000 as it can at SL, then yes, it could produce full rated power at 10,000 feet -- just like a turbocharged aircraft engine that can produce full rated intake pressure at that altitude. That said, turbocharged engines have a "critical altitude" where the turbocharger is no longer capable of producing full rated intake air pressure, and above that point, the engine can no longer produce full rated power.
 
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Bobanna said:
Yes. If you've ever driven in the mountains (think I-70 in Colorado), you'll notice a decrease in acceleration and "pick-up" in the higher elevations, unless you have a turbocharged Saab or something like that.
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Or a Cummins turbo diesel. ;)

I love taking the Dodge and the 182 to sea level. I'm always mildly surprised at how well they both perform. The truck accelerates better with the trailer on, and the takeoffs in the 182 are significantly shorter.

In the mountains with the 12,000 lb trailer I usually see 30+ lbs of boost on the truck computer and have to consciously think about not exceeding the 1350 EGT (well actually the computer will de-fuel at that point and alarm at 1250 which is where I set the alarm to get my foot out of it or downshift - but sometimes you're almost to the top of the hill and it's simpler to just acknowledge the alarm and stay in it for 20 more seconds) and the time limit on operation above 1100 EGT limit in the truck. :) :) :)

Search "Ike Challenge" on YouTube for videos of various trucks pulling various loads up the eastbound side of the climb up to the Eisenhower/Johnson tunnel(s) on YouTube to see what happens to "power" as you reach 12,000 MSL when towing heavy. They have recently switched to a dual-trailer setup with a gooseneck horse trailer pulling a utility trailer and both loaded with water tanks to hit 19,500 lbs, so they can actually give the modern turbo diesels a workout. :) They also test half ton trucks with lighter trailers within their towing capacity legal limits.

They recently did a (controlled somewhat by Ford, the Ford rep demanded to ride along in the Ford truck adding 170 lbs to the cab against the other two where he wouldn't ride along) similar 7% grade test and filmed it pulling 12,500 lb 5th wheels -- and down near sea level the new 2015 Ford walked away from both the Chevy Duramax and the Dodge Cummins in timed tests, which makes me sad... Heh... Go Cummins!...

But Ford hasn't given them a test truck for the Ike Challenge yet. I think Ford knows something about their high altitude tuning or exhaust flow limits and they don't want the test done up here, yet. They'll all keep tuning and working on it.

The Duramax barely accelerated to the speed limit from the rolling 50 MPH start up the 7% grade in Virginia. The Cummins accelerated better. The Ford they had to get out of the foot feed to stay below the speed limit.

Mmm. Yummy horsepower and torque. Tasty. ;) ;) ;)
 
Put a tiger in your tank.

Oh, wait, maybe you need a flower.
 
"power" is driving up to the airport security gate, pushing the little button and when the lady answers you just tell her who you are and the gate opens. .......that's POWER :D
 
With NA engines you almost never get full rated power unless you like flying/driving at sea level which in my case is...never. :D

That's why I like forced induction in whatever I drive and prefer it in planes too. The horsepower rating is almost meaningless for NA. So that 310HP plane you have? If you're lucky the most you will ever see is 80% of that. Go to a hot high field in west texas? Even worse. Like I said I'm a huge fan of turbocharging and supercharging for that reason.
 
Ron Levy said:
I think if you look carefully, you'll find your "200 HP" car engine is rated at redline RPM -- like 6500 or so. When you're cruising along at 55 mph, it's probably only turning around 2000 RPM, and thus is producing perhaps only one-third of rated power. That said, if you take it up to the top of the Sandia Peak outside Albuquerque, at that 10,000-foot elevation, even at rated RPM, it's probably only capable of producing about 130 HP due to the roughly 65% air density -- just like a "200 HP" aircraft engine.
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Absolute nonsense.

First of all, no cars are not "rated" at "redline RPM". Car output power is a marketing figure, nothing else. Production engines are not measured at all, pre-production engines are measured to verify calibrations, but the results are always rounded to the nearest suitable tax bracket anyway.

No modern car produces their peak hp output at redline RPM.

Since around 1999 after Bosch implemented their filling model based ECUs (and rest followed, even US cars have this now), most, even N/A cars, can produce 100% hp up to 5000-6000ft density altitude. Torque will be lower, and the peak hp a bit higher up the rev range, but, again, no-one "rates" them at redline.

Scary thought that a CFI thinks you need a third of available hp to cruise at 55mph. I hope you don't teach anything that has to do with engines... You are NOT using a THIRD of your horsepower cruising at 55mph, just think for a while. You are assuming your throttle plate is wide open in that assumption.
 
mtuomi said:
A
No modern car produces their peak hp output at redline RPM.
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I would agree with everything else you said but it is possible for some engines (NA) to be producing peak HP at redline RPM. I do agree that they are probably in the minority for modern engines.
 
The only way to directly measure power is to have a torque meter on the propeller shaft, as power is the product of torque and RPM. Since most GA airplanes don't have one, we have to estimate power based on power fuel and manifold pressure settings, and performance charts.
 
narchee said:
I would agree with everything else you said but it is possible for some engines (NA) to be producing peak HP at redline RPM. I do agree that they are probably in the minority for modern engines.
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I don't know of any. The current Bosch calibration guidelines(and also common sense) says they should never do that. Most manufacturers work within them.
 
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peerlesscowboy said:
"power" is driving up to the airport security gate, pushing the little button and when the lady answers you just tell her who you are and the gate opens. .......that's POWER :D
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Nah. It's when you ride up in the limo and the people inside are prepped to open it without your driver having to stop. :)
 
SixPapaCharlie said:
I don't quite understand the concept of power I guess.
I have kind of thought full throttle = full power.
Click to expand...

as long as the RPM indicated is what the type certificate says is full power, that's full power.

different applications, different settings, different horse power, will result in different ratings in the type certification.
 
I'm betting he turned around to M6PC before hitting post and said... watch this.

Someone has probably already performed a word search in the FAR/AIM in hopes of having a regulation to post for an answer.
 

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mtuomi said:
Absolute nonsense.

First of all, no cars are not "rated" at "redline RPM". Car output power is a marketing figure, nothing else. Production engines are not measured at all, pre-production engines are measured to verify calibrations, but the results are always rounded to the nearest suitable tax bracket anyway.

No modern car produces their peak hp output at redline RPM.

Since around 1999 after Bosch implemented their filling model based ECUs (and rest followed, even US cars have this now), most, even N/A cars, can produce 100% hp up to 5000-6000ft density altitude. Torque will be lower, and the peak hp a bit higher up the rev range, but, again, no-one "rates" them at redline.
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You're right -- I did not address flat-rating. That does change things significantly. But it was true in the 66 VW in which I learned to drive. :wink2:
 
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