More,power at altitude

[Truckedup]

 I asked a theoretical question on a forum for piston engine aircraft guys about power peak at altitude . Sorta continuation of a discussion I had here.My example was nothing in particular,a non supercharged engine peaking at 5000 rpm. There are variables,just a general answer.
 This answer,does it really hold true?  

      

The power your engine produces at sea level is determined by hooking it up to a dynamometer that absorbs all the power the engine can produce at any given rpm. This is how the data used to plot a typical horsepower versus rpm curve is obtained. In the case of your example engine, it produces all the power it can produce at 5,000 rpm, and presumably does this at wide open throttle.

If you were to put a stop on the throttle that limited its maximum opening some arbitrary point shy of wide open, your engine's maximum power would now occur at some rpm less than 5,000. By partially closing the throttle you have restricted the maximum manifold pressure available, and as a consequence, the maximum power produced.

This is precisely what you do when you take your engine to an altitude above sea level and run it. Just as when it is throttled on the dynamometer, the maximum power it can produce will be achieved at some value below 5,000 rpm.

[revolutionary]

Lots of variables. But the general answer is yes.

You can find short examples by playing with the Wallace Racing Calculators and the long answer by reading some of Prof Gordon Blair and others' work.

[POPS]

In other words:
To get the most heat the fuel must be kept at or near its stoichiometric ratio, (complete burn).  The A/F is a ratio of lbs of air / lbs of fuel.  For methanol the ratio is 6.5.  So 6.5 lbs of air / 1 lb of methanol is the correct ratio and so is 13 lbs of air / 2 lbs of methanol.  The latter combination will make more heat.  To get peak power at altitude you must reduce the fuel volume to get the correct stoichiometric ratio due to the drop in air density.  Less air requires less fuel which = less heat which = less power.  Like the above post points out there is more to it, like compression etc.  A/F being the biggest consideration.
POPS

[Dean Los Angeles]

Air is a highly variable commodity. Sea level doesn't tell you anything. The barometric pressure at sea level in Sibera is 15.22 psi

Air is at 14.696 psi at 1 atmosphere. The weight of all of the air above you weighs 14.696 pounds.

That changes with the weather. Ignoring the weather component, the formula for pressure at altitude is complex.

The short answer is: The higher you are, the less air above you and less pressure available to the engine.
The hotter you are the molecules are spread out and there is less air available.
The higher the humidity, the less air you have because is is displaced by the water.

Running at Denver when it is hot and humid nets you considerably less horsepower then the exact same setup running in Death Valley when it is cold and dry.

This is why you need to verify the barometric pressure, temperature and humidity when you make your dyno run so that you can adjust to the conditions that exist when you run at a different pressure, temperature and humidity. If you don't match the fuel to existing conditions then you will not be running optimal horsepower for the conditions.

[tortoise]

None of the replies answer the question asked. Not opening the throttle fully or lowering the air density will of course reduce power; but will the power peak be at a lower RPM?

If you were to put a stop on the throttle that limited its maximum opening some arbitrary point shy of wide open, your engine's maximum power would now occur at some rpm less than 5,000.

Not necessarily, I'd say. Especially with a motor that is RPM limited, making peak power at redline.

[Truckedup]

None of the replies answer the question asked. Not opening the throttle fully or lowering the air density will of course reduce power; but will the power peak be at a lower RPM?

If you were to put a stop on the throttle that limited its maximum opening some arbitrary point shy of wide open, your engine's maximum power would now occur at some rpm less than 5,000.

Not necessarily, I'd say. Especially with a motor that is RPM limited, making peak power at redline.

 Yes,you have answered my question...The engine in question,not the one in the example,will rev beyond peak power with of course a drop in power.At least at seal level.The loss of power from increased altitude not in dispute.

[Dean Los Angeles]

Closing the throttle is not the same as a change in pressure altitude.

Peak torque is when the full dynamics of the intake system are at the maximum and the air/fuel ratio is also perfect. Intake tract tuning comes into play. Closing the butterfly changes that dynamic differently than the butterfly wide open at a higher altitude.

Peak power comes after peak torque and is a function of rpm and intake efficiency. The difference is that past peak torque the engine is starting to starve for air. The additional power pulses from the higher rpm at peak power make up for that. Past peak power the additional pulses don't make up for the difference and power starts falling off. In a normally aspirated engine air is always the limiting factor. If you run the engine with no load at WOT the only thing preventing unlimited rpm's is air starvation.

If peak power is at 5,000 rpm any reduction in air will reduce peak power rpm to a lower value. If you leave the engine at idle and run the dyno you will discover that peak power is at idle rpm.

[Truckedup]

In a normally aspirated engine air is always the limiting factor

If mechanical design and detonation are not a factor   

 

[JustaRacer]

High DA pushes the powerband to the left.

Non-expert guess:

The speed of sound falls as the air gets thinner.  The speed of sound is how fast the "pulses" travel in the intake and exhaust.  A well-designed engine uses these pulses to improve volumetric efficiency. 

When the pulses are timed perfectly, is where you get max output.  If the pulses are slower, it happens at a lower RPM.

[fordboy628]

Just some additional thoughts based my years of dyno experience......      I have added some comments (in blue) to Dean's post.  Dean, my apologies for hacking up your thought process.   In my opinion.......

Closing the throttle is not the same as a change in pressure altitude.   

Peak torque is when the full dynamics of the intake system & exhaust sytem are at the maximum and the air/fuel ratio is also perfect.   Torque, (the work) is at peak because volumetric efficiency is at its' highest.  V/E is less on both sides of peak torque.  Intake tract tuning comes into play.   As does exhaust tract tuning.    Closing the butterfly changes that dynamic differently than the butterfly wide open at a higher altitude.   On an N/A engine, closing the throttle "disrupts" the airflow path.  This relationship & "disruption" is not linear.

Peak power comes after peak torque and is a function of rpm and intake efficiency.  And exhaust tuning.  The difference is that past peak torque the engine is starting to starve for air.   And become less efficient.   The additional power pulses from the higher rpm at peak power make up for that.  Because of the formula, bhp = torque * rpm / 5252.   Past peak power the additional pulses don't make up for the difference and power starts falling off.  Possibly, very rapidly.  In a normally aspirated engine air is always the limiting factor. If you run the engine with no load at WOT the only thing preventing unlimited rpm's is air starvation.   Be sure to understand those last 2 sentences.

If peak power is at 5,000 rpm any reduction in air will reduce peak power rpm to a lower value. If you leave the engine at idle and run the dyno you will discover that peak power is at idle rpm.

In a normally aspirated engine air is always the limiting factor

If mechanical design and detonation are not a factor    

Yes, of course you are correct to point this out.   But there is limited value to engines that have mechanical design problems and/or problems with detonation.   Certainly no one purposely designs a racing (or other) engine with these issues.   The trick, is to refine/design/tune these problems out.   And that is not as easy as it sounds........
 
Fordboy

[Truckedup]

In a normally aspirated engine air is always the limiting factor

If mechanical design and detonation are not a factor    

Yes, of course you are correct to point this out.   But there is limited value to engines that have mechanical design problems and/or problems with detonation.   Certainly no one purposely designs a racing (or other) engine with these issues.   The trick, is to refine/design/tune these problems out.   And that is not as easy as it sounds........
 
Fordboy

 I'm no expert but I think the vast majority of engines are modified mass production engines with many design limitations when used as dedicated racing engines.Any engine can go into detonation  if,for example,a piston ring starts to leak ,oil get in the combustion chamber causing detonation.
  Ok,most agree the engine power peak will be lower and that affects gearing choices. I'm assuming messing with intake and exhaust lengths will not tune the power peak RPM back  up were it was...

[fordboy628]

Truckedup,

Sorry to hack up your quote......  I've added my thoughts in blue.

I'm no expert but I think the vast majority of engines are modified mass production engines with many design limitations when used as dedicated racing engines.    I agree. Most production engines are designed around "packaging requirements" and other limitations.  Any engine can go into detonation  if,for example,a piston ring starts to leak ,oil get in the combustion chamber causing detonation.   Also true, and I agree again.
  Ok,most agree the engine power peak will be lower and that affects gearing choices.   Yes, peak power will be lowered somewhat.  How much???  This is something you would need to model for an accurate number, and the good software for this is thousands.   I'm assuming messing with intake and exhaust lengths will not tune the power peak RPM back  up were it was...  I would not agree with that.  Again, modeling would be helpful.

Most experienced Salt racers (that's not me) use estimates, guidelines and rules of thumb to work this out.  Perhaps someone more Salt experienced than I can help you out here.  I would head out to the Salt prepared to change out my final drive cogs, if I were you.
 
Fordboy

[tortoise]

If you run the engine with no load at WOT the only thing preventing unlimited rpm's is air starvation.

I strongly advise against attempting to verify this assertion.