Airflow at X psi elevation difference?

[te72]

Paging the smart guys around here, with knowledge of fluid dynamics and whatnot...

Take car A, it is a Turbo car. At sea level, where the ambient air is more dense, does the turbo work less to maintain X psi, as opposed to say 6000' elevation? Or does X psi simply flow more air at the lower elevation?

I could be swayed either way in the argument, but I'm currently under the first understanding (turbo works harder at higher elevation, but power levels are still the same), but the second argument (more air flow at lower elevation, more power at same pressure level) makes sense to me too.

Thoughts?

 

[IndigoMKII]

Well, take this into account.

A car at higher elevation will make less power than the same exact car at sea level.

This might be a good read for you. http://en.wikipedia.org/wiki/Atmospheric_pressure

I know a FI airplane engine will normally maintain power to X altitude whereas a NA airplane engine will maintain BHP til X altitude then slowly drop off.

 

[Poodles]

Paging the smart guys around here, with knowledge of fluid dynamics and whatnot...

Take car A, it is a Turbo car. At sea level, where the ambient air is more dense, does the turbo work less to maintain X psi, as opposed to say 6000' elevation? Or does X psi simply flow more air at the lower elevation?

I could be swayed either way in the argument, but I'm currently under the first understanding (turbo works harder at higher elevation, but power levels are still the same), but the second argument (more air flow at lower elevation, more power at same pressure level) makes sense to me too.

Thoughts?

You're right with the first arguement. You might have a bit more lag though.

Second arguement might be true if you ran without a wastegate...

 

[te72]

Well, take this into account.

A car at higher elevation will make less power than the same exact car at sea level.

Definitely experienced with the perils of high altitude (I live fairly low in Wyoming, at about 6200' or so), and have driven/lived in other places at significantly lower elevations. About the only difference I noticed in the turbo car was the amount of lag increased a bit up here.

Just having a debate with a guy who thinks physics is the end all, be all of any argument. Sure, they play a huge factor, but when you take into account something as complex as a car engine...

You're right with the first arguement. You might have a bit more lag though.

Second arguement might be true if you ran without a wastegate...

Thought I had it right, but was hoping for confirmation, thanks. Definitely have more lag at this altitude. Down in Phoenix/Vegas, my car starts coming on pretty hard by 2000 rpm. Up here, it is about 400-500rpms later for the same feeling.

The waste gate argument... see, that is what I think my friend is not taking into account. If it were an argument of absolute values, then his argument makes perfect sense.

 

[te72]

After doing a bit more research into this this morning, I'm wondering if this is more simple than it seems, or MUCH more difficult. Shall continue reading, but there are a few of you on here I would be most interested in hearing from on the subject.

 

[Poodles]

5psi after the turbo is 5psi regardless of the altitude (since opening the wastegate is fighting a spring).

For anything NA or a boosted application without pressure control (some old turbos, some superchargers) it's a different story. It's why boosted was the way to go for airplanes in WWII.

 

[CyFi6]

So air density does not affect the power output of a forced induction engine as long as the turbo/supercharger can flow enough to achieve the same pressure?

 

[te72]

So air density does not affect the power output of a forced induction engine as long as the turbo/supercharger can flow enough to achieve the same pressure?

This is what I'm thinking, along the lines of the "turbo works harder at elevation" argument.

 

[jetjock]

Physics IS the end all, be all of any argument...

 

[te72]

JJ, the guy I was arguing with was taking it all from the theoretical side, not taking into account the application. Throw a wastegate into the argument and it seems to change the argument as a whole...

His argument was that a turbo engine at X psi will make more power at sea level than the same turbo engine at a higher altitude. If I had access to a dyno, I would take my boost controller off and run the car exactly like I did in Vegas. Granted environmental conditions will play a factor, but if we did it on a day of roughly the same ambient temperature and humidity, and used the same type of dyno, it should play out as close as possible, correct?

 

[jetjock]

I see. Well, although I spend most of my time these days in turbine equipment I've operated enough turbo powered aircraft in the past to say he's wrong.

 

[jonahs_supra]

This is what I'm thinking, along the lines of the "turbo works harder at elevation" argument.

so if the turbo is working hard at higher elevation do to air density. wouldnt this create more heat coming out of the turbo. thus making it feel like less power due to raise of air temp? i dont mean to butt in but im eager to learn lol

good read so far

 

[te72]

I see. Well, although I spend most of my time these days in turbine equipment I've operated enough turbo powered aircraft in the past to say he's wrong.

Thanks again JJ.

so if the turbo is working hard at higher elevation do to air density. wouldnt this create more heat coming out of the turbo. thus making it feel like less power due to raise of air temp? i dont mean to butt in but im eager to learn lol

good read so far

No worries man, we're all here to learn. From how I understood in talking to JJ, it seems that while a turbo works harder at elevation, the air it has resisting its movement is thinner, therefore there is less resistance, leading to less energy being required to spin the turbine. Again, if I understood him correctly, it basically washes out in the end.

Another thing to consider when you start going deeper into the application, if your turbo is working below its efficiency threshold at sea level, it may actually work BETTER at altitude when you spin it a bit faster. However, if you're already in its sweet spot at sea level, you would be pushing things a bit hard at high altitude, leading to higher temperatures going into the engine.

 

[jonahs_supra]

i knew i should have went to college lol
im interested in this...ill do some reading tonight

 

[87targa]

Paging the smart guys around here, with knowledge of fluid dynamics and whatnot...

Take car A, it is a Turbo car. At sea level, where the ambient air is more dense, does the turbo work less to maintain X psi, as opposed to say 6000' elevation? Or does X psi simply flow more air at the lower elevation?

I could be swayed either way in the argument, but I'm currently under the first understanding (turbo works harder at higher elevation, but power levels are still the same), but the second argument (more air flow at lower elevation, more power at same pressure level) makes sense to me too.

Thoughts?

flow and pressure are linked but are two different things.

using thermodynamics and the ideal gas law (PV=nRT), pressure is directly proportional to the number of moles of a gas. meaning, a car will breath the same amount of air molecules no matter if you are at sea level or at altitude, BUT the ideal gas law should account for atmospheric pressure. with atmospheric pressure accounted for, lets say you boost 1 atm at sea level and 1 atm at altitude, say SLC, UT. absolute pressure at sea level would be 2 atm and at altitude would be around 1.9 atm. as you can see, the amount of air molecules you would lose from going from sea level to SLC would be around 5%.

now concerning turbo working. according to compressor maps, the higher you go in altitude the higher the pressure ratio. the pressure ratio for boosting 14.7 psi would be 1.87 (not accounting for pressure drops, etc.) at sea level [(14.7 psi boost + 14.7 psi atmosphere)/14.7 psi atmosphere]. Now up to altitude... 14.7 psi boost....(14.7 psi boost + 12.8 psi atmosphere)/12.8 psi atmosphere.....pressure ratio would be 2.14. the pressure ratio increased thus the turbo is actually doing more work.

so...turbos work harder at altitude but not the same power level for the same boost level. and, not more airflow at lower elevation but more air molecules thus more power at same pressure.

 

[te72]

so...turbos work harder at altitude but not the same power level for the same boost level. and, not more airflow at lower elevation but more air molecules thus more power at same pressure.

So, what you're saying is that if we were to take my car along with a mobile dyno down to sea level, then bring it back up here (along with the same dyno), and try to do the testing under as-close-as-is-realistically-possible conditions, then my car would make more power at sea level, even though boost pressure remains the same?

Now for the fun question, how much of a difference? Would it be appreciable? For what it is worth, between driving here (around 6500') and Phoenix (around 2000'), things feel about the same, except for quicker spool down south.

 

[87targa]

if everything is exactly the same except atmospheric pressure then yes your car would make more power at sea level even though the boost pressure remains the same.

doing some quick math...6500' pressure is about 11.8 psi and 2000' is about 13.7 psi. Percent change for the 2 pressures is 16%. Just from number of moles of air alone, I would predict that from 6500' to 2000' you would have at least a 16% difference in power. Now that percent difference for, say a 100 hp car, would be 16 hp....most people's seat of the pants can't feel a 16 hp change. A 300 hp car would see about a 50 hp change. I believe that a person can't really feel a 50 hp change....i take that back, maybe in a really light car that 50 hp can be felt.

Turbo spool is a function of volume and egt. You are spooling quicker because your exhaust gases are higher. Your egts are higher because you have that much more air molecules to burn which equals more energy. That increase in energy will have an easier time to turn the turbine wheel which in turn will compress the air faster.

 

[te72]

See, this is the same argument I got into with my friend up here. I say that 12psi is 12psi is 12psi. The turbo charger is compressing air, once you introduce a wastegate into the equation, does that not equal things out between atmospheric pressures? This is the argument that Poodles introduced above. Now, if we were strictly speaking of how much air is being sucked in, then your argument is 100% correct. However, after a compression takes place...

As for spooling faster, JJ mentioned that when the air is more dense, it puts more resistance on the turbine's ability to spool, I got the impression that he was suggesting that this effectively balances out whatever power gains might be had.

End result? If there is indeed a 16% increase in power, I certainly can't feel it...

 

[CyFi6]

if everything is exactly the same except atmospheric pressure then yes your car would make more power at sea level even though the boost pressure remains the same.

doing some quick math...6500' pressure is about 11.8 psi and 2000' is about 13.7 psi. Percent change for the 2 pressures is 16%. Just from number of moles of air alone, I would predict that from 6500' to 2000' you would have at least a 16% difference in power. Now that percent difference for, say a 100 hp car, would be 16 hp....most people's seat of the pants can't feel a 16 hp change. A 300 hp car would see about a 50 hp change. I believe that a person can't really feel a 50 hp change....i take that back, maybe in a really light car that 50 hp can be felt.

Turbo spool is a function of volume and egt. You are spooling quicker because your exhaust gases are higher. Your egts are higher because you have that much more air molecules to burn which equals more energy. That increase in energy will have an easier time to turn the turbine wheel which in turn will compress the air faster.

I think the argument is that at higher elevation the turbo will flow more volume of less dense air, making the engine produce the same amount of power. At lower elevation the turbo would flow less volume of more dense air thus making the same power. The wastegate sets pressure by working against a spring, therefore the pressure difference between the manifold and the atmosphere will be greater at higher altitude when boost reaches the wastegate preset pressure. Someone correct me if I am wrong here.

 

[Grandavi]

I think that 12 psi is 12 psi.. however... if you take an ice cream scoop and do one swing and fill it at sea level.. when you are where I live.. it would take a lot longer scoop to get that amount of ice cream.
Its not massive, however, (and this is how it works in my mind) you have to actually blow in 12 psi from a smaller reservoir above sea level than you do at sea level (which is a larger reservoir). Therefore, its more physical work to actually accumulate the 12 psi.

Again.. not a massive difference, but it is harder to get a 10 sec quarter where I live than it is on the coast.
(just think about how in 1980 my 73 celica would overheat going from the prairies to the coast because the air leaned out so much going over the Rocky Mountains. Now ECU's take elevation into account (not sure how, but it adjusts for it) so we dont notice.)

and (according to my bike racing buddy) it has a large effect, but only on supertuners where that 1/100th of a second will kill you. For us on the road.. you arent going to notice it.