Air temp and elevation

[apapada]

[ ADMIN NOTE: Split from this thread http://www.nsxprime.com/forums/showthread.php?s=&threadid=32733 ]

It was a tough track to run that night due to the climate (Hot thin air at elevation). To put it into perspective as Sig had posted a GT3 was only running around the 109mph, an 04 Viper was running around the 114mph, and R1’s and CBR’s running low 120s. We ran neck and neck with an R1 with us losing boost on the top end.

I do realize the importance of ambient air temperature, but fail to understance the impact of altitude/elevation on a FI system in the first place. For NA engines, it makes sense for elevation/altitude to play an important role in performance, but not for FI engines as air is force-fed into the cylinders and 14psi boost at 0 feet is still 14psi boost at 10,000 feet.

 

[Sig]

I do realize the importance of ambient air temperature, but fail to understance the impact of altitude/elevation on a FI system in the first place. For NA engines, it makes sense for elevation/altitude to play an important role in performance, but not for FI engines as air is force-fed into the cylinders and 14psi boost at 0 feet is still 14psi boost at 10,000 feet.

Altitude affects both NA and FI engines, however the impact is greater on a NA motor.

I am sure there are plenty of folks that can provide more reasons and describe them with more accuracy than I, but one thing to consider with altittude and FI:

- 14lbs of boost does not always equal 14 lbs of boost as you stated. CFM is a much more meaningful measure. A good example of this would be to take a look a factory turbo'd car like the Supra. With factory turbos set at 14psi, it probably makes somewhere around 330-350rwhp with a decent exhaust. Take a big old turbo like a T-78 and run it at 14 lbs and you probably have over 600 rwhp. This is due to how much air you can push with a larger turbine and compressor. This does not address the elevation question directly, but is just an example of how 1 psi does not always equal 1 psi.

Altitude kills due to the thinner air. In order to achieve the same level of boost and CFM it does at sea level, a turbo must spin significantly faster. The impact as compared to sea level is increased spool time and the production of more heat. The extra heat will increase the temp of your charge. Netting out to the same effect of rasing the ambient temperature. A better example of altitudes effect would be with a supercharger system because they have a relatively fixed spin rate tied to the pulley size. A blower at sea level with a 6lb pulley will probably only produce 4.5lbs of boost in Denver, CO. This of course equal's less power. With a smaller pulley it too will make 6lbs in Denver but will create much more heat than the 6lb sea-level pulley. The excess heat has to go somewhere... certainly aftercoolers help, but do not completely compensate.

 

[Yellow Rose]

.....but not for FI engines as air is force-fed into the cylinders and 14psi boost at 0 feet is still 14psi boost at 10,000 feet.

Incorrect. The chemists will recognize that Pabs = Patm + Pg, where Pabs is absolute pressure, Patm is atmospheric pressure and Pg is gauge pressure.

Pg is constant, regardless if at sea level or the top of the mountain. This is the "static" pressure inside your tires or inside the intake manifold under boost. Patm varies with altitude. Always. The absolute pressure is the "total" or "dynamic" pressure that the engine sees / feels. Therefore, an engine at sea level responds to x PSI better than if the pulley is making the same x PSI in the mountains.

 

[apapada]

.....but not for FI engines as air is force-fed into the cylinders and 14psi boost at 0 feet is still 14psi boost at 10,000 feet.

Incorrect. The chemists will recognize that Pabs = Patm + Pg, where Pabs is absolute pressure, Patm is atmospheric pressure and Pg is gauge pressure.
[sic]
Therefore, an engine at sea level responds to x PSI better than if the pulley is making the same x PSI in the mountains.

I am afraid you are confusing the issue more than it needs to be. As you stated it correctly yourself, the total absolute pressure is the sum of Pg and the atmospheric pressure. Where you are incorrect, is that a FI'd engine sees the total absolute pressure equal to the pressure generated by the FI system and the atmospheric pressure. That total (14 psi in my example) should not change with altitude, as the FI'd system compensates for the drop of atmospheric pressure by working "harder" as Sig mentioned. Here is a quote better explaining the concept:

"The interesting advantage of forced induction is that it can make the engine totally impervious to changes in atmospheric conditions. If the relative air density is low, a computer controlled boost monitor can dial up more boost, essentially driving the compressor harder so the density of the intake charge is the same. This is especially effective on airplanes. As the plane gains altitude, the engine computer dials in more boost to compensate. The use of superchargers was pioneered during WWII on aircraft engines such as the Rolls Royce Merlin."

 

[KGP]

Uhhh, I'm not sure what Andy said, in fact I got scared when I read as far as Pabs, but here is an analogy that might help:

Pilots well understand that there is something called density altitude. Density altitude is calculated using barometric pressure, tempurature and humidity (dew point). The less dense the air, the faster a prop must turn to create the same amount of thrust (lift is also effected). In a marine enviornment, water tempuratures play a huge roll in performance in terms of water density, decreasing the efficiency of the prop as the water temps rise. A tubro impeller is no different than a prop when it comes to air density, or density altitude. The hot desert at elevation creates some really thin air. What happends beyond that, I have no idea. But, I have to believe that an FI engine will produce more power in cool dense air than when in the desert on a hot day.

Edit: Thinking a bit more (dangerous), would it not be correct to think that even if the same boost level is met (different air density), that because the air moving is in itself less dense, that fuel must be reduced, thereby reducing power?

 

[KGP]

The use of superchargers was pioneered during WWII on aircraft engines such as the Rolls Royce Merlin."

While that might be where many lay people came to realize its existance, it is far from fact. Forced induction has roots back almost to the invention of the internal combustion engine. The first SC patent was issued in the late 1800's.

 

[apapada]

KGP,

the air temperature effect on FI is a no-brainer. It's the altitude reason that got me confused.
Also, "pionnered" doesn't necessary mean "invented". I understand it as just that a concept was given much more attention and more applications were discovered eventhough the concept itself might have been discovered long time ago...

Bottom line the real question is "Does positive manifold pressure (aka boost) increase with altitude". It is my understanding that no, this is not the case, unless you are picky and do consider that you needed additional CFM, which will generate a rising temperature of the intake charge which will raise boost with less O2 particles than the same boost at sea level. For high altitude application like airplanes, I do think that boost increases with altitude as in my quote in the previous post. For Las Vegas, NV (altitude :~2100 feet), I don't think this is the case.

 

[KGP]

Also, "pionnered" doesn't necessary mean "invented".

Ahh, yes. Quite right.

 

[Sig]

Apapada-

You may want to ask that question on factory turbo'd car forum like the Supra forums, Mr2 forums, 911tt forums, WRX, DSM forums, or even the EVO forums. Certainly their are plenty of boy-racer fools on some of those site, but there are also a good number of folks that know their boost.

If SJS sees this thread, I'm sure he will have some good information for you.

 

[true]

The higher your elevation, the thinner/less oxygen the atmosphere is. As your elevation increses, the air pressure decreases. Air pressure (1 atmosphere) at sea level is around 14.7PSI. So basically, earth is 'boosted' The higher you go up, the less resistence to boost you have. I believe at around 10k feet, the atmosphere is around 10PSI. Obvisouly you can still compress the air to your desired boost level, but the amount of 'work' to do so also increases, which increases heat, and decreses efficiency. While it's easy to think in terms of absolute pressure, ie. 14 psi = 14 psi no matter what condition, you REALLY have to take compressor efficiency, density and volume into play. A tiny t25 turbo (stock mitsubishi eclipse turbo size) can EASILY make 20 PSI on a plethora of engines, but the required work is SO much greater then what it takes for a huge T4 turbo to make 20PSI. The T25 probably has to spin 3 times as fast (just a guess), which adds tremendous heat to the intake discharge. Another reason why it is so important to properly match the compressor to the engine -- while a T25 could easily produce 10 PSI on an NSX engine, the turbo would be so far out of its effcieny range, it would also be known as a 'heatercharger'. I also like to think of the difference in compress size using an actual air compressor/tank as an analogy. You could take one of those tiny 10 gallon pancake compressors, and pump it up to 50 PSI along with a huge upright 100 gallon compressor pumpted to 50PSI. They both have the same pressure inside, but the volume and density inside their tanks is not even close. Very basic and obvious, but sometimes that helps people realize that unless you are talking about absolute pressures; like pressures react much different in the combustion chamber because of the many variables involved.

 

[spdntckt]

The higher your elevation, the thinner/less oxygen dense the atmosphere is. As your elevation increses, the air pressure decreases. Air pressure (1 atmosphere) at sea level is around 14.7PSI. So basically, earth is 'boosted' The higher you go up, the less resistence to boost you have. I believe at around 10k feet, the atmosphere is around 10PSI.

I believe there is also the fact that boost is relative to ambient atmospheric pressure.

eg: 14PSI boost at sea level is more dense then 14psi boost at 10k feet.. why.. because the real volume of air being fed is 14PSI (boost) + the 14PSI (ambient)

At altitiude.. you may have 14PSI (boost) + 10PSI (ambient)

the boost gague will only show boost relative to the ambient.. it isnt absolute.

 

[spdntckt]

I'm a scuba diver. if there are other scuba divers here is another analogy


If you go down 33ft under water, you have roughly 2x the pressure surrounding you than you have at the surface. If, at this depth, you were to inflate a balloon to a 4 inch diameter, it would have a certain (measurable) amount of air in it (A certain number of air molecules)

When you rise to the surface, the balloon itself will start to enlarge. Your not putting more air in it, the # of air molecules stayes the same, and the PSI inside the balloon is *exactly the same* as it was at depth. its just that the surrounding pressure on the balloon walls is lowering, and the pressure *differential* between the inside of the balloon and the water outside is increasing (thus causing the balloon to enlarge).

Same deal with engines... if you have 1bar boost with an ambient (external) pressure of 1bar - for the same volume you will have more air molecules than if you had 1 bar of boost with an ambient
pressure of .5bar.

 

[Yellow Rose]

WHAT ?!?!

As you stated it correctly yourself, the total absolute pressure is the sum of Pg and the atmospheric pressure.

Where you are incorrect, is that a FI'd engine sees the total absolute pressure equal to the pressure generated by the FI system and the atmospheric pressure.

Unless I am missing something very fundamental - and I am not - your two statements are saying the same thing, yet you “contradict” yourself. Pabs = Patm + Pgauge. Always and forever.

That total (14 psi in my example) should not change with altitude.....

Yes it will, because the atmospheric pressure - Patm - decrease as elevation increases.

.....as the FI'd system compensates for the drop of atmospheric pressure by working "harder"

Unless there are electronics involved to measure the decrease in barometric pressure as the elevation increases, this is an incorrect statement. A compressor spins at x RPM regardless if at sea level or in the mountain.

Let's say we have a FI system that makes generates a nameplate boost of 10 PSI. Again, to the chemists and engineers, this is 10 PSIG as in gauge. At sea level the atmospheric (Patm) is 14.7 PSI. Therefore, the total pressure the engine sees and feels is 24.7 PSIA (absolute). So the same engine in Denver is now seeing a total of 22.2 PSIA because the air is less dense at 5000' above sea level.

 

[KGP]

gauges and logs

Does any of this explain why a boost gauge might display something different than a stand-alone log might record/show?

 

[Yellow Rose]

It shouldn't. My Autometer gauge shows 0.5 PSI more than the Split Second recording. Weak analogy - two clocks will read different times.