Turbo Chargers at Altitude

[H2power]

I have a few questions about turbo chargers at altitude.

First off has anyone dyno'd the same engine at standard conditions, then dyno'd the engine at altitude. How do the true hp readings compare?

Are their any hp correction factors for turbo engines? How good are they?

Finally happens when a turbocharged engine is run at altitude. Does the turbo spin faster compensating for the reduced air density or somthing like that?

Thanks

 

[patprimmer]

For sake of an example

Say you test with an air pressure of 15 psi at 70 deg F and you set the waste-gate at 30 psi, you get 45 psi absolute pressure.

If you then increase altitude to get 13 psi at 70 deg F, then leave the waste-gate at 30 psi, you get 43 psi absolute pressure, which is about 4.5% less, so will be about 4.55 less power, if all o her things remain equal, which in fact they don't, but at that difference it will be near enough.

If on the other hand, you increase the boost to 32 psi, you then regain your 45 psi absolute, and should get about the same power as the original test.

The change in boost will also change charge temperature and exhaust blow down pressure which will loose a very small amount of power, so you might actually need 33 psi to get the original power.

All this presumes no risk of detonation, as that can prevent the use of extra boost. Also higher altitudes often mean lower temperatures which also might have an effect, but will be compensated for in the dyno correction factor.

If you compensate for altitude by running extra boost, you have longer turbo spool up times, and you need a turbo capable of pumping the extra boost with lower original engine output. Also your intercooler efficiency changes with air density, if all other factors are equal.

Bottom line is, if you increase boost by the drop in atmospheric pressure, you will end up with near enough to the original power.


Regards
pat pprimmer@acay.com.au
eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[evelrod]

Never checked the couple of turbo race engines that I did in the past at anywhere beyond the L.A. area, but---
When I put a BIG camper on my Dodge turbo diesel a number of years ago and went on vacation to the mountains of Colorado, the fuel mileage at cruise (60/65 mph) was ~14 mpg at sea level and at ~12,000' amsl(Cripple Creek area) it INCREASED to ~18 mpg and the power felt, by the sotp, about the same!
No "scientific method" here but, I was surprised.

Rod

 

[SBBlue]

You need to differeniate between turbochargers for gas engines and turbochargers for diesel engines.

Gas engine turbochargers really only function when the throttle is wide open. It doesn't make a whole lot of sense to boost the intake pressure, only to knock it back down with a throttle.

Most diesel engine turbos provide boost all the time. A diesel engine is unthrottled, and there are real advantages to having increased boost pressure even in partial power conditions. I'm not sure just how diesels with turbos for small trucks and cars operate.

Now as to why you would have better fuel mileage at 12,000 feet as opposed to sea level -- if I had to bet I would bet it was because you weren't able to cruise at 65 mph in the Cripple Creek area. Well, you could, but you wouldn't stay at 12,000 feet very long, and you certainly wouldn't stay there long enough to require a fill up.

It would be interesting to know a couple of things. First, with a BIG camper -- how much air resistance and hp loss do you pick up at sea level, as compared to 12,000 feet? Here's something to think about -- the roofcap fairing on a Big Diesel truck that lifts the airstream over the trailer, as opposed to have the airstream smash head first into the top of the trailer, increases fuel mileage in a large truck by about 5%. Its quite possible that a BIG camper could cause about the same magnitude of air resistance, since the cross sectional area would be about the same, as the top portion of a large trailer. The air resistance from that would be quite dependent upon speed, and also on air density.

Second, what type of fuel mileage did you get at 5000 feet elevation? You can drive hundreds of miles at 5000 feet, so you should have a pretty good handle on mileage at that altitude.

I should mention the concept of "turbonormalization". There are some piston aircraft with turbochargers that don't actually "boost" the intake manifold pressure; they are set to keep the intake manifold pressure at 14.7 psi (sea level pressure) regardless of altitude. Now when you outside pressure is 7.3 psi (18,000 feet) you are indeed boosting the intake manifold pressure, but you are not boosting it to levels greater than what it would be at sea level. The P-38 twin engine aircraft had turbos (I think -- maybe it was superchargers) that was able to "turbonormalize" all the way to 29,000 feet.

 

[evelrod]

Just to clarify the mileage deal.---The 18 mpg was an average. At one point I lived in the area and we traveled extensively over the back roads (e.g., Phantom Canyon) and, yes we did stay long enough to need a fill up, but it was in Canon City. Yes I know, that's not 12,000' but, like I said, an average which included up Pikes Peak. The fuel mileage in and around Colorado Springs down toward New Mexico (back roads) yielded 18+. Average max speed at 65 mph but much at lower speeds in lower gears in and around Durango, CO and Chama, NM. Same trip in and around Tucson mountains 15+ mpg and up through Imperial and San Diego counties, again, back roads under similar conditions, 14 mpg. Without the camper (gave it to my son in the Army at Ft. Carson and he "lost" it!) the mileage was then 21 mpg and is now 21 mpg at 200,000 miles on the clock. Mostly freeway stuff at 65 mph. Not to bad for a duelly with 4.10's.

Rod

 

[NickE]

pat- Actully IIRC the P38 had an early turbo-prop design. IE: a true turbine spins the propeller.
nick

 

[evelrod]

Sorry, NickE but, the Planes of Fame P-38 is powered by a pair of Allison V-1710's, a -51 on the right and an opposite rotation -55 on the left. 12 cylinder supercharged 1325 hp @ 3000 rpm. No turbo-props here.
However, the only flying wing in the museum, indeed in the world, uses a form of torque converter to drive the props from "flat 8" air cooled engines as I recall.

Sorry for the subject change.

Rod

 

[NickE]

Well I definately recalled in-correctly there.

 

[carnage1]

I found a lot of information on this subject in the NACA archives. the information in here is amazing.

http://naca.larc.nasa.gov/

 

[PEW]

As I understand from my professional studies (although it was some time ago and I fly turbine engines these days) a turbocharger on an aero piston engine can be designed to do two things, depending on the application required.

It should be borne in mind that we are talking at a constant rpm here, as such aero engines are designed to run like that because of the propellor.

Firstly, a turbo(super)charger can boost performance at takeoff by giving more Manifold Air Pressure (MAP).

Alternatively, it can be designed to maintain engine performance to altitude by keeping the MAP at a sea level figure.

The difference is in the setting of the waste gate. At lower altitudes the wastegate is at least partly open to avoid overboosting the engine. As the altitude increases, the gate progressively closes to maintain boost / MAP. Once the wastegate is fully closed the turbo is producing the maximum boost that it can. Any further increase in altitude results in a reduced power output.

A piston engine will actually produce more power at high altitude for the same MAP, because of the reduced air density. The "back pressure" in the exhaust is less so less HP is consumed in "pumping losses".