relative HP vs DA for forced induction engine

[BillClark]

I need to come up with a horsepower gain/loss percentage for changing density altitude. This piston engine is operated from 500-3000' DA, produces approx. 3000hp from 520 c.i., has forced induction of approx. 45psi manilfold pressure, consumes around 16 gpm of methanol @ 10,000 rpm.
A fairly large error would be acceptable (10%). I have found normally aspirated equations for aircraft but feel they would not be applicable to my application. please help

 

[patprimmer]

How would an engine know if it were in a car or a plane.

A roots or screw type blower is positive displacement and therefore will react as an NA engine as mass of air pumped will drop relative to drop in atmospheric pressure coming in. If the air going into the blower is 10% lighter, the blower pumps 10% less mass of air. This presumes no temperature change, but there is normally a temperature change of the ambient air.

For a turbo, I need to think a bit more about it, but I think the manifold pressure acts against the waste gate spring plus atmospheric. The spring stays constant but the atmospheric varies with altitude, so you pump more of the lower density air to make up the pressure to waste gate set pressure plus ambient air pressure.

Say 15 psi atmospheric plus 30 psi boost equals 45 psi absolute manifold pressure.

Then 10 psi atmospheric plus 30 psi boost equals 40 psi absolute manifold pressure.

Once again, temperature also has the same impact in accordance to the ideal gas laws.

http://en.wikipedia.org/wiki/Ideal_gas_law

A turbo or a centrifugal blower pumps to a fixed pressure not fixed volume if controlled by a waste gate. You have not stated type of blower and boost control system.




Regards
Pat
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[fireslave]

If you have a turbocharger, or a centrifugal supercharger, you should be able to set it up so there is almost no loss in power.

ie, at 1 bar (sea level) you have 3 bar of boost, which means that the total pressure is at 4 times atmospheric.

if you are at a high elevation and the atmospheric pressure is reduced to .75 bar, you can just compensate by using 3.25 bar of boost. this will not result in any more air entering the engine than 3 bar did at sea level.

you will lose power for two reasons though.
1.) the air will be heated more by the compressor because its pressure is increased by a larger amount
2.) you will have slightly more backpressure as the wastegate will be less open the higher you go, because of the increased boost needed to reach the total air pressure going into the engine, which is in this case 4 bar absolute pressure.






As pat said above, if you have a positive displacement blower, power losses would be comparable to what you would expect to loose in a NA application, although the supercharger would probably lose slightly less because
1.) less heat in the intake charge at higher elevation than lower elevation. even if this doesnt add power directly, it allows you to lean out the mixture slightly, which will add power.
2.) slightly less slip in the blower because of the lower pressure.
these two, of course, dont even come close to offsetting the fact that you have less air to work with in the first place.
Overall, the difference in loss between this and a NA setup should be negligible, and should more or less correlate directly to the difference in the amount of oxygen molecules in a cubic foot of air at 500 feet vs. the amount of oxygen molecules in a cubic foot of air at 3000 feet.




I however, do not know the relation between pressure, heat and actual amount of oxygen molecules. Maybe someone has a graph of that, or perhaps a formula.

 

[fireslave]

Ah, and a note on point 2 for the non-positive displacement scenario.

If you have a centrifugal supercharger, there is obviously no wastepaper, and no change in backpressue. the same absolute pressure would require spinning the compressor slightly faster to generate the same absolute pressure, which would leech off more power in its operation. Unfortunately, boost generation isn't exactly linear with RPM, so in order to not overboost at the top speed, you might sacrifice some boost lower in the RPM range.




Actually, you could also do that for a positive displacement supercharger to compensate for an altitude difference. These are linear type devices, so with the right pulley choice, you could have more or less, exactly the same boost pressure at every RPM.

On a street car, I wouldn't change a pulley jst because of 2500 feet of elevation difference, but it may be worthwhile on a race engine.

 

[JSteve2]

Your turbocharger will generally function as a ratio device rather than an absolute boost device (i.e. 30 psi boost at sea level is 3:1, so more like 20 psi boost at 10 psi ambient). However, in your case, with only 3000 feet of density altitude, you can get within 10% by ignoring it completely.

You probably have about 0.7 psi ambient pressure below sea level at that altitude equivalent, so you will notice at most about 5% torque loss. In reality, the turbo has a natural tendency to spin a bit faster (that is just experience, I haven't thought about exactly why) at altitude so you will actually see maybe 3% torque loss (not true for a supercharger that is tied to engine speed, of course). Therefore, if you run the same engine speed, you can figure power loss of 3-5% at that altitude, and since you have a 10% error band, it seems you can just ignore this. If you want to estimate anyway, figure a linear drop from 100% power at sea level to 97% power (95% if you want to be conservative) at 3,000 feet of density altitude and you will be well within your margin.

Now, if the 3000' is a typo and you intended 30,000', then some deeper math will be needed.

 

[BillClark]

it is a screw blower, the drive ratio is always at the maximum (dictated by the rules). to complicate matters low speed output (7000 rpm) rather than peek (10,000+)is the area of interest.
if you havent figured it out yet the engine is in a race car. what I am ultimatley trying to accomplish is to predict changes in engine output relative to atmospheric changes mainly at launch and through a portion of low gear (about 2 seconds and over 1500rpm) so either output can be changed via ignition,compression or fuel to compensate or atleast have a better idea of how the output is effected. the clutch has a very good idea of what the engine is doing and we would rather it didnt make its own decisions (very important). I found this online calculator

http://wahiduddin.net/calc/calc_hp_abs.htm

and was going to start using it until we can get on the dyno early next year but I was unsure of its accuracy in this application and is why I possed the question to the ETF braintrust. i am currently tracking the relationship between fuel flow at the finish line and (like) egt's in diffent conditions with the hope that it may offer some support, confirmation, or clarification to the whole thing.
adjusting the output is somewhat of a crapshoot but I would atleast like to make a better attempt at it than we are now.

 

[patprimmer]

Now we know what we are actually talking about, for all practical purposes, the power will decrease in direct proportion to air density if you adjust fuel to maintain a:f at all elevations.

Pressure and temperature both have an effect density according to the formula in the combined gas laws that can be found by following the link I posted earlier.

Apart from air density, mass of oxygen supplied is very slightly impacted by other variable from location with regards to industry and forests and by humidity levels. The location thing is so minor it's not worth 0ne seconds co0nsideration. Humidity can have a small, but measurable effect.

If you have lower air density you might get back some of the losses by increasing ignition advance slightly.

As pointed out elsewhere, lower air density results in VERY slightly increased blower efficiency and a very small reduction in parasitic losses to drive the blower against slightly lower boost levels.

If you know your power curve, you can adjust the clutch so that the extra rpm at launch corrects for air density.

Different track temperatures and preparation will probably throw your clutch tune out as much as a change of air density will. Certainly on the same track with day time qualifiers and night time finals, the dew comeing down can have a significant effect on clutch tune.





Regards
Pat
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[SomptingGuy]

I thought turbocharger overspeed at altitude was a common problem.

- Steve

 

[patprimmer]

I think that would depend on the size of the turbo relative to the engine and boost levels and the altitude. I dojn't see a real problem for a car at different tracks and different weather.

I would expect for aircraft, you need to compromise boost and response at take off for boost at high altitude.



Regards
Pat
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[black2003cobra]

I would go ahead and use SAE J1349 for a positive-displacement blower application. The basic equation that it is based on, (1D compressible flow through a restriction), should also apply to the air going into the supercharger. You might want to use a lower overall mechanical efficiency though, to account for the parasitic loss from the blower. (SAE J1349 assumes 85% as the default.) I agree with others on the impact from the change in drive power of the blower itself, although it's trivial to calculate, (mdot*cp*?T).

 

[thruthefence]

The wastegates on the small cabin class turbocharged recips generally are controlled by engine oil pressure; A device called a 'variable absolute manifold pressure controller' has a variable orifice that 'traps' oil against a piston in the wastegate actuator,(connected to the valve itself) eg less oil bled back to case, more manifold pressure. The variable orifice is driven by power lever angle, (throttle), and is also linked to an aneroid. Thus, for a given throttle angle, the manifold pressure setting in a climb can be more or less maintained. The pilot doesn't have to keep pushing the throttle levers forward, to maintain power. You lose about an inch for every 1000 feet of altitude, so it cuts the crew's work load.