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  #41  
Old 09-01-2009, 07:15 PM
Bavafa Bavafa is offline
 
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So, let me pose this question to every one, would there be any advantage/disadvantage if you were running it at ROP but throttling back to achieve the same speed/FF ratio as oppose to LOP. Here is a made up example just for clarification: lets say at 10K feet, we and 50 LOP, we are getting 158 MPH and 7.7 FF. Can we achieve the same thing running it at 100 ROF and throttle back to get 158 MPH and hopefully the FF be about the same (7.7)

Is this possible and if so, is there any advantage or disadvantage?

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Mehrdad - RV7A
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  #42  
Old 09-01-2009, 07:26 PM
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Radomir Radomir is offline
 
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100ROP? Probably not.. You'd be burning more than 7.7 in that case at that speed.. Though, I have to say, I'm WAGing this one as I have not confirmed it.. this is more from memory of "transient state of things" as I'm on my way to LOP ops

PS.. I think you meant 158+kts not mph at that fuel flow
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  #43  
Old 09-01-2009, 07:38 PM
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flyeyes flyeyes is offline
 
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Quote:
Originally Posted by Bavafa View Post
So, let me pose this question to every one, would there be any advantage/disadvantage if you were running it at ROP but throttling back to achieve the same speed/FF ratio as oppose to LOP. Here is a made up example just for clarification: lets say at 10K feet, we and 50 LOP, we are getting 158 MPH and 7.7 FF. Can we achieve the same thing running it at 100 ROF and throttle back to get 158 MPH and hopefully the FF be about the same (7.7)

Is this possible and if so, is there any advantage or disadvantage?

Regards
Mehrdad - RV7A
IO360M1B
This doesn't work.

An easy way to think of this is that at peak EGT, the mixture is stoichiometric, which means that there are exactly enough oxygen molecules available to react with every avgas molecule.

If you are ROP, by definition there is more fuel available than oxygen, so some leaves the exhaust valve unburned.

If LOP, some oxygen molecules escape the chamber but all of the fuel is consumed.

The important difference here is that the oxygen is more or less free and unlimited, while you are paying for the fuel, and can only carry a limited amount onboard.

The only way to be certain you've extracted every last bit of energy out of the avgas is too have extra air around so no fuel is wasted (LOP).

Since the concentration of oxygen in the air is fixed at a given density altitude, the only way to be certain that the engine is making its maximum available power is to have extra gas around so no oxygen goes to waste (ROP).

You can cheat on the oxygen concentration (within limits) by compressing the intake air (supercharging) or add an oxidizer like nitrous oxide, but you have to also increase the corresponding amounts of fuel.
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  #44  
Old 09-02-2009, 09:57 AM
Bavafa Bavafa is offline
 
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Thanks James,
I think you hit the nail right in the head... this makes sense. And I go further to contradict/correct myself in the earlier question, if I am ROP and throttle back to lessen the fuel flow? I am starving the engine even more for the air so even more fuel molecules are going to escape out of the exhaust, correct?

I have been flying ROP so far and hoping to give it a try this weekend to see what sort of FF I get.

Regards
Mehrdad RV7A
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  #45  
Old 09-02-2009, 09:20 PM
DENMACRES DENMACRES is offline
 
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YOU ARE NOT LOOKING TO GET ALL TO PEAK AT THE SAME TEMP. BUT AT THE SAME FUEL FLOW.
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  #46  
Old 09-03-2009, 03:19 PM
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flyeyes flyeyes is offline
 
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Quote:
Originally Posted by Bavafa View Post
Thanks James,
I think you hit the nail right in the head... this makes sense. And I go further to contradict/correct myself in the earlier question, if I am ROP and throttle back to lessen the fuel flow? I am starving the engine even more for the air so even more fuel molecules are going to escape out of the exhaust, correct?

I have been flying ROP so far and hoping to give it a try this weekend to see what sort of FF I get.

Regards
Mehrdad RV7A
IO360M1B

If you throttle back, you reduce air and fuel roughly in proportion, so the ratio should stay pretty much the same.

An interesting side effect of this is that if you are lean of peak, you can determine power output by multiplying fuel flow by a constant. Since there is an excess of air (by definition) lean of peak, throttle and manifold pressure readings can be ignored, and you can calculate directly from fuel flow. This same trick doesn't work ROP, since you never can be sure exactly how much fuel is being wasted.
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  #47  
Old 09-03-2009, 04:42 PM
Andy_RR Andy_RR is offline
 
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Quote:
Originally Posted by Bavafa View Post
Can we achieve the same thing running it at 100 ROF and throttle back to get 158 MPH
Three, possibly four reasons why the answer is no.

1 - You will increase pumping work by closing the throttle
2 - You reduce the ratio of specific heats of the working fluid (combustion products). This part is a bit of thermodynamics tech, but basically lean combustion is better than rich from a thermal efficiency PoV.
3 - Heat transfer (loss) to the cylinder head will be increased, since the mass of gas in the cylinder is lower and therefore is made hotter by burning and heating it with the same(ish) amount of fuel energy
4 - If ROP makes you run richer than stoichiometric, then significanly more CO is produced, which represents lost chemical energy.

A combination of all these reasons will give you a degradation in BSFC at the same shaft power level.
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  #48  
Old 09-03-2009, 04:59 PM
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hevansrv7a hevansrv7a is offline
 
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Quote:
Originally Posted by flyeyes View Post
If you throttle back, you reduce air and fuel roughly in proportion, so the ratio should stay pretty much the same.

An interesting side effect of this is that if you are lean of peak, you can determine power output by multiplying fuel flow by a constant. Since there is an excess of air (by definition) lean of peak, throttle and manifold pressure readings can be ignored, and you can calculate directly from fuel flow. This same trick doesn't work ROP, since you never can be sure exactly how much fuel is being wasted.
Doesn't calculating HP from MP and RPM (modified for altitude) have to assume a given SFC implicitly? If so, is it best-power (around .50) or peak or what? So if I am right about that, why won't fuel flow multiplication work as accurately or inaccurately as the standard method when ROP (with a different multiplier than LOP, of course)? If I'm wrong that the standard method assumes some SFC, then how can MP and RPM in two cases, one at best power and one at full rich give the same horsepower (this assumes a CS prop - possibly for fixed pitch the case comes out differently)?
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  #49  
Old 09-03-2009, 05:53 PM
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frankh frankh is offline
 
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Default LOP

Just got back from a (Mapquest equivilent) of a 1235 mile journey...Did it in 6hours both ways and was making a TAS of 158 to 162 kts depending on altitude..All of this was flown at 2400 RPM and whatever MP pressure was available at 12K feet or above.

Fuel flow was about 7 to 7.3GPH. The first part of the trip was on Autofuel WITH ethanol..never taken E10 to 14.5k before but it seemed fine looking at the engine guages.

I do have a Sam James cowl so it might be a little quicker than stock but who knows for sure.

But anyway..we almost had enough fuel to do the trip without refueling.

LOP?..You know it makes sense...

Frank..still got this weired rocking sensation from the autopilot that continually gently rolls the airplane slightly right and left..
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  #50  
Old 09-03-2009, 06:13 PM
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Kevin Horton Kevin Horton is offline
 
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Quote:
Originally Posted by hevansrv7a View Post
Doesn't calculating HP from MP and RPM (modified for altitude) have to assume a given SFC implicitly? If so, is it best-power (around .50) or peak or what?
The power charts from engine manufacturers assume mixture set for best power. The Lycoming data I have shows SFC at best power of 0.49 to 0.5 with compression ratio of 8.5:1. It is about 0.48 with CR of 8.7:1.
Quote:
Originally Posted by hevansrv7a View Post
So if I am right about that, why won't fuel flow multiplication work as accurately or inaccurately as the standard method when ROP (with a different multiplier than LOP, of course)? If I'm wrong that the standard method assumes some SFC, then how can MP and RPM in two cases, one at best power and one at full rich give the same horsepower (this assumes a CS prop - possibly for fixed pitch the case comes out differently)?
If you are running LOP, you are burning pretty much every molecule of fuel, no matter where the mixture knob is set, so there is a fairly constant SFC. You can take fuel flow, multiply by a constant, and get power - this works reasonably well no matter how many degrees LOP you are, as long as the engine is not misfiring.

But, if you are running ROP, you are pushing unburned fuel out the exhaust, and the amount of unburned fuel varies as you change mixture. So, the relationship between fuel flow and power will change depending on how many degrees ROP you are. But, if you always run at XXX degrees ROP, the SFC at that mixture setting should be fairly constant.

Note: even when running LOP, the SFC is not exactly constant, as the power lost to friction is constant at a given rpm, and this becomes a greater proportion of the power produced by burning the fuel at low power settings. So, SFC when LOP will be best at higher powers, and it will get worse as power decreases.
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