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Planebuilder - Fuel economy

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Hrolf Hakonson

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Thursday, December 20th 2007, 5:10pm

Planebuilder - Fuel economy

Grrrrr......

I've been doing various planes in Planebuilder. Efficiency is, in the Planebuilder instructions, suggested to be 20-25% for high-performance piston engines. A while back, RA suggested that that was too high, that a better range was something like 14-18%. Yet, if I use the 14-18% numbers, the SFC is too high, so the plane eats more fuel and I need bigger propellers or more efficient propellers to get the right power. So..... right now I'm confused and puzzled. :(

Red Admiral

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Thursday, December 20th 2007, 6:22pm

I'd adjust the efficiency to give the correct specific fuel consumption. The efficiency given is thermal efficiency but this is only slightly linked into fuel consumption. Jet engines are around 35-50% efficient but specific fuel consumption is far higher.

The specific fuel consumption for the
Merlin is 0.45 - 0.54lb/hp-hr
Griffon 0.42 - 0.46lb/hp-hr
V-1710-111 0.66lb/hp-hr (not sure on power setting)
R-2800-21 0.42-0.44lb/hp-hr
BMW801 0.572lb/hp-hr
DB601 0.473lb/hp-hr
DB605 0.474lb/hp-hr

Gives you efficiency figures between 18% and 24%, depending on cruise speed.

Hood

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Friday, December 21st 2007, 1:01pm

I just leave mine at 22.5% as a roundabout figure. If I'm designing older planes I tend to go down to 20%. I guess as we progress the figure could rise to 23-24% by the early 1940s.

Hrolf Hakonson

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Friday, December 21st 2007, 2:09pm

Maybe, especially for turbo-compound type engines. Though looking at the list, it's not getting much better in most cases (though the R-2800's pretty solid, especially compared to my BMW-801s, gonna have to talk to P&W, I guess.....)

This post has been edited 1 times, last edit by "Hrolf Hakonson" (Dec 21st 2007, 2:23pm)

Red Admiral

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Friday, December 21st 2007, 2:21pm

The decrease in fuel consumption is due mostly to the higher boost pressure allowed by higher octane fuels rather than greatly improved mechanical efficiency.

Turbocompounding doesn't seem to improve things that much. The R-3350 only gets down to 0.38-0.40lb/hp-hr

Kaiser Kirk

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Friday, December 21st 2007, 5:42pm

Quoted

Originally posted by Hood
I just leave mine at 22.5% as a roundabout figure. If I'm designing older planes I tend to go down to 20%. I guess as we progress the figure could rise to 23-24% by the early 1940s.


I do something similar, scaling for engine year. Earlier engines I was using 21-22, now I have some 23s, and I was going to go to 23.5s for OTL 1940s engines. It's crude, but would mimic improving fuels/engines. Looking at RA's data, putting in that R-3350 0.38-0.40 would be in that 25% range, I think? Suggesting that 18-25 is a reasonable ballpark.

Red Admiral

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Friday, December 21st 2007, 7:19pm

Bristol Hercules 0.41lb/hp-hr
Bristol Centaurus 0.42lb/hp-hr
Napier Sabre 0.45lb/hp-hr

BMW-132 0.54lb/hp-hr
Bramo 323 0.574lb/hp-hr
BMW-801 0.504lb/hp-hr

This post has been edited 1 times, last edit by "Red Admiral" (Dec 21st 2007, 9:37pm)

Kaiser Kirk

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Saturday, December 22nd 2007, 12:45am

So that British - German disparity is mostly just fuel quality? In Wesworld fuel quality would be roughly the same across the board outside of wartime.

You said mechanical efficiency had little to do with it, is there much link to litre/cylinder or somesuch?

Red Admiral

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Saturday, December 22nd 2007, 11:43am

Quoted

So that British - German disparity is mostly just fuel quality?


The higher boost pressures allow a lower fuel consumption at higher power levels but is more or less the same at cruising power which is around 33% throttle. The use of sleeve valves would increase the volumetric efficiency and account for some of the lower fuel consumption, however its unknown what power levels the sfc was taken at. A sfc v. power graph is a curve with the minimum point occuring around 60-70% throttle, before you have to set the mixture to overrich in order to suppresss detonation. The sfc is typically the cruising value of around 33% throttle, but the German engines might have been taken at 30% or 40% values. However, I'd guess that its just because those engines are a poorer design. Looking at a few other German engines like the Argus 410 and you get a level of 0.45lb/hp-hr which is much more competitive.

Hrolf Hakonson

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Saturday, December 22nd 2007, 2:59pm

The sleeve-valves would only show a superiority on those engines equipped with them. The BMW radials were relatively low-boost designs, as shown by their issues at higher altitudes.

The other thing to keep in mind is that almost all of the German engines were designed to run on 87 octane fuel, not on the higher octane fuels. A few versions of engines designed to run on 87 octane were modified to run on approximately 100 octane fuel, but the numbers of those engines (DP-601P, DB-605AM, -605ASM, -605D, -605DC, DB-603L) were relatively limited. An example of the compression differences for 87 vs 100 octane fuel: the DB-605AM had a 7.5/7.3:1 ratio with 87-octane fuel; 8.5/8.3:1 ratio with -octane fuel.

Red Admiral

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Saturday, December 22nd 2007, 3:11pm



The graph looks like this. The octane rating of the fuel only has an effect at the upper end of the power level, not where you're going to be cruising.

Hrolf Hakonson

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Saturday, December 22nd 2007, 3:39pm

Where does the upwards curve for 87 octane, vs 100 or 150 octane, appear?

Red Admiral

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Saturday, December 22nd 2007, 5:14pm

Further to the left of the graph. Minimum point looks to be around 0.54 on this graph.

Hrolf Hakonson

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Friday, January 4th 2008, 5:52pm

Tracked down a couple more examples:

Jumo 211 - .451 lb/hp-hr
Jumo 213 - .435 lb/hp-hr

Red Admiral

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Friday, January 4th 2008, 6:14pm

Interesting. I found 0.51lb/hp-hr for the Ju 213

Hrolf Hakonson

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Friday, January 4th 2008, 6:50pm

See here http://books.google.com/books?id=OoFcHOL…pvHKU#PPA209,M1 for the Jumo-213 data.


Also, looking at the Db-601 numbers here http://kurfurst.allaboutwarfare.com/Engi…tasheets_N.html the fuel numbers for that engine should be:

DB-601 - .451 lb/hp-hr

According to this site http://www.fockewulf190.net/uk/fockewu/equipuk/db603u.htm
the numbers for the DB-603 should be:

DB-603A-G - .451 lb/hp-hr
DB-603L/N - .473 lb/hp-hr

This post has been edited 2 times, last edit by "Hrolf Hakonson" (Jan 4th 2008, 6:59pm)