Gallons per hour

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whiic
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Gallons per hour

Post by whiic »

QUESTION 1: What gallons are the reported fuel consumption ("Fuel econ: x ghp") on SHIFT+2 panel? It seems the number is same for US (B, C) and "Imperial" (AVG, Tomahawk) planes (unlike SHIFT+4 and gauges).

Also, another thing I've noticed is that Allison has very different fuel consumption to Merlin.

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Spitfire Mk.I		           Imp	US
2500rpm +0psi (29.9) rich		49	58
1800rpm -4psi (21.8) weak		26	31

P-40
2500rpm 29.9 rich			74 (Imp/US?)
2500rpm 29.9 lean			59 (Imp/US?) (note: used power setting is above permitted for lean)
1800rpm 21.8 lean			23 (Imp/US?)

Pre-amble to question 2:
If we assume gallons on P-40 SHIFT+2 panel are US gallons, we notice that it has somewhat higher fuel consumption that Spitfire at higher power outputs (such as 2500rpm and MP at atmospheric pressure (29.9 inHg or +0 psi)). If it's Imperial gallons, then the difference only increases. Even if we assumed US gallons and run Allison in lean mixture (even though it's not permitted for such "high" MP) it still uses more fuel than Merlin running rich mixture. Of course the engines aren't identical. For example, displacements are slightly different (27L vs. 28L).

On the other hand, at low power settings, the situation is reversed: Allison uses much less fuel than Merlin (regarless whether P-40's ghp are US or Imp).

Some explanation for this increase of fuel consumption on high power output is no doubt the mixture curve and high power enrichment. Spitfire has next to none automatic enrichment for higher power if certain data is to be trusted, where as Allison probably has some.


If I make some rough synthetic calculations, 2500rpm time 29.9 inHg should pump 74750 "units" of air into the engine per second. At 1800rpm 21.8 inHg, it's 39240. Divide former by latter: 1.91. So if the mixture was constant, increasing power from the lower of mentioned power setting to the higher increases not just airflow but fuel consumption by 1.91.

If we however look at Spitfire's gph readings, and divide 49 by 27 we get only 1.88, so airflow increases more than fuel consumption, i.e mixture becomes weaker with added throttle (at least within that power range), so much actually, that even putting the mixture to rich setting, it's still weaker than in the lower power setting.

Doing the same math with P-40, dividing 74 by 23, we get 3.21 fold fuel consumption with same increase (1.91) of airflow through cylinders. Mixture is enrichened by a factor of 1.68 (3.21/1.91). Again, the higher power setting is with rich, and lower with lean. (Running the higher power setting with lean mixture would give 2.57 times the fuel consumption and 1.34 times the mixture enrichment.)


QUESTION 2: Do these calculations seem reasonable? Does Spitfire actually lean out with added throttle so much that even enriching with mixture lever wouldn't maintain the same mixture?
And on P-40, doesn't the enrichment seem a bit excessive? (Especially compared to Spitfire which seems to lean out rather than enrich at all.)

And yeah, these a calculations of mine are very synthetic. I wouldn't have made them if I hadn't had a hunch that something might be wrong before hand. (I noticed that low-power cruise on Spitfire was rather high, compared to how low fuel consumption is during climb.)

Nico081
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Re: Gallons per hour

Post by Nico081 »

I love the level of issues here. The better the product the more professional questions and discussions by competent customers :wink:
This pushes again the quality of our beloved accusim birds :)
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Scott - A2A
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Re: Gallons per hour

Post by Scott - A2A »

2500rpm 29.9 rich 74 (Imp/US?)

This figure is in U.S. gallons.

And yes, the Allison seems to be more thirsty than the Merlin. It may be due to the way the Allison enriches the mixture at power.

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whiic
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Re: Gallons per hour

Post by whiic »

I was just thinking the enrichment might be a bit excessive. That is, a difference of 1.68 between low power lean setting and medium power rich setting.

If the best economy was around 16-17 air to 1 fuel and best power at around 12-13 to 1, there's only about 1.3-1.4 difference between best economy and best power.

That means that if "1800rpm 21.8 lean" was at best economy mixture, "2500rpm 29.9 rich" would already be more rich than best power mixture - that is, "2500rpm 29.9 rich" would already be well into "rich rich". And that's with just 29.9 inches (+0 psi) which is hardly much boost at all. Considering that these engines can be boosted to way above 45 inches, I have my doubts on Allison requiring "rich rich" as low as 29.9 inches of MP. Either the mixture is unrealistically rich at 29.9 inches, or it's unrealistically lean at low power cruise setting. Or only slightly inaccurate at both ends.

___

Spitfire's mixture on the other hand seems to drop rather than enrich when going from low to medium power. Some of such behaviour could be explained with idle mixture enrichment, but is 1800rpm -4psi (26 gal/h Imp) really close enough to idle (~9 gal/h) for that idle enrichment to play a noticeable role...


PS. I could do comparative tests with some other operating condition (such as 2600rpm, +5psi, rich) to see if Merlin would continue to lean out even further with that added power, or if Allison would enrich even beyond the already "rich rich"...
but, unfortunately, I had to reinstall my OS (Task Scheduler on the old installation got corrupted and I lost network connectivity among other things). I'll probably take a couple of weeks off from flight simming, installing and configuring productivity applications and making a reconfigured no-bloat no-games back-up before installing FSX again.

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seaniam81
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Re: Gallons per hour

Post by seaniam81 »

Apples and Oranges. Two different engines and you cannot compare.

n421nj
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Re: Gallons per hour

Post by n421nj »

My thoughts exactly two different engines.
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