Fuel flow to power formula

Some pretty good practical info from Mike Busch here Obsessed With EGT

I read that article and there are a couple of points which are a mystery.

• He states that best SFC (horsepower per fuel flow) is obtained at 16:1. He doesn’t say how many F LOP that might be at, but probably quite a lot (normal mixture is 14:1) and it seems to be known for many decades that all petrol engines deliver the best SFC at 25F LOP which is not that far LOP.

• He states that he doesn’t use EGT for leaning etc. And the other thing – the SR22T spark plug problems – also points at a weird (though not uncommon) way of setting up the cruise power at the TOC (top of climb): just bring back the mixture lever only. On say an IO540, 10k feet, you are doing maybe 60% power, so this is safe (not possible to do damage below 60%, by leaning) but is not safe on a turbo engine doing 100% power, or a NA engine levelling off at 2000ft. In the latter two cases one should reduce power first, and then all that “red box” stuff goes away.

I have GAMI injectors and no way could I fly at 2000ft, flat out, and bring power back to say 65% power (a reasonable cruise setting) only by bringing back the mixture. The engine sounds really rough, because one ends up very far LOP where the combustion is barely sustained. Yet this has been quite a fashion in recent years, despite it being obviously unsafe because the mixture is transitioning through the worst-case CHT (and detonation) region at 100% power.

I think if OAT is lower (lower CHT), you get less knock too because the inside is colder.

From what I understand, if your ICPs are going crazy, your CHT will climb very quickly. I have not experienced this since I just take care to stay plenty rich of peak during periods where my engine is running above 65-70% power. Once I cross this magic threshold, I lean depending on if I want max power or max economy. If I want max power, then I lean to peak EGT. Max economy is just lean until the engine starts running rough.

I don’t get many opportunities to run at 100% hp for any length of time, so don’t have much advice there, other than take care if leaning. Get to know your CHTs, EGTs, and FFs.

@LeSving what is BEP?

Best Efficiency Point.

Peter wrote:

I read that article and there are a couple of points which are a mystery.

Read that article a few times. It just kept bugging me. My conclusion is it’s mostly just words with hardly any substance at all.

Since then, we’ve learned a lot more about the effect of mixture on the combustion event—largely through the research efforts of George Braly at the world’s most advanced piston aircraft engine test facility he created at General Aviation Modifications Inc. (GAMI) in Ada, Oklahoma—and we now know that 50 degrees F rich of peak is very nearly the worst possible mixture for engine longevity.

How many engines did this George Braly destroy to assess this “finding”? At what power ranges? exactly which engines did he test? Under which environmental conditions? At which CHT levels? Why no references? It surely must be a report somewhere.

Also, this picture looks highly misleading to me

It says at 75% power. This is not possible. The whole point of CHT increasing when going richer from Lambda = 1 (or peak EGT) is that the power increases. It increases because the power goes up. More usable heat is produced by the fuel. If it was running at 75% power at Lambda = 1, it would be running at maybe 78 % power at peak CHT, certainly not 75% power. Getting it down to 75% power at peak CHT is obviously done by reducing both fuel and air while keeping the Lambda constant, and you end up with roughly the same CHT as you would at peak EGT. Why? because you produce the same amount of power.

What he probably means is at constant throttle setting (WOT for instance), not constant power. Constant throttle setting is not constant power when the mixture is varied.

Looking at it from a thermodynamic efficiency point of view (again, not the same as MPG), what is happening at peak CHT is you reach peak thermodynamic efficiency of the engine. This can easily be seen by looking at the Otto cycle (Wikipedia)

The efficiency can be expressed in many different ways. One basic way is this (from the same Wikipedia page)

eta = 1 – (T4 – T1)/(T3 – T2)

Increasing CHT means increasing T3, you get more energy out of the fuel in terms of usable heat. All other temperatures constant, this will increase the thermal efficiency.

IMO, all Mr Busch is actually saying is watch the power output, or the CHT may rise to unhealthy levels. But, I mean, as long as the CHT actually is inside the greens, it’s OK, as per the engine manufacturer. He confuse it all with a whole lot of nonsense about “not running ROP”, not running in the pink area, which has nothing to do with it (IMO of course. I may be wrong too of course, but very seldom )

You are not wrong at all, and sadly the Braly/Gami $1000 seminar travelling marketing bandwagon is unstoppable. I got into hot air when I was in PPLIR and said this, when they were organising an event around one of these. I thought it was mostly stuffing a simple idea with BS to get the $1000.

Especially with an NA engine it is simple, due to the obvious constraints of “not enough air available” (WOT) and below that (around 8000ft) you just set 23" or whatever, and lean for peak EGT.

And yes running the engine hotter (CHT) is that modern diesels do, running the block at around +100C. That and the high CR is how those cars to 60mpg+.