CSU Fuel Flow/Range

[Hill Roller]

Hi all,
Just a quick question.
When in cruise with the engine set up as 24/2400rpm leaned, if the throttle was pushed all the way in to say 27+ inches would the fuel flow increase/stay the same? (provided nothing else changes incl. the already leaned mixture). What about range?

I would say that the fuel flow would stay the same as the engine driven fuel pump would still be spinning at the same rpm, however the mixture would now leaner so it would need to be changed (richer) to bring it back to the recommended setting requiring more fuel. I would imagine it would be a marginal increase and would be already be offset by the increased engine efficiency and more efficient prop AoA. Thus range would increase.

Correct. Wrong. Your thoughts?

[27/09]

In a word.... Wrong

The fuel flow will increase as the throttle is opened further, and could increase at a marked rate towards full throttle since there is normally extra fuel scheduled by the fuel control unit/carby at full throttle/high power for cooling purposes. By opening the throttle you are producing more power so it stands to reason the fuel flow will increase.

The engine driven pump has no direct impact on the fuel flow Fuel flow is directly affected by mass airflow and mixture control setting. Mass air flow is affected by throttle position and RPM.

The speed increase will probably be small, fuel flow will be proportionately much higher therfore range will decrease.

[UnderneathTheRadar]

Just to add - the engine driven fuel pump merely fetches fuel from the tanks and ensures that there is fuel sitting at the fuel metering device (carby, fuel injectors) ready to be sent to the cylinders by that device according to the amount requested by the throttle and mixture settings.

It's speed/amount of fuel being pumped is irrelevant in the scheme of things (as long as it is providing enough fuel to actual fuel metering device - it's it not well, then it's likely that things will get exciting quite soon!)

UTR

[OZBUSDRIVER]

Interesting, lean setting at 60% power and go to full throttle. Wouldn't the engine just go way over lean and stumble?

[blackhand]

Lean mixture, throttle to the wall - roundy engines will blow a cylinder for you

[Mark1234]

Fuel pump has nothing to do with it - as mentioned it just provides fuel pressure; RPM has only a part in controling the fuel supply; it serves to set the top limit the rate at which the the engine can process (pump) air.

The throttle controls 2 (and a bit) things:
1) the butterfly position, and hence the actual amount of air going in
2) the fuel delivery system
2.5) any enrichment valves/devices

If it's carburated, opening the throttle allows more air to flow, hence dragging in more fuel. If it's injected it's more complicated, but basically the same principle. It may involve mass flow sensing, FCU's and other stuff, but basically more throttle, more air, more fuel.
Most systems have some form of wide open throttle enrichment.

Provided you're away from the boundary conditions (ICO, or full throttle height), Whatever the leaning, when you open the throttle, more fuel will go in. Enough? Goodness knows. I wouln't risk it. Leaning is specific to a power setting. Chances are you change the power setting you'll stuff up the mixture.

[Joker 10]

Good way to set up detonation, reward hole in crown of piston , engine dead.

Good luck

[blackhand]

Good way to set up detonation, reward hole in crown of piston , engine dead.

Or in a roundy engine the quickest cylinder removal you'll ever experience:ok:

[Chimbu chuckles]

Lots of absolute statements being made here...Ozbusdriver is correct because he started with a specific % hp.

If RPM has nothing to do with how much fuel is delivered to the (constant flow) injectors how come I can lean the mixture and change the % HP/EGTs and CHTs of my IO550 JUST with the prop control?

How come also I can enrichen the mixture dramatically by closing the throttle from full throttle to say 17in MP while leaving the mixture and RPM unchanged? I can go from well LOP to 50 ROP just by closing the throttle in this manner.

Hi Roller it depends.

If you were say leaned 30-50F ROP EGT (peak CHT and worse place to be at 24/24) and did as you suggest there would be a slight increase in FF but a proportionally greater increase in mass airflow so you would lean the mixture and possibly end up back around peak +/- a bit. The CHTs would decrease, get cooler, and your detonation margins would increase. With standard injectors some of your cylinders would be LOP and some ROP and some in the middle and the engine would be running quite rough because all the cylinders are running at different %hp. If you had a set of gamijectors allowing the engine to run smoothly (all cylinders at the same % hp) at almost any mixture you could actually achieve what you are suggesting and lean the mixture/not lose much HP and maybe gain a little range. Without an all cylinder engine monitor and balanced injectors PLEASE do not do this!!!!

If you were already LOP (with balanced injectors allowing smooth engine operation LOP) and you did what you say you would end up MORE LOP and have MUCH cooler CHTs - and your endurance would indeed increase but the % hp might be so low that range actually remained the same or slightly decreased.

If you were leaned to around 100F ROP EGT at 24/24 (a much safer place to be for your engine) and did as you're suggesting you would drive the mixture back towards 50F ROP EGT (peak CHT) and probably damage the engine. If it was a normally aspirated engine you'd be hard pressed causing detonation unless you managed to run the CHTs up WELL over redline and caused some pre ignition first. The more likely outcome would be CHTs high enough to weaken the cylinder metal and over time oval the barrels and lose compression/crack cylinders. This would happen because you are leaning the mixture with the throttle but because you're a LONG way ROP there is plenty of fuel available and when you throw a whole lot more air at that fuel by shoving the throttle open you get a leaner (but still ROP) and hotter burning mixture.

Here is a graph showing the relationship between EGT/CHT et al. MP/RPM are being held constant at 25/25 and the only thing being varied is mixture.

http://www.fototime.com/{B0C6C268-18AC-4B80-83DF-08B1CDCAEDF8}/origpict/tcm_mixture_sweep.jpg

It is entirely meaningless to discuss 'a lean mixture' - precise language is needed - precisely HOW lean - are you are ROP, Peak or LOP?

[bushy]

I believe the chieftain that had a "simultaneous double engine failure" at Whyalla actually had two separate engine failures, and the second one was caused by increased (max) power being used without richening the mixture. The result was a hole in the piston.
Get a chieftain and try it. You will see a definite increase in temperatures.

[Joker 10]

Detonation is usually indicated by high CHT and rough running. Detonation is a result of fuel characterisics ( octane rating ) incorrect fuel/air ratios, high CHT,or excessively high manifold pressure in relation to RPM. Detonation becomes more pronounced and destructive as tempretures increase.

Round engines that are supercharged need special care as it is really easy to overboost and the inevitable result is either a cylinder departs or a piston fails, either way it is not good.

Turbo Charged flat engines are also prone to overboost if the waste gate or dump valve fail.

[Chimbu chuckles]

Yes indeed that is the case...but once again I ask where was the mixture when he shoved up the throttle?

ROP (where he was) things will certainly go from bad to worse. Peak or LOP they will not.

Its called Peak for a reason. Whether it be peak CHT or peak EGT if you are either richer or leaner things will be cooler. In the above graph ICP is Internal Combustion Pressure - that force trying to lift the cylinder head off the barrel. You will see it tracks CHT to such an extent we can use CHT as a proxy for ICP.

I will give you another example - In my Bonanza (before I started restoring it) I typically cruised at full throttle/2200 rpm/50F LOP EGT. On descent I just leave the power set and poke the nose down - at some point on descent I need to reduce power to keep the speed out of the yellow. Initially I do this by reducing RPM but eventually I need to reduce throttle down around 2000' or so. I reduce throttle from about 27in straight back to just above where the gear warning horn sounds...about 16-17in MP. If it was the case, as some posit above, that throttle controlled how much fuel rather than how much air reached the cylinders then the mixture would get leaner and my EGTs (already LOP remember) would cool further...they dont. Yes when I pull the throttle off that much the FF drops but the amount of air available for combustion drops proportionally more and the mixture gets A LOT richer. The EGTs all increase through peak EGT and then reduce again on the rich side if peak. They end up around 50F ROP and as this equates to Peak CHT the rate of cooling of the cylinders actually decreases. They cool at a reduced rate.

So much for shock cooling.

I enter and fly the circuit without touching the mixture/RPM or throttle much - I will generally only reduce throttle a fraction more, after the gear is down and locked, back to about 14in MP.

As I thought experiment only if I decide to go around on finals and do nothing more than firewall the throttle what will happen?

Well I will merely end up back at my original cruise setting. Full throttle/2000-2200rpm and about 30-50F LOP - about 60-65% power. That may or may not be enough power to successfully go around but even if you crash as a result the engine will be undamaged and running fine right up to point of impact...unless you believe 65% power is bad for the engine:ugh:

All three engine controls vary mixture. The actual mixture is the only one that can vary power produced from 0% to 100%. The throttle primarily controls how much air is available for combustion but DOES have an effect on FF too. The RPM primarily controls propeller RPM but in our typical non geared engines it has the same effect on engine RPM and fuel pump rpm.

From a mixture/fuel air ratio point of view varying RPM has a proportionally greater effect on fuel flow than mass airflow. Less RPM leaner/more RPM richer. That is one of the reasons why achieving redline RPM on takeoff is so important.

[Joker 10]

All true for normally aspirated engines, as soon as any form of forced induction is present the equation changes.

[Chimbu chuckles]

The physics and metallurgy does not change merely the speed and severity of the damage if you abuse the engine by lingering in that zone between not rich enough and not lean enough.

There are Turbonormalised engines running happily at 80% power with no adverse consequences and I know of a C310R with TSIO520s that have 4000+ hrs on them with only a single cylinder and 1 exhaust valve changed in all that time.

That is why CASA (C*#($ Against Safe Aviation) not mandating the technology (engine monitors/balanced injectors), for at least commercial piston engine ops, and the training (within the auspices of Engine System and Instruments ground courses at PPL and CPL level) is an obscenity. :mad:

This technology and training would have completely negated Spencer Gulf for starters.:ugh: