A (probably) daft question
 

We all know that in an aircraft with avgas-fuelled engine(s), as we increase altitude, we lean the fuel mixture. To my simple understanding (I'm no engineer), this is done in order that the amount of fuel in the "mix" is decreased in line with the amount of oxygen being ingested from the increasingly less dense air, in order to maintain optimum engine power output and operating parameters.

This has the very obvious benefit of reducing the amount of fuel used, as we again are all aware, with the attendant quite dramatic cost savings.

So, my question is this. Why can't the amount of oxygen (and presumably all the other component parts of "air") be artifically reduced (by some damnably clever contraption) down at sea-level, thereby allowing the amount of fuel used to be reduced ? In effect setting up an artifical altitude, which could be maintained until the aircraft actually passes through the real equivalent.

Reduced pollution, grants for development, positive publicity for GA - cor, what a cracker !

Come on then, shoot me down .... :{

FF :ok:
CEO, FF-OxyThinner Ltd (TM, (c) 2008)

You can; no problem.

But with the decreased air and fuel comes a decrease in performance too.

:}

That is the same as not fully opening the throttle.

Rod1

Such a damn clever contraption exists. It's called the throttle, and it reduces the Manifold Pressure (MAP), which makes the engine anemic just like at high altitude.

Less air and fuel = less power.

The reason you get away with it going up high (and often actually travel faster) is that with the air being less dense up there, you have less of it to push out of your way as you travel through it. If you have less to push out of your way, then you don't need as much power ;)

I hope that helps.

dp

Supplementary daft question: How does a Rotax engine adjust the fuel/air mix with an increase in altitude?:confused:

I think that fully flapped has missed the point when it comes to aircraft performance. As said above engine performance decreases with increasing altitude but TAS increases.

The effect is that the aircraft flys faster but at the same IAS.

Most engines make 75% power up to about 8000ft so the best place to be for cruise is between 7 & 8000 ft this will give the highest TAS for a lowish fuel burn.

Increaseing the altitude to 12,000 will futher reduce the fuel burn but because at 12,000ft the engine can only make 60% power the IAS & TAS will be lower, range is likely to increase but you now have to look at airframe/engine maintenance costs vs lower fuel burn.

Running at best lean setting should save between 15 & 20 % fuel burn over full rich.

A&C, Can you explain the airframe engine maintenance issues verse fuel burn more?

As someone who spends a significant amount of time in the FL120 range, I get an indicated airspeed within a few knots of what it is at FL80 but a massive fuel saving so it pays to be their in that respect. The amount of time a trip takes is negligible difference.

“Supplementary daft question: How does a Rotax engine adjust the fuel/air mix with an increase in altitude?”

The Carbs have a degree of auto adjustment.

Rod1

Damn !!

Another business plan bites the dust ! :ugh:

Makes a nice change from the "my IR's bigger than your IR" stuff, though, doesn't it ? :):)

FF :ok:

Not quite correct, you cannot just keep increasing altitude to get optimum engine power once you reach full throttle altitude thats it, unless you bolt on a blower!.
Range is best at L/D Max A of A, which coincides with lowest altitude for full throttle operation.(which explains your question Bose)

Max endurance is best at sea level.

If that was correct Cessna 150s would be be fuel efficient in the Tropopause!!

Only (pure) jet engines continue to be more efficient with altitude to the tropopause

More to the point, is how come after all this time there is not a system which automatically reduces fuel flow in line with increasing altitude/decreasing pressure?

It's not difficult - even an Austin maestro could do it!

Bose-x
 

I cant see the savings between FL80 and FL120 in terms of fuel burn being "massive" as you put it when you have taken into account the extra fuel burnt in the climb and the reduced TAS.

I can't do the sums for you as I don't know what your airframe/engine cost per hour is but I do know that with the increase in fuel costs the airlines are reducing cruise speeds to save fuel this increases the "airframe" costs but the bean counters have done the sums and found this to be the cheapest way to fly the aircraft from A to B and that points the disussion in your direction.

So please take a look at a 2 hour flight at FL80 and the same flight at FL120 and tell me what the total flying time and total fuel burn is, we also need to know your "dry" hourly costs. Only then will we get a picture for your aircraft, I know that for my aircraft FL80 is about the best, you could well be totaly correct for your type of aircraft.

There is, its called an autothrottle and a flight guidance management computer

All the figures are fed into a computer(by the cost department), the maintanace hours cycle, the major inspection cycle etc and balanced against time in the air and fuel burnt from this is produced a COST INDEX, this figure is then fed into the FMGS or FMS before ech flight to calculate the optimum FL and mach number. Wind and weight are also entered into the computer to enable this calculation. (Then ATC say that level isnt available usually)

A&C according to Flitestar 2hr flight at FL80 and FL120 gives 6 minutes difference. So are you saying that the extra 6 minutes of flight time are where the extra costs come from? My time to climb to FL80 is 11 minutes and to FL120 16.

At FL120 I am burning 28lph and at FL80 burning 33lph.

In the schem of things even allowing great fuel burn for the climb and the extra time for the flight 5lph fuel saving more than covers it?

Rotax 4 strokes (912) use Bing CD carburettors. CD standing for Constant Depression. These use the butterfly valve to signal demand for fuel/air mixture, but the actual metering is done by a slider & needle which is raised by the vacuum applied from the butterfly valve. The mixture at any given setting is determined by the profile and height of the needle, and the size of jet it slides into. At low altitudes the slide would lift all the way up if the butterfly was fully open. As the air gets less dense higher altitudes the slider would progressively open less and less, so the available power decreases but the air/fuel ratio stays the same. Stromberg and SU carburettors work on the same priciple.

I suspect nobody has understood the original question, or why lean when climbing.

When the engine is operating at its most efficient operating point, which is peak EGT or just slightly lean of peak (called stochiometric combustion) each molecule of fuel is getting attached to the correct number of molecules of oxygen.

That is the point at which you want to run an engine - whether it is a car or a plane or a lawn mower.

The carburretor or the fuel injection servo (in this context they both work the same way) dispenses fuel into the engine, according to how much air is going in.

Unfortunately the system doesn't count the # of molecules of each. It would be great if it did that, and keep the ratio exactly right, but that would require mass flow measurement on the fuel, and mass flow measurement on the air.

Now, mass flow measurement on the fuel is easy, because it is liquid, and on a liquid the mass flow is the same as volume flow, and it's easy to make a liquid pump which pumps constant volume per revolution or whatever.

But mass flow measurement on air is hard, because it's a gas and expands and compresses as it feels like. It can be done, but it isn't trivial. There just isn't a "constant mass flow rate pump" for gases. The best you can do is shift the stuff as best you can while measuring its pressure and temperature and calculate the mass flow from that. That's what modern cars do, but the old aeroplane engines never went to that system, for various reasons, some good, some crap.

So, the fuel metering system (whether a carb or fuel injection) meters air by something halfway between mass flow and volume flow, and as you climb and the air gets thinner, there is an increasing error resulting in too little air being let in, relative to the fuel that's going in. So you have to lean the mixture to restore the correct ratio.

A primitive solution would be an altitude compensated carb (or fuel injection) which has a barometer and leans the fuel with increasing altitude. But this makes no allowance for temperature! At higher temps the liquid fuel is still the same "size" (avgas expands only 0.1% per degC) but the air gets "bigger" quite fast....

So, that is why you have to lean the mixture extra as you climb, to hold the engine at peak EGT.

There are some second order effects due to increasing altitude; for example the exhaust back pressure is lower so the pumping losses in the engine are lower, so for a constant MP and constant fuel intake etc, the engine will delivery very slightly more power at a higher altitude. But - unless turbocharged - normally there is such a huge loss of MP that nobody is going to notice this.

The "TAS gain" is a completely different thing. It is to do with the crude airspeed indicator progressively under-reading as the air gets thinner. But a plane pulled along by constant thrust will go faster when higher up because the air is thinner.

I wouldn't trust any performance projections from Flitestar because they come from a generic POH for the type. To use these accurately, one would need to do performance measurements. For example FS assumes 12GPH for my TB20 but in reality I get the target IAS or TAS at 11GPH, by going to LOP.

It would be possible for avgas engines to be like modern car engines, and have gas flow measurement for the air and meter the fuel accordingly to maintain peak EGT. But this would require extra sensors and complexity, and electronics in GA have a poor reliability record, and it's really easy to lean for peak EGT if one has a multi-cylinder engine monitor. You can lean for peak EGT on the ground, or at any altitude, and get the best economy that way. But there are issues with the air cooled engines: to save weight, they have thin sections and generally poor thermal design, and have to be run rich at high power settings especially when the cooling airflow is low, as it is during climb. So, even with proper fuel/air metering, one would still have to go full-rich for a max power climb.

Wow, what a complicated way of saying the oxygen levels decreases the higher you go and you need to reduce the amount of fuel in turn with the mixture control to preserve an optimum fuel air ratio (often quoted at 14.7 to 1) upto full throttle altitude

Actually I constructed my own model in FS for my aircraft for everything including the W&B and it is dead accurate. More than a thousand hours of comparisons to prove it.