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grahama
8th Jan 2009, 15:11
I am sure that I should know this but there is so much excellent technical knowledge on this forum that I will be lazy and ask for advice here!!!

Could anyone tell me simply, for a non-technical artistic type, when at altitude the maximum achievable manifold pressure is indicated well below full throttle setting does increasing the throttle setting waste fuel, alter performance?

Okay tell me how stupid I am being!!

BackPacker
8th Jan 2009, 15:41
Unless you have a turbonormalized or turbocharged aircraft, your manifold pressure can never be higher than ambient pressure, only lower. You regulate this by closing and opening the throttle valve.

The amount of fuel that's sucked into the cylinders is depending mostly on the amount of air that's sucked in (through the carbs). Less manifold pressure means less air is sucked into the cylinders, and so less fuel is sucked in with it. (Yes, it also depends on the mixture settings, but we'll leave that out for now.)

So it doesn't matter at all what the exact throttle position is: fully open to the stops or just wide enough open to equalize ambient with manifold doesn't matter one bit. You can even see this at ground level: the last centimeter or so of throttle movement, when you open the throttle for takeoff, doesn't matter in the RPMs, MAP or fuel flow at all.

ShyTorque
8th Jan 2009, 16:04
Provided that the mixture is correctly set (leaned with increased altitude / reduced air density) there is no fuel wastage, in fact the engine becomes more efficient.

However, if opening the throttle further results in no increase in MAP or RPM then there is no more power to be had compared with that at lower altitude. This is due to the lower mass flow of fuel /air mixture through the engine.

In practice, this means that as the aircraft climbs, the throttle would need to be opened wider to maintain the same power output.

When the throttle is fully open, you will get no more power from the engine - that's when you need forced induction.

Rolls Royce invented a two speed supercharger for the Merlin X engine. The low gear speed was used at low levels and the higher speed for higher altitudes.

IO540
8th Jan 2009, 16:18
If one takes the power v. fuel flow graph for a given engine e.g. the IO-540 and extrapolates one of the constant-RPM lines to the zero-HP intercept, one finds several GPH fuel flow at that point. This fuel flow presumably represents the total of the pumping and friction losses, at that RPM. It is pretty significant, and explains why flying at a lower RPM, say 2200 v. 2400, does produce a significant improvement in MPG.

The engine should be more efficient when operating at a wide open throttle - because it doesn't have to suck quite as hard - but I have completely failed to measure any difference. Maybe I wasn't doing it right.

But there is no waste of fuel. In fact any non-turbo flight above about FL080 is done with a wide open throttle the whole time, and you just set the mixture for LOP or whatever grabs you. The RPM lever does give you a bit of power control: more RPM sucks in more air -> more fuel -> more power. At altitudes near the operating ceiling (20,000ft for me) one is at wide open throttle, max RPM (2575) and to hell with fuel consumption (beggars can't be choosers) so one sets it to 100F ROP.

ShyTorque
8th Jan 2009, 17:27
The engine should be more efficient when operating at a wide open throttle - because it doesn't have to suck quite as hard - but I have completely failed to measure any difference. Maybe I wasn't doing it right.


With a wide open throttle, the volumetric efficiency is improved. In effect, this means the engine works at a higher compression ratio, which improves the efficiency. As you say, pumping and frictional losses are less at lower rpm, too.

A compression ignition (Diesel) engine is very efficient because there is no throttling, only control of the fuel quantity introduced.

dublinpilot
8th Jan 2009, 19:27
Or to put it a slightly differnet way....

If you opened the trottle fully at ground level, you'd get near athmosphere pressure (in inches of mercury).

If you left it there and started to climb, the MP would gradually drop as you climbed (as the outside pressure dropped too). There isn't a sudden drop...it's a continous gradual drop.

dp

SNS3Guppy
9th Jan 2009, 00:34
Unless you have a turbonormalized or turbocharged aircraft, your manifold pressure can never be higher than ambient pressure, only lower. You regulate this by closing and opening the throttle valve.


This is close. Manifold pressure in flight with an open throttle plate will be higher than ambient pressure, assuming an unobstructed induction, because of ram air increase and a slight pressure increase from the propeller.

Rolls Royce invented a two speed supercharger for the Merlin X engine. The low gear speed was used at low levels and the higher speed for higher altitudes.


Two speed superchargers are standard on nearly all supercharged aircraft engines...though many operators today restrict operations to the "low blower" position.

If one takes the power v. fuel flow graph for a given engine e.g. the IO-540 and extrapolates one of the constant-RPM lines to the zero-HP intercept, one finds several GPH fuel flow at that point. This fuel flow presumably represents the total of the pumping and friction losses, at that RPM. It is pretty significant, and explains why flying at a lower RPM, say 2200 v. 2400, does produce a significant improvement in MPG.


Significant differences exist between fuel controlled engines and carbureted engines or those utilizing simple injection. Fuel efficiency and engine efficiency at lower RPM's is largely due to the effects of a constant speed propeller, rather than simply slowing down the engine...but this is largely irrelevant when considering manifold pressure on it's own.

IO540
9th Jan 2009, 09:51
Fuel efficiency and engine efficiency at lower RPM's is largely due to the effects of a constant speed propeller, rather than simply slowing down the engine.

Can you elaborate?

airborne_artist
9th Jan 2009, 11:29
Fuel efficiency and engine efficiency at lower RPM's

For the same power, ICEs are almost always more economical at lower revs, and thus a higher MAP. A CS prop thus allows you to control RPM, and so get a better cruise consumption, without compromising T/O power, for which you'll need high RPM.

Islander2
9th Jan 2009, 12:02
IO540, I imagine what SNS3Guppy was alluding to is that, in most light aeroplanes with constant speed propellers, propeller efficiency is higher at the lower end of the cruise RPM range (because the higher advance ratio gives a blade alpha that is closer to producing max L/D for the prop) and that this gain in efficiency is typically more significant than the gain in engine efficiency from lower friction losses and greater volumetric efficiency. However, for any particular airframe/engine/propeller combination, it is quite difficult to access data that shows the relevant contribution of each to the overall gain in efficiency that is usually achieved by flying at a lower RPM.

mm_flynn
9th Jan 2009, 13:00
In addition, if you are running LOP then a lower RPM effectively advances the spark closer to the ideal point, generating more usable power. So lower RPM helps efficiency due to a combination of

1 - Lower pumping/friction/induction losses
2 - Higher AoA on the prop (hopefully closer to best L/D)
3 - Better ignition timing if LOP
4 - Others... ?


Clearly too slow and you start to get excessive pressures and temperatures, which could cause detonation. If ROP you can get the spark sufficiently 'advanced' to cause pre-ignition.

SNS3Guppy
10th Jan 2009, 01:08
Spark and timing occurs as a function of a fixed magneto position relative to crank position, not mixture setting. The spark fires at the same point BTDC each time. Preignition isn't an issue, though detonation can be. The spark doesn't advance, and there isn't use of a variable timing or advance mechanism on most aircraft engines.

Can you elaborate?


With respect to fuel efficiency at lower RPM's...yes, lowering RPM means lower fuel consumption; the engine isn't turning as fast. However, if one is lowering RPM's by reducing throttle and thus reducing manifold pressure (which is the subject of this thread)...one also isn't getting the engine power...one isn't going to go as fast, climb as fast, and therefore a significant increase in specific fuel consumption isn't necessarily realized...nor is the rate of fuel consumption, or the efficiency, linear.

If one reduces RPM by use of the propeller control, one can't simply point to the lower RPM as the cause of any perceived or actual increase in fuel efficiency, and relative to manifold pressure the point is rather muted. If one is reducing RPM using the propeller control, one is increasing blade angle while maintaining or increasing manifold pressure; one is increasing the effect of thrust imparted by the propeller, and thus increasing the efficiency of the operation, to a point. This really has nothing to do with increases in altitude and decreases in manifold pressure, however, and one can't attribute the increase in efficiency, where this does indeed occur, simply to the decrease in RPM...it's attributable to the change in blade angle, and the change in RPM is a nearly insignificant factor in the matter. The change in blade efficiency and effectivenes, however, is a big player in the matter, and accounts for the change in specific fuel consumption relative to distance traveled.

karl414ac
10th Jan 2009, 01:46
you lose around 1" manifold pressure to every 1000ftabove sea level . a turbo normalized engine will still only produce 29" on the ground at sea level but as you climb the turbo will keep the pressure at 29" by keeping the ambient pressure in the manifold higher than the outside pressure.
a normal turbo engine (turbo arrow etc) will produce higher manifold pressure to increase engine performance . the average GA turbo aircraft will be wastegated to 39" to avoid extra wear on the engine but will be able to maintain engine power to well past 10,000ft barrier until it reaches critical altitude where the turbo cannot keep up with the demand for higher pressure due to the density of the outside air

ShyTorque
10th Jan 2009, 07:30
Could anyone tell me simply, for a non-technical artistic type...

I'll bet poor grahama is totally impressed with all this "non-technical, artisitic" friendly explanation.....:ugh:

As usual it's quickly become a competition to prove who knows the most. :p

mm_flynn
10th Jan 2009, 20:08
Spark and timing occurs as a function of a fixed magneto position relative to crank position, not mixture setting. The spark fires at the same point BTDC each time. Preignition isn't an issue, though detonation can be. The spark doesn't advance, and there isn't use of a variable timing or advance mechanism on most aircraft engines.

While you are correct that the spark fires relative to crank position and nothing (in a typical GA engine) changes this; in a timing sense (i.e. how many msec before TDC) the spark fires is clearly effected by RPM. One of the 'problems' with LOP operation is that the flame front spreads much slower than, say 50 degrees ROP, so the effective maximum pressure (what the timing is trying to position at 14 ATDC) happens later (in time) LOP than ROP and hence at lower mechanical efficiency. Slowing the RPM makes this happen earlier. 2500 vs 2200 bring the peak power pulse about 25 microseconds earlier (3 - 4 degrees at 2200)


At a constant power setting (ie. 200 hp) and 30 in MP (and everything else constant except rpm and fuel flow), I get a significantly better IAS/Gal running at lower RPM than higher, on the order of 10%. How much of this is due to prop efficiency, reduced pumping loss, better timing of peak power pulse - I have no idea - but it works!


----

Back to the more simple question asked. One more point,

If you are not at full throttle and you open the throttle you should always get an increase in MP as you are reducing the intake restriction.

If your question was,

'My POH says I get 75% power at 25/2500 and I am high enough the outside air pressure will only 24 as a maximum MP, should I throttle back to not waste fuel?'

the answer is emphatically no - for all of the technical reasons already posted.

SNS3Guppy
10th Jan 2009, 21:06
I'll bet poor grahama is totally impressed with all this "non-technical, artisitic" friendly explanation.....

As usual it's quickly become a competition to prove who knows the most.


I believe the original poster's question was answered long ago. Does this mean that one should not address the conversation as it continues to evolve?

Having a discussion isn't a contest about who knows the most; it's an analysis of the topic at hand, which is still in play.

ShyTorque
11th Jan 2009, 00:29
Not at all. Yawn...... :8