Need advice - in flight adjustable prop

by John Fleming in 912 / 914 Technical Questions 6 years ago

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Re: Need advice - in flight adjustable prop

by Bill Hertzel » 6 years ago

Bill Hertzel

13 1.95k 7.96k

...I understand that the gauge is displaying the difference in pressure between ambient and that which is behind the throttle plate...

The gauge is Not a Differential Gauge, it is an Absolute Pressure Gauge.
It displays the Absolute Pressure relative to a Perfect Vacuum Not relative to the Ambient Pressure.
It is essentially an Altimeter calibrated in inches of Mercury showing the Pressure Altitude inside of the Intake Manifold of the engine.
It will read ~30" at sea level Engine OFF.
If you were to land in Denver, Colorado at 5000', when you stopped the engine it would read ~25".

This is why it is nearly impossible to do any engine damage at altitude.
There just is not enough air pressure to do any damage and also why the engine does not have a lot of power.
The engine thinks it is at half throttle even when it is Wide Open.

Bill Hertzel
Rotax 912is
North Ridgeville, OH, USA
Clicking the "Thank You" is Always Appreciated by Everyone.

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Re: Need advice - in flight adjustable prop

by John Fleming » 6 years ago

John Fleming

15 92 111

Thanks Guys,

That pretty much has what I already know. What I am looking for is what it actually means. The Letter gives me the graph, and yes, I understand completely not to have the RPM too low, and the prop pitch too high, as it "lugs" the motor - all good there.

What I am looking for is the science behind what the MP is etc, and what is happening inside the engine etc. Does that make sense? I am one of those "why and how" type of guys LOL! I need to know what is happening.

Cheers
John

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Re: Need advice - in flight adjustable prop

by Bill Hertzel » 6 years ago

Bill Hertzel

13 1.95k 7.96k

OK, You asked for it!

Assumptions used for this discussion.
30" of MAP = 15PSI: therefore, 1Psi = 2" MAP
There is no pressure drop across the Intake Valve.
The engine has a Compression Ratio of 10:1
The fuel has a Combustion Pressure Ratio of 15:1
The Stoichiometric Air to Fuel Mixture Ratio (~14.7:1) remains constant at all under all conditions.
(The fact that this number is amazingly close to Sea level air pressure of 14.7 psi is only a coincidence.)
Power and or Pressure levels quoted are only for purposes of explanation and have no relation to any real-world engines.

- - -
Manifold Air Pressure (MAP) is the Absolute Pressure of the gasses within the intake Manifold relative to a perfect vacuum.
When the engine is stopped at sea level the MAP will be the same as the ambient pressure outside of the engine. (~30"Hg or 15psi).

Now the engine is started and the throttle is set to idle.
The throttle Valve in the Carburetor or Throttle Body restricts the intake air flow an the MAP drops to some low level. (~10").
The descending piston acquires a Fuel/AIr load of ~5 PSI Absolute Pressure. A partial vacuum relative to ambient.
The intake valve closes and the piston compresses the fuel/Air charge 10:1 to 50 PSI.
The spark plug fires and the Fuel Air charge heats up producing a pressure increase of 15 times, (~750 Psi) Forcing the piston down during the Power Stroke.
At 2000 rpm this produces 4-5Hp.
About half the power is used up moving the piston through the other 3 strokes and the other half moves air through the Prop. The engine is at idle and barely able to make the Plane Taxi.

Now we go Wide Open Throttle (WOT)!
The throttle Valve in the Carburetor or Throttle Body Opens fully and restricts the intake air flow very little and the MAP Approaches Ambient pressures. (~28").
The descending piston acquires a Fuel/AIr load at ~14 PSI Absolute Pressure.
The intake valve closes and compresses the fuel/Air charge 10:1 to 140 PSI.
The spark plug fires and the Fuel Air charge heats up producing a pressure increase of 15 times, (~2100 PSI) Forcing the piston down during the Power Stroke.
At 5800 rpm this produces about 105Hp. About 5 hp is used to keep the engine cycling, the other 100Hp is used to move air through the Prop. Off you Go!
The engine is at Full power able to support flight!.

Remember that Power is Energy converted to Work over time.
And Energy is proportional to Force(Torque(MAP)) time Velocity (RPM) Squared.

2100/750Psi = 2.8 times the Torque... and... 5800/2000RPM = 2.9 times the RPM
Power = 2.8 x 2.9 x 2.9 = 23.5 time the Power in this example.
4.5hp increases to 105hp with only a 2.9 time RPM change.
Changing the RPM by only a factor of 3 increases the Power by a factor of 27.
The engine pushes 3 times the amount of air, 3 times as fast in 1/3 the time. 3² =27

Now keeping in mind that an engine turning at 5800rpm makes almost 100 rotations per second.
That means the piston goes from Top Dead Center (TDC) to Botton Dead Center (BDC) in roughly 0.005 seconds. (5 Milliseconds).
Also, keep in mind that the Fuel /Air mixture does not instantaneously ignite but takes a small but finite amount of time to fully be consumed.
If the Sparkplug were to fire at or a little beyond TDC the Fuel would not be completely expended by the time the piston reached BDC and the exhaust valve started to open.
Fuel would be wasted and energy lost.

To partially overcome this problem the spark is fired about 20° Before TDC.
Therefore the pressure of combustion is increasing before the piston reaches TDC.
This causes extra forces to be applied to the crankshaft and connecting rods as they continue to compress a fuel load that is already burning.
There is a limit on for much abuse the parts can take.
At High RPMS the compression time is very short and the pressure does not peak until well after TDC.
At Lower RPMS the pressure might peak right near or before TDC. Very Bad!

The MAP needs to be kept lower at lower RPMs In order to limit the ultimate force being applied to the piston as it is compressing the Fuel/Air mixture.

Running the engine at High RPMs and Low MAP is easy on the Engine.
Running at LOW RPMs at High MAP causes extreme forces and ultimately damage to the engine.
- - -
Engine power is ultimately proportional to the amount of fuel burned. It is just physics.
Forget the conspiracy theorist that think we could make a 100hp engine that uses a gallon an hour.
It will never happen.

A modern well-tuned Gasoline engine can deliver about 18Hp per Gallon per Hour.
That is why our 100 hp engines running at a 75hp cruise use just over 4 gallons per hour (gph).
It is just the physics of Gasoline. No way around it. (75/18 = 4.1gph ... 100/18 = 5.6gph)

There is NO Advantage in keeping the RPMs Down!
75 HP is 75Hp no matter how you make it!
5500 RPM at 23 MAP might give you 75HP.
4800 RPM at 26 MAP might give the same 75HP.

Both settings will burn the same amount of fuel. 75Hp Worth!
The 5500RPM setting burns smaller amounts of fuel in more cycles.
The 4800 RPM Setting burns larger amounts of fuel in fewer cycles.

The 5500RPM Setting has lower Cylinder Pressures and a long lasting reliable engine.
The 4800 RPM Setting has extreme Cylinder Pressures and a very short engine life.

MAP is the Grim Reaper.
Keep Your RPMs Up and you MAP Down.
Your engine will thank you!

Bill Hertzel
Rotax 912is
North Ridgeville, OH, USA
Clicking the "Thank You" is Always Appreciated by Everyone.

Thank you said by: Wayne Fowler, RotaxOwner Admin, Andrei Zincenco

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Re: Need advice - in flight adjustable prop

by John Fleming » 6 years ago

John Fleming

15 92 111

HOLY FREAKING CRAP! THAT... is EXACTLY what I was looking for! Man.... thank you for taking the time to explain that to me, and NOW, I have a good understanding of what is going on!

That has painted a perfectly clear picture of what is happening, and how MAP relates to what is happening inside that donk when she is running - Whereas before, I saw it the other way around (MAP was BECAUSE what was happening in the engine)

Now I know, the higher the MAP pressure, the higher the pressure in the cylinder. The higher the MAP, then the more fuel is being shoved into her, and obviously loads of fuel at lower RPM is not going to end well.

Again, thanks so much. That has completely shone a light on it all for me.

Cheers
John

Thank you said by: Grant Farrow

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Re: Need advice - in flight adjustable prop

by John Fleming » 6 years ago

John Fleming

15 92 111

UPDATE!

So, more flying done now with playing with the prop - and I can tell you, the aircraft really gets along when you get it all sorted. :)

On full fine she sits at about 5500 on takeoff and climb out. Hard to tell exactly as the RPM is on the other side of the dash, and I am too busy flying to get a really good look.

Yesterday I had her at almost WOT and coursed up to sit at about 5300, and it was REALLY getting along - around 120.

Anyways, I would like to thank all of those that contributed to this discussion! Nothing beats education, and it's great to get that from very experienced people whom re willing to spend the time.

Much appreciated.
John

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Need advice - in flight adjustable prop

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