Why does MAP increase with power?

[Simonta]

Hi all

I am a low time PPL(H). I know that I am about to be embarassed in a few replies time...

Why does MAP increase with power output? My logic is:

Butterfly opens, more air is drawn in, flow accelerates through the venturi, pressure drops, more fuel is drawn in, more power produced.

If the pressure in the venturi drops, then doesn't the pressure in the manifold also drop because the same mass of air is now in a larger volume?

I've asked several experienced rotorheads and received varying explanations, none of which seemed convincing.

Thanks for any enlightenment.

Cheers

Simon

[dragman]

Sorry, just a quick reply here. The pressure in the venturi may drop as flow is accelerated, but the manifold pressure gauge is measuring the pressure in the inlet manifold just before the fuel/air charge enters the cylinder, it has already passed through the venturi.

The bigger and heavier and fatter the fuel/air charge, the more pressure in the inlet manifold, the more power produced by the engine.

Hope that makes sense for you.

Dragman

[dragman]

Probably should have mentioned too that pressure drops through the venturi as flow speeds up and fuel is mixed with air. Upon leaving the venturi, flow slows down pressure increases.

[RotorRunner]

The engine has a fixed displacement. Therefore it is constantly trying to draft a certain amount of air through the intake. The airflow is being restricted by some means (Butterfly, slidevalve,etc). Due to the engine attempting to draft more air than the butterfly is allowing to pass through, there is a "partial vacuum", or a pressure lower than atmospheric at the intake port. As the butterfly opens more and more, the restriction is lessened allowing the MAP to closer approach atmospheric pressure

[ShyTorque]

Don’t get confused by the venturi; it’s only there to cause a low pressure region, so that the fuel can be drawn from the jet/jets into the throat of the carburettor.

The throttle butterfly plate is situated after the venturi and the MAP is measured after the throttle plate; for a non-supercharged engine at near constant rpm, the position of the throttle is what determines the MAP.

It might help to think of the engine as a fuel/air pump (that’s what it is), powered by the fuel air mixture itself.

The engine is trying to pump / suck all the air/fuel from the inlet manifold, but the atmosphere tries to replace it. If the throttle is closed, the engine wins and the MAP reduces. If the throttle is wide open, the atmosphere catches up and increases the MAP.

Increased power doesn't result in higher MAP, it's more correct that a higher MAP gives more power.

[imabell]

why does the speedo in your car go up when you accellerate?????

why does the altimeter go up when you climb?????

why do your tyres go down when you get a puncture?????

why do you get a hangover when you drink too much????


do do do do, do do do do. twilight zone stuff there my friend.:ok:;)

[k12479]

What RotorRunner said, but its more 'vacuum decreases' rather than 'pressure increases', being approx atmospheric at wide open throttle (unless turbo/supercharged)

[NickLappos]

The power a piston engine produces is made by the explosion of the fuel-air mixture. More fuel-air mixture in the piston means more power, and also more explosions per second means more power.

Manifold pressure measures the density of the air that the engine gets, so it measures the amount of oxygen that the fuel interacts with. More manifold pressure means more mass of oxygen per cubic centimeter. More oxygen per cubic centimeter, more power. The fuel in a carborated engine is automatically mixed at the right proportion with the air, using the clever venturi, so no matter what the density of the incoming air, the fuel is properly measured for the varying density.

A turbocharged engine has an air pump that packs more air in each charge of the piston, so the power is even higher than possible with normal aspiration.

A factor that robs power is moisture in the atmosphere, since dissolved water displaces oxygen, so for the same MAP the power is less. In fog, where the moisture is 100%, the power is noticeably less for the same MAP.


My bad, the ppruners are right, the simple carbs on the Robbie and many simple helos are not fully density compensated, so they require manual leaning. The venturi does a gross job of density adjustment since the pressure drop across the venturi that draws up the fuel is proportional to the mass and velocity of the air flowing through the venturi (1/2Rho V squared!), but the effect is not perfectly compensated, so we must manually fine tune the fuel flow. Density compensated carbs are more expensive, and also harder to maintain - most WWII piston aircraft had them, BTW.

[i4iq]

"The fuel in a carborated engine is automatically mixed at the right proportion with the air, using the clever venturi, so no matter what the density of the incoming air, the fuel is properly measured for the varying density."

So, why do we say that the fuel-air mixture is richer with carb heat supplied... ?

[Chickenhawk1]

cos the air is less dense. 361 cubic inches of hot air contains less 'air' molecules than 361 cubic inches of cold air. Therefore by weight, the fuel hasn't changed but the air has decreased. So a richer mix.

[i4iq]

... so then "the fuel is properly measured for the varying density" is not correct, or am I missing something?

[Chickenhawk1]

Well, the higher the density of the air the greater the pressure drop at the venturi, which pulls in more fuel to keep a balanced mix.

[i4iq]

But if the mixture is richer, it can't be a balanced mix, right?

[Chickenhawk1]

You're right, that's why on the likes of the R22 you loose about 2% ERPM when you do your carb heat check on start up.

[i4iq]

Right.

But I know Nick knows what he's talking about so was wondering what I'm missing...

[ShyTorque]

I would have to contest the statement that a carburettor corrects the fuel mixture for changes in air density. (If it did, there would be no need for a mixture lever).

A carburettor is a dumb device, set up on the ground for one set of parameters.

Modern fuel injection is more sophisticated and does make corrections for changes in air density.

[Chickenhawk1]

Well, and I'm guessing here, but maybe the temperature (and therefore the density) of the exhaust heated air far surpasses the capabilties of the fuel atomiser. That air is much hotter I'd guess than (nearly) anything you'd find in nature. I mean if the air is that thin then the pressure drop will not be enough to vapourise all the fuel. Make any sense?

[NickLappos]

My bad, the ppruners are right, the simple carbs on the Robbie and many simple helos are not fully density compensated, so they require manual leaning. The venturi does a gross job of density adjustment since the pressure drop across the venturi that draws up the fuel is proportional to the mass and velocity of the air flowing through the venturi (1/2Rho V squared!), but the effect is not perfectly compensated, so we must manually fine tune the fuel flow. Density compensated carbs are more expensive, and also harder to maintain - most WWII piston aircraft had them, BTW.

[Chickenhawk1]

QED. Meet you all on the "Helicopter static Charge" thread...:ok:

[Backward Blade]

Nick, I have read a number of your posts and for the most part agree with each and everyone of them...HOWEVER please tell me that you are not infering that a person should adjust the leaning of the mixture while in flight with regards to Robbos. Say it ain't so. Fixed wing obviously different granted.

BWB

[NickLappos]

backward, see post 18 below!

Here is a bit more on the subject from a pretty good website:

"In spite of a number of variations in the different models of Lycoming power plants, there are some general recommendations on leaning offered for Lycoming piston engines except those with an Automatic Mixture Control (AMC) such as the Hiller UH12E, Bell 47 etc."

source: http://brumbyhelicopters.com.au/lean2normal.htm