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Carburettor HEAT!


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ADD HEAT = Reduce Power?

  • SO.. For those who follow...
  • the addition of Carb heat to a piston Engine..
  • will it or will it not reduce the power available.
  • when in flight.
  • AS it is shown on the MAP Gauge the addition of heat makes the MAP increase,
  • on the ground
  • the RPM WILL drop as you check the "carb heat" as part of the start up..
  • SO.. Beside temporiarily enriching the forum mixture.
  • Please add thoughts HERE!

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carb heat makes the air going into the combustion chamber less dense, since cooler air is more dense than warm air. for an explosion, you need fuel and air. when the temp goes up, there is less air in the chamber for the explosion. so, there is a definite decrease in power with carb heat applied.

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Adding carb heat does not increase MAP. If you have a governor and it is on, it increases MAP to maintain RPM, or if your gov is off, the loss of RPM will cause the manifold pressure to increase slightly because at a lower RPM, there is more time for air to enter the induction system. Also, if there is ice, the manifold pressure may increase do to a melting of the ice.

 

Try adding carb heat in an airplane with a constant speed propeller and within the governing range. The MAP won't change one bit.

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Guest pokey
Try adding carb heat in an airplane with a constant speed propeller and within the governing range. The MAP won't change one bit.

 

& if ya do a mag check in the governing range, you will notice that your mags are working flawlessly---no drop at all ! :P

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& if ya do a mag check in the governing range, you will notice that your mags are working flawlessly---no drop at all ! :P

 

This is true!

Edited by RockyMountainPilot
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Adding carb heat does not increase MAP. If you have a governor and it is on, it increases MAP to maintain RPM, or if your gov is off, the loss of RPM will cause the manifold pressure to increase...

 

RMP, just want to clear some things up here. I have read your post many times, but I can't get it. It's been a long time since I flew pistons.

 

Your first statement seems to contradict both of the governor scenarios you give. You say both lead to an increase in MAP. Are you simply suggesting by this that the carb heat does not 'directly' change the MP, but the resulting change of MP is an indirect consequence?

 

Then I'm confused. Does the MP really increase with the govenor on when carb heat is applied? I thought govenor would open throttle, to maintain RPM thus MAP would remain the same.

 

Try adding carb heat in an airplane with a constant speed propeller and within the governing range. The MAP won't change one bit.
Is this not the same as a governed piston helicopter when the collective is taken out of the equation?

 

What am I missing?

 

Sorry for being dumb, I always read this board right after waking up! Sometimes takes time to get the brain working.

 

Joker

Edited by joker
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Adding carb heat does not increase MAP. If you have a governor and it is on, it increases MAP to maintain RPM,

 

 

Assumes of course that you have more MP for the engine to give. Flying along at 7000 feet, you may not. Just a technicality..I know...nit picking.,

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RMP, just want to clear some things up here. I have read your post many times, but I can't get it. It's been a long time since I flew pistons.

 

Your first statement seems to contradict both of the governor scenarios you give. You say both lead to an increase in MAP. Are you simply suggesting by this that the carb heat does not 'directly' change the MP, but the resulting change of MP is an indirect consequence?

 

Then I'm confused. Does the MP really increase with the govenor on when carb heat is applied? I thought govenor would open throttle, to maintain RPM thus MAP would remain the same.

 

Is this not the same as a governed piston helicopter when the collective is taken out of the equation?

 

Carb heat simply increases the temp and reduces the density of the air entering the engine. It doesn't do anything to the pressure. When you apply carb heat with the governor on, you reduce the power the engine can produce as that MAP, so the governor increases throttle thus increasing MAP to maintain RPM. With the governor off, carb heat reduces the power and causes the RPM to fall. Because the engine is not rotating as fast, it is not trying to suck in as much air for a given amount of time allowing the MAP to increase slightly because the outside pressure, which is greater than the MAP, now has more time to push its way into the engine. Again, the carb heat doesn't cause the MAP to change, it causes the reduction in power, which causes the RPM to drop, which then causes the MAP to rise slightly.

 

In an airplane with a constant speed propeller, the governor only changes the pitch of the blades to maintain RPM. When you pull carb heat, the power is reduced and the engine wants to slow, but the governor decreases the pitch of the blades to require less power, thus maintaining RPM. The MAP won't change a bit when carb heat is applied unless you have an obstruction. Some airplanes bypass the filter when carb heat is applied, so the MAP might rise slightly because of less resistance, but again, thats not because of the heated air itself. The R22 and R44 carb heat still goes through the filter.

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RMP,

 

That's a much clearer explanation.

 

As you've probably guessed, my question was largely rhetorical ;) . I wouldn't like anyone to get confused by your first statement there.

 

Yes, manipulating the carb heat only indirectly causes the changes in MAP as a consequence of the power changes. The actual mechanism doesn't do anything to change pressures, if you like.

 

There is just this one last sentence which could also confuse folks:

 

If you have a governor and it is on, it increases MAP to maintain RPM

 

As for the constant speed propellor example, I got it the wrong way round. It's like the helicopter with the collective being adjusted to maintain a set RPM and thus MAP. So the RPM is governed purley by pitch and not at all by throttle (as in a governed helicopter)? Thanks for the clarification on that point.

 

For anyone wanting to read more, this topic has been raised on this board within the past year. A search will reveal a few interesting threads on manifold pressure!

 

Manifold Pressure Thread

 

Manifold Pressure Sucks!

 

Joker

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