Why using carb heat at low power settings

[slbird]

I'm doing my ppl in a 150. My instructor told me to always apply carb heat below 2000rpm because of the lower pressure in venturi. My question is, why does a low power setting cause the pressure to drop?

Thanks for your help

[pdw]

Good question. There's always low pressure in the venturi when running.

When you're descending for example ... you're also less than 2000rpm ... so then really got to watch for the carb ice at the lower power; because suddenly advancing power after being "below" for a while for any reason, if there IS ice presence then it will let you know that its already a problem (engine misses or worse) because of the increased airflow. Ice blocks airflow, but everything may have been O.K. at the slower flow / "low power". The intake icing starts to build but might not cause the engine to miss. Increase that intake airflow to the engine by adding full power (to speed up or whatever) will make it miss then if there's enough there to be a problem.

[photofly]

My question is, why does a low power setting cause the pressure to drop?

When you throttle back to a low power setting you restrict the amount of air that's allowed to flow into the induction system. Since the engine is still pumping air through the cylinders, the pressure in the induction system drops.

[pelmet]

My question is, why does a low power setting cause the pressure to drop?

When you throttle back to a low power setting you restrict the amount of air that's allowed to flow into the induction system. Since the engine is still pumping air through the cylinders, the pressure in the induction system drops.

I'd say because the throttle plate is now in a more closed position. Therefore, less ice is required to create a situation make it stuck in position, attached to the throat of the carb by ice formation. At very low power settings, the circular plate is almost completely closed and just a small space exists between the plate and the throat of the carb.

Throttle at high power:
https://www.google.ca/search?q=carb+ice ... B397%3B529

Throttle at low power:
https://www.google.ca/search?q=carb+ice ... B397%3B529

[gaamin]

Simplified...

The carburetor is a venturi. When air goes through the narrow section, pressure and temperature drop. The lower pressure sucks fuel through the spray nozzle.

The throttle controls the size of the carburetor opening, allowing more or less flow to the cylinders. The narrower the opening, the easier it is to clog. Also, the narrower the opening, the faster the air needs to accelerate through it, resulting in localized even lower and pressure, so a higher risk of having condensation and ice.

The O-200 engine present on many C150 is rather prone to carburetor icing.

"always" put it on when below a certain RPM : not necessarily. A very warm or very dry air mass would not require use of carb heat.

When you use carb heat, you feed warmer air to the carburetor. Warmer means less dense, so it contains less oxygen. Your fuel/oxygen ratio becomes richer, so your cylinders will be cooler, potentially resulting in spark plug fouling. Keep mixture control in mind for a prolonged descent or when on the ground at low RPM.

[slbird]

Thanks for your replies.

My question is, why does a low power setting cause the pressure to drop?

When you throttle back to a low power setting you restrict the amount of air that's allowed to flow into the induction system. Since the engine is still pumping air through the cylinders, the pressure in the induction system drops.

But when you throttle back the engine is also pumping air at a slower rpm..

[slbird]

I'd say because the throttle plate is now in a more closed position. Therefore, less ice is required to create a situation make it stuck in position, attached to the throat of the carb by ice formation.

Then can you clear the ice by changing throttle positions?

[CpnCrunch]

Thanks for your replies.

My question is, why does a low power setting cause the pressure to drop?

When you throttle back to a low power setting you restrict the amount of air that's allowed to flow into the induction system. Since the engine is still pumping air through the cylinders, the pressure in the induction system drops.

But when you throttle back the engine is also pumping air at a slower rpm..

You get a greater temperature drop when the throttle is closed, so you'll get carb ice at higher temperatures.

However you are correct - there is less air flowing through the system at a lower rpm, so if conditions are ripe the ice will form a lot more quickly at higher rpms. The only times I have ever had carb icing have been at full throttle.

As far as I can determine, the temperature drop is about 5-10C at full throttle, and higher than that at lower rpms. At least, that is based on my very limited experience with carb ice. I did a bunch of searching a while ago for research on this, but it doesn't seem to exist. All it says is that the drop can be "up to 30C" (which is presumably at idle rpm).

[photofly]

But when you throttle back the engine is also pumping air at a slower rpm..

There's not really any "but" about it. If the pressure wasn't lower, the engine would draw the same volume of air and fuel into the cylinders and the rpm and power wouldn't have changed. The only way to reduce the engine power is to reduce the pressure in the inlet manifold which is achieved by throttling the inlet.

[pelmet]

Thanks for your replies.

My question is, why does a low power setting cause the pressure to drop?

When you throttle back to a low power setting you restrict the amount of air that's allowed to flow into the induction system. Since the engine is still pumping air through the cylinders, the pressure in the induction system drops.

But when you throttle back the engine is also pumping air at a slower rpm..

But the gap between the throttle plate and the throat is smaller so the speed at which air is moving past the throttle plate is not necessarily slower.

[PilotDAR]

The only way to reduce the engine power is to reduce the pressure in the inlet manifold which is achieved by throttling the inlet.

Yes, but that lower pressure occurs downstream of the throttle plate, any carb ice which was going to occur would occur upstream of the throttle plate, where the air pressure is much closer to ambient (unless you have a blocked intake filter and have not opened alternate air (carb heat). There is a larger pressure drop across the venturi (thus greater potential to cool the air, and freeze the moisture) at high poer settings rather than lower power settings. Just the air is going through faster, and has less time to hang around the venturi in turbulence and freeze there out of boredom.

I use carb heat: To confirm it works during run up, If I suspect carb ice is forming in flight, or if my carb air temp indicator is just warmer than freezing. I do not use it any other time. I do not selected it every half hour in flight, and I do not use it on approach, other than the foregoing circumstances. I have owned an O-200, for 27 years, and an O-360 for 6 years. I have never had a carb icing problem in flight. I belief that instructors like to move controls by rote, so carb heat gets selected whether needed or not on approach.

Decades ago, on a humid day in the 150, I did think I had carb ice. I pulled carb heat - it quit, and I force landed. I had not had carb heat at all. Just at full power, there was enough ummph of induction air to pull the fibreglass insulation mouse nest from the hot air hose, into the venturi, and that was that. This had not occurred during a partial power run up check (I probably just loosened it a little). Since then, and after admonishments during piston engined helicopter training, I do not un-necessarily move engine controls!

Follow the flight manual for your aircraft, and follow your best judgement, but the foregoing is mine...

[photofly]

I was just answering the original question which was "why does a low power setting cause the pressure to drop?" - it wasn't originally a question about carb icing.

[pdw]

I mistook the title for a question ...

[PilotDAR]

Ah, a "use of carb heat" question thread drifted to discuss carb ice. A manageable drift, I think.

[slbird]

I'm still quite confused of what's happening inside the carburettor...

The only way to reduce the engine power is to reduce the pressure in the inlet manifold which is achieved by throttling the inlet.

I know that as outside air passes through the venturi, a low pressure area is created and fuel is vaporized as a result. Is it true that the lower the pressure we have in the venturi, the easier it is to vaporize fuel and the more engine power we can get? So a lower pressure can have opposite effects inside the carburetor and the inlet manifold? How are they related?

I read from FGU that regardless of altitude, the amount of fuel entering the carburetor remains the same for any given throttle setting if the mixture control is unchanged. Can someone explain why does that happen? Is it because for a given throttle setting, the amount (weight) of air entering the carburetor is the same?

Thanks.

[slam525i]

slbird, check your PM