Clearing the engine

asechrest

asechrest

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asechrest
I was up today doing some air work after about a two month hiatus. I did a simulated engine failure from 3,500 ft down to about 800 ft. I recall my instructor "clearing the engine" every 30 seconds or so after pulling it to idle for the maneuver. I did the same, but I'm not sure why.

What exactly are we doing here? I wonder why the engine needs clearing, what the clearing is attempting to prevent, and whether it is actually effective.

Thanks in advance.
 
asechrest said:
I was up today doing some air work after about a two month hiatus. I did a simulated engine failure from 3,500 ft down to about 800 ft. I recall my instructor "clearing the engine" every 30 seconds or so after pulling it to idle for the maneuver. I did the same, but I'm not sure why.

What exactly are we doing here? I wonder why the engine needs clearing, what the clearing is attempting to prevent, and whether it is actually effective.

Thanks in advance.
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When the engine is on the idle circuit they tend to load up. ( go rich and foul plugs)
 
Tom-D said:
When the engine is on the idle circuit they tend to load up. ( go rich and foul plugs)
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Or in some cases, like the Continental engines on older Cessna singles, develop carb ice. At idle power, there may not be enough heat generated to keep it from forming even with carb head full on, so advancing power briefly generates the necessary heat. Otherwise, when you advance power to go around at 500 feet or less, you may end up really making that emergency landing.
 
More a requirement in carburetted engines than in fuel injected ones
 
Ron Levy said:
Or in some cases, like the Continental engines on older Cessna singles, develop carb ice. At idle power, there may not be enough heat generated to keep it from forming even with carb head full on, so advancing power briefly generates the necessary heat. Otherwise, when you advance power to go around at 500 feet or less, you may end up really making that emergency landing.
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Why would idle power make enough heat on the ground and not in the air?
 
Tom-D said:
Why would idle power make enough heat on the ground and not in the air?
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It wouldn't another which is a reason why we check the carb heat during the runup, on a cold day you'll (hopefully) get any ice out using the warmer runup air before you takeoff.

Ron Levy said:
...with carb head full on...
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Geez, I must be getting old. Is that what they're calling it these days? :p :rofl:
 
The idle circuit in a MA3 carb is a fixed oriface it will deliver a constant flow of fuel no matter where the aircraft is. burning that amount of fuel will create the same amount of heat in the air as on the ground.

The only difference in flying and operating on the ground is the MAP, on the ground the prop is moving air, flying (descending) air is trying to move the prop, thus manifold pressure changes, changing the fuel flow. that's the answer to the question I asked Ron.

"Why would idle power make enough heat on the ground and not in the air?"
 
There are two other variables.

1. In the air, there is 65 KIAS air flowing around the engine. It's a lot less on the ground.

2. It's usually colder at altitude.

In a carb, the venturi is the problem, not the throttle. People who claim otherwise forget that injected engines have throttles, too. But not venturis.
 
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A windmilling prop will also generate a larger pressure drop across a closed throttle plate and generate ice faster than at idle on the ground. As already mentioned, there's more airflow through the cowling, too, and the carb body can get colder. And those exhaust pipes can get a lot colder, too.

But opening the throttle for three seconds doesn't heat the pipes much and it certainly doesn't give the heat any time to remove any accumulated carb ice. There are some dismaying statistics on practice forced approaches that turned into the real thing when the carb iced up. AOPA has had some articles on that.

Dan
 
MAKG1 said:
In a carb, the venturi is the problem, not the throttle. People who claim otherwise forget that injected engines have throttles, too. But not venturis.
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The throttle plate can generate ice when it's closed. There's a significant pressure drop, and therefore temperature drop, at the small gap between the plate and carb wall. Injected engines are not immune to icing, just resistant to it.

Dan
 
Tom-D said:
The idle circuit in a MA3 carb is a fixed oriface it will deliver a constant flow of fuel no matter where the aircraft is. burning that amount of fuel will create the same amount of heat in the air as on the ground.

The only difference in flying and operating on the ground is the MAP, on the ground the prop is moving air, flying (descending) air is trying to move the prop, thus manifold pressure changes, changing the fuel flow. that's the answer to the question I asked Ron.

"Why would idle power make enough heat on the ground and not in the air?"
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It doesn't. That's why the carb heat check isn't done at idle.
 
EdFred said:
It doesn't. That's why the carb heat check isn't done at idle.
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If you don't think there is enough heat there, walk up and put your hand on a 0-200 exhaust pipe, that will make a believer out of ya.

we operate the 0-200 on the ground at idle and it heats the carb really well. go cold just before full power for take off.
 
Dan Thomas said:
The throttle plate can generate ice when it's closed. There's a significant pressure drop, and therefore temperature drop, at the small gap between the plate and carb wall. Injected engines are not immune to icing, just resistant to it.

Dan
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If you form ice between the throttle plate and TB wall, it's trivial to get rid of it. Open the throttle and you're done. Immediately. It will never make the engine quit while you can still move the throttle.

It's ice in the venturi that screws you. In effect, you can't open the throttle because the throat has closed off already.

I find it a bit surprising how many pilots don't understand what a venturi does. The car racing folks have it down. There is always a temperature drop across a venturi. A really big temperature drop if it's a good venturi. Even at full throttle -- the difference there is that quite a lot of engine heat is available.
 
MAKG1 said:
There are two other variables.

1. In the air, there is 65 KIAS air flowing around the engine. It's a lot less on the ground.

2. It's usually colder at altitude.

In a carb, the venturi is the problem, not the throttle. People who claim otherwise forget that injected engines have throttles, too. But not venturis.
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What is a venturi?
 
Skylane81E said:
What is a venturi?
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It's a smooth restriction in the carburetor usually ahead of the throttle (but it can be behind), used to suck fuel in proportion to airflow. Higher speed leads to lower pressure and lower temperature. Pressure is the point, temperature is a byproduct.

Injected engines just have a high pressure fuel pump instead. For a carburetor, the fuel pump only puts fuel in the bowl, usually at very low pressure. The venturi allows the throttle to effectively control fuel flow through the air volume, whereas injection has to have some other way to meter it using the throttle (generally, some form of shuttering tied to cam position rather than pressure).

Venturis are more generic than that -- a wind tunnel is another application -- but the carburetor is the most relevant here.
 
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Just a genetic definition is fine, is a throttle plate set to idle not a venturi?
 
Skylane81E said:
Just a genetic definition is fine, is a throttle plate set to idle not a venturi?
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No. It is not smooth and does not return to ambient pressure. Venturis depend critically on laminar airflow. Throttle plates are seldom laminar.

There is an angle of attack effect here just like for a wing. Throttles are always stalled at low position (they may or may not be at WOT). Venturis in a stalled condition do not work.

The throttle also has nothing to do with fuel. It meters air. In a carburetor, it meters mixture, but that mixture is set elsewhere. With fuel injection, the injectors are usually (well) behind the throttle, as it's a lot easier to design an air-only intake.
 
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In a carb it is very much a venturi, even drawing the fuel in. Not a very good one though.
 
oldShar said:
More a requirement in carburetted engines than in fuel injected ones
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Yes, there's a double the threat for FI engines compared to carbureted, but there's still the problem of loading the plugs in FI engines unless you lean way back as well as pull the throttle back, and I'm talking leaning to the point that the engine will choke if you advance the throttle without first advancing the mixture. Better, I think, to leave the mixture at a cruise lean position and clear periodically, even with a FI engine.
 
MAKG1 said:
The throttle also has nothing to do with fuel. It meters air.
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Look at this:

Idling.jpg


That idle port comes into play only when the throttle plate is closed or nearly so, because it generates a low pressure where that air is accelerated past the edge of the plate.

Therefore, it offers a pressure and temperature drop, and that throttle plate is involved in metering idle fuel flow.

Dan
 
Skylane81E said:
In a carb it is very much a venturi, even drawing the fuel in. Not a very good one though.
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We all know where velocity increases the pressure decreases, but in the case of the throttle valve creating a Venturi is not what draws fuel into the intake system when the throttle valve is closed. It is the low pressure in the intake system that draws fuel thru the idle circuit to the intake pipes/spider/what ever.

when that pressure changes so does the fuel flow. It's sorta like sucking on a soda straw, such hard get a lot, suck softly get a little.
 
Tom-D said:
We all know where velocity increases the pressure decreases, but in the case of the throttle valve creating a Venturi is not what draws fuel into the intake system when the throttle valve is closed. It is the low pressure in the intake system that draws fuel thru the idle circuit to the intake pipes/spider/what ever.

when that pressure changes so does the fuel flow. It's sorta like sucking on a soda straw, such hard get a lot, suck softly get a little.
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Yes, I was neglecting the idle circuit.

It's not a venturi. Venturi vacuum is highest at WOT, manifold vacuum at closed throttle.

The venturi effect only works on streamlines. Good luck following those across an idle throttle plate...it's pretty easy on an actual venturi.
 
Neal Howard said:
You don't have a 80-100 MPH cold wind blowing thru the cowling while at idle on the ground ;)
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With a pressurized cowl you will not get 80-100 mile winds in the cowling. There is some thing in there that blocks that, can you tell me what that would be ?

And when it gets thru that restriction it isn't cold anymore.
 
Tom-D said:
The idle circuit in a MA3 carb is a fixed oriface it will deliver a constant flow of fuel no matter where the aircraft is. burning that amount of fuel will create the same amount of heat in the air as on the ground.
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Slightly inaccurate. Same amount of fuel does not mean same amount of heat. You are oversimplifying the combustion process.
 
mtuomi said:
Slightly inaccurate. Same amount of fuel does not mean same amount of heat. You are oversimplifying the combustion process.
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Kinda moot,, there is never the same amount of fuel.
 
mtuomi said:
I thought you meant the idle circuit is a fixed orifice delivering a constant amount of fuel regardless?
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No, read the whole response in post 9.

I often ask the obvious question, and often find we have a bunch of pilots that have different answers.
 
MAKG1 said:
It's a smooth restriction in the carburetor usually ahead of the throttle (but it can be behind), used to suck fuel in proportion to airflow. Higher speed leads to lower pressure and lower temperature. Pressure is the point, temperature is a byproduct.

Injected engines just have a high pressure fuel pump instead. For a carburetor, the fuel pump only puts fuel in the bowl, usually at very low pressure. The venturi allows the throttle to effectively control fuel flow through the air volume, whereas injection has to have some other way to meter it using the throttle (generally, some form of shuttering tied to cam position rather than pressure).

Venturis are more generic than that -- a wind tunnel is another application -- but the carburetor is the most relevant here.
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Actually the RSA FI systems found on most Lycomings uses differential air pressure (inlet vs. manifild) AND differentual fuel pressure (metered vs. regulated) to time, measure and modulate fuel delivery to the injector nozzles. The TCM FI system on Continentials uses differential fuel pressure only to determine the amount of fuel delivered to the nozzles for a given throttle plate opening.

Chris
 
Hiperbiper said:
Actually the RSA FI systems found on most Lycomings uses differential air pressure (inlet vs. manifild) AND differentual fuel pressure (metered vs. regulated) to time, measure and modulate fuel delivery to the injector nozzles. The TCM FI system on Continentials uses differential fuel pressure only to determine the amount of fuel delivered to the nozzles for a given throttle plate opening.

Chris
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The RSA system:

RSA_Models5ad1_5ab1_10ad1_1300.jpg


Note the venturi that generates a pressure drop proportional to the inlet velocity.

Dan
 
MAKG1 said:
No. It is not smooth and does not return to ambient pressure. Venturis depend critically on laminar airflow. Throttle plates are seldom laminar.

Actually a venturi can be anything that causes a change in velocity/pressure. There are ring venturis, slat venturis, single-side-double-bump venturis, etc. A throttle blade in an MA4 at idle allows a limited amount of air to flow from a large space (in front of the throttle blade) thru a small opening (the side of the throttle bore and butterfly) and into another large space (the other side of the throttle blade). The fixed idle orfice(s) are located in the area of low pressure/high velocity formed by this small opening. As soon as the throttle buterfly opens up and the vacuum is lost the fuel in the idle orfice stops flowing due to lack of venturi action.

There is an angle of attack effect here just like for a wing. Throttles are always stalled at low position (they may or may not be at WOT). Venturis in a stalled condition do not work.

The throttle butterfly (throttle valve on the A&P test) is merely a variable opening in the carb throat; it simply allows more or less air into the engine. You have ambient pressure on one side and vacuum from the cylinder intake stroke(s) on the other. At idle it creates a low pressure area at the idle fuel orfice but off-idle it's nothing more than a door.

The throttle also has nothing to do with fuel. It meters air. In a carburetor, it meters mixture, but that mixture is set elsewhere. With fuel injection, the injectors are usually (well) behind the throttle, as it's a lot easier to design an air-only intake.
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The throttle butterfly allows air to be pulled into the carb past the venturi in varing amounts. At idle there is not enough air going thru the venturi (where the main discharge nozzle is located) to pull any gas from the float bowl, hence the need for a seperate idle path. As the butterfly is opened the air volume (velocity) thru the venturi increases the pressure decreases and more fuel is pulled from the float bowl thru a fixed main metering jet. It is the opening and closing of the throttle that varies the amount of air going thru the venturi and therefore the amount of fuel being pulled out of the main fuel nozzle (up to the full-throttle point where you have maximum throttle opening, maximum airflow thru the venturi, maximum pressure drop at the venturi and the maximum amount of fuel allowed by the main metering jet in the float bowl).

In a FI setup the fuel and air take totally different paths; the air goes thru the injector servo and the fuel goes thru a regulator system to the injector nozzles in each intake valve pocket. There is no fuel/air mixing until each intake valve opens.

There is an evil "hybrid" system that is the pressure carburetor...
they provide fuel to the engine using pressurized discharge nozzles (no fuel bowl or float chamber) that use a complex metering system to determine fuel requirements. They have their fuel nozzles AFTER the throttle valve but DO mix the fuel/air prior to the intake plenium.

Chris
 
Note the venturi that generates a pressure drop proportional to the inlet velocity.

Dan

You're right.
How 'bout "inlet vs. venturi" differential pressure?


Chris:(
 
Hiperbiper said:
Note the venturi that generates a pressure drop proportional to the inlet velocity.
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Yup. It's using a venturi as a mass air sensor. It's a nice alternative to throttle linkages, as it would require far less adjustment.

But ice in that venturi would cause some "interesting" and not very nice behaviors. Like, increasing throttle may not increase fuel flow at all, leading to a lean stumble.
 
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