C182P: Carb Temp in Cruise

[Henning]

I've never understood how to use CAT. I have the JPI carb temp probe going into an EDM-700 and it always show ambient minus a few degrees when the engine is running. Of course applying carb heat raises the temperature quickly. The probe is on the piston side of the throttle plate and I would have thought the air would have been a lot colder in the vacuum environment, maybe 50 degrees colder than ambient, which would tell a more useful story.

So if a CAT reads ~ ambient temperature only, as mine does, what is it telling me that my OAT gauge isn't?

The Carb Air Temp probe belongs on the down stream side of the throttle plate as that is where the pressure drop is as well as the fuel atomizing/evaporating both of which cause cooling. If this is registering the same as the OAT, you have something not right in the system.

 

[timwinters]

^^^that^^^

My CAT is probably 20 to 30 degrees below ambient at high power setting (haven't checked the actual different recently) but it's roughly equal to ambient as the engine is idling in the pattern when landing.

 

[JohnAJohnson]

The Carb Air Temp probe belongs on the down stream side of the throttle plate as that is where the pressure drop is as well as the fuel atomizing/evaporating both of which cause cooling. If this is registering the same as the OAT, you have something not right in the system.

^^^that^^^

My CAT is probably 20 to 30 degrees below ambient at high power setting (haven't checked the actual different recently) but it's roughly equal to ambient as the engine is idling in the pattern when landing.

That explains it. Thanks guys. Time to pull out my MA-4-5 manual.

 

[AggieMike88]

My CAT is probably 20 to 30 degrees below ambient at high power setting

This was my experience on this flight that started the thread. CAT was showing about 18, the OAT gauge was about 35-ish at 4500ftMSL

 

[MAKG1]

The Carb Air Temp probe belongs on the down stream side of the throttle plate as that is where the pressure drop is as well as the fuel atomizing/evaporating both of which cause cooling. If this is registering the same as the OAT, you have something not right in the system.

Not just the downstream side of the throttle. In the narrowest part of the venturi. Otherwise, it's just telling you that you're running at high throttle and a vacuum or MP gauge would do it better.

 

[Henning]

Not just the downstream side of the throttle. In the narrowest part of the venturi. Otherwise, it's just telling you that you're running at high throttle and a vacuum or MP gauge would do it better.

No, in the narrowest part of the venturi is where the air will be warmest because the pressure is highest. It's beyond there where the pressure drops and the fuel is introduced, both which drop the temperature significantly.

 

[MAKG1]

No, in the narrowest part of the venturi is where the air will be warmest because the pressure is highest. It's beyond there where the pressure drops and the fuel is introduced, both which drop the temperature significantly.

Review your Bernoulli.

And look in a carb and identify where the jet is.

Flow speed is highest in the venturi. Absolute pressure is lowest. For a symmetrical venturi, pressure before and after the venturi is the same. The lowered pressure is what pulls fuel in and keeps it proportional to air volume, and jets are typically mounted right in the middle.

I think you're confusing venturis with flow valves or orifices. Venturis make poor valves as the flow resistance is very small -- minimum for a given diameter.

 

[Henning]

Review your Bernoulli.

And look in a carb and identify where the jet is.

Flow speed is highest in the venturi. Absolute pressure is lowest. For a symmetrical venturi, pressure before and after the venturi is the same. The lowered pressure is what pulls fuel in and keeps it proportional to air volume, and jets are typically mounted right in the middle.

I think you're confusing venturis with flow valves or orifices. Venturis make poor valves as the flow resistance is very small -- minimum for a given diameter.

Jets themselves are in the bowl typically, but the tubes that feed the fuel are just downstream of the choke.

 

[MAKG1]

Jets themselves are in the bowl typically, but the tubes that feed the fuel are just downstream of the choke.

Which is where the venturi is.

There is little reason to engineer a large distance between choke and venturi. It just makes the carb heavier.

Why do you think carbs have venturis?

The real answer is to lower the pressure even at WOT to suck fuel into the carb.

If they actually raised pressure, they would be unnecessary. They aren't necessary if the fuel is injected under pressure, and FI engines don't have them. A TBI unit is essentially a carb with a fuel injector in place of the venturi.

 

[Cap'n Jack]

No, in the narrowest part of the venturi is where the air will be warmest because the pressure is highest. It's beyond there where the pressure drops and the fuel is introduced, both which drop the temperature significantly.

You have it wrong. The way you stated it, you are violating the conservation of energy. The increase in velocity of the air in the narrowest part of the Venturi comes with a corresponding decrease in static pressure.

 

[flhrci]

Paid a lot of attention to the carb temp today in the 182R I flew because of this thread. It hovered at +10C most of the flight at around 40 degrees F on the ground and I got up to 5500 MSL. In the pattern with carb heat on it hovered just above +20.

Just my observations and input to the thread.

David

 

[Dan Thomas]

Note that the total pressure remains the same. Static + dynamic equals total at any given point in the venturi.

The fuel nozzle is located at the point of lowest pressure. The idle discharge comes into play when the throttle plate becomes mostly closed, at which point the air flowing around the edge of the plate is accelerated, its pressure falls, and fuel is drawn from the idle port. It, too, depends on venturi effect. And that is why icing is at its greatest potential at low power settings. The pressure drop at a closed throttle, which is actually a tiny bit open, is large. So the temperature drop is, too.

Dan