Carb Heat in the Circuit
Joined: Jun 2003
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From: EuroGA.org
ShyTorque
I didn't say a carb measures mass flow (of the air). I said it measures some combination of volume flow and mass flow. It doesn't measure pure velocity either - I am sure air at a given velocity at 100,000ft will suck up a lot less fuel in the venturi than air at 10,000ft at the same velocity would.
If a carb (or a fuel injection system) measured pure air mass flow, that would be great because you could set up a particular engine operating point, e.g. peak EGT, during cruise climb, and just sit there all the way to the aircraft's operating ceiling (as the MP drops off) without any further leaning, and the engine would be running at peak EGT the whole time.
Altitude compensated carbs are an attempt to achieve this.
Car engines that operate at peak EGT have to have a mass flow sensor. This can be done by conventional air flow measurement aided by measurement of pressure and temperature. I don't think any old-style aircraft engine does this though.
The reference to 75% power comes from GAMI's tests on IO540 type engines. I would have expected it to be roughly right for other engines of similar design. It won't be easy to verify because it is said that one cannot tell detonation from the cockpit (pre-ignition is something very different). Yes it will depend on the engine load but that is what power is, at a given RPM. Can you give an example of a current-model engine which can be made to enter detonation at say 50% power?
I didn't say a carb measures mass flow (of the air). I said it measures some combination of volume flow and mass flow. It doesn't measure pure velocity either - I am sure air at a given velocity at 100,000ft will suck up a lot less fuel in the venturi than air at 10,000ft at the same velocity would.
If a carb (or a fuel injection system) measured pure air mass flow, that would be great because you could set up a particular engine operating point, e.g. peak EGT, during cruise climb, and just sit there all the way to the aircraft's operating ceiling (as the MP drops off) without any further leaning, and the engine would be running at peak EGT the whole time.
Altitude compensated carbs are an attempt to achieve this.
Car engines that operate at peak EGT have to have a mass flow sensor. This can be done by conventional air flow measurement aided by measurement of pressure and temperature. I don't think any old-style aircraft engine does this though.
The reference to 75% power comes from GAMI's tests on IO540 type engines. I would have expected it to be roughly right for other engines of similar design. It won't be easy to verify because it is said that one cannot tell detonation from the cockpit (pre-ignition is something very different). Yes it will depend on the engine load but that is what power is, at a given RPM. Can you give an example of a current-model engine which can be made to enter detonation at say 50% power?
Avoid imitations
Joined: Nov 2000
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From: Wandering the FIR and cyberspace often at highly unsociable times
This is now way off the original topic of carb heat, my apology for that.
IO540,
At 100,000ft a typical spark ignition engine or even a turbine would probably not be running at all.
However, to restate what I said, a simple carb does not measure anything. It just senses the difference between ambient and venturi pressures. The fuel jet is exposed to the depression in the venturi and is drawn out as an aerosol spray, mixed with the air and vapourised, either in the inlet manifold or partly in the cylinder. You could look at this from another viewpoint and say that fuel is pushed into the venturi by atmospheric pressure over venturi pressure.
More complex carbs, as you say, can be designed to compensate for altitude to some extent.
The simple carb doesn't know what altitude is, or what altitude it is operating at. Therefore, if the same differential pressure existed at your theoretical 100,000 feet, the same amount of fuel would be drawn out and into the engine because the fuel metering orifice / jet size is the same. However, because the density of the air at that altitude is a fraction of that prevailing at sea level, the mixture would become massively rich at such an altitude, there simply wouldn't be enough oxygen molecules to react with the fuel. The power output reduces with altitude because the airflow through the engine is less, although the engine is theoretically more efficient with increased altitude.
It isn't correct to generalise about when detonation might occur. Your quoted figures are undoubtedly correct for your engine as they will have been derived from complex experimentation by the manufacturer. It is a complex subject that depends on a number of factors such as cylinder size, combustion chamber shape, mixture strength, fuel quality, ignition timing, charge temperature, CHT etc. I agree it is definitely not the same as pre-ignition, I never said it was. It's not the same as auto-ignition either.
I can't give you an example of an engine that can be made to enter detonation at 50% power as I'm not an engine manufacturer, or someone with access to a library of such data. I do have some experience of piston engines, both building them, modifying them, using them and a while back, teaching student pilots about them. From my own experience, detonation may occur at any power setting if the conditions are suitable, certainly at smaller throttle openings than would give 75% power.
For example, in some car engines, you can hear the detonation "pinking" at very small throttle settings if the right conditions prevail. Opening the throttle further may actually result in the detonation no longer occurring.
IO540,
At 100,000ft a typical spark ignition engine or even a turbine would probably not be running at all.
However, to restate what I said, a simple carb does not measure anything. It just senses the difference between ambient and venturi pressures. The fuel jet is exposed to the depression in the venturi and is drawn out as an aerosol spray, mixed with the air and vapourised, either in the inlet manifold or partly in the cylinder. You could look at this from another viewpoint and say that fuel is pushed into the venturi by atmospheric pressure over venturi pressure.
More complex carbs, as you say, can be designed to compensate for altitude to some extent.
The simple carb doesn't know what altitude is, or what altitude it is operating at. Therefore, if the same differential pressure existed at your theoretical 100,000 feet, the same amount of fuel would be drawn out and into the engine because the fuel metering orifice / jet size is the same. However, because the density of the air at that altitude is a fraction of that prevailing at sea level, the mixture would become massively rich at such an altitude, there simply wouldn't be enough oxygen molecules to react with the fuel. The power output reduces with altitude because the airflow through the engine is less, although the engine is theoretically more efficient with increased altitude.
It isn't correct to generalise about when detonation might occur. Your quoted figures are undoubtedly correct for your engine as they will have been derived from complex experimentation by the manufacturer. It is a complex subject that depends on a number of factors such as cylinder size, combustion chamber shape, mixture strength, fuel quality, ignition timing, charge temperature, CHT etc. I agree it is definitely not the same as pre-ignition, I never said it was. It's not the same as auto-ignition either.
I can't give you an example of an engine that can be made to enter detonation at 50% power as I'm not an engine manufacturer, or someone with access to a library of such data. I do have some experience of piston engines, both building them, modifying them, using them and a while back, teaching student pilots about them. From my own experience, detonation may occur at any power setting if the conditions are suitable, certainly at smaller throttle openings than would give 75% power.
For example, in some car engines, you can hear the detonation "pinking" at very small throttle settings if the right conditions prevail. Opening the throttle further may actually result in the detonation no longer occurring.
Last edited by ShyTorque; 29th October 2003 at 01:01.
Joined: Oct 2003
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From: Finding Out on 121.50
I always use 20 Secs Carb down wind Ie BUMFICH (C for Carb)
Then as soon as you configure to start descent ie on base if flying cct then Carb on and power back to "descent setting" ie on PA28's approx 1500 RPM.
If you do a go arround must remember to put Carb Off. I have wondered on many occasions why I have sh
t performance in the climb. Oops Carb is still on. I have however found it don't blow up the engine as I was always told. I guess it don't do it much good though and the aircraft don't perform.
I must admit I never really saw the point of downwind carb as you put it on for a good long stint in the descent and final any way but old habits die hard and I ani't risking it now.
Then as soon as you configure to start descent ie on base if flying cct then Carb on and power back to "descent setting" ie on PA28's approx 1500 RPM.
If you do a go arround must remember to put Carb Off. I have wondered on many occasions why I have sh
t performance in the climb. Oops Carb is still on. I have however found it don't blow up the engine as I was always told. I guess it don't do it much good though and the aircraft don't perform.I must admit I never really saw the point of downwind carb as you put it on for a good long stint in the descent and final any way but old habits die hard and I ani't risking it now.
