does carb ice enrich the mixture?
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I've never really considered this fully.
I was under the impression that closing the throttle would automatically reduce the amount of fuel being sucked in because of the reduced amount of air passing over the fuel jet.
If this is not the case then the mixture ratio is going to vary considerably over the whole throttle range, from very rich at idle to, probably correct, at full power, and would never be perfect at any setting without mixture adjustment.
It would therefore seem a good idea to lean at taxi revs and progressively richen to full revs.
I know this sounds as though I'm re-inventing the wheel. Just trying to understand the real reasons for "set fully rich in circuit, lean only at altitude" "rich first then throttle up, throttle down first then lean" (I hope I've got that the right way round).
Doing this stuff by rote, just because the instructor said so isn't good enough!
Next. Where does the old SU carb with taper needle fuel metering fit into this?
Excuse my ramblings.
I was under the impression that closing the throttle would automatically reduce the amount of fuel being sucked in because of the reduced amount of air passing over the fuel jet.
If this is not the case then the mixture ratio is going to vary considerably over the whole throttle range, from very rich at idle to, probably correct, at full power, and would never be perfect at any setting without mixture adjustment.
It would therefore seem a good idea to lean at taxi revs and progressively richen to full revs.
I know this sounds as though I'm re-inventing the wheel. Just trying to understand the real reasons for "set fully rich in circuit, lean only at altitude" "rich first then throttle up, throttle down first then lean" (I hope I've got that the right way round).
Doing this stuff by rote, just because the instructor said so isn't good enough!
Next. Where does the old SU carb with taper needle fuel metering fit into this?
Excuse my ramblings.
The car where I got carb icing at #5 was a '67 Fiat 124 with a 2 choke Weber (sidedraught, not the later downdraught one). You could manually position the intake to the air filter either against the exhaust manifiold or in cooler air and that's where I had it.
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I've never really considered this fully.
I was under the impression that closing the throttle would automatically reduce the amount of fuel being sucked in because of the reduced amount of air passing over the fuel jet.
If this is not the case then the mixture ratio is going to vary considerably over the whole throttle range, from very rich at idle to, probably correct, at full power, and would never be perfect at any setting without mixture adjustment.
I was under the impression that closing the throttle would automatically reduce the amount of fuel being sucked in because of the reduced amount of air passing over the fuel jet.
If this is not the case then the mixture ratio is going to vary considerably over the whole throttle range, from very rich at idle to, probably correct, at full power, and would never be perfect at any setting without mixture adjustment.
Wikipedia has a good article for anyone interested in the details and different types.
FWIW the SU carb with piston/needle system is much the same as the Bing with its diaphragm/needle usede everywhere in 4 stroke Rotax and some Jab's.
mike hallam.
mike hallam.
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The original question was, does ice richen the mixture? Yes, from most people.
I originally said it would remain the same.
If the ice effectively closes the throttle then what you are saying is the fuel quantity will compensate and therefore reduce, weakening the mixture back to more or less what it should be.
I am not aware of carburettors that modify the fuel flow with a linkage to the butterfly?
Aren't the jets fixed?
The mixture will be enrichened initially as the ice narrows the throat of the venturi. The narrower venturi throat will further accelerate the air/fuel mixture passing through it, resulting in a further pressure drop. Now, with the difference in pressure between the float bowl and the venturi throat being greater, the amount of fuel flow will increase even as the air flowing in decreases. Richer mixture is the result.
But this is often temporary. Eventually as the restriction to air/fuel flow in the venturi becomes more pronounced, the ice disrupts the smooth flow through the venturi as eddie currents form and act to reduce the velocity of the air/fuel mixture through the venturi. With a reduced pressure differential between the float bowl and the venturi throat, the fuel flow will reduce. This results in a leaner mixture.
Testing has shown that ice can form anywhere in the venturi, including on the throttle valve. (butterfly) In fact, ice may form on the throttle valve before forming in the venturi throat. It just depends upon the particulars of the carb design, the air inlet design and the specific conditions under which it is being operated.
When carb heat is applied, everyone knows the hotter air will be less dense. This will result in a richer mixture because the venturi is not sensitive to air density. It only reacts to air velocity through the venturi, which will be a function of volumetric airflow and throat cross sectional area. This will be affected by the position of the throttle valve. Greater velocity creates greater differential pressure between the venturi throat (where the fuel discharge nozzle is located) and the float chamber. (where the fuel comes from) The manual mixture control is simply an adjustable restriction to fuel flow between the float bowl and the discharge nozzle in the most common aero engine carbs. Even with the reduced power available while using carb heat, it's vastly preferable to having even less (or no) power available with an ice blocked venturi!
But this is often temporary. Eventually as the restriction to air/fuel flow in the venturi becomes more pronounced, the ice disrupts the smooth flow through the venturi as eddie currents form and act to reduce the velocity of the air/fuel mixture through the venturi. With a reduced pressure differential between the float bowl and the venturi throat, the fuel flow will reduce. This results in a leaner mixture.
Testing has shown that ice can form anywhere in the venturi, including on the throttle valve. (butterfly) In fact, ice may form on the throttle valve before forming in the venturi throat. It just depends upon the particulars of the carb design, the air inlet design and the specific conditions under which it is being operated.
When carb heat is applied, everyone knows the hotter air will be less dense. This will result in a richer mixture because the venturi is not sensitive to air density. It only reacts to air velocity through the venturi, which will be a function of volumetric airflow and throat cross sectional area. This will be affected by the position of the throttle valve. Greater velocity creates greater differential pressure between the venturi throat (where the fuel discharge nozzle is located) and the float chamber. (where the fuel comes from) The manual mixture control is simply an adjustable restriction to fuel flow between the float bowl and the discharge nozzle in the most common aero engine carbs. Even with the reduced power available while using carb heat, it's vastly preferable to having even less (or no) power available with an ice blocked venturi!
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Eventually as the restriction to air/fuel flow in the venturi becomes more pronounced, the ice disrupts the smooth flow through the venturi as eddie currents form and act to reduce the velocity of the air/fuel mixture through the venturi. With a reduced pressure differential between the float bowl and the venturi throat, the fuel flow will reduce. This results in a leaner mixture.
Anecdote: I was hiking at Point Reyes, on McClures Beach years ago, when I found the wreckage of a small high wing aircraft. The story was that carb icing brought it down, but the pilot made a good forced landing on the beach. The ice soon melted, and he tried to take off, unfortunately a wave caught the seaward wheel, spinning it into the surf. Rich or lean, didn't matter.
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I fly both an injection and carburettor and have to definitely focus on remembering to use the carb heat when I’m flying the carb fitted aircraft.
Avoid imitations
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An SU is a "constant depression/velocity" carb. The throttle butterfly plate is connected to the throttle, but it is only a demand valve because the air piston rises and falls iaw the actual airflow demand ("suction") of the engine - the two components aren't mechanically connected.
This type of carb needs a tapered needle in the jet to meter the amount of fuel mixing with the changes in airflow into the manifold. Obviously, more air = more fuel required. Depending on the diameter of the carburettor, it will use a "standard" jet. The mixture is controlled by a needle which sits in the orifice of the jet itself. For example, all HS and HS2 carbs use a .090" jet. Larger ones use a .100" jet and the very largest (for cars, not aircraft) use a .125". The needle profile, along its length, affects the mixture strength. The thicker the needle in the jet, the weaker the mixture and vice versa. There are over 350 SU needle profiles to choose from.
I spent years fine tuning my SU equipped competition engines by trying different needles and latterly by modifying them myself (by checking the mixture at various points in the engine operation, mainly by looking at spark plug colours and latterly by use of a simple CO meter, then using a rolling road to check I'd got it right).
Some avid racers used to carry alternative sets of needles to race meetings and would fit whatever went best on the day, depending on the prevailing conditions. That was a bit too avid for me - once my engine was tuned, that's more or less how it stayed - until I modified the engine again, then adjustments might be needed.
Fitting my car with an engine using fuel injection and and ECU has made most of my hard earned knowledge redundant.
This type of carb needs a tapered needle in the jet to meter the amount of fuel mixing with the changes in airflow into the manifold. Obviously, more air = more fuel required. Depending on the diameter of the carburettor, it will use a "standard" jet. The mixture is controlled by a needle which sits in the orifice of the jet itself. For example, all HS and HS2 carbs use a .090" jet. Larger ones use a .100" jet and the very largest (for cars, not aircraft) use a .125". The needle profile, along its length, affects the mixture strength. The thicker the needle in the jet, the weaker the mixture and vice versa. There are over 350 SU needle profiles to choose from.
I spent years fine tuning my SU equipped competition engines by trying different needles and latterly by modifying them myself (by checking the mixture at various points in the engine operation, mainly by looking at spark plug colours and latterly by use of a simple CO meter, then using a rolling road to check I'd got it right).
Some avid racers used to carry alternative sets of needles to race meetings and would fit whatever went best on the day, depending on the prevailing conditions. That was a bit too avid for me - once my engine was tuned, that's more or less how it stayed - until I modified the engine again, then adjustments might be needed.
Fitting my car with an engine using fuel injection and and ECU has made most of my hard earned knowledge redundant.
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That thought occurred to me too. I had forgotten the details of how SU type carbs work, thanks for the trip down memory lane. Funny carburetor story- was going for a walk recently when I saw a 60's classic American car full of young people. They couldn't get it started, and the strong smell of gasoline told me it was flooded. I could hear the driver pumping the accelerator as he cranked it. I explained to him that it was flooded and pumping the accelerator was the wrong thing to do. I told him to floor it and hold it floored while he cranked, then let off when it caught. He explained that a relative had let him borrow it, and you needed to pump the accelerator to start it. I explained that now that the engine was hot that was no longer true, and convinced him to try cranking with the throttle open. He lasted a few seconds, then I could hear him pumping the throttle again. OK. On with my walk!
