does carb ice enrich the mixture?
 

interesting view point from a colleague the other day that carb icing enrichens the fuel air ratio, i always thought that it would just reduce the amount of air that could enter the carb. and therefore the ratio would stay the same, any thoughts on this? PA28 in particular.

The air just takes a different route to the carburettor. Yes there is a slight restriction, but not much. Enrichment of the mixture occurs because hot air ls less dense than cold air. How much by I'm not sure. But the biggest effect is a performance reduction due to the reduced density of the charge.

PM

Guess it would cause a rich mixture as the ice in effect would act as a choke.

If ice reduces the air flow wouldn't that also reduce the amount of fuel sucked in by the air? Therefore keeping the mixture ratio about the same?
Followed by what gets completely blocked first, the fuel jet or the air stream?
Lean cut or rich cut?
Interesting think.

Got carb icing in my car once (climbing Beattock southbound from Glasgow;OAT at the airport was +2 and it's about 1,000ft amsl and it was in cloud so very damp); the plugs were sooty and the exhaust was black smoke.

It depends on the particular carb. Most carbs use the airflow velocity or the aiflow pressure (actually the pressure drop) to meter the fuel through the jets. As the ice constricts the flow the air mass flow through the carb drops, but the velocity increases. As the constriction is in the venturi (downstream of the butterfly) the fuel metering "thinks" this means the mass-flow is increasing and so provides more fuel (making the mixture rich).

In a pressure-driven carb the increased velocity increases the pressure drop (manifold depression/vacuum) which the carb "sees" as addtional flow due to an increased throttle setting, so (again) it supplies more fuel and enriches the mixture. If the ice ever did get to block the main jets then you might see a weakening of the mixture, but by that stage the engine would proably be barely running (that's a guess on my part)

PDR

Under normal conditions. The butterfly controls the airflow, the airflow controls the fuel flow. If the butterfly is closed/throttle down, the mixture ratio remains the same, otherwise we would be running rich at low throttle and lean at full power.
Therefore if the ice builds up, as I believe it does, around the butterfly effectively closing the throttle then the same should apply, mixture stays the same, by ratio. The Venturi cross section is reduced the amount of air passing through is reduced. Although the pressure difference from atmospheric may be greater there is less volume of vacuum to fill with fuel.
Take a balloon a foot diameter at two atmospheres in a room, stick a pin in it, it goes pop. take another balloon at twenty feet dia also two atmospheres in the same room it would take out the windows, prob plus a bit! The pressure was the same.
Just my theory.

I don't know about the OPs Q, but when I have carb ice, I get an improvement by leaning when using carb air hot. (O200 in a Jodel DR1050.)

The ice doesn't form around the butterfly, it forms around the venturi. When the air passes through the venturi constriction it accelerates, so its pressure and temperature drop. That causes the ice to condense out. The condensed ice reduces the area of the venturi causing the air to travel faster than it would otherwise do for that throttle setting. To the fuel metering system that looks the same as a higher throttle setting, so more fuel flows causing a rich mixture.

The increased flow velocity causes a greater pressure/temperature drop in the venturi, which reduces the temperature further and increases the rate of ice formation, which in turn reduces the choke area and increases the flow rate, dropping the temperature further and so on causing a runaway condition. That's why once the ice starts to form it can build to dangerous levels very quickly.

PDR

This is accurate. I didn't know the answer to the rich/lean question, but looked it up- the answers I found confirmed everything PDR1 says.

Damn. Can I have another go at it?

:E

PDR

Slight thread drift but for the last three years, I've been flying a 182 with a carburetor temperature gauge. A fantastic instrument. I don't know why more light aircraft don't have them. Just use Carb Heat to keep the needle out of the yellow arc.

The next time you do a runup apply carb heat and then slowly lean the mixture. You should see in a fixed pitch prop aircraft, an initial rise in RPM as the overly rich mixture created by the carb heat is leaned to best power. If you keep leaning you will eventually see s drop in RPM as you lose power from an overly lean mixture.

I've been building and tuning up my own 4 stroke engines for many years and have often experimented with carburettor swaps on them. If you fit a carburettor with a smaller Venturi to any engine (which is sometimes done to improve low speed drive ability and response, but will possibly restrict top end power) you need a smaller main jet, otherwise the mixture will be too rich. This is because for the same airflow, the pressure drop across the main jet is greater with a smaller Venturi. Obviously, the basic principle of a carburettor is that the pressure drop across the main jet is what draws out the fuel.

If ice forms in the Venturi of a carb such that it reduces its diameter, the situation is the same as the above.

Smarty pants.:ok:

This is also why most fuel-injected configurations are reasonably immune to intake icing - they don't actually have a venturi to trigger the ice condensation.

PDR

My apologies.
So it seems from Shytorque that the mixture will become rich under icing conditions.
Seems to make sense as the extra fuel would cool the area further making the situation worse?

BPF.
Leaning the mixture by reducing the fuel and applying heat isn't the issue here.
This is reducing the airflow.

Not really. The fuel itself is warmer than the ice so the liquid fuel would tend to warm things up. But the jets are at the same place in the constriction of the venturi, so other than keeping the jet itself clear the fuel doesn't touch anything that's iced.

Once the fuel is released into the intake air it atimises into afine spray, and then it is supposed to vapourise. If it DOES vapourise it will cool the surrounding air (google "latent heat of vapourisation"), but there is always significant doubt as to whether it does at anything more than low power settings simpoly because it doesn't have time in the brief dash betwixt jet and cylinder.

Vapourisation promotes good fuel consumption and in some cases "cleaner" combustion and emissions, and it can also have an untercooling effect which improves charge density. But in most carburetted setups full vapourisation actually limits the maximum power available because the vapourised petrol occupies far more space in the inlet manifold and cylinder than fine droplets would do.

The power of any piston engine is ultimately limited by the amount of air you can make it breathe, and if you can make the fuel stay in nice concentrated droplets you can actually get more mixture into the cylinders than you could if the volume was cluttered-up with vapourise petrol. Once in the cylinder the droplets (again) have an intercooling effect which can allow you to run a slightly higher compression ratio if you are so inclined.

This has been known for decades, and it's the main reason why the really high performance setups still favoured carburettors over indirect fuel-injection well into the late 80s. F1 cars only really committed to fuel injection when the formula started introducing reductions in permitted fuel capacity.

But I digress...

PDR

The Chevvron microlight is fitted with a carb inlet temp guage and it is excellent.

Same fuel + reduced airflow = yes

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.

Can't recall SUs, at least the earlier ones as fitted to my Mini and MG1100, suffering from carb icing due to their totally different method of operation, but I remember aftermarket 'Fish' carbs by Reece and Minnow doing so; they would sell you an electric element to fit round the throat to heat incoming air.
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.

Carburetors are complex, with many circuits to provide nearly correct fuel/air mixtures at all possible throttle settings, and when changing throttle settings.
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.

We now have a conflict of views here.
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!

I can believe this. Given the many carburetor designs, throttle settings, and different icing conditions, many things could happen. It's somewhat of interest, but the loss of power is the real issue, knowing that it's partly due to a rich mixture is of little help.

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.

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.

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.

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!

Yup, applying carb heat would likely have removed several opportunities for failure!