Carburator Ice ?

[tomgil]

Guys you are great !!!
I have onather question for you !
Why carburator ice is more possible with power reduction then with full power or crussing power setting ?:confused:

[Final 3 Greens]

Carburettor icing is caused by air passing through the venturi and over the throttle valve expanding and local temperature falling.

Heat is lost from the air due to the latent heat of vaporization of fuel, this takes place in the vicinity of the throttle valve, resulting in a local temperature drop up to 33°C (60°F.)

The cumulative effect of both expansion cooling and fuel vaporization reduces the temperature to the point where ice may form on the throttle and in the adjacent passages of the carburettor, even though the ambient temperature is well above freezing.

Each type of engine/installation has particular icing characteristics and generally the lower the rpm, the more likely ice is to form due to a combination of factors. For example, the location of the carburettor in the engine bay may mean that it is kept warmer by a full power or cruise power engine setting and is thus less likey to suffer icing than at lower rpm.

However, this is not a universal rule.

[tomgil]

I'm not gonna teach you about it, looks like you had a rough time just learning to breath.;)
And don't think about it, you gonna get tired.:p
My advise for you is: Go take a walk on a short pier :}

Sorry, this was in response to some guy who was I gess removed.
He made some rather unintelligent remarks !!!:ok:

[ShyTorque]

tomgil,

At low power i.e. with the throttle partly or nearly completely closed, the depression in the venturi is increased. This results in more cooling of the incoming air = quicker ice buildup. Because the venturi opening is very small in comparison with a wide throttle, any ice present has a more rapid effect. This doesn't mean that opening the throttle will clear the ice, as the airflow in the venturi is now adversely affected by a glob of ice stuck to the walls, which puts the mixture awry. The engine may misfire badly or stop as the throttle is opened, especially as this accumulated ice lets go and is suddenly ingested.

The latter point is why carb heat on an aircraft should be used BEFORE ice builds up.

A few years back I had an imported car from warmer climes that had no winter setting on the air intake. After a while in UK, I could predict when carb ice might form. I had a couple of embarrassing and potentially dangerous incidents where, after waiting at idle at a road junction, the damned thing would stall as I pulled out into the main road as it swallowed the ice. Sometimes (weather not too cold, but very humid, with a lot of spray in the air from traffic) it would even cause a severe misfire at motorway speeds. The only cure was to pull over and allow engine bay heat to rise up and melt the ice with the engine stopped, obviosly not an option in an aircraft.

[Final 3 Greens]

it's probably also worth mentioning that carb ice can form on warm days as well as cool days, due to the potential size of the temperature drop.

The other key factor is relative humidity, so you may be free of carb ice on a dry, cold day, but plagued with it on a warm day, with a high dewpoint.

So an aircraft that experiences no carb ice on one day at a given power setting, may experience icing on a different day. That's one of the reasons that pilots flying in humid areas tend to apply carb heat fairly regularly as they cruise and certainly before reducing power.

Some aircraft/engine combinations seem more prone to ice, from personal experience the C150/Continental 0-200 needs careful monitoring, whereas the PA28/Lycoming 0-360 seems to be more resistant.

However, the bottom line is that you can get carb ice on any engine and it can form quickly, so awareness and monitoring is the safest approach.

For more info, you maybe interested in following this link

http://www.caa.co.uk/docs/33/SRG_GAD_SSL14.PDF

[Mike Cross]

Mmmmmmmmmm.................

It sounds convincing but unforunately it's wrong.

The throttle butterfly is downstream from the venturi so the theory that the pressure in the venturi is depressd more at lower throttle openings is wrong, as is the evaporation theory. Pressure in the venturi is higher at low throttle settings, not lower.

When the throttle is wide open more air is passing through the venturi. Since the venturi is usually of a fixed size it has to travel faster, resulting in a greater pressure drop. It is this pressure drop that sucks more fuel out of the main jet so that you get more fuel to go with the extra air and hence more power.

The build up of ice at low throttle openings results from a number of factors:-

The throttle butterfly is acting as a large obstruction between the carburettor and the inlet manifold. Ice crystals formed by water vapour condensing out as a result of the pressure drop (remember how clouds form?) combined with the temperature drop hit this obstruction and stick to it, building up and blocking the airflow. In addition the pressure in the inlet manifold is even lower (otherwise the air/fuel mixture wouldn't pass through). The further expansion of the mixture as it passes through the narrow gap round the butterfly has a further cooling effect.

To exacerbate the situation still further the engine is generating less heat at the lower throttle setting, resulting in the hot air (if selected) being less hot and the ambient temperature inside the cowling being lower, thereby assisting in ice formation.

[ShyTorque]

Mike Cross,

Yes, looking back at my post, my wording was wrong. however, you seem to be forgetting something. Firstly, as you say, the almost closed throttle plate acts as a second venturi, i.e. a further narrowing of the intake pipe. The pressure drop here is much greater than in the main venturi, as the remaining orifice across the throttle plate is very small at idle. Any ice forming here can very quickly block the inlet path.

BUT.. don't discount the cooling effect of fuel flow through the carb. Liquid fuel issuing from the fuel jets is merely atomised, i.e. in aerosol form. Evaporation of the fuel droplets may NOT be completed until after it has entered the manifold, so cooling from this effect (pulling in latent heat of evaporation) continues right through the carb, definitely including the area around the throttle plate, taking heat from it. As the plate has a small mass and can only draw "replacement" heat through its spindle, it cools rapidly.

This is why most cars with carburettors have water-heated inlet manifolds - to ensure the fuel is finally and fully converted to vapour before it enters the cylinders, as liquid fuel cannot burn. Remember how a fridge works? The cooling in that case is brought about mainly by evaporation of a liquid. Your "venturi only" theory would suggest that refrigerator manufacturers have been doing it wrong all these years - perhaps they only need a suction pump and air flowing through a venturi - wot no harmful CFCs? :p

I once had a motorbike that could actually ice up its carburettor without the engine even running. It had a leaky float chamber valve, fuel sometimes came seeping out and when left for a while in suitably humid conditions, ice would form on the outside of the carb - no pressure drop there! Interestingly, the ice looked like pink cake icing as it was mixed with the colour of petrol and 2 stroke oil. It only did this in summer, which ties in with advice given about the worst conditions for aircraft carb icing in UK.

[DubTrub]

Mike, after some thought, and with respect, I have to beg to differ on some points, but please forgive me if I have misinterpreted your post:

I do agree that the "throttle butterfly is downstream from the venturi" but you must recall that you can only get pressure differential with air flow, and it is this pressure differential due to flow that can cause carb icing.

The vacuum within the intake system of a normally-aspirated engine is caused by the intake mechanism of the Otto cycle, in fact the "suck" of the cylinders. The pressure difference between that in the cylinders and atmospheric is, in our circumstances, largely dependent upon the obstruction to airflow between the two, which is the carburettor butterfly valve. This is why, in my case and that of WR, that we get maximum "suck" on our A-65's at closed throttle.

The more closed is the butterfly valve (throttle), the greater the pressure differential (and therefore the pressure drop within the carburettor). If this drop occurs so to cause a sufficient temperature drop for the prevailing relative humidity, then water vapour within the intake mixture can crystalise to form ice, and it will form on any suitable surface within the intake system that is cold-soaked where it can form. The pressure within the carb caused by forward motion is minimal; the intake could be pointed backwards.

All of the above gets worse when fuel vaporisation occurs, because of the latent heat of evaporation of the fuel causes the cold-soaking of all the intake system.

You are indeed correct in pointing out that "when the throttle is wide open more air is passing through the venturi", but of course it is at very close to atmospheric pressure when so doing, so there is little pressure drop. This is why carb ice is rarer at cruise or full-power settings.

F3G: you are correct: "small" Continentals, from the A-40 (yes I have flown one) to the 0-300, are more conducive to carb ice because of the design of the carburettor/intake system in relation to the warm oil sump. In these engines, the carburettor hangs some distance away from the oil sump heat source, and so are succeptible to cold airs. In the typical Lycoming situation, however, the carburettor is located very close to the sump, where it benefits more from the heat of the oil.

shytorque ...Quote: The latter point is why carb heat on an aircraft should be used BEFORE ice builds up. You are also right.

DT

p.s. Mike: I should be very happy to offer you a trip in my A-65 powered machine (as my guest) in order to demonstrate the above; PM me at any time, I am Midlands-based....unfortunately the A-40 is solo only...and in the USA

[edited for typo]

[WelshFlyer]

I've had carb ice on various motorcycles.

People often think it's just planes, but my bike starts icing in the cold.

WF.

[Mike Cross]

We're all agreed that carb ice is more likely at low throttle settings. I was just pointing out that the suggestion that the cause was a greater cooling effect in the venturi is wrong.

There is actually less cooling effect in the venturi at lower throttle settings.

At high throttle settings the pressure differential (and hence the airflow) across the venturi is higher and hence the cooling effect is higher.

With a low throttle setting the maximum pressure differential occurs across the throttle butterfly, not in the venturi and this is where the maximum cooling occurs.

This may be seen as a pedantic point but if the student has some grasp of the subject then trying to explain it by cooling within the venturi is likely to leave him (quite rightly) failing to grasp the reasoning behind the explanation.

My Luscombe has a C85 so I am well aware of the tendency to icing. I suspect one of the reasons small Continentals are prone to it is that the inlet manifold pipework hangs out in the breeze and receives no warming effect from the sump in comparison to Lycomings.

One other thing for the uninitiated. Most carburettors have an accelerator pump which squirts extra fuel in as the throttle is opened. This is to compensate for the lag between the extra airflow caused by the throttle being opened and additional fuel being supplied. The carburettors on small Continentals tend not to have this with the result that (a) The engine can falter if the throttle is opened quickly and (b) You can't prime by pumping the throttle.

[DubTrub]

Mike: your last two para's: absolutely correct.

Most Lycomings have the intake pipes passing through the oil sump, which does warm the intake air quite a lot (unfortunately, this might not help icing upstream of this position, but it does explain some way towards the "carb heat to cold on final" instructional policy on C152, Pa28 and C172..0-300 excepted).

One of the problems is that many Instructors do not recognise this (potentially life-changing) difference with the little old Continentals, and are inadvertently conducting conversions to type for new owners (particularly with the newer requirement for "differences" training) and could be leading owners to applying their C152-training methods to Luscombes, Cubs, Aeroncas, Taylorcrafts, et al. I think VW's are prone to the same? But my experience of these is small.

Regarding accelerator pumps, you are again correct. Fortunately in these circumstances and in my experience, both the Instructor and student only need the thing to stop once for them to learn the lesson...and generally the result is reapplication of power in the appropraite manner...but Carb Ice is so variable and insidious, it is a different matter that might catch out the unaware at any time.

Well there we are. I think that's the limit of my knowledge and experience, non instructor, just a ppl (and I'm slightly at a loss regard your venturi bit, but perhaps for another topic).

Welshflyer: I had the same in my old mini Clubman a few decades ago; it had a "winter" setting for the carb to prevent this, basically a permanent hot air source during the colder months.

[DubTrub]

Mike:

After a fresh look at these discussions (and also a peek at CAA Safety Sense Leaflet 14A "Piston Engine Icing" (http://www.caa.co.uk/publications/publications.asp?cattype=sercat&id=21) I now agree with you on the venturi aspect.

The SSL in fact I believe is misleading, in that it does not IMHO adequately explain the nature of the pressure drop at low throttle settings:

It quotes:

"The most common [type]...is carb icing
caused by the sudden temperature
drop due to fuel vaporisation and
pressure reduction at the carburettor
venturi."

and

"ice which gradually blocks the venturi."

and

"Engines at reduced power settings
are more prone to icing because
engine induction temperatures are
lower. Also, the partially closed
butterfly can more easily be
restricted by the ice build-up."

There is no mention of the greatest pressure drop being caused by the butterfly itself.

What is your opinion of the accuracy of the SSL info?

DT

[BigEndBob]

It seems wrong that people will loose there lives because of carburettor icing. If this was faulty wing bolts on 747's they would all be grounded until the problem is solved. Would you buy a car that needed the application of carb heat every five minutes.

Carburettor icing seems full of myths and misunderstandings.
Aircraft manufacturers in ther manuals won't commit themselves
as to when it should be used.

In 9000 hrs of instructing i can think of only 2 occasions when carb icing created problems, one a summers day practising pfl when learning to fly the engine stopped with carb heat hot in the glide, resulting in forced landing in my selected field and second a clapped out C150 engine stopped three times before reaching the hold on a very wet day. Needless to say i did not go flying.

Basic instructor technique, if you can't think of anything else to do in flight apply carb heat!

Ok there are a lot of engines out there, but surely some fuel additive could be created to eliminate the problem.

[Mike Cross]

DubTrub

I wouldn't knock the SSL's at all, however over-simplification leads to holes in the logic which causes problems when someone subsequently tries to work out what is happening, as in this case.

John Farley has a very good explanation of how he explains lift to novices in the latest issue of "Flyer".

He says "Personally I hate oversimplified or misleading explanations that do not bear close examination. Plus where I come from there is no such thing as a weak student - only an incompetent teacher (at least for something as basic as aerodynamic lift)."

I tend to agree with him.