carb ice question

[mystic_av8r]

Most cars these days are fuel injected, but can someone please explain why cars with carburettors don't fail thru an intake blocked with carb/throttle ice?? As I understand it, the temp range for ice is -5 to +30 degs c, with relative humidity +50%. Most cars spend their entire time in these conditions, and MOGAS is more volatile than AVGAS so the cooling effect should be more. Any ideas??

[parris50]

Most carburettor cars do have carb heat but it works automatically. In the air filter housing there is a bimetallic strip which controls a valve which allows air direct form the atmosphere or air that has passed over the hot exhaust system.

I did have a car where the ducting for the hot air split and in cold weather after a while the car would stop but after a few minutes would run perfectly. I put this down to carb ice.

[eharding]

Buy a copy of Nick Mason's "Into The Red" - it details the comprehensive hooning of his car collection around various circuits, and includes a CD of the resulting noises. Marvellous. Of relevance to this topic is the description of the 1953 V16 BRM, which they never could get to run properly - one of the theories being carburettor icing. 600BHP out of 1.5 litres, running on methanol. Sounds fantastic.

[Figure Of Merit]

Cars DO suffer from carb ice.

I used to own a Fiat 126 which, on cold dark foggy nights, would suffer engine failure on the motorways of the midlands. Even with fully open induction passageways it could barely do 60 mph so it was easy enough to declutch and freewheel onto the hard shoulder. After a 5minute wait the engine's heat would have defrosted the carb and the beast could be restarted and the journey continued for another 10 miles after which the whole proceedure would need to be repeated.

It was fitted with a flap valve to enable selection of an alternate source of induction air that was collected from around the exhaust manifolds. The owners manual advised switching to hot air in the autumn. This wasn't always enough heating to cope in severe icing conditions and the ducting (made from wire re-inforced paper) would frequently have fallen off over the summer months in time for the first chill autumn evening.

Funny how it seems almost quaint now....

[ShyTorque]

I concur,

I brought back my old car from East Asia as a personal import. Because it wasn't for the european market it didn't have an intake hot air device fitted.

It occasionally suffered from carb ice and would cut out in the most inconvenient of places, usually after idling in traffic with the engine not fully warmed up. It would also do it on the motorway at certain speeds, but if I slowed down (as I was forced to do due to the misfire) it would pick up and go again as the engine bay heat got to it.

It occurred in cool, humid, weather and was most noticeable beneath thunderstorms (classic wx for it). It was possible to see the ice in the carb if you were quick enough to get the air filter cover off.

[kabz]

Solihull's finest ... my 1973 Landrover Series III used to randomly stop running on cool mornings ... I guess that was carb ice.

Sadly, I never worked out what was wrong with it, and sold the thing... :ouch:

[Genghis the Engineer]

I gave a talk to a branch of one of the engineering institutions near Birmingham once. I briefly mentioned carb heat as it's fitted to little aeroplanes. The resulting question and answer session left me in no doubt that Engineers (at least ones who live near Birmingham and go to evening lectures) who design and test cars and especially motorbikes spend a lot of time worrying about carb icing and what to do about it.

Interestingly nobody had ever tried the coolant-loop type carb heat you see on a lot of the newer Rotax 912 / 914 installations.

G

[CBLong]

Just to add more anecdotal evidence to the above, the 1978 VW Derby that was my first car also suffered badly with carb ice on winter mornings, until the exhaust warmed up enough to provide sufficient heat... it was quite interesting approaching traffic lights with one hand steering, one hand changing gears, and one hand adjusting the choke, whilst simaltaneously having my left foot on the clutch, my right foot on the brake and my other right foot on the throttle to keep the thing running... :)

Genghis - I don't know anything about Rotax engines but my 1967 Mk II Spitfire has what I would call 'coolant-loop' carb heat - the intake manifold casting incorporates a channel for hot coolant to pass through. See part number 307625 on this schematic:

http://www.rimmerbros.co.uk/spitfire/images/33b.gif

It's never suffered with carb heat, although it has innumerable other problems... :(

cbl.

[FlyingForFun]

Genghis, I'm almost embarrased to ask this, because I should know - could you please describe the "coolant-loop type carb heat you see on a lot of the newer Rotax 912 / 914 installations"? My Rotax 914 doesn't have a carb heat control, and I was led to believe that this was because the inlet air is routed around the turbo-chargers which warm the air sufficiently to avoid carb ice. Is there actually more to it than this?

CBLong, the coolant-loop you're thinking of isn't carb heat - it heats the mixture after it leaves the carb. I believe its purpose is to prevent the condensation of the fuel on the inlet manifolds - the fuel needs to be in a vapourised state when it reaches the cylinders, not a liquid state. But I agree - my Spitfire 1500 doesn't seem to suffer from any carb ice problems. There is very little room under the Spitfire bonnet, though, so I suspect that the air is fairly well warmed by the general engine heat.


On a more general note, I have often thought that part of the reason carb ice is more of a problem in aircraft than cars is because it mainly occurs at low power settings but high speeds - a situation which cars don't often find themselves in. Is there any truth in my theory?

FFF
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[ShyTorque]

FFF,

Carb ice occurs because of two effects.

One is the latent heat of evaporation of the fuel reducing the temperature of the (water vapour bearing) inlet air.

The second is where the airflow drawn through a venturi by manifold depression causes an increase in the velocity of the air past the throttle plate and hence a reduction in inlet air pressure and hence its temperature (Bernoulli's Law) in that immediate vicinity.

If the air reaches its icing point, the obvious will occur. Note that summer air (thunder storm weather) holds more water by weight than drier winter air, so icing in aircraft may be more acute on summer days .

The two effects combined have the potential to cause icing around the throttle venturi. Aircraft are prone to it because they have a forward facing intake duct and operate in air that is conducive to ice formation; namely cool but humid air.

A smaller throttle setting gives a more intense pressure drop than a wider one so yes, lower power makes carb ice more likely. Also, a lower throttle setting makes the consequences more serious because the actual throttle aperture is smaller and is likely to become blocked rapidly. High speed (generally, at least on a fixed pitch prop) = more prop RPM so lower MAP.

The answer is to use carb heat on a regular basis, iaw the Flight Manual. :ok:

[Genghis the Engineer]

I've never taken a Europa apart FFF so yours may well be completely different, but the systems I'm used to use a standard "K&N" type intake filter. Around that is a loose spiral of formed metal pipe within which coolant is passed. Around all of that is a sort of mushroom shaped cowling making sure that the intake air largely passes around this "radiator spiral" to warm it on the way into the engine.

Picture below, which is of the installation on a 912 fitted to a Pegasus Quantum.

G

http://images.photobox.co.uk/public/images/50/82/3705082.s.jpg

[411A]

Interesting to note that thepressure carburetors fitted to many engines (and nearly all large radial engines, except C/W R3350TC series) generally are non-icing.

In flying DC6's for example, cannot remember one time where carb heat was required. Same for twin Bonanza's.

Float carbs however are a different animal altogether.

[av8boy]

November 1987 in a 77 Honda Civic that had never before been outside of southern California and never above about 5,000 feet in beautiful wx. But this time it was Interstate 70 eastbound through Colorado in a humid snowstorm, climbing through about 9,000 feet on the way toward the Eisenhower tunnel. The engine ran rough for a minute or so, then quit. No restart possible. Hitched a ride to the tunnel and called tow truck. By the time the truck arrived at the tunnel and we worked our way back down to the car, the carb ice had melted and the thing started without a problem. Still didn't dawn on me that it was carb ice. It was only after running into the same problem two or three times the following spring in conditions conducive to aircraft carb ice that it dawned on me. Felt pretty stupid...

Dave

[Genghis the Engineer]

411A If you were to reterm it (perfectly legitimately) from carburettor icing to venturi icing your statement makes very good sense.

G

[Dan Winterland]

I bought a FIAT Uno of a mate many years ago. He told me that it had an ignition problem in the past which had been sorted with the purchase of an expensive new ignition module. Needless to say the problem was still there and I quickly realised it was carb ice. There was little that could be done except for waiting for the ice to melt.

Despite the fact there was a hot water manifold on the case of the carb, the hoses weren't fitted. I enquired about purchasing the hoses, but they weren't available in the UK.

[FlyingForFun]

Genghis, I think I'll have to take a closer look the next time the cowling comes off my aircraft!

ShyTorque, thanks for that. If I understand correctly, you're agreeing with my theory on why cars don't suffer from carb ice as much as aeroplanes?

FFF
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[Flash2001]

Most carburetted cars over here had a manifold hot spot to reduce carb ice and reduce the need for manifold wettting before engine attained operating temperature. The Ford Econoline van with 6 cylinder inline engine suffered from appalling carb icing. Under high humidity and temp around 32 F engines would go rough and start belching black smoke and quit. They would start again a few minutes later after the temperature gradients flattened across the engine. Some later carburetted engines had a thermostatic inlet diverter and exhaust shroud to provide true carb heat.

The restart delay would have been longer and more difficult to accomodate in an aircraft as the restart opportunity would have available only after the inevitable off airport landing.

After an excellent landing, you can use the aircraft again!

[ATPMBA]

I once owned a 1974 Ford Pinto (fireball) upon poking around under the hood (bonnet for UK's) it appeared that it had carb heat on all of the time.

[ShyTorque]

ATPMBA,

The RAF's Chipmunks had theirs wire locked "ON" too!