Carb Iceing

[dublinpilot]

We all know the dangers of carb iceing...and I'm sure we have all observed small amounts of it being cleared when doing run up checks/FREDA checks....evidenced by an slight increase in RPM.

Most of us up fly at about 2000ft.

So here is my question- Why does it not effect cars? I know most of the cars today are fuel injected (all?) but this was not the case before cathylitic converters were made mandatory, which is only about 10 years ago.

Lots of cars regularly operate above 2000ft...in some places a lot higher.

So why did we not see lots of cars stopped at the side of the road with iceing in the carburettors? Why didn't cars have carb heat?

After all they use the less controlled petrol instead of 100LL.

Anyone able to enlighten me?

[Field In Sight]

Not sure about devices used in cars but I know my Suzuki Bandit 600cc motorbike has a carb heater fitted for the UK (and maybe european) market.

It is automatic though so I don't need to remember any FREDA checks whilst I'm tear-arsing down the road :D

[sennadog]

I'd guess it's because although cars do operate at this altitude they rarely descend at such a rate on a low power setting.

Assuming you could descend at 500fpm in a car, you would be caning it to get the descent speed or if you were driving normally then you would be in a lower gear with higher revs - both of which are in a higher power setting which is the opposite to what is required to get carb icing - a low power setting.

Plus, I reckon the engine bay in a car would be warmer, in particular the induction area, than an aircraft's.


Now, I'll just sit back and wait for Evo, Beagle or 2Donkeys to come along and blow my theory out of the water.

:)

[dublinpilot]

Know what you are saying Senna Dog....but can't carb iceing occur at any power setting...at any stage of flight....just more likly at low power settings.

However FIS might be right. Maybe carb heat was fitted to cars but was automatic and therefore unseen.

[140cherokee]

Cars can be affected by carb-icing. Early model TR7's were badly affected during the winter as air to the carbs was fed via tubes from in front of the radiator. On the overrun, the needles could freeze solid in the jets and the engine would stop. 5 mins later, the engine would restart without any sign of a problem (sound familiar?)

Later models had a thermostatic flap that took air from above the exhaust manifold when cold.

[Evo7]

Now, I'll just sit back and wait for Evo, Beagle or 2Donkeys to come along and blow my theory out of the water.


I'm flattered, old chap, but I couldn't tell one end of a carburettor from the other. Can barely spell it... :)

[FlyingForFun]

A couple of theories from me here, no real evidence to back any of it up:

Lots of cars have some form of permanent carb-heat. Many cars have the air intake placed just behind the radiator, for example. Some cars, such as mine, route some of the coolant along the inlet manifold (which helps vapourisation as well as removing ice that forms in the manifold itself). In many cases, the air intake position can be adjusted by the driver, the idea being to position it behind the radiator in winter, but somewhere cooler in summer, although I don't know any drivers who do this.

This would not be acceptable in an aircraft though. Pilots don't like being robbed of power, especially on take-off. Then there are certain conditions where it's far too cold for carb ice to form, but where the application of carb heat for any length of time may warm the carbs to carb-ice temperatures. And there's also the fact that pilots like to know what's going on - if an engine is subjected to carb ice conditions, a driver will continue to drive on obliviously while the warm air coming off the radiator keeps the carb throats clear. A pilot, on the other hand, would like to know that carb ice is beginning to form, so that he can climb to cooler air, descend to warmer air, or possibly land.

Interesting thread!

FFF
--------------

[pulse1]

I had several cars in the 70's which would suddenly stop when power was reduced at road junctions on damp misty days. I always put it down to carb icing and it seemed to be more prevalent with the SU carburretor.

[Whipping Boy's SATCO]

Surely this also begs the question, why do we still have carburettors? It seems to me that technical advances in AVGAS engines have been particularly slow.

[sennadog]

I'm flattered, old chap, but I couldn't tell one end of a carburettor from the other. Can barely spell it Evo.

OK, 1 down, 2 to go!

I'm making the assumption that Carb icing mainly occurs at lower power settings as this was one of the questions in Aircraft Technical. I'll consult Thom and see what he says.

[bertiethebadger]

The earlier cars that I remember had an adjustable air intake into the air filter.

In summer, you pointed it into an open space in your car & in winter you swiveled it so that it was taking air from very near to the exhaust downpipe.

[distaff_beancounter]

When I was young (a long time ago :D ) & more lunatic about fast cars, than I am now, I had one that had been modified by the previous owner.

I won't bore you with the detailed techie stuff 'cos I don't understand it :( ). Anyhow it had been fitted with twin carbs, plus enormous air inlet ports, direct into the cabs.

On several occasions, it suffered from carb icing. This was usually when going over high ground. The only solution was to slow down, to decrease the effect of ram air into the carbs. On a couple of occasions I just had to stop, & leave the engine idling (which was all it would do) & wait for the heat from the engine block to melt the ice.

YES, I learnt about carb ice from that car! :rolleyes:

(But it did go like s*** the rest of the time :D )

[2Donkeys]

OK, 1 down, 2 to go!

Couldn't let an offer like that go through :)

There seems to be an assumption in some of the earliest postings that carb icing is in some way related to altitude. Whilst this is not entirely false, it is an odd way to think of the problem.

The real determinant of Carb Icing risk is the ambient temperature of the air, and its water content.

Carb Ice risk is at its highest when air starts of fairly cool, and with high moisture content (note that the warmer the air, the higher its moisture retaining capability)

As the air passes through the throttle butterfly, it expands (and therefore cools). If the cooling effect is sufficient, the ambient temperature is not too high, and there is enough moisture in the airflow, then ice will form around the butterfly and adjacent structure.

So, the worst possible day for carb icing will be one in which the weather is moderate to warm (increasing the water-bearing capacity of the air), and humid.

The worst time in flight is when the throttle is partially closed, because this maximises the expansion of the air passing through the butterfly, increasing ice formation.

This is why we worry about a long low-power descent from altitude on a warm summers day.


As we have heard, cars with carburetors either tend to pre-heat the incoming air by passing it close to the radiator, or have specific heating systems to avoid the problem. We tend not to do the same thing on an aircraft because warm air carries less oxygen and reduces the power of our engines. Small piston engines in aircraft typically need all the power they can get. Chipmunk owners will be familiar with the historic custom of wirelocking the carb-heat on though.

Motorbikes tend to be less sophisticated than cars, and lack the same carb heating options. Carb icing on bikes is therefore far more common.

Worth noting that Icing is not purely restricted to Carbs though. Injection systems are also known to ice, but under rather more extreme circumstances not normally applicable to cars :D

[FormationFlyer]

Dont forget there are two types of icing....

The point of 100LL vs Petrol is more to do with anti-knock qualities of the fuel....and moisture content...this is fuel icing and can occur in the carb around the jets.

Carb icing can occur (pretty much) at an OAT and any power setting. Check the CAA nice graph.

It is more common at low power settings though due to the increased drop in temprature in the carb through the butterfly valve.

As for cars....AFAIK I think FlyingForFun is right - many cars have built in 'carb heat' due to the design of the air intakes...(well before fuel injection).

Why dont all a/c engines have fuel injection - well you must remember most of the hire fleet were built between 1960-1990 a period when fuel injection in cars was not hugely popular - only in the 1980s did cars really start getting fuel injection as standard.... so it is no great surprise that most a/c are still old style carb designs...but yeah - on the face of it there has been very little change in aircraft propulsion technology used in light a/c for the last 50-70 years.

[Who has control?]

We don't see carb icing in cars because cars don't have carbs. They have to have a computer to control the injection system as the cat requires a precise amount of fuel to meet emission standards.

The trouble with cars now is that is is TOO hot under the hood. The skin temperature of a cat. is approx 500-600 deg. Cel. and a lot of cars nowadays have an undershield to keep to noise in.

The problem is now trying to find a source of cold air into the air induction system to keep the power up, rather than hot air to stop ice forming.

[QDMQDMQDM]

I had several cars in the 70's which would suddenly stop when power was reduced at road junctions on damp misty days. I always put it down to carb icing and it seemed to be more prevalent with the SU carburretor.

I had this on a Citroen Visa. Late, cold, misty night cruising down the M11, no traffic, so contant throttle setting. Sudden loss of power, no warning. By the time the AA came, I turned the key and it started fine and ran normally. The patrolman reckoned it was carb icing.

I suspect the constant changing throttle in a car helps prevent it.

QDM

[dublinpilot]

2donkeys- my reference to altitude was simply to preclude someone saying that airplane operate at altitude, and cars on the ground. I was simply making the point that both can operate at similar altitude. I have experienced carb ice on the ground before :(

Who Has Control-look at my original question. I specifically mentioned that modern cars were fuel injected...and my question was about cars older than this...ones that had carbs.

Interesting responses..thanks to everyone who contributed.

Looks like cars with carbs either suffered from carb ice or more commonly were fed hot hair all the time!

[strake]

S'funny, I was wondering the very same thing myself only the other day.
So, I asked "Google" and this is what came up:

"I have an '85 Ford Ranger with a problem. Often in temperatures below 50 degrees, usually when there's moisture present, my truck will begin to run rough. It loses power, like it's not running on all cylinders, black smoke comes out the tailpipe, and it sometimes stalls. It usually runs fine until about 35-40 minutes into a highway trip. In warmer weather, there's no problem. My mechanic has said the carburetor is OK. He's replaced the EGR valve, the throttle positioning sensor, plugs, plug wires, rotor and distributor cap. None of that helped. A friend of mine mentioned carburetor icing. Is that possible? -- Harry

Ray: Not only is it possible, Harry. It's extremely likely. In fact, I'll bet my brother's salary on it.

Tom: There's something called the "nozzle effect," which says that when things pass through a nozzle, they cool down. You may wonder why this happens. So do we. Only Bernoulli understands it.

Ray: But we know it does happen in all carburetors. And as the cool, wet air passes through the throat of your carburetor at high speed, it cools off. And when it cools off, it freezes. And when it freezes, it blocks new air from getting through (it looks kind of like my brother trying to swallow one of my mother's meatballs).

Tom: That's why your car runs lousy and black smoke comes out of your tailpipe (black smoke indicates fuel-rich conditions, i.e., too much gas, not enough air).

Ray: And eventually, as I'm sure you know, so much ice builds up that the truck won't run at all -- until you stop, let the heat of the engine melt the ice in the carburetor -- and start up again.

Tom: So what can you do about this problem, Harry? Well, you could move to Honolulu, where there's never been a case of carburetor icing reported to the CDC (the Center for Dying Carburetors).

Ray: My second choice would be to reattach or replace a little paper hose that probably fell off your truck last year. That hose is there to prevent carburetor icing.

Tom: That hose carries warm air from around the exhaust manifold right to the carburetor. And aside from helping your engine run more efficiently in cold weather, because it prevents the gasoline from condensing back into a liquid, it also prevents carburetor icing. That should do it, Harry."

[fu 24 950]

Evidenced by a slight increase in RPM.
During a run up , carb heat to hot to make sure it is working, the increase in RPM is not due to carb ice (clearing), but due to hot air in the system. Back to basic's

[dublinpilot]

Fu 24 950

Back to basics for you i think :)

For fixed pitch prop.

Set rpm as per checklist

Apply carb heat and rpm falls (due to hoter-less dense air in carb).

Remove carb heat.

If RPM returns to original level-no carb ice present.

If RPM increases above original level then ice cleared (at least some of it) :rolleyes:

QDM as my old maths teacher would say! ;)

[fu 24 950]

I STAND CORRECTED, I think i will stick to turbine's and keep my mouth shut

[dublinpilot]

Laugh out loud!:D :D

Though you were going to try and confuse me by bringing in constant speed props!! ;)

I'll get my difference training soon!!

[2Donkeys]

FWIW, for constant-speed props, the indication of carb icing (in the first instance) is a drop in manifold pressure. This again will behave in the same way as RPM in dublinpilot's description of fixed-pitch ops.

[FNG]

I have experienced carb icing in my 1977 Alfa Spider which has twin Dellorto carburettors, and am pretty sure that I had it on a Triumph Vitesse I used to own which had two Strombergs. The Alfa does have a knob to alter the direction of the air intake but I usually forget to move this.

This is really a sneaky way of getting an ad past Paul: anyone wanna buy my lovely Alfa? Goes like the clappers and hardly ever gets carb ice really. Very pretty and, er, red.

All PPLs should drive old cars because we appreciate machines which, despite being impractical, thirsty, noisy and smelly and requiring to be twiddled with by men with spanners and inventive approaches to invoicing are nonetheless highly entertaining.

[A_Pommie]

Carb icing can occur on any petrol engine, as it is caused by 2 cooling effects, firstly as the air speeds up through the venturi its pressure drops and this causes cooling secondly as the fuel is atomised it is cooled by evaporative cooling. The way this is avoided in modern car and bike engines is to run with permenent carb heat supplied by heating the inlet manifold with coolant from the liquid cooling system.

[distaff_beancounter]

I would be interested FNG, except I have already got a nice red 1976 Alfa Romeo Spider 2000, that is not for sale :D

I have never had carb icing on this Alfa, but then I only drive it in nice weather, when I can pose with the lid down :cool:

So, its not been out of the garage much this summer. :(

[Thumpango]

Once had a Fiat Panda and if ever a car needed Carb heat this was the one! Was forever losing power on the climb.

[Fokker-Jock]

Fiat panda?

Loosing power in climb?

Come on, you're driving a Fiat!!!

Has nothing to do with carburator icing :D

[Volume]

Most ´modern´ carb equipped cars (build in the late 80´s) had automatic carb heat, mostly a flap actuated by a vacuum driven diaphragm actuator switching from free air intake to an intake close to the exhaust muffler or in many cases to the exhaust muffler heat shield. Vacuum actuation automatically worked more with low manifold presure (more prone to icing) and less at full throttle. An additional temperature sesitive valve vents the vacuum line at intake air temps above 30 °C, this valve is installed in the air filter housing, closely in front of the carb for most japanese cars.
On older cars (i know from a 60´s Renault 4) you have to switch manually from ´summer´to ´winter´.
The ´winter´ setting (permanent carb heat) costs a little performance and causes slightly more fuel consumption. This is compensated by the higher air density in winter.
In Airplanes we always need full power, so permanent carb heat is no good idea.

I know Carb icing frequently on the Volkswagen Beetle (the old one with these cheap rubbish paper type air hoses that always broke when getting older and let in fresh air in the hot air duct...) which results in total stopping of the engine at idle. No problem while driving with more power.
On the 1987 Honda Civic the effect of carb icing was even more interresting, it happened frequently on cold, misty mornings at slightly above freezing point. This high tech carb had lots of valves, jets and pneumatic actuators for best air fuel mixture at all power settings, unfortunately some of the compensating air jets were affected by icing, causing the second stage throttle valve actuator to open when idle, which results in quite interresting characteristics. It always started after about one minute and went normal after just 2 or 3 minutes when the exhaust was hot enough to preheat the intake air.
This also made the choke ineffective, which only worked on the first stage of the carb, so the engine was running at 2500 rpm idle, but had no real response to the throttle when you wanted the engine to speed up.

Good old days, when you didn´t need a degree in electronics to make your car run again ...:D

[Circuit Basher]

Used to have an Escort Mk3 which progressively lost power in the middle of the Scottish Highlands during a whiteout at 2230 on a Sunday evening until coughing to a halt (somewhere around 10 miles from the nearest phone box and this was before the days of mobile phones - there'd have been no signal anyway!!). Thought that it was wet plug leads, so sprayed WD40 everywhere - lo and behold it started! Went to dealers at next service and asked them to change plug leads (it was a company car!) - they did so, but said there was nothing wrong with them. Went through details of what had happened and they diagnosed carb icing (which had cleared during the stationary bit whilst I sprayed the WD40).

Back to flying - the more experienced amongst you may now yawn and turn away ;) . I also hold a Bruneian PPL (from when I worked out there for a few months). Despite average daily temps of 30-35 deg C with up to 98% RH, one of the major threats is carb icing (worse than UK). I know it's in Safety Sense, etc, but the average guy (or guyess!) wouldn't normally think of carb icing in that temperature!

[FNG]

Distaff Beanounter, you are lucky to have a garage for your Alfa. Alas, I now have no garage for mine, which is the main reason for selling the car. I fear that left outside it will go funny as quickly as an Auster or Cub parked in a field (except that, unlike an Auster or Cub, it is unlikely to have its wings eaten by cows)