Why isn't the carb heated all the time?

[rich_g85]

This question popped up yesterday but I forgot to ask my instructor, so here you go..

I just wondered why couldn't the carb heat either A) stay on constantly or B) the carb be modified so that there was always a flow of warm air to/through it, removing the need to 'remember' (not that I have an issue with remembering!) to switch it on when reducing power, during the FREDA check, etc.

Perhaps the answer to this will become obvious when I've done the work for the Aircraft Technical exam, but in the meantime could anyone explain this for me?

Thanks,
Rich

[flybymike]

Cold air is denser than warm air and gives more power and better performance with less likelihood of engine damage due to knocking/pinking/pre detonation etc

Having said that I believe some aircraft engines do have a permanent warm air supply to the carb

[Flight Inspector]

Carb heated air is thinner (less density) and that's what you need to have a better (optimized) combustion. The thicker, colder or denser the air the better your combustion. (If the mixture is right.) See it as flying always 2 or 3000 feet higher than actual altitude. The air is les dens, so you need to lean, to have an optimum mixture. Performance will be less with Carb Heat on. You’ll notice when you switch it on while checking for Carb Ice. Why it is still on modern light airplane’s, its cheap. Other solution is the injected engine. This will have the Carburetor remove and there is no change of Carb Ice.

Hope this helps. Gool luck with the training.

[tomtom_91]

Economy - I remember thinking this a few weeks ago!

Tom

[Iwasoneonce]

Applying carburetor heat sends a warm air through the carburetor, which has the effect of giving a rich mixture, hence the RPM drop. Also when carburetor heat is applied the air is unfiltered. IIRC.

[Deeday]

I believe some aircraft engines do have a permanent warm air supply to the carb

I've heard of a mod for Rotax engines that takes warm fluid from the cooling system and makes it circulate around the carburettor body, so that the body gets the heat (and so preventing any ice accretion), instead of the air/fuel mixture.
Now that's a smart solution, always-on and costing no power.

[IanPZ]

Yeah, the rotax engines in microlights (912UL) have the water jacket extended to the carb bodies. This stops ice forming on the air intakes, but at the same time does not sufficiently heat the air going in to impact density that much.

It's still possible for them to ice up, but much more unlikely, I am told.

[Nearly There]

Ive often wondered why aviation still has these uneconomical and very outdated engines, and there not cheap, I heard someone talking £10k+ for a reconditioned lycoming recently!

[Rod1]

The Rotax 912 is very common in Group A / SEP aircraft as well as micros. There is an electrical system which heats the body of the carb which does a similar job to the water jacket. It is available on Rotax 91X and Jab engines.

Rod1

[FlyingStone]

As explained, use of carburetor heat increases the temperature of air in the carburetor venturi, which means that density of the air before the venturi (where mixing of fuel and air takes place) drops (if I remember correctly the mathematical equation is Rho = p / (R * T) ). Don't take me for granted, but I believe the rise in carb temperature with carb heat is usually around 10-15 degrees Celsius, which basically means that density (and therefore engine power) drops for around 5% - provided the air-to-fuel ratio remains the same. Not much of importance on low-level cruise, but could be lethal at high-elevation takeoff/go-around.

I think at this point it should be noted that ANY aircraft engine can suffer from icing, it's just the type of icing engine is prone to that differs from engine to engine. Most people immediately think of carb ice when mentioning engine icing, but that is only one reason. It's important to know that ice can form on the air filter (cold and humid air), which prevents air from going into intake system and this type of icing doesn't affect only carbureted engines, but injected too. This is also one of the reasons why carb heat isn't ON all the time, since the design was made to be easy to use and carburetor heat and alternate air switch were incorporated into a single lever/knob now called carb heat so by applying carb "heat" the engines also "breathes" air without a filter blocking the way.

As said, the technology is here for a few decades (classic gasoline injected engines) and is improving (direct injection diesel engines with accurate mass air flow sensors which makes possible to set the optimum mixture), many manufacturers even put injected engines into training aircraft (C172SP, DA20-C1, ...), but the price and "reliability" seem to be on side of carbureted engines. Ah, new technology in aviation - seems to take forever to get it certified and almost impossible to convince aircraft owners to buy/use it

[BackPacker]

Obviously the Rotax 912 is (partly) water cooled so there's coolant available to heat the carb bodies. On an air-cooled Lycosaurus you would need a different solution.

This stops ice forming on the air intakes

Actually, it doesn't. Ice can still form due to the venturi effect and the evaporation of the fuel. But ice can't stick to the carb walls anymore. Either it contacts the carb walls and melts, or is sucked into the engine itself. Both of which are harmless.

Other solution is the injected engine. This will have the Carburetor remove and there is no change of Carb Ice.

Another issue is that the carb heat mechanism also ensures that the intake air bypasses the air filter, thus enabling an alternative air intake path when the air filter is clogged up (for instance by impact ice). That's why you try to minimize carb heat application on the ground - the unfiltered air might have sand particles or other foreign matter in it that's been thrown up by the prop blast. You could of course decouple the carb heat and alternate air intake functionality, but that would mean you'd get another lever in the cockpit.

[POBJOY]

Heated Carbs

A "oil heated carb" (jacketed) was not uncommom on pre war engines, however they are expensive to produce and the carb would need to be "heat soaked" to work.Their purpose is to prevent a build up when the throttle is closed under normal conditions.However in severe conditions if a build up occurs it would still need another source of hot air to clear it.
The same principal can use the coolant as the heat source on a liquid cooled engine.

[englishal]

Ive often wondered why aviation still has these uneconomical and very outdated engines, and there not cheap, I heard someone talking £10k+ for a reconditioned lycoming recently!

Don't be ridiculous. Who's ever heard of a Lycoming costing 10k to rebuild !!!

Ours cost 28k to zero time....

[lotusexige]

He did say £10k+, a big + though

[Genghis the Engineer]

Originally Posted by englishal

Don't be ridiculous. Who's ever heard of a Lycoming costing 10k to rebuild !!!

Ours cost 28k to zero time....

Or about the price of two brand new Rotax 912s!

(This is one reason why I try to avoid owning more than 1/10th of any one light aeroplane!)

G


N.B. Standard practice on most Gypsy Major installations is also to leave carb heat on all the time.

[Big Pistons Forever]

Originally Posted by Silvaire1

Lycomings run the intake tubes through the oil sump and for that reason they are not particularly susceptible to carb ice. Small Continentals do not, and carb heat should be used liberally in icing conditions.
.

While in general your statement is correct the engine installation has a significant effect on the propesnsity to suffer from carb icing. The two worst aircraft for carb icing I have ever flown were the BN2 Islander and the PA 23 Apache, and both have Lycoming engines.

The smaller carburated Continentals are getting rare in the training fleet as almost all training aircraft in use today are fitted with the lycoming O 235 or O 320. The C 172 and Pa28 series in particular seldom develop ice so it is common that students complete their training without ever actually expereincing carb ice, thus I think it is doubly imprortant that proper procedures for detecting and dealing with carb ice are taught.

[funflier44]

When I did my flying training on RAF Chipmunks back in 1965 and when subsequently flying them on holding postings the Carb Air was always "wired HOT".

[bingofuel]

If I recall, on the chipmunk it was common to wire the selector in the 'warm' position. I don't think it had a 'hot' setting and warm had minimal effect on power

[T21]

Carb icing occurs when the ambient air is moist and between plus 5 and plus 15 degrees. If you have carb heat on all the time you can raise the temperature of colder air up to the above range. If you get icing then you do not now have extra carb heat to clear it.

[SNS3Guppy]

Carb icing occurs when the ambient air is moist and between plus 5 and plus 15 degrees.

Actually, no. That's structural icing. Not carburetor icing.

Carburetor icing can occur at much higher temperatures, because the carburetor experiences a significant temperature drop at the venturi throat. Carburetor ice can happen on hot summer days, and often does.

I just wondered why couldn't the carb heat either A) stay on constantly or B) the carb be modified so that there was always a flow of warm air to/through it, removing the need to 'remember' (not that I have an issue with remembering!) to switch it on when reducing power, during the FREDA check, etc.

The purpose of carburetor heat is to move the carburetor air temperature out of the icing range. This is properly done using partial carburetor heat, and a carburetor air temperature gauge. Given that most light airplanes don't have a carburetor air temperature gauge, manufacturers take a ballpark shotgun approach and have the user apply full carburetor heat, or no heat. This is surgery using a meat cleaver.

Carb heat, when not needed, is detrimental. It reduces engine power.