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Carby heat - Why do we need to do it manually

Guest Ginger

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As a very low hours student I am surprised about the need to manually apply carby heat.


Besides adding to pilot load, being manual it is subject to being forgotten and the potential cause of accidents. I know of one carby heat related incident of engine failure on landing.


In my long life I must have driven cars at least a million miles and never experienced engine failure that I could contribute to carby heat.


My runabout L series Subaru, carby heat is applied via an 8mm feed from the hot water in the engine cooling system. It also has a bi-metalic strip controlled flap which can direct hot air from around the heat exchanger when the air inside the air cleaner gets too cold. It is a 20 year old car and still works perfectly. This is probably common in other makes.


I once had a G series Citroen with an air cooled flat 4. It had carby heat applied via diverted exhaust gas and a heat exchanger under the carby. I must admit though, the aluminium heat exchanger burn out when the car got very old - still inteligent design should have fixed that.


Another person also told me the old air cooled old VW flat 4s were subject to carby icing. Inteligent design should have fixed that too.


At the very least', Rotax engines could have something like the Subaru and the aircooled motors such as the Jabaru could use the automatic bi-metalic strip idea. A complimentary carby thermometer would be great as well.


So to summarise, why do we have to fiddle around with a manual carby heat control ???



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Well, i can only speak for myself, and i wouldn't like any automated system playing around with my carby and reducing the power of the engine whenever it got cold feet...And also, your aren't likely to reduce altitude at over 500 feet per minute in your car, and it doesn't nessacarily have to be cold for carby ice to develope.. someone with a bit more technical knowlage will surely chime in with a better answer for you...cheers, good luick with the training



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I was plexxed by your question so i did a bit of research...Hers's what i found, it may help you and others understand the carb ice problem.. Point 5 answers your question i think...The texts relates to a Corvair powered Pietenpol tha crashed some years ago in the US.


1) Ice can form on warm days. Anytime a gas expands from high pressure to low it will consume energy from its environment. In this case, the gas is the air the engine is consuming and the pressure drop is from ambient to manifold pressure, about 30"map to 12"map. The energy it consumes is any form of available heat. Most of the heat comes out of the air. This temperature drop is instantaneous and can easily be more than 40F. Shoot a thermometer with a CO2 extinguisher and learn.

















2) Carb ice forms at the pressure drop point, which occurs at the




restriction to flow. At a reduced power setting, the throttle plate is the




restriction, not the venturi. At 2,200rpm and 12"map, the throttle plate is




barely cracked open. Right at this crack is the idle fuel port, a tiny hole.




A minute film of ice could cover it in an instant. The engine will stop




running because at power settings like these, most of its fuel comes from the




idle circuit.








3) A certified four-cylinder engine of 7 to 1 compression and 190cid




swinging a 25-pound metal prop generally will sputter for a while when it is




experiencing carb ice. By contrast, a 9.5 to 1 six-cylinder 164cid engine




with a 6-pound prop will quit nearly outright. Many Lycoming pilots said




their engines gave warning. Lycoming carbs are bolted to the oil sump and




experience the onset of icing at a slower rate.








4) Icing has nothing to do with rpm; it results from the pressure drop.




Granted, an A-65 Cub with a certified prop is very unlikely to ice at




2,200rpm, but this is because 2,200rpm usually is associated with nearly open




throttle on this plane, and consequently very little pressure drop in the




carb. However, any motor experiencing a large pressure drop in the carb is prone to




ice, regardless of rpm. The motor in N1777W had a static rpm of 2,650. Any




motor that has a prop that will allow a similarly high static rpm will be




running low map at an rpm like 2,200. A manifold pressure gauge provides




useful information that a tachometer by itself does not.








5) . Remember that at a




reduced power setting, the restriction is the throttle plate. And when




operating at reduced power, there is a large pressure drop at the plate, with




its accompanying temperature decrease. Fuel flows out of the idle port in a




mist. Misted fuel is still a liquid, not a vapor. 100LL under these




conditions remains a mist until reaching the combustion chamber. Contrast




this with auto fuel, which by design will vaporize readily under these




circumstances. It is a fact of physics that when the fuel changes from a




liquid mist to a gaseous vapor, it takes further heat from the surrounding




air. This is the cooling one feels when gas evaporates off the skin. This




additional temperature drop can produce icing when the same engine under




identical circumstances would not ice with 100LL.







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or you could just use a fuel injected engine!

There is some debate about icing in various types of fuel injection systems. Most systems I have used are a direct mechanical injection system which, as Rog says, is not prone to icing. I was reading the other day about Throttle Body Injection systems which, although rare, can ice up if they have long induction paths. Do may RAA type aircraft have these systems ? Are they required to have a heat source ?


One site that covers this is http://www.ellison-fluid-systems.com/faqs/carburetor_heat.htm which mentions experimental aircraft in particular so I am assuming they must be used a bit.



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Guest Flyer40

Ginger, I think a better question is, why are carby engines being built at all in the 21st century? There was a discussion about it here a while back.


There is an inherent conservatism in aviation which is generally a good thing, but it has the side effect of sometimes slowing progress. Engines are a good example of that. Thanks to LSA regs in the US, the engine with the fastest sales growth is the 60 year old carburettored O-200. I can't think of any other industry where 60 year old technology is still on sale.


Anyway, I just read in the March issue of Kitplanes that one aviation carby manufacturer has ceased production after it was unable to get product liability insurance. It's been on the cards for a while. Insurance was the reason carby engines were just as expensive as injected engines.


Perhaps the insurance companies are going to drag aero engine makers into the 21st century. Kicking and screaming if necessary.



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Guest RogerRammedJet

Flyer, your post is the very thing that I was subtly hinting at. One does have to wonder why, when the cheapest of cars have fuel injected engines, we need to have a discussion about carby ice in aero engines in other than the "vintage" aircraft area.





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For those of you who get the Kitplanes magazine, I would refer you to the September 2006 edition. On page 47 an article starts called "Budget Injection." It is an article about mechanical fuel injection and personally I found it informative. I hope this can add to our knowledge base and understanding.



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I worked on mechanical fuel injection in 1956, horrible, but no doubt it is far better now.


Carbies are cheaper to make, cheaper to fix and have stood the test of time. We could have full time carbie heat which is what some cars had, but we need every last bit of power we can get at take off and climb.


Having flown cessna's and other GA plane I can see no need to do away with carbie heat. The load on the pilot is minimal.


If you are going to argue to do away with any thing which needs a pilot input, you must be for automatic landing gear controls, auto control of flaps, radio tuning, stall prevention and a host of other things. Some have been found to be incompatible with safety, some would be too expensive for recreational flying and others would mean we might as well employ someone else to fly our planes.


Far more usefull would be auto control of engine cooling and auto control of mixture, which is already done by Bing carbies.



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I think there are bound to be problems with anything "automatic". I don't have a problem with carburetted or fuel injected engines. If anything fuel injected engines can be more of a hassle - especially when hot.


What's so hard about using carby heat when it is needed?


My aircraft has a carby temp gauge which is very helpful. Remember engines run rough with carby ice, but they also can also run rough when carby heat is used when not required.



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Guest Flyer40



I agree that using carby heat is not much of an imposition on the pilot in the routine pre-descent circumstance. But the accident data shows that the hard part is in recognising when you need carby heat at other times. Carby ice can be insidious, the warning signs can be easily missed and it's getting people killed. In my view that's unacceptable when the option exists to eliminate that risk.


I've seen aircraft conk-out due to carby ice just before lining up for takeoff (grass runways with early morning dew are a bad combination). Imagine what might have happened if a takeoff had been attempted with the carby partially blocked and the engine making much less than normal power.


Sure injected engines are hard to start. That's not ideal, but I would prefer that to the risk of being killed.



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A very good question.


I guess the answer, much as others have stated is for several reasons.


  1. Simplicity, whilst some of the automotive solutions aren't overly complicated they're certainly not as simple as a manual system.
  2. Controllability, you really do need the ability to decide when to use heat and when not to, and this leads to 3.
  3. Performance, as there is a notable horsepower reduction with carb heat applied you need to be able select heat off for take off and missed approaches. The POH's take off and climb performance figures are based on full power or rated power.
  4. Certification, a manual system is quite probably a lot less drama for certification purposes.
  5. Litigation, much better from a manufactures point of view to have an accident/incident caused by pilot inability than a manufactures component failure.
  6. Testing, it is a simple exercise to test the functionality of a manual system during pre-fight checks.
  7. History, it is a system that has stood the test of time and is universal to all types.


However there is also merit in an automatic system, countless accidents have been caused by undiagnosed carb ice. However with good training and few occurrences you will find the operation of a manual system not overly taxing. The key to it is to try and induce carb ice at some stage whilst in a safe environment so you get to see what it's all about.


Good luck with your training.





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"Sure injected engines are hard to start"They are?



I think the reference was to "hot" injected engines can be hard to start. I've had a few experiences with the CT4 (mechanical fuel injection system) where it's taken a little while to get it going when it's hot - mostly due to me not being precise with the "warm start" procedure which is very different to the cold start procedure, the peak time for a warm start to be a pain is between 30 minutes and 1 hour from shutdown as heat soak starts to affect fuel lines etc.


That being said, I also had similar "hard to start" experiences with the Rotax in the Sportstar when either really hot or really cold.


The point about "automatic" everything is not too far off - look at the FADEC systems available now in aircraft like the Cirrus and Colombia, they control everything engine and prop (if CSU fitted), all you get is a "power control lever" and an on/off switch. Don't be surprised to see fuel injected engines with FADEC in your new ultralight within the next 10 years - which takes us back to a previous point I made about supply and demand - the flyers of the future will demand these innovations and manufacturers will be driven to supply them.



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Just to add my pennys worth, I have owned a couple of cars, (many moons ago) which had these wonder devices fitted to the carbs.... and all I will say is that if anyone ever tells me that their aircraft is fitted with such a device, I will not even look in it's direction, let alone try and fly in the thing. There was also a very similar set-up fitted as an automatic choke on some vehicles, (also many noons ago) and it was worse than useless, most owners having to buy manual adaptor kits to put it back the way it should have been in the first place, (the inteligent design). Thankfully we were all saved as fuel injection began to take over.





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Yes, hot fuel injected engines can be harder to start! I've seen plenty of hot Pitts Specials grinding away without starting for several attemps. Even with hot start techniques.


Agreed about carby ice on the ground. I've had it in a C172 at Bathurst and one of the things I'd do there before take off was to put carby heat on for a short time before lining up, then turn it off again before take off.


I'm pleased that I have a gauge because the thing that surprised me was how roughly an engine will run when carby heat is not required and it is used!



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Hot injected engines are not hard to start......they DO require a different technique.


Automated carby heat......why bother, hand control works fine!


Also.......you think its cockpit or pilot workload......I think with most RAA machines there is not a great deal of pilot workload when compared to the run of the mill GA machines.







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Guest Flyer40
"Sure injected engines are hard to start"They are?



Yeah Rog. Aside from what's been said about hot starts, some pilots find the cold start in the newer 182s a bit challenging. There is a specific technique required, and hence the high likelihood of error. Aux power sockets are a common mod in these aircraft due to the frequency of flat batteries.



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Guest jimbo_xyz

I agree with the comparison to the old automatic chokes.


Manual is often eaiser.


I say bring on Diesel engines. Even less to worry about with no mags.


Can't wait until this becomes a real viable option.



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Guest Flyer40

Perhaps I should clarify, what I meant was that a Lycoming with mechanical injection (or at least the 540 used in the 182) needs a specific technique to start, hot or cold which can be difficult to get right. I reckon that's pretty poor for a brand new aeroplane, but I'll take that over carby ice.


I'd like to know more about your 2100 Don.



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I have a EFI Jabiru 6cyl with 240hours and have never had a starting problem hot cold etc or icing problem. My mates EFI Jabiru 6cyl with 230+ hours has also never had a starting or icing problem.


Cheers Helmut.



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