Carby Heat

[asra]

This one for Des and Russ ( as requested in the "Beach" topic).As previously posted, I own a gyro with EA81 and a webber dual throat carb (which are renowned "ice-makers"). The original carby heat mechanism was a bit of exhaust tube opened up and "tacked" to the exhaust pipe on one bank of cylinders. I found out early on that this was not effective, and was getting ice while taxying to bring the blades up to speed. It was alright on climb out, but induced ice on descent with a trailing throttle. The first time it really "got" me was at about 20 hours solo, when I descended gradually to the strip to do a low level strip run and climb out. I did the run, but lost power on climb out. I landed straight ahead into a faily rough paddock. I decided it was too risky to taxi the 7-800 metres back, so got the ute and trailer and picked it up.I modified the heat exchanger and set about deliberately inducing ice to test. I tried 3 more mods, and while they were better, they did not totally eliminate the problem. About this time, I saw an article in the gyro news on the subject. I think it was by Anthony Walker (SA), and he claimed that he was using an electrically heated spacer between the manifold and carb that had solved the problem for him. Logic and learning told me it wasn"t the answer, but hey! if it worked, well and good. If some-one could convince me that flying along with a clothes peg clipped to the lobe of my left ear would cure carby ice, then that"s what I would do.This was just before the 98 Nationals (which were to be held at the Traegars). So jumped in the car and beetled over (only 2500km round trip) to have a look. Saw all sorts of innovations. Found Anthony"s gyro and stood to the side waiting until he fired up for a flight. When he landed, whipped over real quick for a look as soon as he parked it. Sure as eggs, the carby looked like a cold can of Fosters. While the device was good enough to cope with icing in the low air moisture of the inland, I doubted it would stand up under the pressure of moisture laden air next door to the Great Southern Ocean (where I live).AS everybody knows, there are a couple of factors that are the root cause of carb icing. Firstly, the change of state of petrol from liquid to gas is exothermic (it absorbs heat). It will absorb heat from the air, or surrounding objects. The temp drop due to the change of state is in the order of 15 - 18deg C. (don"t hold me to the exact figure, as I"ve been nearly 40 yrs out of motor mechanics, and the grey matter is considerably "browner" these days, but Tim or some-one can probably give you three decimal point accuracy. It"s the principle that matters). The second factor that enters the equation is the temperature drop due to lowering of air pressure thru the venturi (same amount of air, now occupying a bigger space). The temp drop is in the order of 3 - 5 degC, and the temp drops are cumulative. So you can see that there is a potential for temp drop of about 23 degC. If the ambient temp is 20C, then it is possible for it to be 3 degrees below freezing down the neck of the carb. The heat can be extracted from the air, but once it lowers, then it will extract heat form the surrounding carb. Many people have made carb heat (or ice preventers) by heating the body of the carb (electrically like Anthony did, or by a coil running oil or rad water). The big problem with any of these solutions is that the part of the carb with the least thermal mass is the throttle valve. Once the throttle valve starts to form ice, its "all over, red rover". And none of these solutions heats the trottle valve. The only real practical way to warm the throttle valve is thru heating the air that passes over it. Thats the way Lycoming and nearly all carburetted petrol engines manage the carb ice problem. It also explains why "slide" type carbs like Mikuni/SU etc are less prone to icing (greater thermal mass of the slide vs throttle valve). But they are not ice proof.Back to my experiments. I hypothesized that the reason my heat exchangers were not 100% effective in severe icing conditions was because on trailing throttle in a long descent, and with the heat exchangers hanging out in the air, they were getting some cooling effect (enough to lowere their effectiveness). I didn"t do temp measurements to verify the theory, but the results were telling me that the heat eaxchangers were not totally effective (with the air temps and moisture contents where I fly). The one thing I did note, though was that the temp of the engine coolant (in the closed loop inside the engine) did not fall below thermostat openeing on the long descents. So my solution was to draw the air for the carb intake thru a heater core (plumbed into the cooling system inside the closed thermostat loop). The carb heat control is a choke type knob thru a cable to a heater tap. The added advantage is that the carb air is drawn thru the same restriction, whether the heat is on or off, so there is no problems with idling. Never had any more ice in the 10 yrs I ran the system.The usual disclaimers, and FWIW"s.CheersFred

[GraemeMonro]

Thanks for that Fred. Your time in writing and posting is appreciated.That has given me something to think about.Graeme.

[Des Garvin]

Thanks Fred for your explaination. Just a confirming question did you use the heater take of points on the motor that is usally used for the heater when installed in the motor vechical? I visulize that you did. Cheers Des Garvin.

[asra]

That"s correct, Des. Sorry for the length of the post, but it was as short as I could make it and still address the issues. The way I did it was not necessarily the only way (or even the right way )_ to do it. But you have to address what is actually going on to get a result. Otherwise, it"s like putting a band-aid on a heart attack.CheersFredPS. Why didn"t I just change the carb? I like the way this one runs and starts. One other thing worth considereing. Magoo used to have a device for switching carb heat on and off. It was a bi-metal strip attached inside the carby throat. AS far as I can recall, when the temp dropped below a certaijn temp (2 or 3 degC) it would generate a current that activated a servo that operated the air flap for the carb heat. It worked real neat. Not sure if it stock item for subaru or not. All the 81"s that I have been involved with have been bastardised by blokes like myself for use in a gyro. I imagine Tim would know if it is a stock item. You could probably make something up with a modern electronic temp sensor (with the probe inserted into the venturi) and have it switch for you. Some of the rocket scientists on the site could probably work it out for you. The reason I still fly the 81 is that I can understand it.

[Tim]

Magoo used to have a device for switching carb heat on and off. It was a bi-metal strip attached inside the carby throat. AS far as I can recall, when the temp dropped below a certaijn temp (2 or 3 degC) it would generate a current that activated a servo that operated the air flap for the carb heat. It worked real neat. Not sure if it stock item for subaru or not. It was a stock item. All the early 1800"s had a manually controlled lever with two positions and was operated from the air-cleaner snout, unless you fitted a cable to it. The valve referred to was on all later engines up to the fuel injected ones. It was as stated a bi-metal strip valve that opened and closed depending on air temperature inside the air-cleaner. A constant vacuum line ran from the manifold to it, and from the other side to a vacuum operated flapper valve in the aircleaner snout. It was fully automated and did not need pilot input other than to periodically check its operation.It did have one flaw on underpowered machines - When run at full throttle there was not enough vacuum to operate the valve. In such cases, fitting a vacuum "ball" (reservoir) and non-return valve from an early Commodore solved the problem.Some pilots by-passed the bi-metal valve and fitted a small 1/8" tricklon tap in the vacuum line so that they could switch it on and off from the pilots seat.

[asra]

Thanks Tim. Vacuum was always available right when you didn"t need it (with the exception of carby heat). Those old enuff to have owned a FJ/FE/FC/FB Holden will know what a pain in the **** the vacuum powered windscreen wipers were. With your foot flat to the floor going like the clappers (as we all were in them days), the wipers would wipe about once a minute. Sitting at the stop sign with the engine idling, about 10,000 wipes per minute.CheersFred