Carburettor Heater - need help


Hi All,

Hope this is in the right section.

I am working on a project to heat the body of a carburettor (Bing 84) in order to prevent carb icing.  No commercial solution exists for my carburettor.  My engine (for paramotoring) is air cooled so can't use water from cooling system.

My idea was to strip out the nichrome heating element from a car cigarette lighter and use that.  The heat needs to be applied to the metal casing of the carburettor, preferably being hottest near the air intake side of the carb.  I decided the best place was where the air filter is attached - a 12mm long metal throat with a 35mm diameter The trouble is, I don't know I can transfer the hundreds of degrees of temperature from the nichrome to the metal body of the carb without just shorting out the battery.  If I am to have a layer of insulation between the nichrome and the carburettor, what can I use that will conduct heat but not electrical current, and withstand around 300 degrees of heat?  I also thought about reducing the current and spreading the heating over a larger portion of the metal casing, which would allow the temperature to be lower.

I have also been experimenting with motorbike hand grip heaters that run of 12v but they don't generate enough heat - I need to be pumping in around 8-10A as the internal cooling effect of the fuel vaporising rapidly causes sub zero conditions inside the carb even when the outside air temperature is well above freezing.  I have a 7Ah li-ion which I would only switch on when required (maybe have a latching push button on throttle control).

It may not be doable but its an interesting project nonetheless and something I am going to play around with.  Any ideas?

Thanks, Dug




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Buy some insulated ni-chrome wire, and wind that directly on your carb body.


Steve
dugaldcurtis (author)  steveastrouk5 years ago

Hi Steve, you can get that stuff with insulation on it? Perfect, thanks!

Wrapping it on directly will save me all sorts of problems.
How much power do you really need ?

Why not draw the incoming air over the exhaust manifold, and pre-heat it ?

Steve
dugaldcurtis (author)  steveastrouk5 years ago

Thanks for the suggestions. There are a couple of reasons I would like to avoid joining two parts of the engine or adding what would have to be a fairly large scoop to try to collect warmer air:

The exhaust is on rubber mounts due to the amount of vibration going on. Although the prop is well balanced, it is a single cylinder two stroke and vibration is unavoidable. Even the mounts themselves can break away over time. If I had any sort of connection between exhaust and carb it would eventually break, and the pieces would go into my prop and cause all sorts of in-flight problems.

The black air intake silencer that you can see attached in the second set of pictures actually works very well in reducing noise in flight and I would miss it if it went to make way for something that could sit between the prop and the exhaust. The only place I could collect warm air (due to direction of air flow) is between the cylinder head and the prop, or between the exhaust and he prop (very little space). To get a pipe down to the cylinder head area would be tricky, and I am not sure how it would affect performance.

The other point to consider is that warm air going into the engine will lower performance. Perhaps not by a large margin but every hp counts when you are trying to take off. A solution that I can switch on and off would be best, or an always on solution that heats the carb body.

Its frustrating having so much heat in the exhaust and cylinder head and not being able to access it but I'd be nervous trying to attach anything to fancy to the engine because in flying, everything that can go wrong will go wrong. I appreciate that collecting engine heat if possible would make a lot more sense than electrical heating but I can't see a way to do it that is within my skill set.
What's you maximum flight time ? If we knew that we could work out power availablity better.

The colder the air at the inlet the more efficient the combustion process, so you really only want to just stop freezing/

Steve
dugaldcurtis (author)  steveastrouk5 years ago
Hi Steve,

I can stay up for 3 hours maximum but realistically would only stay up for 2 and most flights probably average 60-90 minutes.

I am still unsure whether it would be better to have an hour of constant heat or say 20 minutes of intense heat. There are arguments for both I suppose. The humidity is generally quite patchy up there and it is usually the case that if you can get through 5-10 minutes of icing it will go away by itself (as you have flown through the patch of humid air). The times when I have had it I really could have used some instant and very intense heat for around 10 mins. 2 heating modes would be good - perhaps two circuits with differing resistance?

Batteries are pretty expensive and I am sure that my motor's stator coil assembly puts out a current (although it is unregulated). I will have to do some more research and check exactly how much it puts out and how to fit a regulator. Perhaps I could have it providing constant heating and then engage a a battery for the more intense heating as required...
I'd make it thermostatically controlled for a start - no heat, unless its actually cold. I'd like to experiment to judge how much heat you actually NEED to do this reliably. I'm not sure pulling energy from the magneto is a good idea though - they ain't wound for current.

Add a small alternator ? Say off a motorbike, or moped ?

Steve
dugaldcurtis (author)  steveastrouk5 years ago
It seems it actually has a three phase alternator already - it just isn't currently used for anything and the wires are taped up. I would need to add a regulator/rectifier and then test whether I get the required amps.

Thermostatically controlled is a good idea except that inside the carb it is *always* a constantly freezing cold temperature - the question is whether the air is humid enough for condensation and then ice to form (which I guess would theoretically increase temperature fractionally). I don't suppose there is any way to reliably detect humidity? That really would be cool :)
Nope, you wouldn't need a rectifier - AC heats just as well as DC.

How much current can you pull out of it ? What voltage ? Wiring a three phase load for it would be no problem.

You CAN measure humidity/ dew point very easily - there are off the shelf sensors for it
dugaldcurtis (author)  steveastrouk5 years ago
I am told the alternator spits out 20-40 volts depending on rpm. I think that the current might be an issue however as someone on a paramotor forum mentioned that around 2.5A is the maximum to expect (although that is assuming a rectifier/regulator is attached).

Perhaps adding a small battery into the mix would allow me to use a lot more amps for short periods, and it would slowly recharge when not in use.

Thanks for the heads up on the humidity sensors - I found this one http://www.mindsetsonline.co.uk/product_info.php?products_id=267

I think it would be best if the humidity sensor ran on its own circuit and just lit up an LED on my throttle control when humidity was looking high but I retained control on when the heat is actually applied. Or I could have a three stage rocker switch - on-off-auto.

If I am to use a battery I would presumably have to get a rectifier/regulator unit. Do they restrict current at all? Is it correct to say that whilst the volts generated by the alternator increase with rpm, the amps are fairly constant?
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