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Discol

Under Heavy Twins, there's some comments about carb icing being worse with alcohol fuels. I found this the other day in a 1934 'Automobile Engineer'. And when ethanol in fuel is discussed  the topic of Cleveland Discol often crops up.

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... but I'm not convinced that the fuel is actually vaporised. Consider fuel injection - the petrol is introduced as a very fine ("atomised") stream of droplets.

If you are getting vaporisation then I think there's something wrong. My 16H used to vaporise the fuel if left for 10 minutes or so when warm and it was a pig to start then (but fine other times).

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I have suffered carb icing 3 or 4 times over the last 2 years ,perhaps its my fault for daring to ride  when its not sunny and dry!.  Usually its just a few minuits wait for the carb to unfreeze by the side of the road. Last time it was in an unlit narrow lane pitch dark except for the annoyed motorist that could not get by.

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I think that without vaporisation  the fuel just passes through  and you end up with wet plugs and tired legs.  If you are brave enough to plunge a lit match into a full can of petrol  its likely to go out. Not recomended .  When I used to fly powered models ,my assistant (idiot)  put a match to an empty fuel can that had been lying open and empty for a week.  A very loud bang and the can flew better than my plane. A tiny bit of vapourised fuel.  After that I built  rocket and pulse jet powered planes that mostly blew up. 

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As stated, liquid fuel doesn't burn, and is one of the reasons that fuel companies supply 'winter' and 'summer' fuels - winter grades have a lot more 'light ends' (propane etc) in them to allow vehicles to start when it is cold. Alcohol fuels are very poor at evaporating when cold, so the highest concentration in regular use is E85, eg.15% petrol, which is needed to get engines running from cold in anything other than a hot climate.

Fuel supplied via the inlet port will vapourise both in the port and on the back of the inlet valve prior to the induction stroke and of course within the cylinder under compression where the temperature increases quite markedly.

Most port injected EFI engines are developed to allow the fuel injector 'spray' - (the droplets of fuel are usually around 8 -15 microns(0.04-0.06thou) diameter) - to target the back of the valve/s with minimal impact on the port walls and the injection pulse is timed so that  the fuel spends as long as possible there*. It is necessary to do this to keep hydrocarbon emissions down, especially with a cold engine when the catalyst has yet to reach operating temperature. Engines with 2 inlet valves will have 'twin spray' injectors to target each valve. As carburettors cannot do this, they compensate by having a 'choke', i.e. chuck a lot of fuel in so that at least some will evaporate and allow combustion to occur and the engine run.

Direct injection engines have all of this evaporation within the cylinder, allowing better cylinder filling ("dry" air is denser across the whole inlet stroke as less heat is transferred) and higher compression ratios are possible too as the compressed charge is cooler. It is for this reason that direct injection engines are more thermally efficient than port injected ones.

* Competition engines will inject onto an open inlet valve to promote better cylinder filling, but they don't have to worry about emissions regulations.

Hope that this makes sense,

George

 

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