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Gasoline engine modification - ethanol fuel

  1. Dec 2, 2007 #1
    Hi guys,

    Im doing a group project on a conversion of a small motorcycle engine(suzuki dr125s) so that it can run on ethanol(have not decide on what proportion, maybe going to run it at 85% or more). The modifications and the reasons behind it :

    1) Cleaning/changing the fuel tank,filter,gaskets and tubes - Because of the corrosive nature of alcohol.
    2) Increasing the carburetor's jet diameter, add weight to float - Increasing jet diameter to make the mixture more richer because stoichiometric value of alcohol(9:1) is is lower compare with petrol. The weight of the float needs to be added because alcohol is more denser than petrol.
    3) Increase compression ratio - Because ethanol+petrol = high octane number enables high compression ratio.
    4) Advancing the ignition timing and change to a hotter spark plug - "Alcohol burns at a much more even rate. It is harder to ignite and requires more time to completely vaporize and burn."

    First question :

    On the modifications of the ignition system, ive got it from a website :


    Can anybody tell me what chemical properties of ethanol that makes the burning rate of alcohol different from petrol or why is it hard to ignite and needs more time to vaporise and burn?

    Second question :

    This motorcycle uses a magneto+CDI system. From what i understand,as the flywheel is rotating(the rotor on the magneto assembly will rotate too) a magnetic strip on the rotor will excite the pulser coil and signals will go into the CDI module.The capacitor in the CDI block will discharge 150-300volts into the primary coil(part of the ignition coil) .This will create an output of about 20000volts from the secondary coil to power the spark plug. Altering the position of the pulser coil to advance or retard ignition timing is easy but i dont have any idea change the timing curve for a CDI system. Does anyone have any idea on how to do it?
  2. jcsd
  3. Dec 2, 2007 #2


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    Science Advisor
    Gold Member

    I stand to be corrected, but I don't think the rate of combustion of alcohol is the reason it's harder to ignite; I imagine it's just because petrol vapourises more readily. This sometimes necessitates things like fuel pre-heaters, or (as you alluded to) hotter spark plugs.

    The primary reason to advance the timing would be to take advantage of the wider knock margin you'd experience with alcohol over that of petrol.

    I have no idea about your CDI system, but anything which advances the pulser coil will do the trick nicely.
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