04-14-2007, 06:20 PM
Join Date: Oct 2006
Location: Honolulu, HI
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WI/ methanol faq
OK guys, I wanted to add a faq if at all possible to this part of the forum. That way the info is right there and people can just read it and find the info that they want. If it looks good to the mods, can we make this a sticky to help people out some.
There are two reasons blower cars tend to detonate. One is high inlet air temps, the other is higher cylinder pressure (which in turn further heats the charge). There are a couple of ways to deal with this. One is higher octane fuel, which is not practical for day to day use given what is available at the pump. Another is making the AF ratio very rich, which is used by all blower tuners. The extra fuel acts to cool the intake charge. This costs power, but not to a great degree. Another way is to retard timing under boost. This is also used by nearly all tuners. Timing retard also costs hp and has somewhat limited effectiveness at preventing detonation. Lowering engine temp by better cooling helps, to a small degree. Lowering CR is quite effective. Based on both theory and practice, if you have the choice between lower CR and more boost, or a higher CR and less boost the former will unequivocally produce more hp. The explanation is somewhat long winded and I don't have time to go into it now. The only downside is decreased fuel efficiency under light load (where a street auto engine spends 95% of its' time).
Lowering intake air temps can markedly decrease detonation. This is done by the use of intercooling. The resistance to flow through the intercooler will decrease apparent blower efficiency and hence cost power, but this is more than compensated for by the denser intake charge (due to cooler IAT) and the ability to run more boost without detonation. As noted above, extra fuel also cools charge temperatures. Methanol is a fuel. It has a much higher heat of vaporization than gasoline. So, using methanol instead of gasoline as the "extra" fuel is more effective than using extra gasoline. Methanol also has an octane rating of ~99, so it will raise the octane of the charge in amount proportional to the quantity compared to gas. Just like mixing a little race fuel with the pump gas.
This brings us to water. Water has an even higher heat of vaporization than methanol. It is not a fuel and has no "octane" at all. It cools better than methanol due to the higher heat of vaporization. It also slows the combustion process after ignition, this dramatically reduces detonation and is primarily why the most effective injection fluid is at least 50% water. Why not all water? Well, that does work. So does straight methanol, but not as effectively. What practice has shown is that a 1:1-2:1 water:methanol mix works best. I don't know the theoretical explanation for this. I suspect it has to do with water displacing air. Of course, methanol does too but it allows less gas to be used which will partially compensate. Water increases the need for a strong spark because it is so effective at slowing combustion. That's one limiting factor in it's effectiveness and maybe why some people report better results with methanol. BTW, the combustion slowing effect will require some of timing we already took out to be added back in.
So, max hp for a given octane fuel is obtained by lower CR, higher boost, water/methanol injection, intercooling, careful tuning, and a strong spark.
Methanol has extra added benefits when used. Methanol has a badass flame travel speed.
Higher heat of vaporization (pulls more heat out of intake)
Higher specific heat. As water passes thru the combustion process it carries heat with it. That is heat that could be doing work.
Makes for dead space in combustion
Slows flame travel speed. More ignition advance is needed for both flame propagation AND flame degree. More of the mixture is started BTDC in order to balance as much mixture burning right ATDC so your doing more negative work. On top of that mixture is not burned as close to TDC where change in volume is happening fast relative to crank angle.
Carries a good heating value
Superior flame travel speed. Even better than gasoline
Adds to total heat addition to the process, instead of taking from it
Its heat of formation of the products is less than that of what pure excess water's specific heat
Hard on parts?
All in all, very effective way to add horsepower and torque. The real secret of water/methanol injection is in the tuning. When just the injection is added, a little bit of power will be taken away, but when tuned for the extra spark advance, the power is added back in and more.