Methanol Injection Explained -- Air/Fuel Raios, Mixtures, Latent Heats, etc...
This is from another forums, and I found very easy to understand (not that I didnt already know). Iam sure there is something like this on the srtforums but it wont hurt to have more usefull information.
Not everything applies to the Srt4 or 2.4t motor just keep that in mind.
Sticky Maybe ?
and thanks Razathorn
As one of (if not the, I don't really recall for sure) the first supercharged guys to do water/methanol a few years back, my setup has basically "just worked" for a long while now. Now that I've moved to 11 pounds w/ 06 tsx cam in 100+ degree heat, I find myself needing to make adjustments to keep the detonation fairy and her pings of doom from claiming my 75k oem motor. For a while, off and on, I've dropped little nuggets of explanation in chat threads and random threads -- but I haven't really created a technical resource for those who really want hard info on what is going on behind the scene. This thread aims to fill that void, confuse some of you, and enlighten (hopefully) the rest of you that want to understand things better. If you get confused, ASK questions -- you won't be the only one, and hopefully we can turn this thread into something really useful for others.
First, before we start, we need to get vocabulary out of the way. Without knowing the following terms, you won't get very far here.
An air/fuel ratio describes how many parts of air and fuel are present during combustion.
Stoich comes from stoichiometry. Stoich, for our purposes, describes the condition where combustion of the air/fuel mixture leaves no remaining oxygen or fuel. I sometimes refer to it as perfect combustion, but don't confuse that with being what you should want at all engine operating conditions.
A ratio that defines how many parts fuel to oxygen you need for stoich mixture. The Stoich point of gasoline is 14.7:1, or 14.7 parts oxygen to one part fuel. The stoich point of methanol is 6.4:1.
Loosely defined for our goals, it describes, via a percentage, how much fuel or oxygen was needed for stoich combustion. A lambda value of 1 is stoich. A lambda value of .85 means 85% of your fuel was burned, and 15% of it was left over. A lambda value of 1.15 means 15% of your oxygen was not used, and 85% of it was.
Lean describes a condition when too little fuel is present in your combustion event to be stoich. Rich describes a condition when too much fuel is present in your combustion event to be stoich. A rich mixture has a lambda less than 1.0, a lean mixture has a lambda greater than 1.0. Rich/lean is also used to describe having too much or too little fuel in relation to what is desired, not just in relation to stoich. For instance, even though 14.7:1 is stoich for gasoline combustion, and by definition is not lean, it is far too lean for wide open throttle to be safe, and should be richer.
Your wideband oxygen sensor. It is called a lambda sensor, or air/fuel sensor, because it detects and informs the engine computer the lambda of the combustion event that has just happened -- in other words, how close it was to stoich. Kmanager converts the lambda values to "gasoline air/fuel" ratios for you by multiplying the lambda by the stoich point of gasoline. For example, .85 lambda is .85 * 14.7 = 12.5:1 a/f. The lambda sensor, engine computer, and kmanager are totally unaware of what fuel you are running. If you were to run pure methanol, and tune your car to a perfect stoich air/fuel ratio of 6.4:1, the lambda sensor would see 1.0, and kmanager would report this to you as 14.7:1. It is important that you realize that the air/fuel ratios you know and love are simply a calculation based upon empirical measure that is NOT actually parts of fuel to parts of oxygen. It is also important to realize that when you run a combination of methanol and gasoline, your air/fuel number scale in your head that says "11.5 is safe for boost, 13.2 is not" is still 100% valid and useful -- just think of it as "this is the air/fuel ratio I would be running if I was running purely on gasoline." 11.5:1 gasoline air/fuel ratio literally means 11.5/14.7 = .78 lambda, or 78% fuel was burned, 22% remained unburned (and absorbed heat).
Methanol is a simple alcohol with a stoich point of 6.4:1. It melts/freezes at -98C and has an octane rating of around 120. It has a relatively low flash point, so mixing with water, in addition to adding more cooling benefit to your water/methanol mixture, adds safety by increasing its flash point.
An engine noise -- not good. Knock generally falls into two categories: spark knock (detonation) and rod knock (or other physical problem with the motor that makes you tear it down).
Detonation is far less harmful than it sounds, but leads to things more dastardly such as pre-ignition or engine damage if left unchecked, especially if the detonation is severe or the engine is already operating near it's physical limits. Detonation is an event that happens AFTER normal ignition of the compressed air/fuel ratio as the cylinder is moving down on the power stroke. Detonation occurs when the air/fuel mixture transitions from burning (granted, very fast) to exploding (hence the word detonation). Ideally, the air/fuel mixture should burn across the time when the piston is moving down on the power stroke. If for some reason the mixture cannot sustain normal combustion under the heat/pressure it is being exposed to, and explodes, this is called detonation. It delivers a large amount of force to the piston very quickly for a very short period of time producing an audible sound referred to as a "ping", and hence "pinging" is another term to describe a motor that is detonating. It heats up the combustion chamber quickly. Normally, when you encounter detonation, it is near the end of the normal combustion event. This is especially true for detonation you encounter from running too much ignition timing -- you can literally think of this as the burn beating the piston. Denser mixtures burn faster -- that's why more boost dictates less ignition timing. Detonation can be caused by too much ignition timing, too little octane, too hot of intake temperatures, or too lean of a mixture.
Pre-ignition almost always leads to detonation, but... pre-ignition also has a terrible habit of destroying motors. Pre-ignition is when the air/fuel mixture is ignited BEFORE the normal ignition point (dictated by ignition timing) by means other than the spark plug firing. Pre-ignition causes the piston to compress an expanding mixture, which causes a huge strain over a long period of time compared to detonation (which happens quickly and is over). The mixture usually detonates when pre-ignited. Potential ignition sources could be an overly hot combustion chamber, glowing carbon embers, a glowing spark plug, and any other residual heat in the combustion chamber that shouldn't be there. The problem with pre-ignition is you simply don't know if the detonation you are hearing is from pre-ignition until your motor blows. Rest assured, most detonation encountered is not from pre-ignition -- however, if your motor detonates enough, it could raise the combustion chambers so much that your engine pre-ignites, which tends to put the piston on the ground, and we all know that isn't where it goes, now is it! That is why you address detonation, in addition to the fact that severe detonation can damage the motor as well.
Octane is a rating used to describe a fuels resistance to detonation. It is also a compound, but we don't care about that.
The point at which the spark plug fires before top dead center on the compression stroke to ignite the air/fuel mixture. More ignition means you ignite the mixture sooner. The idea is to expose the power stroke (piston moving down) to as much of the force of combustion as possible while not detonating and not compressing an expanding mixture by starting the burn too soon. Ideally, you start the burn while still compressing the mixture because it takes some time to burn. If you have a high enough octane fuel, you can actually run too much ignition timing and compress an expanding mixture -- this puts enormous stress on your rods and rod bearings and is akin to mini pre-ignition. You CAN over-advance the motor with water/methanol. Don't advance ignition timing without being on the dyno to see if it adds more power. If it doesn't add power, take it out, it's just adding stress. Ideally, you should be just below peak power ignition for a long lasting boosted motor tune.
Latent heat, for our purposes, describes the amount of heat absorbed by a liquid as it changes matter states to a gas when vaporized.
-Why do we run rich?
This is something many folks take for granted -- you run rich, or fuel enriched, under load. More so for boost, less for n/a, but not at all for cruise. Why? It's simple. We don't want our motors to blow up. Why would it blow up? Because it gets hot in there damn it! Running rich means there is extra fuel left over that didn't get burned. This fuel absorbs heat. This keeps the engine from being damaged. At cruise (in vacuum), the power produced by the engine is tiny -- you don't need to run richer than stoich to protect the motor, and catalytic converters need stoich combustion exhaust to work correctly.
-How exactly does fuel cool the combustion event?
The unburned fuel is vaporized (evaporated) -- it changes from a liquid to a gas. This change in matter state, from a liquid to a gas, takes heat with it. Think of when you sweat -- liquid exits your pores, wind blows across your skin, the sweat evaporates, and your arm gets cooler. The problem with blindly adding fuel to cool down the combustion event is that it makes the air/fuel mixture richer... and after a while, it doesn't want to burn, and it stops making power (and other problems as well). The amount of heat absorbed by a liquid as it changes to a gas is described via its latent heat.
Methanol, an alcohol, is a fuel that your engine can burn. It takes more than twice as much methanol to make the same power as gasoline, which is directly related to it's stoich point being about half that of gasoline. It literally takes half as much oxygen to burn an equal mass of methanol as it does gasoline, and that directly represents it's energy potential. Ethanol is an alcohol also (om nom nom), and has a similarly low stoich point, and similarly lower power output, which is why e85 folks have to run huge fuel systems compared to gasoline folks. The good thing about methanol is that it has an octane rating of around 120, and cools about THREE TIMES as well as gasoline when vaporized in a rich mixture. I know that sounds wonderful, but just wait until we get to water. By adding a little bit of methanol, you can raise the octane of your mixture a decent amount, and any remaining fuel is partially methanol, and thus will remove more heat than if it was purely gasoline.
Water is a.... wait, we drink this stuff, it shoots out of our pores when we're hot... perhaps we could use this for cooling?! Indeed! Water removes TWICE as much heat as methanol, and OVER SIX TIMES the heat of gasoline. Want to know the really cool part? (Oh man, I sincerely apologize for that pun, it wasn't intentional, but now that I see it, I'm not removing it because it hurts sooo good.) What's really cool is that any water injected into the mixture does not burn. ALL of its heat removing goodness is left there to absorb heat, regardless of the air/fuel mixture you used.
-The proper mixture of water/methanol to inject:
This is debated a lot, but it generally ends up being 50/50 by mass, not volume. Sorry guys who are mixing your own and filling two jugs equally, you failed -- you should have used a scale. Methanol is about 74% the weight of water, so if you thought you were running 50/50 and you measured by volume, you're actually running around 37% methanol to water. DOH! Don't worry though -- if you're not detonating, then you're in the money, because all that extra water is there to cool the combustion event and perhaps the boost charge as well, especially if it's hot enough and has some distance to travel (see turbo piping and high boost supercharger.) Ideally, in my opinion, you'd run the perfect mixture that had just enough methanol to up your octane rating enough to ward off detonation with the cooled combustion event -- this is because the water cools better than methanol. If you run 35/65 (-30F washer fluid, what I used to use), and it prevents all detonation, moving to 50/50 would just add octane while removing cooling ability. This is why I never messed with mixing before I went to 11 pounds. Now that I'm here, one 100F+ day and I detonate a little -- time to add more octane. By running 35/65 instead of 50/50 when that was all I needed, I cooled the combustion event (and to some degree, the charge) quite a bit more. Remember that the methanol that cools is what is left over, so more water means more cooling, not only because it cools better than methanol, but because it isn't burned. The methanol is still burned, and the remaining methanol from running rich is all that is left to cool! This is why I'm so against people running more methanol than they need -- not only is running pure methanol dangerous, it's not required most of the time. Any methanol you can replace with water means more cooling and less flash danger. Seems obvious now, but good luck convincing some folks.
-The proper ratio of spray to fuel:
Generally speaking, you want to be around 20 - 25% spray to fuel. any more and you could potentially (so I've heard) wash water down the cylinder walls and contaminate your oil sooner than you change it. Personally, I don't put too much faith in this since we squirt a lot more fuel in there, and the methanol burns too, so it should (in my head) just depend on how much water you're shoving down there. I have about a 25% spray to fuel ratio on average in my power band (less than 25 at 8500 rpm, more than 25 at 6k rpm.) You can determine the proper amount by calculating the fuel you use by multiplying your injector size * 4 then factoring in your duty cycle. Injectors are rated at 43psi from rc, and we run around 51 pounds of fuel pressure, so the injector flow is a little under what it's rated, but that changes with your boost curve, so close is good enough. For example, if you have a duty cycle of 60% on 650cc injectors in the middle of your powerband (say 7400 rpm), that's 650 * 4 * .60 = 1560 cc/min of fuel, so your methanol jet should be spraying at 1560 * .25 = 390 cc/min. If you are unsure about your jet, it's easy to flow test -- just take an empty oil bottle and point your jet there, trigger your pump, let it go for 30 seconds, let the foam settle, then multiply the ml by two for ml/min and there you have it (one ml is one cc, for those who didn't know).
-The proper air/fuel ratio to run:
First, understand that I'm about to use "air/fuel ratio" totally incorrectly here just because we're used to the gasoline air/fuel numbers. When you added the methanol, kmanager no longer reports the actual air/fuel ratio, but you can tune with the numbers just as if nothing has changed. You can run a much leaner air/fuel ratio now because the water removes 6 times as much heat as the fuel you were leaving behind, and the methanol will allow for a leaner mixture without detonation, and any remaining methanol will remove heat twice as well as the gasoline that remains as well. I generally tune for low to mid 12s on my car. I could go leaner. I don't. There's not much power leaner than low to mid 12s, so why risk it. Hell, just tuning n/a cars, 12.8 makes for practically peak power. Just to give you an idea how much cooling the water alone is providing, I'll calculate the air/fuel ratio you'd have to run with gasoline alone to compare to the heat removing properties of the water.
Latent heats in J/g
Water: 2257 J/g
Methanol: 1100 J/g
Gasoline: 350 J/g
Mass in Kg/m3
-If you are running 1560cc of fuel to 390cc of spray at 50/50 mass, that means about 44% of your volume of 390cc is water. That leaves 172cc of water to remove heat. If you were running
a 11.7 air/fuel ratio gasoline only combustion event, it has 80% fuel burned, and 20% remaining (.8 lamda.) This leaves .2 * 1560 = 312cc of gasoline with the 650cc @ 60% duty example. Using the above charts you can see that water weighs 1 gram per cc (you should know this from school), and gasoline weighs .737 grams per cc. 172cc of water weighs 172 grams. 312cc of gasoline weighs 230 grams. This means the water removes 172 * 2257 = 388,204 joules, where as the gasoline removes 230 * 80,500 joules. The water removes 4.82 times as much heat as the gasoline.
This means that if you wanted to remove as much heat as running just the water alone, you'd have to have 4.82 times as much fuel left over. With the 650cc @ 60% duty @ 11.7 a/f example from above, that means that 1560 * .8 = 1248cc of fuel is consumed and 4.82 * 312cc = 1498cc would need to be left over. That would put your air/fuel ratio at 1248 / 2746 = .45 lambda or an air/fuel ratio of 6.62:1. Not only would that not run well, if even run at all, you would need 820cc injectors, or larger!
-Water cools best.
-Methanol increases octane.
-Extra octane from a higher methanol percentage comes at the expense of losing the heat removing properties of the water you replaced with methanol.
-Pick the mixture that works for your requirements.
-You can continue to tune with gasoline a/f as reported by kmanager.
Pistons on ground = bad
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Last edited by bitts; 08-18-2008 at 09:39 PM.