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Old 04-28-2012, 10:47 PM   #11 (permalink)
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The thing about having 2/3 the BTU/vol is that you use more volume (much larger jets) when you run alcohol in a gasoline engine. So the BTU per stroke is the same-ish, or more if you're making substantially more power on alcohol.

What I'm thinking is that since the alcohol's explosive reaction is so much faster from start to finish, that the metal of the engine may not have as much time to soak up the heat before the expansion of the cylinder area allows it to begin cooling and the upstroke starts pushing it out the exhaust. Merely conjecture on my end, I have no idea.

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Old 04-28-2012, 11:04 PM   #12 (permalink)
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Something similar is gunpowder. I think alcohol works better with high compression. I believe alcohol burns slowly which would be why it can handle much higher compression ratios than pump gas.

Fast burning gunpowder versus slow burning gunpowder. If you use two powders of greatly different burn rates you can actually feel the hottest portion of the gun barrel move out towards the muzzle, versus the slower powder burning quicker and the hottest portion of the barrel would be nearer the breech of the gun.

I remember my brother loaded some 7.62 X 54 R ammo with 20 MM cannon powder that he got for $4 a pound surplus. The powder burned so slowly it would not completely burn in a 30 inch barrel. The chamber end was fairly cool while the muzzle was much warmer than a few rounds of normal burn rate powder.

That was what I thought about when he described the temp difference between the cylinder head and the exhaust. I would imagine it also had something to do with the actual amount of fuel that you have to deliver to the combustion chamber with alcohol, with that volume of fuel actually contributing to the cooling of the combustion chamber.

Christ, I think it is actually the alcohol burns slower, and the much larger volume of fuel keeps the combustion chamber cooler by absorption, but it is really only a guess on my part.

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Old 04-28-2012, 11:14 PM   #13 (permalink)
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Actually, that makes much more sense. If the fullness of the initial combustion event doesn't occur instantly at TDC, the burn rate increasing while the piston falls helps to evenly distribute the heat through the entire engine, rather than concentrating it at the head and uppermost area of the cylinder.

The extra liquid fuel vaporizing and cooling the chamber probably has a larger effect, but both parts seem relevant to the effect.

Guess that's something I'll have to look up sometime.

If alcohol burns slower than gasoline, does it have the same torque production effect as diesel (smoother torque application through the entire expansion cycle)?
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Old 04-29-2012, 02:36 AM   #14 (permalink)
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Cool Alcohol, again

-I think we're working around to saying similar things. A gallon of alcohol does NOT have the BTUs of a gallon of even WalMart gasoline: 76,100 BTU's for ethanol and 114,000 BTUs for gas.

-Pushing more alcohol into an engine burining only alcohol will actually decrease efficiency by preventing burning as the combustion chamber is designed around a certain amount of fuel. Raising the compression ratio works to a point but one source I came across said that 14:1 was about as high as one could go. He figured that alky had 2/3 the power of gas and raising the compression ratio to 14:1 would bring the efficiency up to about 90% of a gallon of gas, which brings it down to a matter of economy. Oh, one site also pointed out that the pumps that have ethanol added actually LOWER the enrgy value of each gallon of gas, which makes sense.

-I have read (Mother Earth News, of course) of modified airplanes and motorcycles crossing the country on pure alcohol.

-Three items for consideration: 1) what about emissions? 2) has an engine ever been specifically designed to run on alcohol? 3) does it have to be an INTERNAL combustion engine?

-I'm going to bed.
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Old 04-29-2012, 09:20 PM   #15 (permalink)
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There have been engines designed specifically to alcohol, but the only ones I know of are the Ford 1.6MHO (Euro spec) and many aircraft enignes, as well as military "multifuel" engines.

Different fuels have a different "ideal" ratio, so to say that putting more fuel into a mix will decrease efficiency is a fallacy. If the ideal ratio were stoich (14.7:1) for alcohol AND gas, then you could say that's true, but it's not. (I'm not sure the ratio for alcohol, and it changes for different types of alcohol content).

Because of differences in burn rate, etc, the BTU value of different fuels doesn't necessarily determine the usable power output of the fuel unless other variables (engine type, design, etc) remain constant, and even then, there are variables beyond simple control such as heat soak that will determine output. Keep in mind that useful output is not the same as BHP. An engine that has a smoother torque application will accelerate faster and spend less energy as vibration, even with the same peak HP figure/location.
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Old 05-01-2012, 06:37 PM   #16 (permalink)
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Smile Just a few items

Fellas,

-I’m neither a chemist not a physicist nor do I play one on TV but something doesn’t pass the common sense test.

-Given: a gallon of gas produces 114,000 BTU, or thereabouts, under ideal conditions.
-Given: a gallon of +/- 180-190 proof ethanol produces about 76,000 BTU also under ideal conditions.

-Then 10% of a gallon of alcohol will IDEALLY produce 7,600 BTU and 90% of a gallon of gasoline will IDEALLY produce 102,600 BTU, the combined BTU rating is 110,210 BTU, about 97% of the rating of a gallon of gas. Since the alcohol will be burning under less than ideal conditions one may infer that the actual total BTU produced will be lower at a given stoich mixture.

-On the other hand, as Christ properly points out, there are other variables to consider. According to what limited research I was capable of understanding, the addition of alcohol will actually LOWER the stoich ratio, but the electronics in the ECU automatically compensate for this. Alcohol, as noted before, also has other effects on engines such as lowering the cylinder temperature thus, perhaps, aiding in more efficient combustion. As Old Mech pointed out the mixture can have adverse effects on emissions. One assumes that there must be some positive effects as well or the government wouldn’t be pushing 10-15% alcohol mixtures.

-Christ, RE: post # 11, can we infer that alcohol has a higher explosive reaction (“rate of detonation” is the term used in my explosives class). Is there a way to confirm this? I also wonder if lower initial residual temperature will, over time, be a true effect. It’s a poor analogy, but a coal stove isn’t very warm during the first minutes of burning, but does finally heat up. Hence the cooling systems built into any internal combustion engine.

-Mech, as an old soldier, your #12, to me, leaves a few things out. Norma lists about 47 gr of 203-B for a 150 gr round nose FMJ. If you were to put 47 gr of #200, their fastest powder, the cartridge might well detonate and demolish the rifle. On the other hand, if you loaded 47 gr of MRP, which is designed for large case capacity in relation to bore size, you might have something close to a squib load. At the very least the velocity of the 150 gr bullet would be very low. Yes, 20mm powder is fairly slow burning. It is also possible that the primer your brother used just wasn’t powerful enough to get the powder going. It sounds as if he had a “progressive burn” where more powder was actually burning the further down the barrel the round went. Friction probably also played a part due to low initial velocity from the chamber. I worked with a black powder riflesmith who taught me how to calculate the length of barrel needed for a given bullet/powder combination. He would start with a known over length barrel and then cut it back an inch at a time until unburned powder particles showed on the snow (or sheet). That signified that the proper sized “combustion chamber” had been achieved. Personally, I’d only use 20mm powder to: a) reload 20mm or b) fertilize my garden. Gunpowder (not black powder) makes great fertilizer. Just don’t smoke while applying it.

-To (finally) wrap up, I’d love to know what a proper alcohol internal combustion engine would look like: cylinder head and piston dome shape; number/size of valves; bore vs. stroke, etc.

-Thanks, guys. This is fascinating!
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Old 05-01-2012, 09:46 PM   #17 (permalink)
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NVSmith, I generally refrain from using firearms and reloading analogies on a fuel economy forum. The old M91 had a 30 inch barrel, might even have been close to the 20MM but probably shorter. The bullets were cast lead, we have enough accumulated for about 150k 150 grain 30 cal rounds. It did take magnum primers to light the powder and the muzzle velocity was so low the sights needed to be set for 400 meters at 50 yards.
Plenty of unburned powder residue in the barrel. Bottom line was we are just cheapskates .

For super light loads use something like bulls eye or unique. Even the recoil was more of a push than the normal sharper hit of a conventional load.
Kind of like the standard Soviet surplus in a carbine, a semi flame thrower, not to be used in dry brush.

I think the Indy engines run about 16 to 1 compression. Even at 50% of atmosphere manifold vacuum, that's still 8 to 1 which is about the max compression of the early pollution control choked engines of the mid 1970s.
Even the cars sold as E85 capable do not (as yet) have the compression needed to run most efficiently on alcohol, but I thin in the next few years you will see the throttle control pass from a butterfly to more control by cam timing, possibly both lift and duration, individually or separately, on both cams.

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Old 05-01-2012, 09:57 PM   #18 (permalink)
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GDI is taking place, as well... much higher compression ratios can be utilized, and that means that alcohol and gasoline could both be sufficiently used. A simple computer could compensate the fuel flow based on what's in the tank at the time.
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Old 05-01-2012, 10:36 PM   #19 (permalink)
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take any old engine on a hot summer day, and run it on gasoline. Lets say it makes 100 horsepower.

Now we take the carb off, hog out the main jet, and run a LOT of alky through it. The engine can make a LOT more horsepower.

Why?

Alcohol has a much higher latent heat of vaporization, and it burns at a MUCH higher fuel rate then does gasoline.

What this means is if you pour a lot of alcohol at the engine, the alky supercools the air, letting a denser air charge into the engine. In addition, you pour even more alky, and you get more air.

Further, alky has oxygen contained in it (why they use alky to replace MBTI) because it "oxygenates" the fuel.

Add it all up, and if you are willing to burn LOTS of it, you get more power.
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Old 05-01-2012, 10:37 PM   #20 (permalink)
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ESPECIALLY on a hot summer day.

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