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Fuelman -> RE: Airplane fuel in a Nitro Truck? (3/28/2005 2:51:37 AM)
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OK Cummins.
This has been discussed a number of times over the years here on RCU. I will give you my spin on it, others have a different opinion as to why this occurs, but regardless of what the reason is, it can and does happen in many engines. In car engines, its easily noticed.
Since you're probably a car guy, I'll use one of my car examples.
Engine was a box stock Fantom 18 drop in replacement in a TMax.
Using a fuel that contained 20% nitro and 12% oil we were running it as hard as we could (with correct tuning, not lean and not rich) and getting about 340-350 degrees. We ran a couple quarts, real hard and the engine was performing flawlessly and reliabaly.
Just to prove a point to a couple friends that airplane fuel could be run it a car engine, so I grabbed a gallon of my 20% nitro, 18% oil airplane fuel. After about a half tank re-adjusting the needles to accept the higher oil (you must richen the needles when moving up in either nitro or oil), and getting it into its sweet spot, the engine was again running flawlessly and reliably. After running it real hard, on the same course as before, same afternoon, we were running temps from 395 to 415. We were still turning close to the same track times as before and the truck was a bit slugish on that much oil but it still performed beautifully. We ran it that way for over a quart more, no problems.
So why does this happen? My spin is that the higher amounts of oil are doing a couple things; More oil means less methanol in the same air fuel charge, since methanol has a high latent heat of vaporization which helps cool your engine, you get less cooling from the methanol, because less of it is there. Second, oil will not vaporize like methanol and nitro will, it remains a liquid and liquids are not compressable in the combustion chamber. What happens when you compress a liquid? heat is created. Higher amounts of oil in the mix act as if its increasing the compression ratio.
Now, this does not necessarly mean it happens in all engines this way. There are a lot of other variables that can affect the above. For instance, most airplane engines are much more tolerant to higher oil contents than car engines. Oil quality and type also have a role to play, compression ratio, engine load, etc....
On the other side of things, in one test I performed with a competitors brand of fuel 15 nitro, 17 oil, with a Super Tiger 75 engine, I noticed that it ran hotter (30 to 40 degrees hotter) than with my 15 nitro, 20 oil fuel. Why? well there are the other variables such as oil quality and friction and flashpoint differences probably causing the difference. This test also did not have the same difference in oil contents, we only had a 3% oil difference here, the car engine was 6%. Also, the car fuel had the same oil used in both the car and airplane fuels (my brands) where the airplane comparison was different oils because of different fuels.
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