toprudder
Posts: 167
Joined: 10/4/2005
From: Raleigh,
NC, USA
Status: offline
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quote:
ORIGINAL: downunder
quote:
ORIGINAL: toprudder
The oil, after combustion, carries a lot of the heat from the engine out of the exhaust.
I've always had a problem with this idea because of the tiny amount of oil that comes out the exhaust with each stroke. Basically, what comes out must exactly equal what goes in. So to use rough but probably reasonably accurate figures this is how I figure it. You have an engine that uses 10 ounces of fuel in 10 minutes running at 10,000 rpm (nice round figures  ). Or in other words, an ounce of fuel per minute. But 20% of that fuel is oil so in one minute only 0.2 ounces of oil passes through. In the meantime the engine has turned 10,000 times which means that every rev only 0.2/10,000 or 0.00002 (or if you like, 5/100,000) ounces of oil comes out the exhaust per rev. For those of us who prefer metrics this is 0.0006cc.
Whichever way, this is a tiny volume of oil and can't absorb enough heat to make any difference.
Well, when you put it like that --  I have always used that same logic when people talk about how additional oil will raise the compression ratio and cause detonation. This was something Clarence Lee used to say a lot.
All I can say it that the oil comes out of the exhaust HOT.  Sometimes adding an additional 2% of oil is all it takes to make hot engine run cool. Most four-strokes (not YS) depend on the oil film to CONDUCT heat from the piston to the sleeve. There is no fuel/air stream in the crankcase to cool the the piston from the bottom. (This is not true on YS which used the bottom of the piston as a pump for the supercharging effect). The small amount of blow-by is how the oil gets there. I have opened up my OS Surpass before and found a large amount of oil in the bottom end.
Another thing I hear is that adding oil reduces the amount of combustibles in the fuel, which will reduce power. That is not true, IMHO. You will simply need to open the needle valve slightly, to get the same ratio of COMBUSTIBLE fuel/air mixture. A tank will not last quite as long, however. Most people won't notice.
Another observation I've had is that adding nitro will allow you to use a higher percentage of oil, and the combination will give you more power AND run cooler. I had a helicopter a LONG time ago that would overheat terribly with 10% nitro fuel. I did everything I could think of, including trying different brands, and finally I bought a gallon of Missle Mist (which IIRC was 25% nitro). At this point I did not care if I was going to burn it up. It ran great, and no hint of overheating. I've also had the same observation in pattern, with both two-strokes and four-strokes, that adding nitro does not necessarily make the engine run hotter, as long as the compression is correct for the nitro content. I've used as much as 40% nitro in four-strokes, including an OS 1.20 Surpass. 25% seemed to be the best amount for me. Now before everybody flames me  I read an article many years ago that explained this. Nitro actually burns cooler than methanol, BUT, it expands to a much larger volume than methanol when it burns. Most people say that heat = power and power = heat, but that is not true in an internal combustion engine, it is all about expansion inside the cylinder.
I can dig up that article if anyone is interested.
Bob R.
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All Forums >> Glow Engines, Gas Engines, Fuel & Mfg Support Forums >> RC Fuels >> RE: 4 Strokes 
RE: 4 Strokes - 
, there's a good coating of oil all over every surface inside the crankcase so some at least must seperate out in there. Actually I suspect most of it might if only from the centrifugal action of the swirling. Then there's the turbulence in the cylinder which must seperate out more oil to get that film on the liner. There'd also be a film covering the combustion chamber but that would be fairly static because obviously there's no moving parts to wipe it off then replace it. Same goes for the piston crown to some extent which could explain why you can get a build up of carbon on the head and piston.