Went flying yesterday with my neighbor (pvstjr). He had two planes. A UCD 60 with a OS120AX and a Twist 60 with a OS91FX. After one of the flights of the twist 60, Immediately after shut off, I reached down to pick up the plane and place it on the starting stand. The end of the crank was cold and then I felt of the crankcase and it was cold also. The OAT was around 70 degrees F.

The twist 60 is a "Cheek Cowl" type plane with basically an exposed engine.

I was surprised at the cold crankcase after a 15 minute flight. I had touched the engine immediately after shut off so there was no post run heat soak.

I had a Slow Poke with a OS 52 but failed to feel of the engine. On the Slow Poke, though the engine is exposed, the bottom of the crankcase is basically enclosed. My friend placed my plane on the starting stand.

The crankcase in a glow engine should be cold, because it spends all it's heat evaporating the methanol that passes through it.
Methanol has excellent internal cooling properties, much different from diesel and gas engines.
If the case gets hot, it is a sign that the engine spends heat where it should not, and consequently it runs at reduced power levels.

Interesting posts. (after all these years I haven't yet felt the bottom of a just flown crankcase)

Pe, the explanation is both reasonable and understandable.

Now, what about a four cycle??

As my engines are cowled I will not be tempted to do the 'temperature test' ------ too much work

in a four stroke, the crank case is not cooled by the fuel and will run a bit hotter. Some raw fuel with oil that has condensed at the cylinder walls will however find it's way down from the combustion space to the crankcase, and do the cooling trick. It is amazing how much waste is expelled from the case vent, so there must also have been quite a lot of methanol involved in the blowby process.

I would imagine that the crankshaft passage is the coldest running area by far because liquid fuel is changing to a vapor here. If the temp could be measured in this area of a running engine, I would guess it would be close to freezing.

Mike

Pe' Your explanation of the fourstroek crankcase also applies to Diesels, on my Fox .74 the crankcase runs pretty warm, I attribute this to no (or very little) evaporation occurring in the crankcase.

With the exception of a YS, the typical fourstroke crankcase would be getting nothing but hot oil and gasses that blew past the compression ring. Of course, the intake tube would be very cold and have moisture condensing on it. I always have to flush my Enya crankcases with oil after a outing because the blowby crud is so corrosive and will attack lower end parts quickly. I learned this lesson the hard way after being dogged with several bearing failures because of severe rust.
I seem not to have this problem with my YS .91AC because its lower end internal parts are always being washed with fresh fuel while running. I would guess that the YS requires extra lube because it is contiueously SP? being washed away with a fresh charge every cycle.

Text from my book "Motorboken" printed in Teknologisk forlag, Oslo. ISBN: 82-512-0133-0 :

The carburator and intake canal works as a "freeze machine" due to gas velocity and vacuum make cold and still more colder with atomized fuel. There are not possible to evaporate the fuel inside by heat of surface such as in intake canal. After the fuelmix are entered in the crankcase, the gas will be expanded and make cold again under induction period by vacuum. The fuelmix get a bit heated up by crankcase compression, but not enough time to evaporate since there are still cold fuelmix inside. In fact are 3 thing who are entered in the cylinder after valve (4 stroke engine) / transfer port (2 stroke engine): fuelmix are allready evapored by heat by compression, liquid drops small diameter from 1/20 to 1/2 mm depent on carburator construction, a thinn fuelfilm at surface at canal before valve/transferport. With increased air temperature the mixture will be better such as in fair weather. Heat by piston will transfer heat to fuelmix under compression period before ignition starts

In fact the speed of gas are more higher than speed of hurricane!!!

How can it be possible the speed of gas are more higher than speed of hurricane?

Let us take the engine with bore at 18 mm and 16 mm stroke, also a .25 size engine and we are selecting the carburator with 5 mm diameter venturi to run the engine as diesel engine since we need more gas velocity to improve fuel atomizing cause the dieselfuel has higher viscosity than methanol/nitro for glowplug engine. The engine are running at 15000 rpm.
The result will show here:

1. Calculate piston speed in meter/second: 2 x 0.016 metre x 15000 rpm : 60 seconds = 8 meter/second

2. Calculate piston area and venturi area: Piston: 0,785 x 18mm x 18mm = 254,34 mm2, Venturi area: 0,785 x 5 mm x 5 mm = 19,625 mm2

3. Calculate speed of gas in meter/second: 8 meter/second x (254,34 mm2 : 19,625 mm2) = 103,68 meter/second (373,24 km/hour / 231.92 Miles/hour)

Also speed of gas are faster than speed of hurricane: 373,24 km/hour / 231.92 Miles/hour...

How big are vacuum inside in the engine under induction period.
1. Calculate vacuum in bar: (8 meter/second : 4) x (8 meter/second : 4) = 0,4 bar vacuum

There are enough vacuum to suck fuel from fueltank and make air more cold.

Formula from: Formeln für technische berechnungen by J. Phyl, 1980.

Provided the engine is not lean and bearings are smooth for very little friction the 2 cycle crankcase will be cool and carb near freezing.............

Don't know about freezing but it will be close to the dew point of the methanol at the negative pressure in the venturi

Whenever I run a new engine on the test stand I always feel the crankcase underneath the carb are after a minute or so to make sure it's still cool. Any heat there would indicate too close a fit between the crankshaft and crankcase. Try this with a bushed engine and it'll still feel cool which is an indication of how little friction there is with a bushed crank. The crankcase itself is (or better be!) always cool for the reason given by Pe. After shut down there'll be a heat soak down from the cylinder and then it'll warm up some.

Some time ago I was running in a rear induction OS Max-H 80 and just for curiousity I dropped my thermocouple down past the carb into the elbow where it turns towards the drum valve (no chance of the drum valve chopping the thermocouple into bits ) and the temp was about 20F below ambient (which from memory was around 70F).