Air cooled engine in cold weather ?

[UNC017]

Originally Posted By: Nick R
What IS the relationship between air temperature and engine temperature in small air cooled engines? I mowed our lawn today at 33 degrees. It took 3 pulls to start, but it ran perfectly for the 3 hours it took to finish. And then I dump some stabil in it, and put it up for the winter.


For a given load, its more or less a fixed offset. You put a certain amount of heat energy into the metal per second of operation, and then the cooling fins have a thermal resistance in getting the heat from the metal to the air. Energy departs the engine as at a rate determined by that thermal resistance (fixed) and the difference between the metal and air temps. When the metal gets hot enough that the rate of heat flow out equals the heat flow in, the engine temperature stabilizes. So holding everything else fixed, the difference in temp across the cooling system is constant so if you lower the air temp around the engine, and the engine metal temp lowers the same amount to a first approximation.

One thing to consider is that air is less dense at higher temps so the cooling system becomes less efficient. That means that the spread between the air temp and the engine temp actually gets a little bigger as the surrounding air gets hotter and smaller as the air gets colder and denser.

 

[boraticus]

OK...

Thanks for the physics lesson....

So at what temperature is one to switch from burlap to the finer weave tweed?

 

[JimPghPA]

K$B RC model airplane engines made an engine about twenty years ago that would not run properly because the cooling fins were too large even for use on summer days. They had a recall where they were sent back to the factory and the fins were reduced in size, and then they ran ok for a K$B which is not saying much since that brand was a low end engine to start with.

So yes Virginia there is such as thing as running too cold, especially for a two cycle that uses compression ignition.

 

[JimPghPA]

Originally Posted By: 440Magnum
Originally Posted By: Nick R
What IS the relationship between air temperature and engine temperature in small air cooled engines? I mowed our lawn today at 33 degrees. It took 3 pulls to start, but it ran perfectly for the 3 hours it took to finish. And then I dump some stabil in it, and put it up for the winter.


For a given load, its more or less a fixed offset. You put a certain amount of heat energy into the metal per second of operation, and then the cooling fins have a thermal resistance in getting the heat from the metal to the air. Energy departs the engine as at a rate determined by that thermal resistance (fixed) and the difference between the metal and air temps. When the metal gets hot enough that the rate of heat flow out equals the heat flow in, the engine temperature stabilizes. So holding everything else fixed, the difference in temp across the cooling system is constant so if you lower the air temp around the engine, and the engine metal temp lowers the same amount to a first approximation.

One thing to consider is that air is less dense at higher temps so the cooling system becomes less efficient. That means that the spread between the air temp and the engine temp actually gets a little bigger as the surrounding air gets hotter and smaller as the air gets colder and denser.



True, but also as the air becomes more dense at colder temperature the engine will intake more air molecules, and the flow of more air molecules in the carb will also pull more fuel. So the engine will be able to produce more HP when wide open, and also produce more waste heat that it has to get rid of.

 

[rmattingley]

My 2cents...I know my snow blower has a casing that uses the exhaust to heat the incoming air in the carb. My generac 33hp engine also has a vent that is used to pull warm air when it is below 0 out.

 

[boraticus]

On snow blower engines, warm air is directed toward the carb to prevent icing. Throwing a burlap bag over the engine's cooling air intake will not accomplish that. It will reduce effective cooling of the entire engine.

If there is an issue with air cooled OPE engines running too cold, one would think that manufacturers would put louvers over the intake fans on their snow engines.

For those who advocate the "burlap solution", please explain to us why no such accessory is required on our snow blower engines?

Anybody here throwing a burlap sack over their snow blower engines during operation?

I've owned snow blowers for decades and run them in temps close to -40C. Not once did I have a concern about the engine running too cold and not once have I ever had an engine problem on a snow blower.

Sack the sack!

 

[Donald]

Even my ancient 1970 Montgomery Wards snow blower runs fine in cold weather. I have let it sit outside after blowing snow and some melted and froze and the blower was frozen and would would stall the engine when engaged. But engine started fine.

Get some good oil if you are worried. Try Amsoil 4 Stroke. Its made for OPE.

 

[Propflux01]

Originally Posted By: boraticus
I was under the impression that the VW had that apparatus to capture air from the engine to heat the inside of the car?

The fan shroud has two round 'vents' that can be seen from the back of the engine on the left and right side of the fan housing (Shroud). They are connected, via metallised cardboard tubes, to a box that surrounds the front two cylinders header pipes. At the front of that box is a valve that, when opened, allows exhaust heated air into the cabin. If the valve is closed, the heated air is dumped overboard through vents at the top of the box. The thermostatically controlled flaps I described earlier are located just above the cylinders, at the bottom of the fan shroud assy, and have nothing to do with the air deliverers to the vehicle cabin.

 

[steve20]

Originally Posted By: boraticus
On snow blower engines, warm air is directed toward the carb to prevent icing. Throwing a burlap bag over the engine's cooling air intake will not accomplish that. It will reduce effective cooling of the entire engine.

If there is an issue with air cooled OPE engines running too cold, one would think that manufacturers would put louvers over the intake fans on their snow engines.

For those who advocate the "burlap solution", please explain to us why no such accessory is required on our snow blower engines?

Anybody here throwing a burlap sack over their snow blower engines during operation?

I've owned snow blowers for decades and run them in temps close to -40C. Not once did I have a concern about the engine running too cold and not once have I ever had an engine problem on a snow blower.

Sack the sack!


Sounds like you've never had problems in the 40 years you have owned anything?

 

[edhackett]

Quote:
If there is an issue with air cooled OPE engines running too cold, one would think that manufacturers would put louvers over the intake fans on their snow engines.

For those who advocate the "burlap solution", please explain to us why no such accessory is required on our snow blower engines?

Anybody here throwing a burlap sack over their snow blower engines during operation?

I've owned snow blowers for decades and run them in temps close to -40C. Not once did I have a concern about the engine running too cold and not once have I ever had an engine problem on a snow blower.


The OP is talking about a lawn tractor, not a snow blower.

Ed

 

[boraticus]

"The OP is talking about a lawn tractor, not a snow blower."

Would you care to elaborate on the significant differences between the engines on both machines and in particular, what would cause a lawnmower engine to run so much cooler than a snow blower engine?