Outdoor Power Equipment (Lawn Mowers, Snow Blowers, Chain Saws and more) Discussions

is it better to run the generator once a month or can it be stored until it is needed.I read it should be run monthly under load so the generator head stays magnetized.Is this true?

You're both right ... or you're both wrong, take your pick   It's a whole lot simpler if you just trace the heat path I'm not saying anything here you don't already know, just stating it differently.

In an air cooled engine the moving air extracts heat from the fins, which extract heat from the block [I'm including the more "solid", unfinned, parts such as the cylinder housing with the term block here]. The fins will be cooler than the block under normal conditions.
This is where the engineer who designed this engine has to know his stuff, and most of them do. Anyone who has had a mouse build a nest under the cooling fins and seen what happened to the engine can tell you that.  Sizing the fins and the fan (flywheel fins) and designing the tins is critical, but we don't have to worry about it since the engineer already did.
When the engine is shut off, there's no moving air and less heat will be absorbed by still air. If the fins are still cooler than the block, they'll continue to absorb heat while giving up less, so  their temp will climb until some sort of balance is reached with the block. (this can include the actual engine block absorbing heat from the heads or jugs depending on the configuration) The block can only cool down as:
(1) It is giving up heat to the air around it and to the fins.
(2) No new heat is being produced.

In a water cooled engine the radiator (appropriately named, isn't it!) gives off heat to the air; the same principle applies except for the extra step of transferring the heat to and from the water that connects (heat path, not physically) the radiator to the block. That has a fan also. Shut that fan off and the radiator/water temps climb for the same reason - the water is absorbing heat from the block but the radiator is no longer absorbing as much heat from the water as it did because it is no longer ... radiating. So the water can get hotter and start spitting out the pressure relief on the cap.

Update after reading this: Remember that the various parts of a running engine will be at different temperatures. Once that engine is turned off, the parts that are bolted together will tend to stabilize. Parts that were cooler become hotter, and other parts will cool down. Most likely the combustion chamber will be the hottest and the tips of the fins the coolest.

When I was tuning/testing my twin cylinder air cooled two stroke Yamaha motorcycles, I'd use an infrared thermometer to check the heat on each cylinder/head/exhaust pipe etc. to see if the readings on each side of the engine were reasonably close.  While the engine was running, the heat remained consistently higher than shortly after shutting it down. 

I've never seen an occasion when any part of the engine actually returned a higher temperature reading after shutting it down.   Finned parts of the engine cooled more rapidly than say, crankcase areas.  Exhaust pipes cooled the quickest due to having the highest temperatures vs. ambient temperatures. 

That makes sense. Totally finned jugs with no tins so you'd have direct convection. I'm sure you noticed that stopped at a light on a hot day! I know I did. Something like an Onan twin, even some B&S single cylinders, would surround those fins with tins that trap heat when there is no air being forced through them.

I stand by my assessment that it is best to idle down an engine that has been working hard for a few minutes before shutting it down. Exhaust valves and seats get very hot when the engine is run hard under a large load and if when you shut down the engine the exhaust valve is closed a lot of heat is trapped in the valve and seat. Idling down for a few minutes allows the exhaust valve and seat to cool a fair amount before stopping the engine.

carlb wrote:
I have no problem with the idea of allowing the engine to idle for a bit after a long hard run.  It's something I normally do with a riding mower for instance.  After cutting, I'll disengage the mower blades, lower engine rpm and casually ride the machine back to the storage shed.  If nothing else, the lower engine speed will continue to circulate oil which may allow the cooling of hot spots on cranks, bearings, cylinders and valves for instance.  Don't think it would hurt any with a generator.  Particularly if the generator had been operating near maximum output.  Leaving it spinning with no load would also likely assist in cooling the power head which would be beneficial for bearings and electronic/electrical components.

I do not agree however, that an engine will continue to "produce" heat once it's shut down.  When combustion and movement stops, so does the production of heat.

Agreed that the engine won't continue to "produce" heat after shutdown. That is, the hottest point in the engine (wherever that may be) will only get cooler. However, things near it may warm up.

I had an interesting example of the getting-hotter-after-removing-the-heat-source phenomenon a few years ago. We were injection molding some solid round b a l l s. Maybe 1.5" in diameter. The plastic material is injected at about 450-500F. The mold is cooled with roughly 80F water. A typical injection molded part is quite thin, relative to its surface area. These, however, were extremely thick, much more so than would be typical.

The plastic would be injected, and the parts would sit in the mold to cool, for quite some time (say a minute), so the outside would become firm enough that they could be ejected from the mold. During that time, the surface of the part is exposed to metal being cooled by 80F water. The center, however, is still very hot from the 450-500F plastic that you'd put in.

The interesting thing was that, when the parts were initially ejected from the mold, you could easily hold them in your hand. The outsides were warm, but OK. But wait 20-30 seconds, and you had to put them down, or you'd burn your hand. You'd removed them from the steel, which was drawing all the heat away. So as the remaining heat from the center (still at, say, 300F, to pick a number) continued to conduct outwards, the outer surface of the parts heated up a *lot*. This is in contrast to a typical-geometry injection molded part, which, to the touch, only gets cooler once it comes out of the mold.

Different context, but same phenomenon. No longer introducing any additional heat source (engine shut down), but remove some of the active cooling (airflow from flywheel), and the temperatures of some of the cooler areas will climb as things equalize.

Guys we could go round and round with this,    Say-we run a engine at full load for a few minutes, then to be nice guys, we remove the load to allow full airflow over our hot engine.....

Has anyone heard of the effects of "shock cooling".....  I have seen a lot of Franklin, Continental, Pratt and  Lycoming jugs replaced due to cracking from shock cooling...

More food for thought...

Friiy

I've hear aircraft guys around here refer to it as "super cooling" the engine.  Shock cooling is probably the same thing.  

Not particularly familiar with the phenomenon.  What's the cause and effect of shock cooling?   I assume it has something to do with shutting the engine down too soon after landing.


RedOct:

A similar but much simpler example of what you're talking about is taking a boiled egg out of the cooling water too soon.  Seems fine at first but is soon too hot to hold.  Simple matter of heat dissipation. 

Shock cooling refers to cooling the outside quicker than the inside,   the outside shinks and splits/cracks over the larger inside..

Friiy