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I'm not that kind of "Engine"er...
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IDEngineer:
Not electronic, but we have an amazingly wide and deep range of expertise and experience on this site.

This has always bugged me: Why do cold engines stall easier? If you start a cold gasoline or diesel engine, a very small load can outright stall it. But if you wait until the engine is warmed up, the same load won't affect it. Why? The fuel is the same, the air is the same, the compression is the same, the rotating mass (flywheel/harmonic balancer plus all other components) is the same. Why does the engine's temperature make a difference?

I've tested this on everything from small yard engines with hyper-simple ignitions, to old-time diesel tractors with purely mechanical injectors and fuel pumps, to late-model engines with fully electronic ECU's. They all behave the same: Once warmed up they have MUCH more resilience to loads whether applied instantaneously or ramped up slowly.

Anyone know why?
Ranayna:
I'm also not an engineer. But i can at least confirm this behaviour with my lawnmower engine: It it's cold i need several pulls to start it, if its warm a light pull will do it.

I can think of two factors at play here.
One: Cold lubricant is not as fluid as hot lubricant.
Two: The cold engine absorbs some of the heat energy created by the combustion, so compression is likely less on a cold engine.
themadhippy:

--- Quote ---Why do cold engines stall easier?
--- End quote ---
total guess,when cold everything's tighter so the frictions greater
IDEngineer:

--- Quote from: Ranayna on December 19, 2022, 07:59:57 pm ---The cold engine absorbs some of the heat energy created by the combustion, so compression is likely less on a cold engine.
--- End quote ---
Well, strictly speaking the compression (ratio) would be the same. But I get your point: Some of the heat energy being absorbed by the cold metal might reduce the power output of the engine. Not sure the amounts of thermal energy extracted by that cold metal would be enough of a percentage to cause the stalling effect, but it's a very interesting possibility.
IDEngineer:

--- Quote from: themadhippy on December 19, 2022, 08:00:14 pm ---total guess,when cold everything's tighter so the frictions greater
--- End quote ---
Hmm... would the friction really be greater? Thermal expansion/contraction would affect everything, so while the cylinders would contract when cold so would the pistons and rings.

EDIT: Or perhaps contraction of the block would actually enlarge the cylinders. That would reduce friction when cold because the cylinder walls and pistons would be contracting away from each other.

EDIT2: And perhaps the resulting increased losses (blowby) through the cylinder-piston interface could contribute to this stalling behavior. If the load is great enough the path of least resistance is the blowby past the rings instead of pushing the pistons and continuing to rotate the crankshaft.

I'm glad I'm not the only one who doesn't have a quick answer for this effect.
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