I'm not that kind of "Engine"er... - Page 1
Forum section: electronics / general

author: IDEngineer
author_rank: Super Contributor
timestamp: December 19, 2022, 07:47:59 pm
content: 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?

author: Ranayna
author_rank: Super Contributor
timestamp: December 19, 2022, 07:59:57 pm
content: 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.

author: themadhippy
author_rank: Super Contributor
timestamp: December 19, 2022, 08:00:14 pm
content: Quotetotal guess,when cold everything's tighter so the frictions greater

author: IDEngineer
author_rank: Super Contributor
timestamp: December 19, 2022, 08:05:15 pm
content: Quote from: Ranayna on December 19, 2022, 07:59:57 pmWell, strictly speaking thecompression(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.

author: IDEngineer
author_rank: Super Contributor
timestamp: December 19, 2022, 08:07:12 pm
content: Quote from: themadhippy on December 19, 2022, 08:00:14 pmHmm... 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 actuallyenlargethe cylinders. That wouldreducefriction when cold because the cylinder walls and pistons would be contractingawayfrom 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.

author: Monkeh
author_rank: Super Contributor
timestamp: December 19, 2022, 08:09:48 pm
content: Quote from: IDEngineer on December 19, 2022, 08:07:12 pmGreater when cold due to the lubricant, really. However: Materials are not the same. Pistons, cylinders, and piston rings are not the same material.Cylinder temperature will also affect the combustion pattern.

author: IDEngineer
author_rank: Super Contributor
timestamp: December 19, 2022, 08:14:30 pm
content: Quote from: Monkeh on December 19, 2022, 08:09:48 pmAs long as they all have the same polarity coefficient of thermal expansion, though, the effect should be the same even if not linear.QuoteTHAT is a very good point. Head temperature will affect it too, perhaps even more than cylinder temperature (since combustion occurs when the piston has moved up past most of the cylinder). So there simply may be less power extracted from the fuel because at cold temps combustion is less complete.I have to say, though, it "feels" more sudden than "you only have 70% of normal power". The engines don't lug down, they just STOP.

author: bdunham7
author_rank: Super Contributor
timestamp: December 19, 2022, 08:35:50 pm
content: Quote from: IDEngineer on December 19, 2022, 07:47:59 pmThe differences you notice between a cold engine and a warm one are going to be mostly due to the temperature of the air inside the engine interacting with the fuel. A gasoline engine needs to vaporize a significant portion of the fuel and a diesel engine needs the air to be heated by compression to ignite the fuel. Each has their own separate issue with temperature. In the case of a gasoline engine, the fuel ratio need to be enriched during cold operation to compensate and that can be a tricky and imprecise process, especially with carburetors. Port fuel injected engines have much less of an issue with this. There is also a secondary issue involving required ignition voltage and the conductivity of the surface of the spark plug insulator, but that's not much concern on modern automotive engines. Diesel engines, especially older models, will be limited by the amount of time the cylinder temperature remains above the ignition point of the fuel. If you keep injecting fuel after the has moved far enough down that the cylinder temperature has dropped, that fuel won't burn and you get white smoke instead.I would take exception to your extreme and universal characterization of the differences between a hot and cold engine--unless you are talking about select primitive models or extreme subzero temperatures. Operatingfrom idlesome primitive engines of either type might hesitate or stall with sudden loads, and older non-turbo direct-injection diesels can be difficult to rev up when very cold. However, if you park a modern vehicle of either type by the side of the road overnight at 35F and then in the morning you start the engine and let it run for only 5 seconds before putting it in gear and applying full throttle, it won't stall or sputter--it will just take off and go down the road. You might have slightly reduced power, and in the case of a diesel you might have some smoke, but it will go. Large diesels might be a bit more problematic and I certainly don't recommend treating any vehicle this way unless you are taking it to the junkyard.