Post B1rRXIc7JuZpBWAHsO by amelia@misskey.bubbletea.dev
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 (DIR) Post #B1rRXIc7JuZpBWAHsO by amelia@misskey.bubbletea.dev
       2026-01-02T02:07:49.871Z
       
       0 likes, 1 repeats
       
       (1/2) okay its complicatednah its not the compression, that's a fixed number that represents the ratio of volume between cylinder at top dead center and bottom dead center (piston full up or full down).actual PSI inside the cylinder during different operating scenarios can differ (cam profiles, boost, different air temps, and so on)what i may have explained confusingly is: idling is hard on engines because theyre designed to run at a RPM that moves the vehicle.oiling itself isnt an issue because the oil pump runs off the crank. the utilization of fuel is the issue. fuel (specifically in the cylinder as an air-gasoline mixture) combusts most efficiently and completely at a certain temperature.when you're idling, the cylinders are generating much less heat than moving operation.that only matters for one of the same reason that engines tuned to run on a shitload of ethanol in their fuel (which is more cooling than standard gasoline) experience a performance benefit. the cooling (or heating) that's happening immediately in the cylinder has an effect on performance, fuel usage, and wear, even when the rest of the block is at a stable operating temp. at idle, the air-gasoline-mixture movement cools the cylinders away from optimal range faster than the rate of combustion heats them up into the optimal range. heating the entire block sufficiently (by reducing the flow rate of the cooling system) to get the right temp for complete combustion would be unsafe for other components, so the cooling system (as part of its job) locks the engine into this awkward position at idle.we also can't just run less fuel at idle, as running too little fuel actually causes the cylinders to run way ABOVE the right temp. this is because the gas part of the air-gasoline mixture is doing most of the cooling. when the injectors (or carburetor(s)) turn the gas from liquid into vapor, its temp drops significantly. it's the same thing that a can of compressed air does, just at a higher starting temperature.RE: https://misskey.bubbletea.dev/notes/agwwzshmov