How does the oil pump works in terms of the pressure that it is using to pump the oil? For instance, at idle speed is the pressure less than at moving speed? This question just occured to me after my mom told me how a couple years back she forgot to turn off her engine while it's idling for like 2 hours, but the car now still runs fine. So I was wondering if it was starving from oil at all.

 

The oil pump runs at engine speed. The faster the engine turns, the more volume and pressure the pump will deliver, up to a point. There is an oil pressure regulator valve built into the pump to bleed off excessive pressure at high engine speeds. This reduces the risk of a burst oil filter shell which would result in a massive oil leak and serious engine damage due to lubrication failure.

A healthy oil pump provides all the lubrication needed by an idling engine. When you think about it, an idling engine is doing almost no work. Piston speeds and bearing loads are very low, so lubrication requirements are also low.

No harm was done by idling the engine for two hours.

 

Thank you Daniel for the input. Does this relationship between oil pump pressure and engine speed apply also to the transmission fluid pump and tranny speed (AT/Manual)?
From what you described can it be deduce the only bad part about starting a cold engine is the first few initial seconds that takes the oil to fully circulate/lubricate dry engine parts (transmission if this theory applies) and the excess fuel being burnt from running rich, but after that the car is pretty much in it's least wear & tear stage, aside from being off? This include the assumption that the engine fan is working properly to dissipate heat while the car is not moving.

 

The 5-speed does not have an oil pump of any kind. The moving parts are lubricated by splash. I don't know enough about A/T internals to respond to that part of your question.

Lubrication is vital, but there's actually more to the story. Metal parts expand as their temperature increases. This problem is aggravated by the fact that modern engines are made of different metals. Each material has its own Coefficient of Thermal Expansion. Aluminum parts expand more than iron or steel, per degree of temperature rise. Important part-fit dimensions are optimized for normal operating temperatures. For example, pistons are an undesirably loose fit in the cylinders immediately after a cold start. They actually wobble around as they go up and down. This is called Piston Slap and is audible on some engines after a cold start on a winter day. Parts with an ideal fit wear less than parts with a sloppy fit.

 

A/T transmissions have a pump to generate hydraulic pressure. There is a pressure regulator for the line pressure, and the excess fluid runs to the TC & external heat exchanger. The cold, thick fluid puts additional strain on the pump and may cause harsher shifts. Warming up the fluid is one of the reasons for the heat exchange in the radiator (aka. cooler). Stable viscosity, over a large temperature range, is one of the advantages to synthetic ATF. (Particularly when a true cooler is added, and it takes longer for the fluid temp to rise.)