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Anyone know how an oil pump draws a prime?
That is one way of doing it, and a usually excellent way to boot.
I once got so worried about start-up oil pressure on an engine I'd just built that I made up a small accumulator with a solinoid valve run from the ignition. The idea was to isolate about 100cc of oil under pressure when the engine was turned off and re-introduce that back into the system when the ignition was turned on next time. Supposedly 40% of engine wear occures when the engine is cold but that wasn't why I did it, the engine was required to go on full load immediately on startup and I hoped the instant oil pressure would protect it better. It did produce almost instant start-up pressure but I've got no idea how much it would affected engine life, that engine has only done 200 Hrs in the last 10 years or so. Mark.. "Dave Skolnick" wrote in message news:VIDDc.1384$fd3.81@lakeread04... JAXAshby wrote: genee/rickie claim that oil pumps have to spin for some time to draw a prime. I am trying to visualize how an oil pump draws a prime, particularly on oil at say 15 degrees. In order to draw a prime the pump would have to evacuate the air above the oil and below the pump, correct the difference in air pressure on the evacuated side vs ambient air pressure leaves no more than a few pounds of pressure total (can't be more than 14.7# total, for that is atmospheric pressure). correct Then the oil would have to vaporize and then be drawn into the pump, then to be compressed back to liquid to then be pumped to the bearings needing pressure oil. The pumped fluid doesn't vaporize. At this point, the ambient air pressure through the vent (e.g. the PCV on an engine crankcase) is higher than the pressure in the line evacuated by the pump. The differental pressure (even if only a couple of psi) pushes oil up to the impeller or positive displacement device in the pump. The differential pressure must be greater than the weight of the oil in the line to prime the pump (ambient air pressure inlet pressure plus weight of pumped fluid). The greater the difference, the faster the prime and the greater the available suction head (distance pump can be above steady state fluid level). I believe in most conditions that the effect of viscosity is limited to "how long" and not "if" priming takes place. This assumes the pump can run dry indefinitely and ignores second-order effects like friction of the fluid against the line. dave -- ----- news_bucket e-mail address goes to a blackhole. Sorry. Send e-mail to "respond" at the same domain. |
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