oil pumps-pressure reliefs, and positive displacement

Not open for further replies.
Sep 28, 2002
I just received a flame email from a fellow member that will remain nameless. He took exception to something that I stated ...and to his credit..brought it up to me privately instead of flaming on the board. OTOH ..he's still a flamer and since I'll keep him nameless. I'd like you to critique his assertions versus my knowledge and experience. Here's the meat of my assertion. Oil pumps are typically positive displacement pumps. XX rpm ..XX volume. I've never heard of a centrifical engine oil pump. Vaned, "crescent" ..and perhaps some other vane/rotor type, sure. All have a pressure relief typically built in to limit pressure under all conditions. Here's what I said:
What is the oil pressure relief?
Basically, within certain limitations, it sets your MAX oil pressure. Oil pumps are typically "positive displacement" type pumps. If it turns one rpm ..X amount of volume MUST be pumped. So, if you've got straight 50 weight and it's -40 out ..you're going to probably break something if the oil pump trys and move the full volume of oil. They have a built in relief valve that bleeds all excess volume via (typically) spring pressure ..back to the suction side of the pump. So the pump turns ..the max flow goes to the engine that "can" go there at that max pressure/at that temp-viscosity (let's take mine, 58 psi, for example) and the rest of the volume (that is causing the high pressure) is shunted to the pickup side of the pump.
Granted this is a simplified and not necessarily complete explanation ..but I don't see any error in my assertions. This is the email I got (edited to protect the flamer): I suggest you stop giving advice and go back to school. So, if the oil pump is mechanically connected to the crankshaft, if the oil pump is a positive displacement pump as you so mistakenly pontificate, why does the hot oil pressure drop at idle? True, because there is a drop in viscosity, oil more easily leaks out all the bearing surfaces, but, if you'll pull your head out of your butt for a second, you'll also realize that the same seal condition that occurs at the bearing journals occurs at the oil pump where the pressure is generated! Positive displacement? Please guy, you are dating yourself. (as far as years of experience goes) Next you'll probably tell me that pistons are positive displacement and don't leak down too! Yeah sure, blowby doesn't exist! You should apply for a BITOG administration job! You'll for sure, . . . raise the bar! Please either allow this guy to quietly simmer in shame ..or publically humiliate me. [ July 10, 2004, 06:56 PM: Message edited by: Gary Allan ]
I'm with you on this one Gary. The oil pressure drops when hot because it is able to flow through the engine much easier. If your pumping in say 1L/min through a hole X diameter, when the oil is cold, it won't flow through that hole as easily as when it's hot. Sounds simple enough to me. Dave
Gee, that writing style could only be. . . Gary, you're dead on correct. I looked at a diagram of the oil pump for my car (a 2003 Toyota) and it's very definitely a positive displacement type device. I've heard of fuel boost pumps that are non-positive displ, but not an oil pump. If your brain surgeon/diplomat friend has an example of a non-postive oil pump, I'd sure like to see a diagram and explanation of how (and where) it functions.
You should apply for a BITOG administration job! You'll for sure, . . . raise the bar! [/QB]
To the person who sent Gary the email with this comment, my reply to Him is simple: We as Administrators/Moderators are just that. We are not experts in the field of oil and we do not claim to be. Our job is to Moderate the Board. If we make a post it is our opinion on a topic or subject and we take our slam dunk when we are wrong and a smile if we are correct. [Dual]
I'm with you too Gary. Sure, an ideal positive displacement pump may not exist, but to say a pump is not positive displacement because it has a very small amount of leakage through clearances is ridiculous.
Your flamer simply has it wrong. Yes, oil pumps are typically positive displacement devices, and all of the one's I've taken apart have a maximum pressure relief valve as you describe. This valve sets only the maximum pressure and has nothing to do with the fact that a thin oil at idle might not result in enough pressure to activate the valve. Honestly I do not even see what the flamer was trying to get at. The basic fundamentals of an engine pressure lubrication system are pretty straightforward! John
Yes, engine oil pumps (and many hydraulic pumps) are simple positive displacement pumps where two gears push the oil through and out the other side at a given volume. They do NOT create pressure. Pressure is created by resistance to flow, either from the bearings, the curves in the tubing and ducts, or the filter. The amount of this resistence is varied by the viscosity of the oil. The thicker the oil, the more resistence. When the resistence reaches a certain point it goes back through the pressure relief valve. And, yes, a moderator's job is NOT to be an expert but to keep things friendly and neutral, get rid of the nuts, and maintain order. The ones on this board do a great job of this. I run and moderate a maintenance engineering board in Spanish. I don't have to be an expert on vibrational analysis, thermographics, or many of the other things discussed to maintain order. I just have to respect their opinions as long as they are expressed properly.
I think I recognize that writing style. If true, he wasnt being nice to you. He is banned primarily I believe for his warm and charming personality. Next he will be telling us a Roots Blower isnt a positive displacement device. It operates in essentially the same manner as a oil pump only pushing air or air/fuel mixture instead of a liquid. A Oil Pump moves volume. Without a restriction their is no pressure. Gene
Thank you all. I'm well versed in most forms of pumps ..and have taken many an automotive oil pump appart (well, a few) ..as well as auto trans ...and a variety of centrifical pumps of varried configurations. I had no doubt in my assertions ..aside from the "rhyme and meter" of my rhetoric (gear vs. vanes, etc.). But my arguing with this individual would have served no purpose. Independent review was in order. Then again, obviously, even your review and confirmation of my assertions was probably a waste of time as well. So ..I THANK YOU ...and I "beg your pardon" at the same time. I think my pride got the better of me and I enlisted you guys to bolster it. Shameful, I know. [No no] [Cheers!] [Patriot]
Originally posted by Gary Allan: I think my pride got the better of me and I enlisted you guys to bolster it. Shameful, I know.
Without pride there is no progress... Gene
Not open for further replies.