Oil Pumps, The hows and Whys

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Pressure versus Flow The subject of pressure versus flow can be confusing because it's hidden inside the engine. So let's consider this principle in something we all use daily - the water system in a house. The maximum amount of flow is determined by the size of the line into your house if you have city water, or by the size of your pump if you have a well. The upper and lower limits of pressure are regulated by the city or by two electronic pressure switches on your pump. Everything works fine, and you have water when you need it. But have you ever opened every faucet, including those outside? If you did, the pressure drops considerably. The maximum flow available has not changed. With all the faucets open the resistance is very small, but the flow is maximum and the pressure is slim to nil (Figure 1).  - Figure 1  - Figure 2  - Figure 3  - Figure 4 Pressure, relative to your house, is created by the resistance to flow, i.e., the faucets being closed (Figure 2). When you have total resistance, all faucets are closed, the upper pressure is limited by the city or the electronic switch and the flow is zero. Now let's consider pressure vs. flow inside an engine. The size of any oil pump is designed to supply the correct amount of oil to meet the engine requirements. It produces a specific amount of flow at a given rpm. The resistance to that flow produces the pressure. There is never a time that duplicates all the faucets closed (maximum pressure/zero flow). The resistance to the oil flow is from the bearing and lifter bore clearances. When an engine is new, the clearances are tight and the pressure is good (Figure 3), just like a faucet that is cracked open enough to produce a small stream of water. As the bearings and lifter bores wear, clearances increase, resistance to flow decreases and oil pressure starts to drop (Figure 4). When the pressure drops, we get our first signal that something is wrong in the engine. With the increased clearances/decreased resistance, the flow from the oil pump is at its maxi- mum. This is like opening all the faucets in your house. Pressure is down, but flow is at a maximum. If we used a flow meter instead of a pressure gauge, we would see a gradual increase in flow as the bearing clearances increase with wear. But flow meters are more expensive and more bulky than oil pressure switches, making this method impractical. A few comments about the relief valve in an oil pump. The spring pressure behind the valve determines when it will open. If it is designed to open at 60-psi, it does not have an effect on anything below 60-psi, unless it sticks open. If it does stick open, the pressure is low at idle, but builds up to 60-psi at the point the valve opens. If an engine has excessive clearances in anything but rod bearings, the oil pressure will be uniformly low throughout the rpm range. If the rod bearings have excessive clearances, the oil pressure will be low at idle and will get worse as the rpm increases. Rod bearings turn in a circle rather than on an axis, so they are subject to centrifugal force trying to pull the oil out of the bearing. The amount of loss will vary depending on the actual clearances in every bearing and lifter bore. This gives you an idea of how much flow increases with a small increase in clearances, and how pressure can drop when we exceed the flow available from the pump. Science of the Pump The oil pump is the heart of the engine, supplying lubricating oil to various moving parts of the engine. Most pumps consist of four parts: A main body; A cover assembly; A gear-type assembly (one driven, one idling); or rotor-type assembly (one inner, one outer); An oil pressure relief valve (usually in the pump, but sometimes in the engine block).  - Figure 1  - Figure 2 Figure 1 shows the most common pump designs. Although they look different, their function is the some: to supply lubricating oil to various moving parts in the engine. To do this, an oil pump regulates oil flow (volume) and oil pressure. A lot of people think that the pump produces the engine's oil pressure by itself. Actually, the pump creates only the flow of oil. The restrictions in the oil passages of the engine produce the pressure. The faster the pump turns, the greater the flow of oil to the engine. The more oil flowing through a restricted area of the engine, the greater the pressure. Oil pressure is directly related to the clearances in various parts of the engine, as the clearances increase, so does the capacity for more oil flow. But when the engine's need for more oil increases beyond the pump's ability, pressure drops. Because most pumps can produce more than 150 psi, a pressure-regulating valve is installed in the oil pump or the engine block The valve is loaded with a closely calibrated spring, telling oil bleed off at a given pressure (oil pressure pushing against the relief valve unseats it and allows excess oil to bypass it). The valve assembly is designed with very tight clearances, and must work freely in order to keep the engine's oil pressure constant. Foreign material entering this area can easily jam or hinder the operation of the pressure relief valve. Also affecting engine oil pressure: Operating speed of the engine; Viscosity of the oil. Crankshaft-Driven and Chain-Driven Pumps Most automotive oil pumps are driven by a gear on the camshaft and turned at half engine speed. However, many now engines are using crankshaft-driven pumps. These pumps are gerotor style and the inner rotor fits over the crankshaft. With this setup, the inner rotor is loose in the pump before installation. You'll also notice that the rotors are thinner; they don't have to be as thick because the pump is turning at engine speed. Another pump type that's unique is found on the 2.6L four-cylinder Mitsubishi engine. The pump and the counterbalance shafts are driven by a chain at twice engine speed. In most oil pumps, the shafts get their oil directly from the pump. In this case, the oil for the shaft goes from the pump to the engine's main oil galley, then back to the pump to oil the shaft. If there's an oil leak or loose bearing, the shaft is starved for oil and the pump and engine self-destruct quickly. Crankshaft- and chain-driven pumps require great care in installation and priming. The gasket surfaces must be clean, and the gaskets and the pump need to be precisely aligned. Be sure to prime these pumps according to their manufacturer's instructions. We strongly recommend that you prime the whole engine with an external priming device before start-up. Oil Pressure Problems The first reaction to on oil pressure problem is often to replace the oil pump-sometimes unnecessarily. Since existing problems within the engine can't always be fixed by installing a new oil pump, you should carefully evaluate the situation before doing so. An engine could have oil pressure problems for many reasons: An air leak (usually where the screen assembly is mounted to the pump). Check for loose connections, cracks, or holes. Since air is compressible, any air inside the oiling system can cause a fluctuation of pressure. Too much oil in the crankcase (leading to air in the oil) Excessive clearances within the engine (rod and main hearings, corn bearings, seals and oil plugs). Oil leaking post the mounting pad area (usually caused by tightening the mounting bolts wrong). Malfunction of the relief valve assembly. As we mentioned before, the main function of the relief valve is to regulate oil pressure within the engine by keeping a constant flow of oil to the engine. (See Figure 2.) There are three different relief valve problems (usually caused by foreign material that is drawn into the pump from the engine) that can cause oil pressure problems: The relief valve is stuck in the closed position. It won't let extra oil pressure release itself inside the pump, so the pressure builds and ruptures the oil filter. The relief valve is stuck in the open position. The pump will release too much of the pressure, creating low oil pressure and possible bearing failure. The relief valve is sticking, so movement of the valve within the assembly bore of the oil pump may be restricted. In this case, the valve isn't necessarily stuck in a fully open or closed position, and the result is erratic pressure. Since oil pumps are manufactured with such close tolerances, it doesn't take much foreign material to cause problems with relief valve operation. And its easy for did, bits of wire, etc., to get into the oiling system, even With the screen assembly in place. So. be sure to find the root of your oil pressure problem before you decide to replace the pump. Oil Pump Screens The oil pump screen smooths the flow of oil into the pump. it usually keeps out debris that can lock up the pump. It is the only thing in an engine that assists the pump in it's function. The other engine parts rely on the pump to help them. If we could take a screen apart, it would be easy to clean. Unfortunately, we can't. Therefore it is impossible to clean it completely. Any debris left inside has the potential of locking up the pump. Close examination of a used screen assembly after attempts to clean it may reveal a dark brown stain, which is usually a varnish type coating. The most common screen mesh has a .040" square hole between the wires. Oil flow is directly proportional to the area of the hole. If the varnish coating is .0051 thick, our square hole is down to .0301 on a side. This is a 25% reduction on a side, and a 44% reduction in the total area and flow [(.040")2- (.030")2/(.040")2 = 43.75%). The second statement references, "usually keeps out debris." There are two styles of screens that allow debris to enter an oil pump.  - Figure 1  - Figure 2 The first (Figure 1) has a valve in the center that opens if the oil is too thick or if the screen is restricted. The second type (Figure 2) has eight gaps approximately 3/161 x 1/2" which allows oil to flow if the mesh will not. Oil pump screens should always be replaced. There is not a more economical way to reduce oil pump and engine failure from ingested foreign material. Period. ----- Common Misconceptions on Oil Systems OIL PUMPS PRODUCE PRESSURE. Oil pumps produce flow and the resistance to that flow produces pressure. The pressure relief valve limits the maximum pressure but does not do anything until the pressure has reached that point. THE INLET SCREEN WILL KEEP DEBRIS OUT OF THE PUMP. Some of the new ones will but the majority have a 1/2" ID valve in the center or they have eight 3/16" X 1/2"slots around the edge of the screen. The valve or slots are there to allow cold thick oil into the pump. If you use thick oil with the new screens, you have a good chance of starving the engine of oil and ruining it. I HAVE LOW OIL PRESSURE SO I'LL PUT IN A HIGH PRESSURE PUMP.The low pressure is usually caused by a leak or excessive wear in the engine. If the original pump has a 50 psi bypass and the engine will not get over 30 psi, then putting in a pump with a 75 psi bypass will not change a thing. You will still have 30 psi. You have to fix the leak or rebuild the engine. SAME AS #3 EXCEPT I'LL PUT IN A HIGH VOLUME PUMP. The high volume pump does increase the flow so you will have some increase in pressure. However, you still have the original problem of a leak or worn out. The high volume pump just delays fixing the real problem. High volume pumps are for increased demands on the oil system such as higher RPM usage, racing, remote filters and or coolers, etc. -------------------------------------------- by Gary Cross [ January 10, 2003, 02:10 PM: Message edited by: BOBISTHEOILGUY ]
 
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Thanks, Bob, great article. Just a note on how those gear pumps actually pump oil. Look at the spot where the two parts of the oil pump touch. Now, visualize the part with the shaft turning...as you visualize a gap opening, that's where oil is pulled into the pump. There's an inlet port that lines up with this gap on this side of the pump, and the gap is carrying oil around the housing. As this particular gap that we've been watching moves around the pump body that gap approaches the outlet port. The two parts of the pump begin to squeeze together, the outlet port is there, and the oil is pushed through the outlet port into the oil system of the engine. The pic of two gears needs the body of the pump in the pic. The pump's body has a figure-eight space machined into it just the right size for the two meshed gears. One of these gears is driven by the engine, and the other has a shaft just mounted in bearings. The pump works on the same principle--as the meshed parts of the gears separate they pull oil in from an inlet port. The oil is carried around the housing in the gaps between the gear teeth. As the original gear teeth we've been watching begin to mesh again the oil is squeezed through an outlet port. Ken
 
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Quote: "SAME AS #3 EXCEPT I'LL PUT IN A HIGH VOLUME PUMP. The high volume pump does increase the flow so you will have some increase in pressure. However, you still have the original problem of a leak or worn out. The high volume pump just delays fixing the real problem. High volume pumps are for increased demands on the oil system such as higher RPM usage, racing, remote filters and or coolers, etc." When these HV pumps are used the sump capacity or engine oil volume must be raised. At sustained Hi-rpm these will pump more oil than the engine will allow do drain back . Many performace motors have lived a short life using these type pumps without using a larger capacity oil pan. The tolerances in these cast iron pumps are usually very close. Abrasives in the oil will do them in very quickly and I would assume the newer aluminum pumps would last a shorter time than a cast iron pump when using too high a VI oil for the ambient and or having abrasives in the oil. Something to think about when using a passive drive while you clean type engine flush.
 

BOBISTHEOILGUY

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I pulled my 302 ford oil pump apart on the bottom and thought you might like to see how this pump looks while actually running. So, I cut out a piece of plexiglass, and put over the bottom and video'd the pump running. Running Oil Pump [ May 27, 2003, 11:51 AM: Message edited by: BOBISTHEOILGUY ]
 

MolaKule

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The principle by which these pumps operate is "Variable Displacement" since the volumes bewteen input and output are constantly changing.
 
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'Stralia
quote:
Originally posted by MolaKule: The principle by which these pumps operate is "Variable Displacement" since the volumes bewteen input and output are constantly changing.
Not quite sure if I get you Molakule. These are positive displacement pumps, which shift (minus leakage and entry effects) a constant volume per revolution. They cannot change their displacement. i.e. "dead-headed", things get broken
 
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So, if an engine is worn and the pressure is low many people move to a higher weight oil to compensate for the low pressure. At least that seems to be the recommended plan of treatment in many cases. What are your comments relative to a higher weight oil as I do not believe I read any comments on this potential solution to low pressure.
 

MolaKule

Staff member
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They are indeed positive displacement in that they have a total "positive" constant volume; however, the <b>volume</b> changes during the <b>cycle</b> to create a low pressure area at one position and a high pressure area in another.
 

BOBISTHEOILGUY

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Here is a bottom view of a 302 ford oil pump. Notice that there is a pressure relief valve that when the pump see's too much backpressure, it will send the oil back into the input side of the pump to be pumped back out the output.  -
 
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Northern California
To anyone: Some questions concerning flow and pressure. If a positive displacement pump pushes a certain anount of oil down the oil lines would thicker oil be pushed in the same amount but at higher pressures until it comes past the restriction? Wouldn't that make the oil get to the top of the engine at the same speed regardless of thicker or thinner oil? Considerig the oil pump by-pass, how would the by-pass valve effect this? Would the oil be by-passed back into the pan and only the remaining fraction of the pumped oil go up through the oil gallerys? I guess these questions could be posed with both cold oil and hot oil.
 
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Pennsylvania
quote:
Originally posted by White 03: To anyone: Some questions concerning flow and pressure. If a positive displacement pump pushes a certain anount of oil down the oil lines would thicker oil be pushed in the same amount but at higher pressures until it comes past the restriction? Wouldn't that make the oil get to the top of the engine at the same speed regardless of thicker or thinner oil? Considerig the oil pump by-pass, how would the by-pass valve effect this? Would the oil be by-passed back into the pan and only the remaining fraction of the pumped oil go up through the oil gallerys? I guess these questions could be posed with both cold oil and hot oil.
correct me if Im wrong, but viscosity does affect rate of flow in a fixed 'pipe' given same input force....I cna look up the old formulas if you want
 
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QD, viscosity does affect the amount of pressure loss in a pipe. Problem with thick oils is that the pump shoves so many c.c.s per rev at the outlet port. The greater pressure drop forces some of it out the bypass valve rather than at the bearings.
 
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Great Post. It is really hard to get information on interpreting oil pressure readings. Two points, 1) I was told that a large difference between cold and hot oil pressure at highway speed, say 20+ psi, was an indicator of problems in the lower end. 2) My apparently successful experience with a thicker oil to improve oil pressure. I was getting only 36 psi hot at 2000 rpm on my truck with 5w30 and spec is 40-60 psi. I moved up to a 10w40 and now get around 45 psi. Not sure why it was low. Engine has 104,000 miles, some of them hard. I suspect the bearing clearances are a tad wide, but not too bad as the pressure is consistently low throughtout the rpm range (thus, as noted above, the rod bearings are not too bad). Also possible that the oil pump relief valve spring may be weak. Anyway, the 36 lbs was probably sufficient, but it gives me a warm fuzzy feeling to have it in the spec range without going extreme on oil thickness. As noted above, the thicker the oil the more it will tend to dump out the relier valve. However, the thicker oil also will not dump out the higher clearance main bearings either and thus hopefully will get more oil to the top end. I think I should be very happy to be where I am and would be a fool to, say, try for the upper range of the specification by using something like a 20w50. Although somewhere along the line, when the engine is only partly warmed up, my oil surely is at a 50-weight's viscosity or more. I suspect point 1 above is true and wondered if anyone had comments regarding my experience (#2 above). Thanks.
 
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