The filter outlet restriction doesn't change the dP across the filter. ... I disagree, in a model it could with a capacious, low efficiency (low R value) inserted into a small engine model; but as I mentioned later in the discussion likely not in practice. Must use care when discussing a bench model and using the term never.
There is no way that the outlet pressure can be higher than the inlet pressure. If it was, the oil would be flowing backwards through the filter. ... . I never stated that, did I? I think I wrote outlet restriction
Again, there is no way the fluid pressure on the outlet of the filter is higher than the pressure on the inlet. The pressure on the inlet has to be higher than the outlet in order to make fluid flow from the inlet to the outlet. Basic fluid dynamics. Again, I never stated that, did I intimate that in some way?
There is not way an oil filter is more flow restrictive than an engine's oiling system. I find that most oil filters are only about 1/15th the flow restiction of an engine oiling system. I think you meant less. If you had a filter canister with no element - just the centre tube - would not there be NO delta p? Do you have examples? How was this data obtained?
Hard to say exactly what the Subaru service manual oil pump spec really means. I've addressed this before when that spec sheet has popped up in previous discussions. Yes, the "discharge pressure" seems low for oil at 80C, but maybe those oiling systems are relatively low flow restriction. Says at 5000 RPM that the pump discharge is 49.7 qt/min (12.4 GPM). That's not out of the question, but yes I agree is that actually the flow rate seen in the engine with the pump pressure relief set to 85 PSI as shown in the specs? If the engine does flow that much oil volume, then that's one reason why Subaru shows the OEM filter to have a high 23.2 PSI bypass valve setting.
Yes, this does beg for more information from subaru.
Thanks for the continued discussion. Just thinking and learning here.
Does the bypass relief valve pressure matter much?
- Thread starter AJB0009
- Start date
To add ... that's because the bypass valve in an oil filter (or one built into the engine block) only operate based on the dP across the filter. The total oil pressure at the outlet of the PD pump doesn't effect the operation of the filter bypass valve.
As my previous posts touched on, what determines the dP across the oil filter is: 1) The Flow vs dP performance of the filter, 2) The oil viscosity, and 3) The oil flow rate going through the filter. The graph in post 62 shows the result of that on a specific oil filter (PureOne PL14006).
You can have the same oil viscosity and oil flow rate going through the oil filter in all these cases regardless of the oil pressure level of the inlet to the filter/oiling system. The oil pressure seen on the dash gauge is essentially showing what the "total engine + filter flow restriction" is causing the PD oil pump to produce in terms of oil pressure in order to force the pump output flow through the oiling system.
If the inlet oil pressure gives an indication of the total oiling system flow resistance. In these cases, all conditions (oil viscosity, flow rate and same oil filter) are constant, and the lower inlet pressure just means the whole oiling system is less flow restrictive.
Filter inlet @ 100 PSI, and filter outlet @ 90 PSI. Filter dP = 10 PSI
Filter inlet @ 80 PSI, and filter outlet @ 70 PSI. Filter dP = 10 PSI
Filter inlet @ 60 PSI, and filter outlet @ 50 PSI. Filter dP = 10 PSI
In all case, the same oil viscosity and flow rate is going through the same oil filter, and the dP is identical in all cases.
Keep in mind that with a PD pump (I think that's the part you're not considering), that the pump will be forcing the same flow through the filter and engine as long as the pump is not in pressure relief. So the inlet and outlet pressure across the filter (or even the whole oiling system) does not effect the dP across the oil filter. Like I said in post 82 above, the only factors that determine the dP across the filter are: 1) The Flow vs dP performance of the filter, 2) The oil viscosity, and 3) The oil flow rate going through the filter. The filter outlet restriction and what goes on down stream of the oil filter has no effect on the dP across the filter. Only those 3 factors I listed effect the dP across the filter.
Maybe I misunderstood. See response above.
An oil filter having 1/15th the flow restriction of the engine oiling system is less ... 15 times less.
There would also be a dP across a filter with no media like you say ... because everything that oil flows through causes some level of dP. The base plate hole, the center tube both cause their own small dP as oil flows through them. I think there is even a section in ISO 4548 that tests just the dP across the filter without any media so the dP caused by the base plate and center tube can be distinguished from the dP of the media.
PS - The way you respond to my posts makes it very hard to respond in a manner that can be followed. If you just do a reply to my whole post, then do a carriage return of where you want a quote to break up the post, then respond to those individual quotes, it makes it much easier to respond.
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To clarify on this statement, it doesn't matter what the restriction is down stream of the filter - R1 being the filter restriction, and R2 whatever restriction is after the filter. That's why finding the "Flow vs dP" of an oil filter with a bench test is an accurate indication of what the Flow vs dP will be on an engine - if you know the engine's pump flow and what the oil viscosity is.
Example using filter in the graph:
Flow path of a constant oil volume (5 GPM at 11.5 cSt viscosity) from pump to atmosphere, with associated dP in PSI:
Pump Outlet (P=80)--->Filter (dP=1.9)--->Oiling System (dP=78.1)--->Atmosphere (0 gauge pressure)
You will see the same dP of 1.9 PSI across the filter if the oil viscosity is 11.5 cSt and the flow rate is 5 GPM, regardless if the oil filter is in a flow bench test, or on an engine if those flow conditions are exactly the same.
I have been using Electrical circuit equivalent (Ohms law and to Kirchhoff's rules) to model
simple flow (also in loudspeaker cabinet design), so I have been using constant pressure in the model (high rpm operation pump against bypass setpoint) and not varying flow (other than changing R values). Also I have been doing this in my head at limits - some arguably unrealistic. I should play around with some of the online tools available to model this. I think I recall seeing a link in another thread. That will certainly be easier on my aging gray matter!
Thank you for the continuing discussion.
If you look at the oil filter as a resistor in a simple electrical circuit only comprised of a battery (the pump), the filter (R1) and the engine oiling system (R2) where R1 and R2 are in series, then:
- R1 is the flow resistance of the filter.
- R2 is the flow resistance of the whole engine oiling system.
- The oil pressure is like the voltage created by the battery.
- The oil flow going through the filter and engine is like the current flow though R1 & R2. Flow to ATM is like current going back to the negative battery post (no voltage/pressure potential left).
Think of the oil pump as a variable voltage source causing variable current to flow through fixed resistors R1 + R2.
As the voltage (pressure) goes up, then the current (oil flow) must also increase.
The voltage source can only go to a max voltage - that's the same as the oil pump going to max relief pressure.
In the simple R1 + R2 series electrical circuit driven by the battery, the separate voltage drops across both R1 and R2 will go up and down as the voltage and current goes up and down. Just like the separate pressure drops (the dP) across the filter and the engine will also go up and down as the oil pump pressure & flow output goes up and down with engine RPM. The PD pump pressure must go up to force the increasing oil flow output through a fixed flow resistance of R1 + R2, with each having its own resulting dP across it.
If you take a resistor out of an electrical circuit and put it in an electrical test "bread board" and measured the voltage drop across it as you changed the voltage source (which chances the current through it), then it would act similar to an oil filter in a flow test bench to measure the dP across it when you changed the oil flow rate through it.
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