Oil pressure guru's, fluid dynamics help needed!

[RandyP]

Hi everyone, I;ve been trying to find an answer to my question below in vain. Found this forum and hope someone here can give some professional insight.

I have an engine with a constant volume oil pump, that feeds the oil to the motor using 1/4" oil line. With this 1/4" oil line it has an overall oil pressure of 10lbs at idle and 60lbs at 3500rpm. The pressure release valve is rated at 65-80lbs.

I know overall pressure in a motor is related to bearing clearances.

My question and confusion is. When I switch out that 1/4" line with a 5/32" line or even a 3/8" line overall oil pressure stays the same.

Does this mean it does not matter what sized line is used between the three?

I know the "potential" flow of the larger lines is obviously more but since they all register the same lbs at any given RPM I'm thinking the flow is not being restricted by the lines but something downstream of it. Please let me know your thoughts.

Alsom does pressure drop still play a role in the smaller or larger line?

Thanks Randy

 

[PUMPDESIGNER]

You have stated it correctly as:
"I know the "potential" flow of the larger lines is obviously more but since they all register the same lbs at any given RPM I'm thinking the flow is not being restricted by the lines but something downstream of it."

That restriction downstream is the bearing clearance.
Your supply lines are all large enough to properly feed the bearings sufficient oil AT THIS TIME.
As the bearings wear the "system curve" will change, then the lines may become a factor.
You could be deceived at some time however IF the lines themselves cause restriction. If the lines cause a restriction, then flow rate will decrease below what the bearings require, and yet your pressure would remain high because of the restriction in the lines, when in fact the bearings would not be getting sufficient flow.

PUMPDESIGNER

 

[RandyP]

Thanks for the insightful reply pumpdesigner.

Maybe you can help with one more question regarding the same concern. I tested the output of the pump at idle (950rpm) using the 5/32 line and the 3/8 line. Both lines delivered roughly .72 gal/min. If I remember correctly the 5/32 line was measuring roughly 6lbs while the 3/8 line measured roughly 2lbs. I couldn't test at higher RPM for fear of damaging the motor but if it makes any difference pressure can go all the way up to 65-80lbs before the pressure release valve kicks in.

Do you think using the smaller line could still be a problem or would you still need more information? Thanks Randy

 

[PUMPDESIGNER]

I have insufficient information to work it further.
The only way to really calculate it all is to have both the pump curve and the system curve, and you are probably not set up to obtain all that through testing or calculation.

However the larger line is always a safer way to go.

PUMPDESIGNER

 

[RandyP]

Thanks Pumpdesigner,

Hope you can answer what I hope is the last simple question. If the smaller line wasn't up to the flow rate in this system what would you see happen to the pressure? Thanks Randy

 

[TD2K]

Your oil pump will be some sort of PD pump which means it WILL deliver the 0.72 gpm at 950 rpm regardless of the line size you attach to it. This flow rate is directly proportional to speed, run the engine 10% faster, the flow rate goes up by 10%. Doubt the speed, the flow rate goes up by double. The pressure drop through the line is proportional to the square of the flow rate. So, if you increase engine speed by 10%, you increase flow by 10% and the pressure drop across your line increases by 1.1*1.1 or 21%.

Within the engine oil distribution system, you need a certain pressure to push the oil through the passways and final clearances. That's the oil pressure you would measure right where the line connects to the block. This pressure also is proportional to the square of the flow rate the pump puts out.

So, the pressure back at the pump is the sum of the pressure at your engine plus the pressure drop through the line. If the pressure gets too high, your overpressure valve will open to dump oil back to the sump.

If you put a smaller line between the block and the oil pump (I'm assuming the pressure release valve is on the pump), the pressure release valve will begin to open sooner. Any oil that flows through the valve (to prevent excessive pressure) is less oil going to your engine since the pump only puts out a fixed amount (which is a factor of the engine speed at that time).

 

[RandyP]

Thanks TDk2,

You've described everything right on. The one thing that confuses me is the pressure remained the same no matter which line I used. The pressure gauge is located inline of the lines between the pump and the input of the block.

What confuses me is I know the larger lines can flow more than a smaller one. Pressure will drop in a smaller line. But I never measured this to be true in this application. Pressure remains the same at all RPM's up to about 3500RPM where the pressure relief valve starts to open, this was true of all line sizes tested.

I am trying to see if the smaller line is sufficient. What would you expect to see if it wasn't? Thanks Randy

 

[PUMPDESIGNER]

As long as the oil lines cause little restriction it matters not what size they are. If you install 1" lines you will not have a difference either.

What would change the pressure is if the bearing clearances become larger, OR if you installed an extremely small line say about 1/32", in which case the pressure at the pump would increase while pressure inside the block would decrease.

PUMPDESIGNER

 

[automatic2]

Randy, you can answer all your questions if you consider this, the pressure is max at the pump outlet, and min at the point oil exits the bearing. You will see varying degrees of pressure drop between these two points, dependant on the formula of flow x flow restriction. You seem to querying if there is adequat pressure delivered to your engine at a point after the hypothetical supply line. This would likely be at the engines main oil gallery. For a true answer, you would have to gauge at this point. Depending on the engine, you may have many, or few opportunities to tap into the main gallery. You can, and many do, gauge at points other than the main gallery, but the value (accuracy) of the gauge depends on fixed parameters, ie: constant line size. Altering those parameters negates the value of the gauge.

But, more specifically to your last question. If the supply line size was reduced to the point of starving the engine, you could indentify a substantial pressure drop across the length of the supply line. If the supply line was straight (no bends), then the drop would be linear over the length of the line. Depending on the point along the line that you gauge, if you saw pump pressure close to the outlet of the pump, you have little to base assumption on in regards to your query. If on the other hand you saw pump pressure at 75% of line length, you could safely say that little pressure drop is happening. With all that being said, without a flow/pressure test, if you have noticed no pressure drop with a change in line size, you can be assured that you have not restricted flow rates. You can be comfortable with this assumption at the same percentage that your gauge point is along the length of the supply line.

 

[budt]

Randy;

The main thing to keep in mind is "Pumps produce FLOW not Pressure" PRESSURE is "RESISTANCE TO FLOW"

The biggest pump in the world discharging to atmosphere theoretically has no pressure. A hand pump with fully restricted output can reach any pressure until you give out or it blows up.

Lines give resistance to flow. Even a 36" pipe with a 1 GPM flow will have pressure drop. Always know if there is flow there is pressure drop. When flow ceases pressure drop also ceases.

In your engine there is a higher pressure at RPM because flow increases while restriction (resistance) remains the same. The larger lines could carry more flow but only saw the pump output which was well satisfied by the original line size.

If you never forget "Pressure is Resistance to Flow" you will have less problems with reading pressure gauges and figuring out what is happening.



Bud Trinkel CFPE
HYDRA-PNEU CONSULTING, INC.
fluidpower1@hotmail.com

http://www.fluidpower1.us