GEROTOR pump, dry sump engine pressure problem 1

[fabiom75]

We have built a race engine. It's a 2jz-gte supra 3 liters engine and we have applied a dry sump kit.
We are actually using a peterson gerotor 3 stages pump rated at 18/20gpm (at how many rpms?)
We have a 5 gal tank in the trunk and a -16an line going to the pump and -12an line pumping oil inside the engine.
2 scavenging lines keeps the tank always at at least 2/3 of the level.

Here is the problem:
hot oil, idle 1600rpms, 28psi of oil pressure.
everything is ok so far.
Increasing the engine rpms we see an incease in flow and pressure. We are actually using evne a flow meter gauge.
Everything is ok up to about 6k rpms engine speed where we have about 117psi of pressure and 16gpm of oil flow.
At this point we have problems:
keeping the engine STEADY at 6k rpms we start to see a drop of oil pressure BUT NOT a drop in flow and this could be caused MAYBE by the increasing of oil temperature and loss of oil viscosity.
If we increase the engine rpms we have an even more accentuated loss of oil pressure but still absolutely no loss of oil flow.

The actual pump ratio is 52% so cavitation due to excessive pump speed should be possible. These pumps are rated at well more then 5k pump rpms. At 6k engine rpms we are at about 3k pump rpms.

Cavitation due to small inlet line should not be the case as well. We are using the -16an line as per pump instructions. The oil line have no tight bents.

The actual gerotor is 1.1 inches wide. The stock oil pump is 0.8inches BUT spinning at 100% of engine speed.
The external of the actual pump is about 2.75inches. While the stock pump was 4.3inches.

Could be the actual pump undersized?
Is there any way to calculate the oil volume that the stock pump was flowing based on the rpms/gerotor size?

Any help would me really appreciated.

 

[fabiom75]

We have checked for oil leaks and air leaks.... all is perfect...

I wonder if the problem is that this engine really wants more oil then usual.... the crank have been modified by us: it was a striaght shoot crank. We have added an hole on the rods journals going from one side to the other of the journals and we have even made the mains full groove from half groove...I wonder if this is the reason why the oil pump with a ratio of 50/60% is not sufficent...

 

[patprimmer]

An oil pump is positive displacement, so apart from leakage past the clearance inside the pump, one turn of the pump pushes a fixed volume of oil.

Pressure is a result of resistance to flow.

To change the pressure you must change the resistance.

The things that effect resistance if the pump is turning (say by being disconnected from the engine drive and being connected via an outside source) are static bearing clearances, oil viscosity and oil passage sizes.

Once the engine is turning, centrifugal force is applied to oil in the crank passages and the big ends act as an eccentric oil pump. This pumping action is dependant on the effective clearance (which changes with localised temperature changes while running) and effective film thickness on the loaded and unloaded side of the journal.

The pressure at any two points in the system will vary depending on supply to the system and restrictions before and after that point.

Most measure their oil pressure between the pump and the main gallery, but the critical points are the individual entrance points to the journals and the differences between them.

Splash feed gives enough lubrication so long as heat is not an issue. Splash feed depends entirely on film strength and has no hydrostatic component. I believe (I read it somewhere) that oil film strength is much greater than hydrostatic effect as seen at a bearing journal in an automotive engine.

A modern engine runs at high enough loads and speed to create enough heat to melt a splash fed bearing in seconds.

Pressure simply promotes flow. Flow cools bearings. Hot bearings heat the oil and reduce the film strength and thickness on the loaded side. Hot journals expand and reduce clearance as the caps cool more than the crank. Tighter clearance then reduces oil flow, which reduces cooling etc etc.

Do you have any signs of overheating at the bearings. If not, what is the problem other than numbers on a gauge that is not measuring at the most important places and is only measuring a parameter that is separated from the real parameter by several degrees of separation.

Just a thought, can oil pressure gauge pickup or sensor location be effected by venturi effect.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[thruthefence]

If this is a dry sump application, is the scavange side adressed? Isn't it possible that poor scavaging would lead to the afore mentioned airation? Someone also mentioned poor design of the oil reservoir. Just dumping the oil back into the tank would also contribute , I would think

 

[SMOKEY44211]

Get the non groove main bearings in and your problem is solved. There have been a number of SAE papers written on the subject. Often you can get copies through your local library. I think once you digest all the reasons why that style bearing is more desirable it will help you to understand the problem you are experiencing with the engine you are working with now and others you will come across in the future.--------Phil

 

[MikeHalloran]

Good catch, Smokey.

Fabiom, go back and read your engineering textbook chapter about hydrodynamic lubrication and you'll understand where your oil pressure went.



Mike Halloran
Pembroke Pines, FL, USA

 

[fabiom75]

Smokey, Mike and all the others,
this forum is an amazing place!

Now I am pretty sure that the problem resides in the full grooved mains.
I have been searching about this topic on google but I have found almost nothing... No real comparison/benefits/pro/cons between full, 3/4 and half groove...
Beisdes we are NOT using full groove bearings BUT we modified teh block itself: we made a 360 degrees groove all around the main: on teh block and on the cap. This way we didn' tmodified the breaing to keep the original strenght of the bearing itself but we allow a constant oiling to the bearing due to the 360degrees groove that we made.

Any suggestion and idea about this?

 

[Supercar1]

Very interesting thread. An unheard of oiling problem with the indestructible Supra engine! Maybe if you figure this out then our wimpier engines could benefit from this too.

Could there is some king of flow restriction after the pump, like the filter sandwich/adapter? Try installing that pressure sensor right after the pump and see what it reads there. I am betting it will read 117 psi all the way to 10K rpm.

I am also doing a dry sump project on a similar engine. My pump is rated 12 gpm at 2500 RPM. It is geared down 60%.

This is how I was sizing my dry sump pump:

http://supercar-engineering.com/tmp/DrySumpPump1.mpg

Actually, this does not show the real procedure when I had both of the pumps on the electric drill at the same time. They filled two large trash cans pretty quickly. The dry sump pump filled up its can with about 2.1 times more water. After multiplying 2.1 by the 60% gear ratio that is 26% more oil flow than the stock pump, so I am pretty happy.

I have not ran the actual engine yet. Hopefully later this month. Wish me luck! And please report your findings. Thanks.

 

[fabiom75]

ok I have some news:

It really seems like the pump is not flowing enough oil for this eninge. More exactly. For all the mods we made to this engine.

Ok here are our thoughts:

we made some experiemnts.

1) we brought the pumps rpms higher. We used a ratio of 1:1. We wanted to see and log with teh telemetry what the CAVITATION looks and sound like. We reached it easily at 6k engien rpms. Well teh way you read cavitation on the telemetry and the sound is completely different from what we had before.

2) we measure the oil pressure right after the oil pump. Using the 55% ratio with no cavitation. Brought slowly the pump at the rpms level were we had teh problem and logged. There is a costant drop in oil pressure with no cavitation at all that is pretty much accentuated after 6500rpms.


After reading the article thet GregLocock suggested me (

http://forums.autosport.com/showthread.php?s=&threadid=90890

) I think we went on the right track.
Seems like in that test at about 6500rpms there is a big difference in oil flow with HALF GROOVE vs FULL GROVE.
A drop in flow means that the oil is having trouble to pass thru the oil holes. This even means that the pressure will most likely go higher.
That is where we start to have problem. it could be a coincindence but I am not sure.

Actually there could be many reasons why this supra block wants more oil

We moved from a stock cross drilled crank to a striaght shoot crank. This already helps teh flow at high rpms due to the resons that all of us knows (centrifugal force with cross drilled makes the oil flow problematic at high rpms)
Beisdes we modded it: a normal striagh shoot crankhave have one hole going from the main journal to the rod journal. We added a pass thru hole to the rods journals. NOT SURE how much this could help as probably the centrifugal force wont allow much oil to pass thru that hole and go to the opposite side of the bearing. But it could help a bit.

Plus we added a FULL GROOVE on the MAIN caps and the block. this should help to feed the rods journals all the time. (we made the full groove on the main caps and the block. The main bearings are grooved for 1/4 plus they have holes all around 360 degrees. We didn't wanted to mod the bearings to avoid problems with weakness.)

All of these mods could be teh reson why this engine wants more oil.

Right now I have in front of me a new oil pump with dual pressure stage. Rated at 25gpm.
We will isntall it this week and I will let you know.



Any help and thought even about the full grooves would be VERY VERY appreciated

fabio