dry sump oil separators

[v114]

The engine is a V-twin with a dry sump oil system. The engine crankcase is basically sealed
and runs at between 5 and 10 inches of vacuum generated by the scavange pumps.
The breathing to atmosphere is done at the remoted oil tank. The tank is vented
to atmosphere through the filler cap. We are restricted by the fact that the oil
tank on these motorcycles are only approximately 3 qts capacity and we are only
running 2 qts of oil in order not to overflow the tank during operation.

This basic setup works well in all but sudden high rpm throttle bursts. At
those instances, the sudden flow of oil from the scavange back to the tank
causes oil to come out of the vent in the tank.

What solutions are out there for proper venting of oil tanks? Is an air/oil separator the solution? We are considering regulating the flow from the scavange to the tank.

 

[patprimmer]

It is more likely a blow by problem than an oil flow problem , unless the return line position and tank internal design is bad.

The problem can be masked by bigger breathers and/or oil separators in the breather.

There are many OEM and aftermarket oil separators out there. Some are as simple as a piece of tube with some Scotch Brite stuffed inside.

Regards

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[v114]

patprimmer,

Running the engine at steady high rpm does not exhibit the tank overflow problem. I can run the engine speed up while the bike is standing still and the tank oil level seems to be fine with no violent disturbances. If blowby was the answer, would I not see it during steady high speeds? How can we be sure? What is the test?

The oil tank on a harley is designed for much less oil volume going through it than we are using here. That said, our customers will be using a stock tank. Basically, the tank is not well designed for this use since the return outlet is aimed almost at the vent. Do you think that a larger volume between the scavange outlet and the tank could dampen these pulses?

 

[patprimmer]

Load will be a big factor re blow by.

High speed, steady state part throttle will not generate as much blow by as accelerating to high speed at wide open throttle.

The return to the tank should be high in the tank and aimed at a curved wall so it strikes at a tangent and swirls around without splash.

It would be better if it were well away from the breather.

There should be baffles to direct splash and surge away from the breather.

Regards

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Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[sprintcar]

Back in my days at Harley, the biggest oil leak problems were simply due to overfilling the oil reservoir tank. This led to oil migrating out the breather tube to the air filter when expansion (heat and aeration) raised the level in the tank. "It's gonna leak anyway so I might as well add more oil" was usually the root culprit. The FXR got a relocated vent tube and a change to the other end so vapor was sucked directly into the carb, along with an oil capacity label on the tank. Dyna series solved most of the issues by putting the oil beneath the powertrain where it belongs.

Consider a balance line between the tank and the engine. This can be jetted to maintain partial crankcase vacuum.

Otherwise - as Pat said - the return line shoud inject the oil into the tank against a curved wall. This helps remove some of the air from the oil (deaeration) which keeps the volume lower.

TESTING: Make a polycarbonate window in the tank so you can watch what happens. If you can't duplicate it on the dyno, rig up a helmet camera for the test track. We had windows in the primary cover to watch the wet clutch oil patterns and the aeration change was spectacular.

Timing has an awful lot to do with the outcome of a rain dance.

 

[v114]

Sprintcar,

Can you describe the balance line a little more? Right now, we run a line from the engine to the tank but with a 1 psi check valve with direction of flow being from engine to tank to prevent overpressure of crankcase for safety.
The jetted balance line you describe will work differently. How will it help the tank from blowing oil?

Thanks

 

[sprintcar]

Check your check valve. 1 psi may not seem like a lot but you need to find actual cracking pressure under operating conditions (heat, vibration). You may also find that after time the chk valve could freeze open or closed due to contamination, sludge, etc. It sounds like you are using this line to vent pressure spikes from c'case and still maintain c'case vacuum.

Somewhere in any oil system is a vent to atmosphere - typically going into the intake (carb/inj) manifold to burn blowby hydrocarbons. This usually creates a slightly lower pressure for the vent. Without this, all the pressure must go into the tank, then to atmosphere. There may be a much greater flow of air than you may have allowed for - thus the velocity is pulling oil with it and exiting the oil cap. Try running a 1/2" hose from the very top of the oil tank up as high as you can get it. This way oil will have a chance to drop out of suspension and flow back into the tank. Use a small in-line gas filter at the terminal end to keep dirt out. I'm guessing this is a race application since you are free venting.

Remember that scavenge pumps are positive displacement so if you have a 3 gpm pressure feed and (2) 3 gpm scavenge sections, you have to contend with 6 gpm total flow, 3 of which will be oil mist. Unless this is redirected to the c'case, that's what you have to vent off. Using an air balance line with a jet inside will let you pull some of this air volume back into the system while still maintaining negative c'case pressure.

Timing has an awful lot to do with the outcome of a rain dance.

 

[v114]

Sprintcar, Thanks for the feedback. This engine is still in development and will be primarily a street engine when finished.

Your suggestion of a 1/2 inch hose from the vent cap (to slow velocity) is already being used now. That empties into a catch can right now which is how I know if the oil is coming out of the tank vent or not. Once complete, this vent must connect to the intake system and the vent gases burned off like other applications.

I like the balance line idea. This would mean to eliminate the 1 psi check valve and replace with a jet to regulate vacuum.

You mentioned your experience with Harleys. In the end, would it be safe to say that with a stock oil bag (3 qts), this engine may be able to run only 2.5 qts because of the greater need for an air gap at the top of the tank to prevent oil from escaping?

I don't see any other tricks at this point.