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Oil cooler bypass plunger versus vernatherm

pecanflyboy

Well Known Member
I'm hoping to put a bunch of information into one place, as I've slowly pieced together this puzzle after a lot of reading. I'm not here to debate whether the oil bypass plunger is better than the vernatherm. Instead, I want to share the information I've gathered into one place. Much of this is opinion mixed with fact (isn't it always! Ha!).

What is the oil bypass plunger? Without the vernatherm, Lycoming uses a viscous PRESSURE plunger to control the oil to the oil cooler. The philosophy is that when the oil pressure is high (cold oil), the oil stays in the engine until the pressure reduces (hot oil), and then it is routed through the oil cooler. This is a simple device that uses a spring to exert force on the plunger. When the cracking PRESSURE is high enough to push the plunger off the seat (cold oil), most of the oil stays in the engine and does not go through the cooler. When the pressure is below the cracking pressure (hot oil), the plunger seals on the seat and most of the oil is sent through the oil cooler. The plunger seems to seal better than the venatherm and does not wear out and become less efficient over time. Apparently, the plunger can send as much as 30% more oil to the cooler resulting in more oil cooling.

The vernatherm uses the same kind of oil porting, but is activated by temperature and not pressure (unless there is a blockage in the oil cooler). Lycoming Service Instruction No. 1008C explains how to remove the plunger system and install the vernatherm. It also shows all the part numbers.

https://www.lycoming.com/sites/default/files/SI1008C Bypass Valve Installation.pdf

Temperature moves the sealing surface into and out of the seat by extending and contracting the venatherm. While this is a proven and automatic system, users have reported that a new vernatherm can let as much as 30% of the oil bypass the oil cooler when fully extended (hot). The sealing efficiency is also affected by the aging of the mechanism. So, you will read about a user whose oil temperature has become higher than normal, installs the plunger system after removing the vernatherm, and immediately has cooler oil temperature. The oil that was bypassing the cooler because of a poor vernatherm seal is now being routed into the cooler system.

There is an issue with the plunger system that must be addressed in cooler weather. When the engine is warmed up, all of the oil will be sent to the cooler regardless of temperature. This will result in cooler than desired oil temperature during cold OAT. The solution is to install a ball valve in the oil cooler line that is controlled from the cockpit using a push-pull cable. A good way to do this is to install the control near the throttle/prop/mixture controller. Full forward opens the ball valve allowing full flow to the oil cooler. On takeoff, all of the knobs are pushed fully forward. Oil temperature is now manually controlled and must be monitored by the pilot. I?ve not yet found installation pictures of this ball valve, so any advice is greatly appreciated. It has also been noted that you don?t want to completely block flow to the oil cooler. The ?closed? stop should be adjusted to allow a small amount of oil to still flow.

The decision comes down to replacing the vernatherm or removing it and installing the plunger system for a fraction of the price. When working properly, the verantherm provides automatic oil temperature control. The plunger system is cheaper, maybe more efficient, requires the pilot to manually manage oil temperature, and adds a ball valve and fittings to the system that are potential failure points.

If you decide on the plunger system, here are the parts you will need. Most you can find at Aircraft Spruce:

Plug (may already be installed): 62417
Plunger: 62415
Spring: 69436
Gasket: MS35769-11
Push-Pull: A-700-BL-0960
Ball Valve: McMaster 4256K52 (3/8? NPT, PTFE seat and packing, brass body, stainless ball)

Any help with ball valve installation greatly appreciated.
 
With a plunger, all oil is routed to the cooler regardless of temperature. The plungers only purpose is to bleed off any pressure that exceeds the design limit. The volume of oil it bleeds off will be nill or very minimal unless the cooler is blocked or the oil is REALLY cold and can't flow through the cooler fast enough. It provides no temp regulation. Your logic of cold oil cracking off the plunger and thereby reducing flow to the cooler doesn't really apply untill really cold. Like sub-zero. Above this, thereis no temp related regulation.
 
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Last year I was going to try this after removing vernatherm for testing. Bought spring and plunger parts, but discovered the lycoming/superior plunger would not fit my ECI 360. Tried to order an ECI plunger, but discovered they were never manufactured! Oh well. Installed new vernatherm instead.
 
With a plunger, all oil is routed to the cooler regardless of temperature. The plungers only purpose is to bleed off any pressure that exceeds the design limit. The volume of oil it bleeds off will be nill or very minimal unless the cooler is blocked or the oil is REALLY cold and can't flow through the cooler fast enough. It provides no temp regulation. Your logic of cold oil cracking off the plunger and thereby reducing flow to the cooler doesn't really apply untill really cold. Like sub-zero. Above this, thereis no temp related regulation.

I agree, and I guess I didn't say PRESSURE often enough when referring to the plunger. Per the Lycoming Service Instruction 1008C:

"This Service Instruction includes a procedure to install a thermostatic oil cooler bypass valve as a replacement for a pressure-operated bypass valve."

The plunger is a PRESSURE bypass and has no automatic temperature control. The plunger is pushed off the seat by oil PRESSURE if the oil is cold enough to restrict flow through the cooler, or there is a cooler blockage. Once open, most of the oil (more than the vernatherm) flows through the cooler, hence the described pilot operated ball valve.
 
Pics and diagrams

Search Dan Horton. I believe you will find oil circuit diagrams and ball valve pics have been posted.
 
Once open, most of the oil (more than the vernatherm) flows through the cooler, hence the described pilot operated ball valve.

The plunger is an overflow only, not flow control. When the plunger is closed, all oil is flowing through the oil cooler. If pressure into the cooler exceeds the spring rating, the plunger opens enough to get the pressure back down. Oil escaping through the open plunger bypasses the cooler. The vernatherm works fundamenatlly the same, though it can vary from partially open to fully closed and also has a pressure overflow spring/plunger built into it. The closed plunger will flow the same amount (i.e. ALL) of oil to the cooler as a fully closed vernatherm (vernatherm expands and closes an orifice to force oil to the cooler.

Larry
 
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What is the oil bypass plunger? Without the vernatherm, Lycoming uses a viscous PRESSURE plunger to control the oil to the oil cooler. The philosophy is that when the oil pressure is high (cold oil), the oil stays in the engine until the pressure reduces (hot oil), and then it is routed through the oil cooler.

No. The seated plunger blocks a bypass port. With the bypass post blocked, all oil flows through the cooler. The plunger is pushed up off its seat when oil viscosity is high, i.e. when it is difficult to force all the oil through the small oil cooler passages. Unblocking the bypass post allows some oil to flow directly to the filter. In this regard, the plunger is a pressure relief valve.

Note that viscosity (resistance to flow) is the determining factor, not necessarily temperature. For example, 15W-50 Aeroshell would result in a closed bypass port at lower temperatures, as compared to straight 50W.

The vernatherm uses the same kind of oil porting, but is activated by temperature and not pressure (unless there is a blockage in the oil cooler).

The plunger and the vernatherm control the same bypass port, although from opposite sides.

Temperature moves the sealing surface into and out of the seat by extending and contracting the venatherm. While this is a proven and automatic system, users have reported that a new vernatherm can let as much as 30% of the oil bypass the oil cooler when fully extended (hot).

I doubt it, but will cheerfully recant if shown the measurement method and data demonstrating 30% leakage.
 
I mis-wrote. When the spring forces the plunger to the seat, all of the oil goes to the cooler.

The plunger was not designed to regulate temperature. It is a safety valve, if you will, that will allow oil to by-pass the oil cooler system if the pressure in the system is high due to blockage, or very cold oil is restricting flow.

DanH has a great thread on the veratherm at:

http://www.vansairforce.com/community/showthread.php?t=71409&highlight=vernatherm

Avweb has a great discussion of the vernatherm and the oil bypass plunger:

https://www.avweb.com/news/maint/182617-1.html?redirected=1
 
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