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On "Accusumps"


(This is another blog in a series of thoughts/ideas that I commonly see, created as a placeholder for future reference so I don't have to re-write it each time. I totally understand the limited audience and for such a post, but I invite feedback from those who have interest in this arcane subject...)

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This was posted in response to someone inquiring about PDM control, pressure sensors, switches, valves to manage an Accusump in his race car...

You're making this wicked too complicated...and significantly failure-prone.

An Accusump (AS) is a passive hydraulic/mechanical oil pressure accumulator (thus, "Accu"sump). Its purpose* is to provide oil flow when the pickup tube in your engine becomes unported due to side loads oil "slosh" or flow away from it.

First, note that an AS is a "BandAid" for a design flaw in your road race engine. That design flaw is the unporting of the oil pickup. The ultimate resolution for that problem is a dry sump system but that is typically not allowed in most limited-prep production-based classes. So the next step is to stop the sloshing some other way, be it oil pan gates, trays, windage, anything you can do to keep the oil around the pickup tube. Oil pressure is engine life.

Second, absent those fixes, note that in the case of high-RPM, high-speed, high-g, sustained oil pickup starvation that the oil flow has to be provided RIGHT NOW, long before the oil pressure begins to drop, otherwise you'll toss a rod bearing toute-suite. Only a passive system can provide that.

When used for this purpose properly, an Accusump will get charged once the engine starts running and engine oil pressure is ramped up. Best use is to close the manual valve and start the engine and let the engine get running right (instead of initially stealing oil flow from the engine to charge the AS). Once established and the engine running fine then the manual valve is opened and the AS is allowed to charge.

From this point forward, during normal usage the AS valve is left open so that in any cases of the oil pressure dynamically dropping below the engine's current oil pressure - which, of course, is the AS's current oil pressure - will result in the diaphragm/spring of the AS pushing oil back into the engine.

This is a passive, dynamic, RIGHT NOW supply of oil. No computing, no sensors, no fiddling with switches, none of that. It will sit there constantly ensuring that your engine will be supplied with oil, regardless of the situation of your oil pickup.

Of course, as you can imagine there's a limited time and amount of that goodness. AS's can only be so large, and if the track corners you're negotiating are so long, and the pickup situation so dire, that the system cannot supply the volume long enough or if there's not enough time beween transitions for it to fully recharge, then eventually it's "time to pay the piper" and you're out of oil pressure. No amount of switches, sensors, and computing power can fix that, and you're back to Point #1 where you need to fix the root cause for the problem (after you rebuild the engine).

As for the valve part, I prefer the manual valve, with some kind of reminder. The purpose of the valve is so that it gets shut off before engine stop and the accumulater retains that oil pressure so that you don't have to recharge it next time you use it. After engine start you open the valve and it works like the above. I don't like to use an open system that allows the AS to recharge on engine startup (or opening the valve to precharge) as that startup oil needs to go to the engine first before we use it to charge the AS (or run it through the cooler, or whatever). This is likely why some people suggest using the AS as a pre-oiler, which is totally fine as long as you understand any properly-built engine doesn't need a pre-oil at every startup...and you close the valve on startup...

Some people will use an electric solenoid valve on their AS presumably because they don't want to forget to turn the manual valve on after start. Understandable, but that adds more (electrical) parts/systems to fail. If you and/or crew keep a checklist (or a red "flight" flag to look for before pulling away, along with your fire system's) then it shouldn't be needed.

Bottom line, don't try to get fancy and use sensors switches and data acq to hinder the simple operation of a passive, straightforward "solution" to a basic mechanical problem. All you're doing is adding unnecessary non-value complications and failure-prone complexity.

And that's all I have to say about that. - GA

*Many will argue that a primary purpose of an AS is to provide oil on engine startup. I call "sales garbage tosh" on that. Many engines in world history - probably billions every day - survive engine startup without an Accusump-like system to provide "startup" oil. Your engine probably isn't anything special in that regard.

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