Oil CBS Function

E93Dude · 03-27-2021, 09:09 AM

I did not want to hijack another recent threat on oil changes, and take it off on a tangent, hence the separate post.

There has been A LOT of discussion about oil chage intervals, but I can't find any post that describes how the BMW's "CBS" (condition based service) actually determins when its time to change your oil. I don't think its trigged off mileage. Its it based solely on monitoring oil level? Does anyone know from a technical perspective how the CBS evaluates oil to determin when its degraded enough to warrant an oil change?

Brian86 · 03-27-2021, 11:26 AM

The oil level sensor has two capacitive circuits.

One circuit measures the capacitance between electrodes A and B to determine the oil level. The other circuit measures the capacitance between electrodes B and C to determine how contaminated the oil is.

My understanding is that as the oil degrades the permittivity of the oil also changes, which can be read as a changing capacitance between the electrodes.

It's actually pretty old tech. Stationary industrial and rotary machinery has been using similar quality sensors since the 60s or 70s.

Chappers 71 · 03-27-2021, 11:38 AM

All you need to know

https://www.google.com/url?q=https:/...u3ddFCNwzTE747

Phil325i · 03-27-2021, 12:40 PM

And this: BMW-OZS sensor.pdf

E93Dude · 03-27-2021, 06:40 PM

All of these replies are vey informative - thanks. I hate/love it when i *think* I've searched for something, and then others snap their fingers and come up with the answer.

I understand now how the vehicle senses oil level. I also understand now that capacitance changes in the oil fluid determine level of degredation. What I still don't understand, is why a change in capacitance = bad oil. What changes occur in oil that reduces efficacy, that can be evaluated by a delta in capacitance? I realize there were some really smart engineers and chemists in Germany who can vouch that this technique works...and I'll take it on faith that its good to go.

This is a really down-in-the-weeds sort of question (imagine being my parents when I was 9...). No one need answer if folks don't want to bother. I'll go spend time time digging through Bob the Oil Guy....

Brian86 · 03-27-2021, 08:54 PM

Oil's behavior as a dielectric is an extremely well researched subject. Oil is used to insulate high voltage equipment. Every transformer can on every telephone pole on earth is full of transformer oil.

Somebody made the link 50 years ago that degradation in transformer oil changes it's voltage withstand and permissively. It's not a very big leap to start using sensitive capacitive circuits to measure oil quality.

As to what changes in the oil, I'm sure the change in the oil molecules themselves, as well as the ingress of fuel, and loss of the waxes that make motor oil multi-viscosity changes the indication, since each of those components will have different permissively and dielectric withstand.

Honestly it was probably more trial-and-error. They took some new oil and tested it, then took some known bad oil and tested it, and came up with the limits from that.

Brian86 · 03-27-2021, 08:57 PM

This is all wasted knowledge on my part. I bought my car well past the 300,000km where the oil CBS system halts.

Now I just run it 10k between changes and top it up every 3,500is to replace what leaks and is consumed.

E93Dude · 03-27-2021, 10:05 PM

Quote:

Originally Posted by Brian86

This is all wasted knowledge on my part. I bought my car well past the 300,000km where the oil CBS system halts.

Now I just run it 10k between changes and top it up every 3,500is to replace what leaks and is consumed.

To a large degree its wasted knowledge on me, too. I change the oil every 8-10K based on odometer readings. My CBS has never triggered since I got the car in 2012. This was more about me wanting to understand what - in BMW's mind - consitiutes degraded oil. I don't beat the hell out of my car on the track, so I've never really worried about oil condition. Fixing oil leaks, however, has been another problem entirely...

Brian86 · 03-28-2021, 06:15 PM

As far as I can tell the CBS system works exceptionally well.

According to the BMW service records my N52 got the CBS based oil changes of 15-18k for the first 150k miles of it's life. It's just about to turn 300k now so the long CBS intervals don't seem to have degraded the engine in the slightest.

03-27-2021, 09:09 AM	#1
E93Dude Lieutenant 369 Rep 423 Posts Drives: 2007 E93 MT6 Join Date: Sep 2018 Location: The Old Dominion iTrader: (0) Garage List 2007 BMW E93 [0.00]	Oil CBS Function I did not want to hijack another recent threat on oil changes, and take it off on a tangent, hence the separate post. There has been A LOT of discussion about oil chage intervals, but I can't find any post that describes how the BMW's "CBS" (condition based service) actually determins when its time to change your oil. I don't think its trigged off mileage. Its it based solely on monitoring oil level? Does anyone know from a technical perspective how the CBS evaluates oil to determin when its degraded enough to warrant an oil change? __________________ BMW: Doing Amazing(ly cheap) Things With Plastic (crap) Since the E90... Last edited by E93Dude; 03-27-2021 at 09:12 AM.. Reason: Because I f'ing wanted to.
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03-27-2021, 11:26 AM	#2
Brian86 Captain 638 Rep 766 Posts Drives: e36 M3, e91 325xi, N50 Xterra Join Date: Dec 2020 Location: Pittsburgh iTrader: (0)	The oil level sensor has two capacitive circuits. One circuit measures the capacitance between electrodes A and B to determine the oil level. The other circuit measures the capacitance between electrodes B and C to determine how contaminated the oil is. My understanding is that as the oil degrades the permittivity of the oil also changes, which can be read as a changing capacitance between the electrodes. It's actually pretty old tech. Stationary industrial and rotary machinery has been using similar quality sensors since the 60s or 70s. __________________ Sometimes a bolt is just a bolt.
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03-27-2021, 11:38 AM	#3
Chappers 71 Major General 5480 Rep 5,427 Posts Drives: Beige G31 40ix m sport Join Date: Feb 2016 Location: Kent, The Garden of England iTrader: (0) Garage List 2018 G31 xdrive 40i ... [4.00] 2006 E92 335i SE [9.50]	All you need to know https://www.google.com/url?q=https:/...u3ddFCNwzTE747 __________________ G31 40ix M Sport E92 335i. E36 328is coupe E39 540i V8 6SP manual E34 3.6 M5. E34 525i sport. VW Jetta Mk2 GTI 16v. 1679cc 1967 resto-cal beetle
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03-27-2021, 12:40 PM	#4
Phil325i Brigadier General 615 Rep 3,194 Posts Drives: E92 325i MSport Coupe Join Date: Apr 2012 Location: UK South East iTrader: (1) Garage List 2009 BMW 325i Coupe [6.00]	And this: BMW-OZS sensor.pdf __________________ E92 pre-LCI 325i - Ohlins R&T; H&R spacers; M3 strut brace; Swift thrust sheets; 3 x chassis braces; diff brace; N53 V-brace; 034 subframe inserts; BMS clutch stop; BMS CDV; RE g/box mounts; Delrin shift bushes; Saikoumichi OCC; Cyba scoops; BMW Perf Exhaust; HEL s/steel brake hoses; M3 rear spoiler; Recaro Sportster CSs; M3 white dash LEDs; LED Angels; LED side repeaters; BMW Perf black grille; CSL reps; SSDD carbon diffuser; Monster Wrap black roof/clear front
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03-27-2021, 06:40 PM	#5
E93Dude Lieutenant 369 Rep 423 Posts Drives: 2007 E93 MT6 Join Date: Sep 2018 Location: The Old Dominion iTrader: (0) Garage List 2007 BMW E93 [0.00]	All of these replies are vey informative - thanks. I hate/love it when i think I've searched for something, and then others snap their fingers and come up with the answer. I understand now how the vehicle senses oil level. I also understand now that capacitance changes in the oil fluid determine level of degredation. What I still don't understand, is why a change in capacitance = bad oil. What changes occur in oil that reduces efficacy, that can be evaluated by a delta in capacitance? I realize there were some really smart engineers and chemists in Germany who can vouch that this technique works...and I'll take it on faith that its good to go. This is a really down-in-the-weeds sort of question (imagine being my parents when I was 9...). No one need answer if folks don't want to bother. I'll go spend time time digging through Bob the Oil Guy.... __________________ BMW: Doing Amazing(ly cheap) Things With Plastic (crap) Since the E90...
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03-27-2021, 08:54 PM	#6
Brian86 Captain 638 Rep 766 Posts Drives: e36 M3, e91 325xi, N50 Xterra Join Date: Dec 2020 Location: Pittsburgh iTrader: (0)	Oil's behavior as a dielectric is an extremely well researched subject. Oil is used to insulate high voltage equipment. Every transformer can on every telephone pole on earth is full of transformer oil. Somebody made the link 50 years ago that degradation in transformer oil changes it's voltage withstand and permissively. It's not a very big leap to start using sensitive capacitive circuits to measure oil quality. As to what changes in the oil, I'm sure the change in the oil molecules themselves, as well as the ingress of fuel, and loss of the waxes that make motor oil multi-viscosity changes the indication, since each of those components will have different permissively and dielectric withstand. Honestly it was probably more trial-and-error. They took some new oil and tested it, then took some known bad oil and tested it, and came up with the limits from that. __________________ Sometimes a bolt is just a bolt.
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03-27-2021, 08:57 PM	#7
Brian86 Captain 638 Rep 766 Posts Drives: e36 M3, e91 325xi, N50 Xterra Join Date: Dec 2020 Location: Pittsburgh iTrader: (0)	This is all wasted knowledge on my part. I bought my car well past the 300,000km where the oil CBS system halts. Now I just run it 10k between changes and top it up every 3,500is to replace what leaks and is consumed. __________________ Sometimes a bolt is just a bolt.
	Appreciate 0 Quote

03-28-2021, 06:15 PM	#9
Brian86 Captain 638 Rep 766 Posts Drives: e36 M3, e91 325xi, N50 Xterra Join Date: Dec 2020 Location: Pittsburgh iTrader: (0)	As far as I can tell the CBS system works exceptionally well. According to the BMW service records my N52 got the CBS based oil changes of 15-18k for the first 150k miles of it's life. It's just about to turn 300k now so the long CBS intervals don't seem to have degraded the engine in the slightest. __________________ Sometimes a bolt is just a bolt.
	Appreciate 0 Quote