: Technical Oil Question



soon to V
06-30-04, 09:57 PM
Curious about how the Oil Life is calculated. I know there is an oil temp sensor on the car, and I understand oil life has something to do with how many heat cycles the oil has been through, but how is GM calculating this percentage? Is dirt being sensed in either air stream or oil stream? I'm sure mileage and age are probably being computed, but I wouldn't think that would be good enough. So I'm curious about this and also how the Germans may be doing it.

Any knowledge?

StealthV
06-30-04, 10:06 PM
The oil life monitor is a pretty sophisticated algorithm.

http://www.gm.com/company/gmability/environment/news_issues/news/oillife_how_041603.html

http://www.gm.com/company/gmability/environment/news_issues/news/oillife_qa_041603.html

http://www.gm.com/company/gmability/environment/images/oillife.gif

GM "The GM Oil Life System will automatically adjust the oil change interval based on engine characteristics, driving habits and the climate in which the vehicle is operated."

More info straight from the General --> http://www.gm.com/automotive/vehicle_shopping/suv_facts/300_env_emissions/6_oil_life_system.html

And another GM link --> http://www.gm.com/company/gmability/environment/news_issues/news/oillife_monitor_041603.html

StealthV
06-30-04, 10:12 PM
Ask me a tougher question.... :)

StealthV
06-30-04, 10:19 PM
Still working on my V spreadsheet, in the meantime, here's the one for my SS. Check out column U "Miles to Oil Change" and you'll see the distance the oil life algorithm is calculating before a change. It is interesting to see it fluctuate from 4,000 miles all the way up to 15,000 miles depending on all the factors. Since my SS has an hour meter, I try to perform an oil change every 100 hours and disregard the miles or oil % calculation - the curse of my large equipment background. :)

Silverado SS Excel Spreadsheet (http://www.nitro-nights.com/2003ss/silverado_ss3.xls)

soon to V
06-30-04, 10:39 PM
Wow, Good stuff. Big thanks. No further questions. :)

BeelzeBob
07-01-04, 12:15 AM
The "German" approach involves the use of an "oil quality" sensor. It is a variable capacitance device I believe that can sense the acidity of the oil as well as the presence of some contaminants and moisture. It isn't perfect, however, as some additives in some oils can "fool" the sensor and the reliability of the sensor is not great. It also adds complexity and wiring to the car as it is another sensor to wire, plumb, diagnose and maintain.... Also, while a sensor can provide an accurate (to some degree) reading on current oil quality it cannot predict oil life based on the drivers schedule, climate or driving habits. It just looks at the current oil status.

The GM oil life algorithm is based fundamentally on counting engine revolutions as the basis for oil life degradation and then adds degradation factors for engine load, oil temp, soak times, ambient temps, run times, etc. It takes into account all the operating regimes that affect oil life and then predicts how much of the oil's life is remaining given the driving schedule todate. It continually predicts the slope of the oil degradation and adjusts the slope (shown as the percent oil life remaining) based on the latest driving schedule.

The advantage to the GM system is that it is all software in the PCM using input from existing sensors used to run the fuel injection system....so there are no parts or wiring added...and software is VERY reliable. The PCM that the software is running in is obviously very robust so the system is basically "free" , adds no mass or complexity to the car and is as reliable as the basic fuel injection system. It can also predict the oil life based on the driving schedule, using the rate of oil degradation recorded todate, instead of just telling the existing oil quality. The system can be applied to any engine very easily once the basic parameters affecting oil life are mapped for that engine.

The oil life algorithm is very very accurate at predicting the oil quality and the oil life. It has been correlated on many many GM engines over the years comparing the modeled engine oil life to actual oil samples taken from engine during many many types of mileage accumulation.


Using hours alone to guage oil life is not necessarily the best thing to do with a gasoline engine in a passenger car. Short trips, winter weater, city driving vs. highway driving, etc. will all dramatically affect the oil life...not just hours the engine runs. In HD equipment the engines are run for long periods of time and are usually run hard and/or on a steady HD schedule so the oil life degradation can be modeled or predicted with hours....but not a passenger car use duty cycle.



There are several reasons oil needs to be changed that must be taken into account whether the oil life algorithm is used, a sensor for oil quality is used or a change interval based on seat of the pants engineering is devised.

The additive package for anti-wear (the ZDP) is sacrificial and gets used up with time. This is one reason that so called long life oils (like Amsoil) do not really allow longer change intervals. The additives get used up with engine revolutions..i.e...metal to metal contact...and all oils have about the same amount of ZDP...so no oil can "last" longer than another for this reason. This reason for changing oil is the reason that the GM oil life algorithm is fundamentally based on engine revolutions and it is the factor that is usually the least important for a change interval...in other words, the additive package does last a long time in modern oils and modern engines so , all other things being minimal, the additive depletion rarely occurs before the 12,500 mile cap currently on the oil life algorithm.

Additives for anti-oxidation, detergents, dispersants, etc...all get used up also. This is important as oil temp comes into play and short run and cold starts happen.

Oil gets contaminated with water, gas, by-products of combustion. Sulfur in the gas combines with the water and other blowby contaminats to form sulfuric acid in the oil. Cold starts put a tremendous amount of fuel and water into the oil (raw fuel condenses on cylinder walls and is scraped into the crankcase by the rings and water condensing from combustion also gets scraped into the crankcase). That is why the oil life algorithm adds significant deterioration factors to the model for oil life when cold starts and short trips are added.

Oil is oxidized due to heat. Heat plays a huge factor in the oil life. Synthetics do not oxidze as quickly as conventional oils at very high temps which is one reason synthetics are required for engines that can run at oil temps above 305 F and is the basis for Amsoil's claim of extended drain intervals. If the only reason to change oil (obviously it isn't....) was oxidation caused by heat then Amsoil's claims might have some merit...but they are the same in that respect as other synthetics.

Contamination due to soot from combustion and from silicon from dust and such that might enter the engine when driving in a dust environment.


So.....you can see that there are a myriad of reasons to change oil that are accurately predicted by the GM oil life algorithm.

Understand that older engines still need shorter change intervals compared to later, more modern engines. Modern engines with roller followers, gerotor oil pumps, no distributor gears, filtered air PCV systems, etc... do not deplete nor contaminate the oil nearly as quickly as older engines with flat tappet followers, spur gear oil pumps, distributor gear driven oil pumps, etc... and the newer engines are also MUCH more tolerant of depleted oil. So the longer change intervals allowed with the oil life monitor are real and believable and perfectly fine for the engine...but the concept is not to be applied to older engines necessarily.

StealthV
07-01-04, 02:10 AM
Using hours alone to guage oil life is not necessarily the best thing to do with a gasoline engine in a passenger car.
Engine hours is perfectly acceptable and preferrable to basing it on a pure mile basis like the vast majority of people do with their 3,000 mile intervals. Unfortunately, hour meters are a rare thing on vehicles.

Perhaps the onboard oil life monitor is accurate for the purpose it was intended; I don't trust it to give me the best interval for longevitiy of my engine. The oil life monitor was designed to push the envelope of oil change intervals so GM could put their "environmentally friendly" stamp on the company, not maximize engine life.

My professional engineering career is in the field of developing technology to enable customers to better manage their engines through optimizing oil change intervals, preventive maintenance, analysis of oil samples, etc. We spend millions of dollars a day on research alone on how to best maximize engine and other component lives. We analyze thousands of oil samples everyday in our own labs. One thing remains constant - every time old oil is exchanged for fresh oil, good is being done to the engine. The more often the better. The oil interval becomes a matter of cost as most people can't afford to change oil every day. For optimum engine life, shorter than "recommended" intervals are the best thing to do.

Take a peak at my spreadsheet on the maintenance tab, the lowest amount of oil life remaining at an oil change was 28%. The computer calculates the life assuming it has plain old oil in my SS engine. Since it only gets Mobil1, my oil in reality probably has over twice that life remaining with the longevity advantages of synthetics. Therefore my 100 hour or less interval on my SS is ultra conservative, especially considering the Mobil1.

Using the onboard oil monitor on the V, the oil change interval will probably be in the 6,000 to 10,000 mile range which works out to 133 to 222 hours at a typical 45 mph mean velocity. My V will never ever see an oil change at those long of mileage or hour intervals. It had its first oil change at 300 miles and will get its second at 1500 miles. After that I'll start stretching the interval to the 2,000-2,500 range. Cost effective? No. Peace of mind when running it to the red line with 100,000 miles on it? Priceless.

Instead of a worthless oil life monitor, I would of rather had zero wheel hop, a hand parking brake and a precision shifter.

:hide:

97Deville
07-01-04, 05:12 AM
Engine hours is perfectly acceptable and preferrable to basing it on a pure mile basis like the vast majority of people do with their 3,000 mile intervals. Unfortunately, hour meters are a rare thing on vehicles.

I commute 75 miles each way, a solid hour of hiway driving, twice a day/ five times a week. Contrast to my dad who travels 15-20 min 3 times a week. Changing the oil based on hours, he would never have to change it (and he doesn't) My oil level goes down, his goes up, so he goes for a drive and it comes back down again. LOL



Perhaps the onboard oil life monitor is accurate for the purpose it was intended; I don't trust it to give me the best interval for longevitiy of my engine. The oil life monitor was designed to push the envelope of oil change intervals so GM could put their "environmentally friendly" stamp on the company, not maximize engine life.

What an incredible dedication you have to your SpreadSheet. Look at how tight your calculated measurments are to the computer's calculations.

I find it amazing that people trust the computer to regulate the air, fuel, ignition, based on temperature, barometric pressure and altitude among other things,,, but they just cant bring them self to trust it to calculate oil change

97Deville
07-01-04, 05:23 AM
btw

My dad's car is one of those "low milage cars"

Im over 350,000 miles. Just Drive It!!! (never had a head gasket changed)

BeelzeBob
07-01-04, 12:54 PM
I will respectectfully disagree with your first statement for the reasons I mentioned in my post before. Hours ALONE has no way to comprehend the driving schedule of the engine. Not that hours isn't possibly a good indicator under certain conditions...i.e...industrial equipment that is started and run on a dedicated, predicable schedule, marine use where the engine is always working hard, tractors and such where the engine is run at a very regular load cycle, etc. Personally, I use an hour meter on my lawn/garden tractor to determine change intervals as the engine always runs for at least 2 hours when it is started and it is run at a specific RPM.... Cars, however, have such a varied duty cycle with varying loads, speeds, run times, cold soaks, hot soaks , etc. that useing hours alone is a very poor way of determining oil life. It may work IN YOUR SPECIFIC CASE based on your oil measurements, etc.....but....it would NOT work for everyone and is a poor recommendation for recommending oil change intervals for the general public.

Imagine the grandma or housewife....er...houseperson....that cold starts in the winter, drives 1 mile to the school, drives 1 mile to the grocery store, cold soaks for an hour, drives 1 mile home....etc..... It would take a long time to accumulate your "hours" yet the oil would need changing much much much sooner than in my car that runs on the interstate for an hour each time it is started regardless of the weather. Hours alone just does NOT hack it.

Remember, GM is responsible for ALL drivers and possible use schedules for the vehicles we produce and responsible for recommending the change intervals for oil and fluids to provide the best possible balance between engine life and environmental issues and maintenance issues. The first thing that ANYONE working on maintenance intervals will tell you is that there is NO SINGLE MAINTENANCE INTERVAL FOR EVERYONE. Period. Miles, hours, days, etc....just do not hack it. Any comphrehensive maintenance schedule MUST take into account a myriad of factors beyond simple hours. That was what drove the initial development of the oil life monitor. There were Drs. of Chemistry working with the engineers that developed the system and millions of dollars were spent developing the correlation for the various reasons requiring an oil change with a measurable, quantifiable, predictable metric. The oil life monitor was the result. It was developed, tested, and validated on a myriad of vehicles on a myriad of driving schedules for the specific purpose of predicting oil life in the specific vehicle it is installed in. There is absolutely NO WAY that anyone can provide a more accurate way of recommending an oil change for that vehcile....regardless of lab samples, hours recommendations, etc.

I appreciate the experience you have in regard to oil analysis and such but I would still trust the oil life monitor developed by GM for that specific vehicle long before I would trust the recommendations of someone who knows little about the makeup of the engine or vehcile and recommmends changing oil based on a single specific parameter. You know that there is much more to changing oil than hours......I hope.

Interestingly, during our research and development on the oil life monitor back in the mid 80's and early 90's we tried various labs around the country for oil sample analysis to backup our facilities at the GM Research Labs and basically gave up after getting poor repetition on sample analysis, inaccurate analysis, etc.... We did all our research on samples in house, finally, to avoid some of the problems we encountered with outside labs. Not sure of their ultimate capability but I would personally be reticent to trust my oil analysis to an outside lab based on my experience.

As stated, if oxidation were the ONLY reason to be changing the oil then temp and hours might be a good indicator and you might be able to make the statement that "Since it only gets Mobil1, my oil in reality probably has over twice that life remaining with the longevity advantages of synthetics." Oxidation, however is RARELY the reason the oil needs changing and that is really the ONLY place that synthetic oil surpasses conventional oil for performance. Contamination, ZDP depletion, etc. all are much more commonly the reason for an oil change and synthetic provides ablsolutely NO ADVANTAGE there. That is why the oil life monitor is basically the same deterioration for synthetic or conventional oil. In a case like dirt, dust and metallic contamination (pentane insoluable contaminants) the synthetic oil has absolutely no advantage and synthetic does not have any higher "dose" of the ZDP in the oil to guard against wear. So it really isn't any "better"....it will just live at higher temps than conventional oil. That is why I personally use it in my air cooled, backup generator that runs oil temps of 320 on a hot day!!! Good use for synthetic.


The oil life monitor was designed to accurately model oil life. Period. Not push the limits of the oil or the engine. The decay rates and degradation factors calibrated for any specific vehicle are designed to provide an accurate predicition of oil change intervals for that vehicle with a sufficient safety factor so that the oil is nowhere near "used up" totally when the oil life indicated is zero. In fact, to insure that there is adequate safety factor built into the oil life monitor the durability cars that GM runs to validate the vehicles on an accelerated mileage accumulation schedule over a WIDE variety of driving and climatic conditions usually run 2 TIMES the oil life interval for actual changes....meaning that the oil is actaully changed every other time the oil life monitor says to. The oil is sampled at the "first" change recommendation before resetting the oil life monitor and then sampled and changed at the second change recommendation. The engines look perfectly fine at the end of 100k or more on this schedule.


I have seen Northstar engines run in HD livery service in livery fleets around Detroit and LasVegas that had the oil changed per the oil life monitor and the engines ran 300K with no problems at teardown at the end of the schedule. Same for 4.5/4.9 engines that ran in taxi cabs in Pennsylvania for 250-300K. So the oil life monitor is accurate and it does NOT minimize engine life.

Understand that if the grandma that "housewifes" the car to death in the winter never takes any long trips and always cold starts for short trips, the oil life monitor may tell her to change oil in as little as 500 miles on very severe duty like that. Simple hours would never pick up on that.

I , too, have done a considerable amount of development of oil life and engine performance over the years (32 and counting) and personnally worked with the development of the oil life monitor...so...from what I know of it and my experience with it I would not trust any other form of oil change recommendation.







Engine hours is perfectly acceptable and preferrable to basing it on a pure mile basis like the vast majority of people do with their 3,000 mile intervals. Unfortunately, hour meters are a rare thing on vehicles.

Perhaps the onboard oil life monitor is accurate for the purpose it was intended; I don't trust it to give me the best interval for longevitiy of my engine. The oil life monitor was designed to push the envelope of oil change intervals so GM could put their "environmentally friendly" stamp on the company, not maximize engine life.

My professional engineering career is in the field of developing technology to enable customers to better manage their engines through optimizing oil change intervals, preventive maintenance, analysis of oil samples, etc. We spend millions of dollars a day on research alone on how to best maximize engine and other component lives. We analyze thousands of oil samples everyday in our own labs. One thing remains constant - every time old oil is exchanged for fresh oil, good is being done to the engine. The more often the better. The oil interval becomes a matter of cost as most people can't afford to change oil every day. For optimum engine life, shorter than "recommended" intervals are the best thing to do.

Take a peak at my spreadsheet on the maintenance tab, the lowest amount of oil life remaining at an oil change was 28%. The computer calculates the life assuming it has plain old oil in my SS engine. Since it only gets Mobil1, my oil in reality probably has over twice that life remaining with the longevity advantages of synthetics. Therefore my 100 hour or less interval on my SS is ultra conservative, especially considering the Mobil1.

Using the onboard oil monitor on the V, the oil change interval will probably be in the 6,000 to 10,000 mile range which works out to 133 to 222 hours at a typical 45 mph mean velocity. My V will never ever see an oil change at those long of mileage or hour intervals. It had its first oil change at 300 miles and will get its second at 1500 miles. After that I'll start stretching the interval to the 2,000-2,500 range. Cost effective? No. Peace of mind when running it to the red line with 100,000 miles on it? Priceless.

Instead of a worthless oil life monitor, I would of rather had zero wheel hop, a hand parking brake and a precision shifter.

:hide:

BeelzeBob
07-01-04, 12:59 PM
BTW....I have nothing against frequent oil changes nor synthetic oils....

I just want to make the point that synthetics are good for basically one thing...very high temperature operation. High meaning higher than the typical passenger car will ever see. The CTS-V can push the oil temps high during track use but in normal traffic and everyday driving the oil temps are quite low so the synthetic is just there for the HD use intervals.

Change the oil as often as you want. Changing it sooner than the oil life monitor says to will just wear out the drain plug threads, though. It will not give any longer engine life nor improve the condition of the engine at high miles. The oil life monitor change intervals will easily provide adequate lubrication protection for hundreds of thousands of miles of use.

soon to V
07-01-04, 10:52 PM
Just wanted to say the continued discussion has been very enlightening and most helpful in my understanding of things. This will also help in those detailed discussions of competitive advantage between certain makes of cars; but I'll keep it to cars as I don't want to open the can of worms between oil makes! Thanks again!