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Oil Life Monitor --
How Does It Know?
How long will oil last in an engine? What reduces the oil’s effectiveness? When should it be changed?
Lubrication engineers perform a number of tests to answer these kinds of questions. Vehicles are operated under prescribed conditions, and periodically a sample of the oil is taken into the laboratory for analysis. When the condition of the oil is no longer satisfactory, the mileage is noted. From controlled testing like this, engineers in the past have determined two sets of mileage numbers, one number for normal driving and the other for severe conditions. Severe conditions can mean that the vehicle is driven hot (for example, pulling a trailer up a mountain) or is driven such that the oil never warms completely (for example, trips less than 5 or 10 miles in a winter climate). It is then up to the owner to decide whether their own driving is normal or severe and to change the oil accordingly. Now, science and technology have found a way of taking the guesswork out of the picture. GM is installing an oil life monitor in an increasing number of new vehicles. Using a simple indicator lamp or readout on the instrument panel, this system notifies the driver when to change the oil. The February and March 2000 issues of TechLink explain how to reset these monitors.
Here’s information on how an oil life monitor works.
Additives
Straight oil is not an ideal lubricant in an engine. A package of additives is needed to give the oil properties it does not naturally have or to enhance its natural properties. Some of the tasks accomplished by additives:
- viscosity modifiers, to keep the oil the proper thickness over a wide range of operating temperatures
- anti-oxidant, to keep the oil from thickening
- corrosion inhibitors, to protect engine components
- anti-wear
- anti-foam
- detergents, to suspend solid particles.
What Makes Oil "Wear Out?"
If you were to start out with a crankcase full of fresh, clean oil, and drove the vehicle for a period of time, eventually the oil would have to be changed. During this time, what can change fresh oil into "worn out" oil?
First, dilution. When gasoline is burned in the combustion chamber, the by-products include a lot of water. Some of this water can find its way into the crankcase through piston ring blow-by. If the engine is cold, and if combustion is not perfectly complete, a small amount of acid is formed. It, too, can blow-by into the oil. You don’t need to be a top-notch scientist to realize that water and acid aren’t good things to pump through the lubrication system of the engine. If an engine is run long enough for the engine oil to warm, the water
and acids will evaporate and not accumulate. But, during very short trips in cold weather, water and acids can enter the engine oil and cause the oil to "wear out." Second, the degradation of the oil and its additives. We mentioned earlier that a number of additives are put into oil to improve its performance. If these additives are degraded or decomposed, the oil is no longer capable of doing all of its jobs properly. Oil with
degraded additives can become thick and dark. Additives become degraded by exposure to extreme heat. There are two places a lot of heat can reach the oil. One is near the combustion chamber. Oil at the top piston ring is exposed to very high temperature. And some bearing surfaces can also put a lot of heat into the oil at high operating temperatures. So, degradation of additives from high temperature operation is the second factor that can cause oil to "wear out."
How Can Operating Conditions be Used to Predict Oil Life?
Using carefully controlled laboratory tests, it’s possible for lubrication engineers to measure how long it takes to dilute engine oil during cold operation. And it’s possible to measure how long it takes for high temperature
to degrade the additives. We usually think of measuring time in hours and minutes, but for an engine, the
amount of revolutions it has run is also a good measure. So for the purposes of oil life, time is measured in engine revolutions. Engineers like to talk in terms of models. A model is a way to describe something
mathematically. It’s possible to create an oil life model that very carefully matches the results of analyzing the oil in a laboratory. The oil life monitor, then, is based on a model. A computer chip in the Powertrain
Control Module is loaded with a certain number of engine revolution counts. The count for each engine/vehicle combination is determined by testing. As the engine runs, each revolution is subtracted from the remaining count in the oil life monitor. When the count reaches zero, the instrument panel light comes on. But, here’s the clever part. When the various input sensors detect that the engine is running under either cold or hot conditions, it subtracts extra counts (penalties) for each engine revolution. So, the conditions
that cause the oil to "wear out" make the counter run down faster. When the oil is changed, it’s necessary to
reset the oil life monitor (see the February and March 2000 issues of TechLink) and the countdown begins again.
NOTE: Synthetic oil resists "wearing out" better than mineral oil, so the oil life monitor
is set to account for this, but only on vehicles that are specified for synthetic oil from the
factory -- the Corvette, for instance. Using synthetic oil in other vehicles is certainly not
harmful, but the oil life monitor will continue to count down as though the engine contained
mineral oil.
- Shirley Schwartz contributed to this article
