This past November General Motors announced the expanded inclusion of its oil change indicator on 90% of their production over the coming five years. The Oil-Life System is currently in use on some models and will be added to all new 1999 Chevrolet Silverado and GMC Sierra full-size pickups, GM's best-selling models, featuring the powerful Vortec family of engines. At that level of usage GM calculates that one year's production of new oil-life monitor-equipped vehicles, coupled with longer change intervals, will allow a decrease in waste oil of approximately 40 million quarts (37.8 million litres) per year compared to the same fleet not equipped with the system.
The patented engine oil change technology involves computerized monitoring of engine revolutions, operating temperature, and other factors to optimize the change interval selection. The typical recommended interval for gasoline-fueled passenger cars and light-duty trucks is 3,000 miles (4,800 km) or three months, whichever first occurs, when outside temperatures are below freezing and trips are short.
These conditions are considered severe duty. For ideal driving conditions, relating to long trips with mild outside temperatures, the interval can be expanded to 7,500 miles (12,000 km). Starting with the 2000 model year on certain vehicles, GM will raise the maximum mileage allowed for Oil-Life System-equipped vehicles to between 10,000 and 15,000 miles (16,000 and 25,000 km), depending on vehicle brand and engine.
The development of the Oil-Life System began over a decade ago by researchers Shirley Schwartz and Donald Smolenski, both of the GM Research Laboratories. They discovered, through various investigations, that oil degradation, in general, followed pathways influenced by service and environmental conditions. The extremes of these conditions, as shown in Figure 1, are high-temperature, high-load on one end and low-temperature, low-load on the other.
In between is the large operating domain representing the majority of driving conditions. The basic design of the Oil-Life System was intended to characterize extreme operating conditions and most points in between.
While the Oil-Life System does not actually monitor any single quality or physical property of the oil, it does incorporate the use of a highly sophisticated mathematical model. This model applies the known influence of oil service temperature and revolutions to characterize the remaining life.
The influence of temperature, in particular, has a marked impact on oil life as is shown in Figure 2. The almost parabolic nature of the aging rate with temperature emphasizes the importance of this as dependent variable. On the other hand, time or running time (in the absence of mileage or engine revolution data) was not found to be a particularly good indicator of oil life, since it did not adequately distinguish between periods of extended idle and periods when engine speed was high.
The onboard calculation of oil age was simplified by using penalty factors (as opposed to equations). A penalty factor is an indication of the rate of oil aging at a given operating temperature. For any given oil, higher penalty factors are associated with faster oil-aging rates. The model uses engine revolutions as a basis for measuring duration of service. A simplified schematic diagram of the model is shown in Figure 3.
Gathering the data to show correlation of on-board measurements of oil-change intervals to laboratory oil analysis is a slow process, requiring months or years. For example, a typical short-trip service test can last two years. During the original research program approximately 130,000 kilometers were accumulated in determining the constants for the mathematical model and another 160,000 kilometers were logged in testing vehicles equipped with the Oil-Life System.
In Figures 4 a-c the results of four analyses, as indicators of oil degradation, are plotted as a function of distance traveled for the three test vehicles. The four oil analysis tests are:
- Total Acid Number (TAN)-Concentration of acid constituents in the oil from oil oxidation and combustion products.
- Total Base Number (TBN)-Depletion of overbase detergent additive.
- Differential Scanning Calorimetry (DSC)-Approximates the remaining life of the antioxidant (residual oxidation induction time).
- Pentane Insolubles (PIN)-Concentration of carbon soot and sludge.
The point at which the oil-change indicator signaled an oil change is also shown. All oil analyses results are plotted, including those data points regarded as "outliers", that is, points with values differing by more than approximately 20 percent from the trend lines from all data. It is interesting that DSC data followed a rapid decay curve from the outset. Also interesting is that both TAN rises and TBN falls at an apparent increased rate near the point where PIN rises (about 16,000 km). Even though not all of these changes occurred with each vehicle and there was usually some oscillation in the data, it was still possible to use these generalizations as guidelines to characterize the oil aging process.
From these studies and other subsequent investigations it is clear that there are distinct benefits to drivers of vehicles equipped with the oil-life monitoring systems. For those who neglect to change their oil on a regular basis, the system provides reminders that a change is due. If they need the reminder and follow through with an oil change, they'll protect their engine from premature wear. And, drivers who thought they needed to get their oil changed every 3,000 miles (5,000 km) or so, might be able to go longer between changes. This will save them money, time, and perhaps more importantly, precious natural resources.
Ref: Schwartz, S. E. and D. J. Smolenski, "Development of an Automatic Engine Oil-Change Indicator System," SAE Paper 870403.
