Maintenance and oil changes are two of the more burdensome responsibilities that go along with owning a car — and they also represent a significant cost factor in the operation of commercial vehicles. This is why DaimlerChrysler engineers decided to search for ways to substantially lengthen maintenance intervals.

One of the more useful — if admittedly irritating — indicators on today’s instrument clusters is the maintenance-interval warning display in Mercedes-Benz passenger cars, and its Flexible Service System (FSS) counterpart in the brand’s heavy-duty commercial vehicles. These instruments notify drivers when their vehicles are due for the next maintenance check and, more important, for their next oil change.

The quality of engine oil declines with every kilometer driven, and it is this relationship between a vehicle’s use and oil quality that determines maximum service intervals. Engine oil that has lost its lubricating capacity can cause a critical operating situation, which can eventually lead to engine damage.

In the past, robust technologies and advances in materials and production methods led to longer maintenance intervals for vehicles. And now, with the help of intelligent software, the effect of operating conditions on parts subject to wear and tear, and especially the impact of such conditions on engine oil, are factors that can be incorporated into service interval calculations.

The software does this by analyzing a variety of parameters, such as oil temperature and the number of times the vehicle has been started. Using a mathematical model, the software then calculates whether it is possible to determine the service interval based solely on the number of kilometers driven. Alternatively, it can calculate whether more conservative driving justifies a kilometer “bonus” that would lengthen the interval, or if use under exceptionally tough conditions would warrant an oil change before the normal interval has elapsed.

“You have to remember, however, that it’s difficult to reliably lengthen service intervals using model-based maintenance calculation systems alone,” explains Rainer Mäckel. At DaimlerChrysler Advanced Development, Mäckel’s team in the Reliability and Diagnosis lab is working on a system that would allow direct monitoring of oil quality in the vehicle as a means of lengthening the maximum interval between two oil changes.

The engineers here are focusing on commercial vehicles, including vans and heavy-duty trucks like the Actros. That’s because up to 40 liters of oil might be required to ensure sufficient lubrication of diesel engines, depending on engine type and vehicle use. In other words, the material costs alone for an oil change are very high — not to mention the costs that a commercial vehicle operator incurs when the vehicle is out of service for required maintenance.

> Sensors that measure three parameters

The lab team’s work began with what is known as a “QLT sensor,” which it integrated into the oil cycle as a probe that measures three parameters: the oil’s quality, level and temperature.

Whereas the latter two parameters can be measured directly, oil quality is gauged by determining a single summary value. Known as “permittivity,” this value is a measurement of the current flow and voltage behavior of engine oil that serves as a conductor between the sensor’s metallic inner and outer tubes. Permittivity can be measured as soon as an alternating current is sent through the two tubes.

If the oil has been polluted by the introduction of water or diesel particulates, for example, it will become more polar, which will increase its permittivity. This means that an increase in permittivity is actually an indicator of increasing wear and tear. Impurities caused by diesel fuel, which also reduce oil quality, cannot be identified with sufficient reliability by measuring permittivity, however.

> Measuring viscosity as the second indicator of quality

“This is why our approach also involves determining the viscosity of the engine oil as a second quality attribute,” explains Michael Pulvermüller, head of the Operating Materials Monitoring project. Measurements here are made using a “runout viscosity meter,” which is basically an oil tank with a small opening at the bottom, through which oil flows at different speeds, depending on its viscosity. The time it takes for the oil to flow through is a direct indicator of its lubricating capacity.

The engineers have now tested the principle in a vehicle. The method they use for this is actually quite simple: When the vehicle is in motion, the oil swishes around in the oil pan — and the more viscous it is, the slower its movement will be. The engineers monitor this swishing oil with the help of the oil sensor and determine its viscosity on the basis of their observations. “This means that we don’t need any hardware components other than the oil sensor; we simply use the data already available in the vehicle in a more intelligent manner that enables us to determine oil viscosity,” Pulvermüller says. The engineers have also succeeded in demonstrating that changes to viscosity can be reliably measured using a sensor that comes as standard equipment in Sprinter vans — they did this by comparing their results with a lab analysis of the same engine oil.

“We’ve successfully demonstrated the technical feasibility of onboard oil quality monitoring,” reports Alexander Bodensohn, director of the Reliability and Diagnosis lab. “Now we are going to be spending the next few months working intensively together with our partners from Truck Advanced Engineering and Commercial Vehicles Series Development to get this project ready for series production with the Actros.” The engineers believe that using the system could lengthen the intervals between oil changes by approximately 25 percent.

Viscosity - the second indicator of oil quality - can be determined by measuring the speed at which the oil level in the QLT sensor changes.


Engine oil lubricating capacity can be measured in a laboratory using a “runout viscosity meter.” The speed at which a specific amount of oil flows through an opening at the bottom of the measuring instrument serves as an indicator of the fluid’s viscosity.


A rocking device operated by a motor swishes engine oil around the swinging oil pan. Two QLT sensors dipped into the oil measure the oil’s level, temperature and permittivity, a summary measurement of quality. The measurement results appear on a monitor.


A rocking device operated by a motor swishes engine oil around the swinging oil pan. Two QLT sensors dipped into the oil measure the oil’s level, temperature and permittivity, a summary measurement of quality. The measurement results appear on a monitor.


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