The good oil

Oil change intervals can be extended by around 25 percent by means of a special sensor. Researchers from Daimler AG have developed a practical system that allows them to monitor oil quality directly on board a vehicle. This helps reduce operating costs, especially in commercial vehicles.

The longer engine oil remains in use, the more it is susceptible to impurities. Its quality is impaired and it gradually loses its lubricative effect; this can even lead to engine damage. Timely oil changes are thus indispensable. However, this is offset by material costs with up to 40 litres of oil in a truck's diesel engine, and loss of earnings due to downtime during maintenance. An oil change should therefore take place as early as necessary, but as late as possible.

Practical measurements

In view of the many external conditions to be taken into account, calculating the precise maximum service interval on the basis of mathematical models alone is difficult. The software must combine and assess various parameters: the engine oil temperature, the frequency with which the engine is started, mild or harsh conditions of use, and the particular circumstances of the vehicle's operation and environment. In order to determine the ideal moment for the next servicing, the Daimler researchers therefore use a special sensor that provides clear readings. This sensor, which is integrated into the oil circuit and registers the characteristic parameters, allows the engine oil to be monitored directly.

Potential put to effect

To evaluate the quality of the oil, its so-called permittivity is calculated by means of an AC potential applied between the interior and exterior pipes of the oil-filled sensor. This parameter is a measure of the extent to which the oil can transmit the applied electric field. If the engine oil is contaminated by water or soot particles, it polarises to greater extent and its permittivity increases.

Slow flow

However, not all impurities can be registered with sufficient precision via the electric field. The researchers use viscosity as a further quality marker to detect any diesel fuel that may have found its way into the oil.

In the laboratory, they use an orifice viscometer – a container with a small opening through which the oil flows out. The time required for this process – the 'efflux time' – which is dependent on the oil's flow characteristics, is a direct measure of its viscosity. This in turn gives an indication of the oil's remaining lubricative quality.

The Daimler researchers can also measure viscosity while the vehicle is in motion by observing the oil's side-to-side motion in the sump. The more slowly the oil moves, the higher its viscosity. This movement is registered by the oil sensor and the viscosity calculated on this basis.

One single sensor, and the intelligently processed information which is already available on board the vehicle, are sufficient to determine the various parameters of the engine oil. This onboard oil quality surveillance is currently being prepared for series application in commercial vehicles. The resulting precise calculation of due times for maintenance stops will allow o