• 5min

Do vehicles with automatic start-stop system require special oils?


The automatic start-stop systems in vehicles place much greater demands on the engine oil. The constantly interrupted lubrication and cooling of the components in the engine through their short-term shut-off requires high-quality oils that must meet a high-performance requirement profile. We explain the specific requirements, why they arise and whether specific lubricants are actually required. 

Advantages and disadvantages of the start/stop systems

Start/stop systems reduce the idle time of the engine. Particularly in urban traffic and in traffic jams, this reduces fuel consumption and as a result CO2 emissions.

But the technology has its drawbacks, too, some of which the motor oil has to compensate for. The drawbacks of the start/stop systems include significantly higher wear, static friction during the starting, much higher engine temperatures and increased formation of deposits in the oil circuit due to downsizing.

Why is wear increasing so much?

A thin film of oil, called a hydrodynamic lubricating film, separates two components. Each time the engine is stopped and restarted, this oil film breaks down and the two friction partners touch. This operating state is referred to as the mixed friction range. Each motor is operated more or less briefly in this range. Wear is particularly high, especially when a poor quality of oil is used.

Another point is the forces that act until the moving parts in the motor return to their rotational movement after a stop. In order to overcome the static friction between the friction partners, high forces are required in relation to normal operation without automatic start/stop system. This can shorten the service life of timing chains, starters or the battery.

Why is the temperature rising so much?

We need to go into more detail to explain this. Downsizing is regarded as a key factor by car manufacturers in reducing fuel consumption. But there is a drawback here: More power with less displacement means a greater thermal load on the engine and also on the oil. And if the volume of oil in the engine is also reduced, the oil really begins to sweat. These engines are usually also turbocharged. This is the only way to achieve the high power densities that are common today.

If, for example, you now approach the end of a traffic jam after driving on the freeway and the start/stop electronics switch off the engine, the flow of motor oil also stops. Especially in components subject to high thermal loads (this includes e.g. also the turbocharger) can have fatal consequences. As the components are no longer cooled by the motor oil, the temperature in the bearings increases again after the engine is switched off. The extremely hot turbine wheel releases its heat to the shaft and other components. And these heat up to temperatures of over 300°C. This can lead to carbonization of the motor oil and increased deposits in the oil circuit.

Requirement profile for the motor oil in start/stop systems

A motor oil that meets the requirements for start/stop operation of vehicles and thereby guarantees maximum wear protection must produce an extremely strong and resilient lubricating film. In addition, the oil shall contain active substances that offer increased wear protection in the mixed friction range.

So start/stop systems require special oils due to the increased engine load?

No, special oils for engines with start/stop system are unnecessary. There is no need for such products because it is not individual technical components that determine the usability of a lubricant, but only the approvals of the manufacturers. The vehicle manufacturers specify the exact condition of the lubricant in their motor oil approvals. The effects or requirements of the automatic start/stop system will be taken into account during development. LIQUI MOLY’s motor oil formulations significantly exceed the specifications of the most modern oil specifications. Our oils ensure the highest possible lubricating film stability and their special wear protection technology ensures a long and low-wear engine life, even under the most adverse conditions.

Simply put, our motor oils are so good that we do not need any special lubricants for engines with automatic start/stop system. We have long since taken into account the high demands that automotive manufacturers place on modern engines with this type of automatic transmission. The lubricants in the Top Tec and Special Tec product ranges, as well as the High Tech smooth-running and the Longtime High Tech lubricants, are particularly suitable for these vehicles. These oils have always been suitable for the high requirements of vehicles with start/stop technology, even before this technology even existed.