Automatic Start-Stop: Good and Bad?
Dr Boris Zhmud, Head of R&D, Bizol Lubricants
Engines with automatic start-stop have become a de-facto standard in passenger cars and light-duty vehicles. A start-stop system automatically shuts down and restarts the engine to reduce the amount of time the engine spends idling. This is most advantageous in the city traffic where fuel economy gains from this technology are typically in the range of 3-10 percent.
One unfortunate consequence of frequent start-stops is that the tribological stress upon some critical engine components, first of all bearings, increases significantly. While starting the engine corresponds to only an infinitesimal fraction of the engine service life, it accounts for upto 75% of engine wear and wear-related failures! With the start-stop technology, the number of engine starts gets on average a tenfold increase, which is the highest in the city traffic.
At the same time, the majority of present-day crankcase lubricant specifications largely overlook the impact of start-stop technology on engine health. As a result, the lifetime of bearings in engines with automatic start-stop may be halved as compared to equivalent engine configurations without start-stop.
The situation is further aggravated by use of ultralow viscosity motor oils which often compromise wear protection for the sake of fuel economy. The most intense wear occurs at camshaft bearings, though balancing shaft bearings, main bearings, and connecting rod/gudgeon pin systems are all affected as well. Both oil manufacturers and automotive OEMs are working hard to resolve these problems which are often inherited from the pre-start-stop engine era design. Thus, the increased use of electric gear oil pumps helps maintain hydraulic oil pressure in the subsystems after the engine has been switched off. Another possible, though expensive, solution is the use of roller bearings both for camshafts and balancing shafts as seen in Jaguar Ingenium engines.
All major lubricant companies are also intensively working on updating their product formulations for new engines, and there are already a few products on the market claiming improved performance with start-stop engines. BIZOL has also developed new formulation approaches to address the aforesaid challenges using an innovative COMB LubriBoost™ technology which allows a twofold increase in lubricant film strength and prevents lubricant film collapse even if there is no relative motion between the mating surfaces.