The purpose of this experimental study was to investigate the relative influence of the physical characteristics of monograde and multigrade lubricants on engine oil consumption, taking into account different engines and different operating conditions. A bench test program on FIAT FIRE 1000 and BMW S14 gasoline engines was carried out using an automatic oil consumption measuring equipment.
The interaction between simulated driving conditions (highway, hill-road) and oil consumption was studied using lubricants differing in volatility and rheology: the results showed the importance of volatility in limiting oil consumption; the influence of viscosity was more complex, as it depends on the engine; the role of VII polymers was also pointed out. The hill-road for FIAT and the highway for BMW were identified as the most severe driving conditions.
Moreover the dependence of oil consumption on engine running conditions was investigated in detail, obtaining a map of oil consumption as a function of speed and intake manifold vacuum: a different behaviour was observed for both engines thus indicating BMW as more critical regarding oil consumption through the oil ring package, and FIAT as more critical regarding oil consumption through the valve seals.
The role of the valve seals in limiting oil consumption was also examined on FIAT: the presence of both exhaust and intake valve seals was confirmed as really important for oil consumption control under every running condition and even after extended running times.
Formulas were established on the basis of the experimental results relating oil consumption to the volatility and rheological properties of the lubricant.
During the whole experimental study, both the test engines were run for almost a thousand hours, thus giving the oil consumption trend during the engine life.