Engineering
Tribology of Journal Bearings Under Transient Start-Stop Conditions
Quick fact
A journal bearing can experience more wear during a single engine start than during hours of steady highway driving, because the protective oil film is squeezed out before the shaft reaches hydrodynamic speed.
Why this is interesting
Your car's engine automatically shuts off at every red light to save fuel. But each restart may be wearing out its most critical bearings—a hidden cost of stop-start technology.
Read the full explanation
Understanding Tribology of Journal Bearings Under Transient Start-Stop Conditions
Think of a journal bearing as a shaft (the journal) spinning inside a sleeve (the bearing). During normal engine operation at constant speed, the shaft's rotation drags oil into a wedge-shaped gap, building up enough pressure to lift the shaft completely off the bearing surface—this is called hydrodynamic lubrication, where there is no metal-to-metal contact. But when the engine is turned off, the shaft stops and settles onto the bottom of the bearing, and the oil film drains away. When the engine is restarted, the shaft begins to turn from a standstill. At that moment, the speed is too low to build a hydrodynamic film, so the journal and bearing rub directly against each other. Only as speed builds up does the oil film reappear and separate the surfaces. This transient period, from zero speed to full speed, is when most bearing wear occurs—it happens every time the engine starts and stops.
A deeper explanation
The transition from rest to full speed is governed by the Stribeck curve, which maps the lubrication regime to the bearing's speed and load. At low speeds and high loads, the regime is boundary lubrication, where the oil film is thinner than the combined surface roughness, allowing asperities (microscopic peaks) to touch and weld momentarily. As speed increases, the film thickens, leading to mixed lubrication—partial contact—and eventually full hydrodynamic lubrication, where a complete film separates the surfaces. The wear that occurs during boundary and mixed regimes is responsible for most of the bearing's cumulative wear. In start-stop engines, these transient events happen far more frequently than in traditional engines, so bearing wear accumulates quicker. To mitigate this, engine oils contain anti-wear additives like ZDDP, which react with the metal surfaces to form a protective tribofilm that reduces direct metal contact. Additionally, bearing materials are chosen for their ability to tolerate brief boundary conditions, and surface textures may be optimized to retain oil during stops.