Starting and Stopping: The occurrence of boundary lubrication in an IC engine is likely during starting and stopping conditions. This is due to the fact that during these phases, the relative velocity between the surfaces is low or zero, resulting in insufficient hydrodynamic lubrication. Let us understand this in detail.

Boundary Lubrication: Boundary lubrication refers to the lubrication regime where the lubricant film thickness is very thin or absent, and the surfaces come into direct contact with each other. In this regime, the lubricant forms a protective layer on the surface, reducing friction and wear. However, the effectiveness of boundary lubrication depends on the presence of additives in the lubricant, which can form a protective film on the surface.

Explanation:

1. Starting Condition: During starting, the relative velocity between the surfaces is low or zero. As a result, there is insufficient hydrodynamic lubrication, and the lubricant film thickness is inadequate to prevent direct metal-to-metal contact. This leads to the occurrence of boundary lubrication.

2. Stopping Condition: Similar to starting, during stopping, the relative velocity between the surfaces reduces gradually until it reaches zero. As a result, the lubricant film thickness decreases, and boundary lubrication occurs.

3. Maximum Power Condition: In the maximum power condition, the engine operates at high speeds and loads, resulting in high relative velocities between the surfaces. This promotes the formation of a hydrodynamic lubrication regime, where the lubricant film thickness is sufficient to prevent direct contact between the surfaces. Therefore, boundary lubrication is less likely to occur during this condition.

4. Constant Speed Operation: During constant speed operation, the relative velocity between the surfaces remains relatively constant. As long as the lubricant film thickness is maintained, the hydrodynamic lubrication regime will prevail, preventing boundary lubrication.

5. Idling: During idling, the engine operates at low speeds or is completely stationary. Since the relative velocity between the surfaces is low or zero, boundary lubrication is likely to occur.

Conclusion:

In an IC engine, boundary lubrication is likely to occur during starting and stopping conditions, where the relative velocity between the surfaces is low or zero. During maximum power conditions, constant speed operation, and idling, the lubricant film thickness is maintained or the relative velocity is higher, resulting in a hydrodynamic lubrication regime.