Suppose an engine at 80°C comes in contact with a coolant at 50°C. In which direction will thermal energy transfer? How long will the thermal energy transfer continue?

That depends on what kind of environ meant it is in  in space it has the same force that was applied when the velocity was gained

Answer(a)   the matererials that are in contact, the two materials and the nature of their surfaces/// (b)/// the force pushing the two surfaces together, pushing the surfaces together causes the more of the asperities to come together and increases the surface area in contact with each other