🔍 Key Concepts

• single-acting four-stroke vs single-acting two-stroke loading on crankshaft journals and main bearings • how often gas pressure load acts on the crankpin/main journal during each cycle for 2-stroke vs 4-stroke • effect of reversal or change in direction of load on formation and maintenance of an oil film in bearings

💭 Think About

• Think about how many times per crankshaft revolution each engine type has a power stroke, and how that affects the direction and steadiness of the force on the crankshaft journals. • For a good hydrodynamic oil film, is it easier when the bearing load is steady in one direction, or when it changes direction over the cycle? Why? • Which engine type causes the bearing load to alternate more clearly between compression and relatively unloaded (or opposite) conditions, helping fresh oil to be drawn into the loaded zone?

✅ Before You Answer

• Be clear which engine type (two-stroke or four-stroke) has a power stroke every revolution vs every other revolution. • Consider in which case the direction of the main bearing load tends to reverse or at least vary more over the cycle, helping oil distribution. • Eliminate any option that focuses only on piping complexity instead of the direction and timing of bearing loads and how that affects lubrication.