🔍 Key Concepts

• Valve timing in a four-stroke diesel engine (intake, compression, power, exhaust) • Piston positions: TDC (Top Dead Center) and BDC (Bottom Dead Center) • Why intake valves often open slightly before or slightly after these dead-center positions

💭 Think About

• Think about when you want fresh air to start entering the cylinder: should the valve start to open exactly at TDC, or slightly before/after, to get better filling? • As the piston moves from TDC to BDC on the intake stroke, does it make sense for the intake valve to close exactly at BDC, or a bit before/after, to keep air flowing? • In real engines (not the ideal textbook diagram), are valves timed to open and close exactly at dead centers, or do they lead/lag those positions for efficiency?

✅ Before You Answer

• Identify which options have the intake valve open during the entire intake stroke (from just before/after TDC to just before/after BDC). • Eliminate any option where the valve timing would clearly cut off air too early or start intake too late to fill the cylinder properly. • Remember that practical engines usually use valve lead and lag, meaning timing is advanced or retarded slightly from exact TDC/BDC.