Exhaust stroke thermodynamics: cylinder pressure compared to atmosphere During the exhaust stroke of a four-stroke engine (exhaust valve open, piston moving up), how does the in-cylinder pressure compare to atmospheric pressure? Choose the correct option.

Given data

• Exhaust valve open; piston travels toward TDC expelling burnt gases.

Concept/ApproachA slight positive pressure above atmospheric is needed to drive gases out through the exhaust port and manifold. Pressure waves and valve timing may cause small fluctuations, but the mean pressure is slightly above atmospheric during most of the stroke.

Step-by-Step reasoningExhaust blow-down begins near BDC with high cylinder pressure; as the piston rises, pressure reduces yet stays a little above ambient to maintain mass flow.

Final AnswerMore than atmospheric