Intake stroke fundamentals — cylinder pressure relative to atmosphere During the suction (intake) stroke of a four-stroke engine, how does the instantaneous in-cylinder pressure compare to atmospheric pressure?

Introduction / Context:
The intake (suction) stroke draws fresh charge into the cylinder. Pressure differential across the intake system drives this flow and is key to volumetric efficiency.

Given Data / Assumptions:

• Throttle-controlled SI engine or unthrottled CI engine operating at typical speeds.
• Ambient atmospheric pressure acts at the intake entry.
• Valves and ports provide finite flow resistance.

Concept / Approach:
As the piston moves down from top dead center with the intake valve open, the cylinder volume increases. Because air cannot fill the expanding volume instantaneously through restrictive passages, in-cylinder pressure falls slightly below atmospheric. This pressure drop drives air into the cylinder.

Step-by-Step Solution:

Verification / Alternative check:
Manifold absolute pressure (MAP) sensors show values below atmospheric during intake, especially on throttled SI engines at part load (e.g., 30–60 kPa absolute vs. ~101 kPa ambient).

Why Other Options Are Wrong:

• More than atmospheric: occurs in boosted engines, not typical unboosted intake.
• Equal to atmospheric: only approximately true at specific moments.
• Half atmospheric or undefined: not representative of real operation.

Common Pitfalls:
Confusing boosted intake (above atmospheric) with naturally aspirated part-load conditions.

Final Answer: