Four-stroke petrol (spark-ignition) engine — suction stroke pressure: During the intake (suction) stroke, the pressure inside the cylinder is ________ atmospheric pressure.

Introduction / Context:
The intake or suction stroke draws fresh air–fuel mixture into a petrol engine cylinder. Understanding the pressure level relative to atmosphere explains how throttling, volumetric efficiency, and breathing losses arise in spark-ignition engines.

Given Data / Assumptions:

• Conventional naturally aspirated four-stroke petrol engine with a throttle plate.
• Intake valve open, piston moving downward during suction.
• No supercharger or turbocharger raising manifold pressure.

Concept / Approach:
During the intake stroke, the descending piston increases cylinder volume. Because the throttle and intake system cause a pressure drop, the instantaneous cylinder pressure is slightly below atmospheric, creating the differential that draws mixture into the cylinder. This sub-atmospheric manifold/cylinder pressure is typical of throttled SI engines, especially at part load.

Step-by-Step Solution:

Verification / Alternative check:
Manifold vacuum gauges on petrol engines read below atmospheric during idle/part load, confirming sub-atmospheric intake/cylinder pressure on the suction stroke.

Why Other Options Are Wrong:

• Equal to: Only an ideal, frictionless, unthrottled flow would approach this; practical engines show vacuum.
• Above: Requires boost (supercharger/turbo) or intake ram effects, not assumed here.

Common Pitfalls:
Confusing petrol SI with diesel CI; diesels often operate unthrottled, but cylinder pressure during intake is still close to atmospheric unless boosted.

Final Answer:
below