Four-stroke petrol engine: direction of piston motion during the suction (intake) stroke Consider a conventional four-stroke spark-ignition engine. During the suction stroke, the inlet valve opens and the fresh charge is drawn into the cylinder as the piston moves from top dead centre toward bottom dead centre. Is the statement “the piston moves upwards from BDC during suction” correct?

Introduction / Context:
The intake (suction) stroke is the first of the four events in a four-stroke spark-ignition cycle: intake, compression, power, and exhaust. Understanding piston motion and valve events during intake is foundational for grasping volumetric efficiency, manifold tuning, and throttle response.

Given Data / Assumptions:

• Four-stroke petrol engine with poppet intake and exhaust valves.
• Throttle controls manifold pressure; spark timing is irrelevant to the intake stroke.
• We use the standard sign convention for crank angles: TDC at the top, BDC at the bottom.

Concept / Approach:
On the intake stroke, the inlet valve opens (typically slightly before TDC). As the piston travels downward from TDC toward BDC, cylinder volume increases and its pressure drops below the intake manifold pressure. This pressure differential draws the fresh air–fuel mixture into the cylinder. Therefore, the piston moves downward, not upward, during suction.

Step-by-Step Solution:

Verification / Alternative check:
Valve-timing diagrams and PV plots show the intake stroke associated with decreasing pressure and increasing volume, confirming downward piston travel during suction.

Why Other Options Are Wrong:

“True” contradicts the kinematics of the four-stroke cycle.Two-stroke engines do not have a discrete suction stroke in the same sense; scavenging/intake occurs differently.Cold start and throttle position affect pressure levels, not piston direction.

Common Pitfalls:
Confusing the intake and compression strokes: compression is the upward motion from BDC to TDC after the intake stroke.

Final Answer:

False