Pressure History in a Petrol (Spark-Ignition) Engine – When Is the Intake Mixture Pressure Lowest? Considering the suction (intake) stroke of a typical carburetted or port-injected petrol engine, at which point is the in-cylinder mixture pressure generally the lowest relative to ambient?

Introduction / Context:
During the intake process in spark-ignition engines, the piston motion, valve timing, and flow losses create a pressure profile below atmospheric. Recognizing when this pressure reaches its minimum aids in understanding volumetric efficiency, throttling effects, and the benefits of variable valve timing.

Given Data / Assumptions:

• Throttle-controlled SI engine operating at part load.
• Conventional valve timing with intake valve closing slightly after bottom dead center (ABDC).
• Flow losses in the intake tract and across the valve are present.

Concept / Approach:

As the piston moves down on the intake stroke, cylinder volume increases, drawing mixture through the intake valve. Due to throttling and valve/port restrictions, cylinder pressure remains below manifold pressure and below ambient. The pressure typically continues to fall and remains at its lowest near the end of the intake stroke when piston speed is still significant and flow inertia sustains a pressure deficit. After the valve closes ABDC, backflow ends and the pressure begins to recover during the early compression stroke.

Step-by-Step Solution:

Verification / Alternative check:

Indicator diagrams show minimum pressure near late intake. With tuned intakes or VVT, timing shifts the exact minimum, but it remains around the end of the intake event.

Why Other Options Are Wrong:

At the beginning of intake, pressure is closer to manifold pressure and not yet at minimum. At end of compression, pressure is high. “None of these” and “midway through exhaust” are unrelated to intake minimums.

Common Pitfalls:

Assuming minimum occurs at the very start; in reality, inertia and valve timing extend the low pressure into late intake.

Final Answer:

end of suction stroke