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The figure below shows part of a petrol engine, in which air flowing under atmospheric pressure passes into a constriction, where it mixes with petrol. The mixture then flows into a combustion cylinder. Explain what causes the petrol from the petrol chamber to the air steam in the constriction when the piston is moved downwards.

Answer Text:
Air velocity at the constriction is higher. This makes the pressure to drop. Since the atmospheric pressure remains to be high, it pushes the petrol into the constriction.