induction

Induction is the phase of an internal combustion engine cycle where atmospheric air, or an air-fuel mixture, enters the cylinder as the piston moves downward and the intake valve opens. This intake stroke draws gas into the combustion chamber, establishing the raw material for the power stroke that follows. The quality and volume of inducted charge directly affect engine output, efficiency, and emissions.

In naturally aspirated four-stroke engines, induction relies on atmospheric pressure pushing air through the intake manifold as the piston creates a partial vacuum. The intake valve typically opens near the end of the exhaust stroke and closes after the piston reaches bottom dead center, with some overlap allowing scavenging of residual exhaust gases. Valve timing, manifold design, and cylinder head porting all influence how completely and how quickly the charge fills the cylinder.

Variants and complications

Boosted engines use turbochargers or superchargers to force a denser charge into the cylinder, increasing mass flow and power output. Two-stroke engines handle induction through ports rather than valves, with the piston itself controlling when intake ports open. Direct injection engines meter fuel separately from the intake charge, allowing precise control of mixture strength and combustion timing. Carbureted engines mix fuel during induction; fuel-injected engines inject atomized fuel directly into the intake port or cylinder.

Induction efficiency suffers when deposits accumulate on valves and ports, reducing flow area and disrupting charge motion. Low intake temperatures improve charge density but can cause cold-start condensation and fuel wetting of cylinder walls. High intake temperatures reduce density and increase the risk of preignition in spark-ignition engines.

The term derives from the physical action of drawing or leading gas inward. Induction is distinct from scavenging, which refers to the removal of exhaust gases, though overlap between intake and exhaust strokes means both processes occur simultaneously during transition periods. Understanding induction is fundamental to tuning intake manifolds, selecting camshaft profiles, and optimizing engine breathing for a given displacement and RPM range.