Overview Of The Air Induction System

The purpose of the air induction system is to filter, meter, and measure intake air flow into the engine. Air, filtered by the air cleaner, passes into the intake manifold in varying volumes. The amount of air entering the engine is a function of throttle valve opening angle and engine rpm. Air velocity is increased as it passes through the long, narrow intake manifold runners, resulting in improved engine volumetric efficiency.

Intake air volume is measured by movement of the air flow meter measuring plate or by detecting vortex frequency on engines equipped with L type EFI. On engines equipped with D type EFI, air volume is measured by monitoring the pressure in the intake manifold, a value which varies proportionally with the volume of air entering the engine.

The throttle valve directly controls the volume of air which enters the engine based on driver demand. Additionally, when the engine is cold, it is necessary for supplementary air to by-pass the closed throttle valve to provide cold fast idle. This is accomplished by a bimetallic or wax type air valve or by an ECU controlled Idle Speed Control Valve (ISCV).

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