ICE-37 — Idle Control

ASSUMPTION — Idle target speed varies by engine; a qualitative framework is given.

In this lesson you will learn:

• The goals of idle control
• Load disturbances
• The role of closed loop

1. What Is Idle?

Idle is the minimum stable speed with no pedal input. The goal is to keep the engine running with low fuel use and emissions.

FACT — Idle speed is a balance between fuel economy, emissions, NVH and load acceptance (A/C, power steering).

2. Load Disturbances

Sudden loads can drop idle speed:

• Air-conditioning compressor engaging
• Electrical load (lights, fan, seat heating)
• Hydraulic power steering (older systems)

FACT — The ECU anticipates these loads (load signal) and increases air/fuel to prevent a speed drop.

3. Closed Loop

FACT — Idle control is a closed loop: it measures engine speed (crank sensor), compares to target, and corrects throttle angle and/or spark advance to hold speed stable despite load changes.

4. Summary

• Idle is the stable minimum no-throttle speed.
• Load disturbances can drop speed; the ECU compensates ahead of time.
• Closed loop holds speed at target.

5. Sources

• Bosch, Gasoline-Engine Management and Diesel-Engine Management

Next Lesson

• ICE-38 — Engine Start.