How to set-up a knock-sensor signal-conditioning system

October 30, 2014 // By Yvette Tran, Texas Instruments
Engine knock occurs in engine cylinders because of improper ignition timing or faulty components. Modern cars incorporate knock-sensor systems for engines to minimize knocking, which can maximize engine lifetime, increase power, and improve fuel efficiency. This article discusses engine knock basics and how to set up a knock-sensor signal-conditioning system.

Basics of engine knock

Engine knock, or detonation, is uncontrolled ignition of pockets of air and fuel mixture in a cylinder in addition to the pocket initiated by the spark plug. Engine knock can greatly increase cylinder pressure, damage engine components, and cause a pinging sound.

In normal combustion, an internal-combustion engine burns the air and fuel mixture in a controlled fashion. Combustion should start a few crankshaft degrees prior to the piston passing the top dead center. This timing advance is necessary because it takes time for the air and fuel mixture to fully burn and it varies with engine speed and load. If timed correctly, maximum cylinder pressure occurs a few crankshaft degrees after the piston passes the top dead center. The completely ignited air and fuel mixture then pushes the piston down with the greatest force, resulting in the maximum torque applied to the crankshaft for each cycle.

Today's engines are designed to minimize emissions and maximize power as well as fuel economy. This can be achieved by optimizing the ignition spark timing to maximize the torque. With this timing control, the spark plug ignites the air and fuel mixture from the ignition point to the cylinder walls and burns it smoothly at a particular rate. Deviations from normal combustion, such as igniting too soon, can cause engine knock and, in extreme cases, result in permanent engine damage. Other causes of engine knock include using the wrong octane gasoline or defective ignition components.

Signal-conditioner interface

Modern cars have a knock-sensor system to detect engine knock for each cylinder during a specified time after top dead center called the knock window. A typical system consists of a piezoelectric sense element and signal conditioner. The sensor detects vibrations and the signal conditioner processes the signal and sends a voltage signal to the engine control module. The module interprets the knock signal to control timing and improve engine efficiency. Knock

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