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Configuring knock-sensor signal-conditioning system

Posted: 19 Nov 2014 ?? ?Print Version ?Bookmark and Share

Keywords:Engine knock? ignition? knock-sensor? signal-conditioning? TPIC8101?

Improper ignition timing or faulty components sometimes result to engine knock in engine cylinders. Modern cars incorporate knock-sensor systems for engines to minimise knocking, which can maximise 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 centre. 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 centre. 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 minimise emissions and maximise power as well as fuel economy. This can be achieved by optimising the ignition spark timing to maximise 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 centre 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 sensors typically are mounted on the engine block (figure 1).

Knock sensor mounted to an engine block

Figure 1: Knock sensor mounted to an engine block.

Coefficient descriptions:
VIN = Amplitude of input voltage peak
VO = Output voltage
AIN = Input amplifier gain setting
AP = Programmable gain setting
ABP = Gain of bandpass filter
AINT = Gain of integrator
TINT = Integration time from 0.5 ms to 10 ms
AOUT = Output buffer gain
C = Programmable integrator time constant
VRESET = Reset voltage from which the integration operation starts

The simplified diagram in figure 2 shows the TPIC8101 dual-channel, highly-integrated, signal-conditioner interface from Texas Instruments that can be connected between the knock-sensing element and engine control module. The two internal wide-band amplifiers (figure 3) provide interface to the piezoelectric sensors. The outputs of the amplifiers feed a channel-select mux switch (figure 2), followed by a third-order anti-aliasing filter (AAF).


Figure 2: TPIC8101 block diagram with coefficients.

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