Knock Detection

[Tom Cloud]

>>To my knowledge, when a spark ignites a compressed fuel air mixture, a=
 flame
>>front moves across the chamber.  Behind the front is burned gases, In=
 front
>>of the front is unburned gases.  The pressure is the same throughout the
>
>thank you Gary, I knew I didn't have it quite right ...
>
>I do have a zipped copy of a TI publication called "Engine Knock Detection
>Using Spectral Analysis Techniques with a TMS-320 DSP" that  I'll send
>to anyone that wants it -- it's available (or was) from the TI web
>site but I don't have the URL.  The zipped file (SPRA039.PDF) is
>248 kbytes.


here are some tidbits from the article:


ABSTRACT
An efficient method of detecting engine knock using spectral analysis is
presented. Multi......

INTRODUCTION
What Is Engine Knock?
Modern engine control systems are designed to minimize exhaust emissions=
 while
maximizing power and fuel economy. The ability to maximize power and fuel=
 economy
by optimizing spark timing for a given air/fuel ratio is limited by engine=
 knock.
Detecting knock and controlling ignition timing to allow an engine to run at=
 the
knock threshold provides the best power and fuel economy. Normal combustion=
 occurs
when a gaseous mixture of air and fuel is ignited by the spark plug and=
 burns smoothly
from the point of ignition to the cylinder walls. Engine knock, or=
 detonation, occurs
when the temperature or pressure in the unburned air/fuel mixture (end=
 gases) exceeds
a critical level, causing autoignition of the end gases. This produces a=
 shock wave
that generates a rapid increase in cylinder pressure. The impulse caused by=
 the shock
wave excites a resonance in the cylinder at a characteristic frequency that=
 is
dependent primarily on cylinder bore diameter and combustion chamber=
 temperature.....

Knock Sensors
Implementing a knock detection/control strategy requires sensors to monitor=
 the
combustion process and provide feedback to the engine controller. Knock=
 sensors
can be classified in two broad categories: direct and remote measurements.

Direct Measurements
Pressure sensors measure the pressure inside the combustion chamber of a=
 running engine.
This direct measurement of the combustion process provides the best signal=
 to analyze
to detect engine knock.  However, each cylinder requires its own sensor, and
individual sensor costs are still relatively high. As a result, pressure=
 sensors are
used primarily in research settings. Currently, Toyota is the only=
 manufacturer
that installs pressure sensors in production engines.......

Remote Measurements
Remote measurement sensors use vibrations transmitted through the structure=
 of the
engine to detect knock in the combustion chamber. The signal received by=
 remote
sensors can be contaminated by sources other than engine knock, which=
 increases
the difficulty of signal detection. This is especially true at higher engine
speeds in which background mechanical vibrations are much higher,=
 effectively
reducing the signal-to-noise ratio........

Two types of remote sensor are being used today: tuned and broadband. Tuned=
 or
resonant sensors are used in many low-end knock detection systems. Either
mechanically or electronically, the sensor amplifies the magnitude of the=
 signal
in the frequency range of the knock-excited resonance (sometimes called the
fundamental frequency). A limitation to this approach is that a different=
 sensor
can be required for each engine type, due to variations in the=
 characteristic
frequency......Broadband sensors have no resonant peaks below the 20-kHz=
 operating
range of the knock-detection system. One sensor works equally well for any=
 engine
configuration. Some type of postprocessing is required to identify the=
 characteristic
frequency, placing an additional burden on the signal conditioning part of=
 the
system.......

Knock Detection Overview

Spectral Signature
When engine knock occurs, a shock wave is generated inside the combustion=
 chamber.
The shock wave excites a characteristic frequency in the engine, which is=
 typically
in the 5 kHz=967 kHz range. Cylinder bore diameter and combustion chamber=
 temperature
are the main variables that affect this fundamental frequency. Variations in=
 the
fundamental frequency for a given engine configuration can be as much as =B1=
 400 Hz.
Larger diameters and/or lower temperatures result in a lower fundamental=
 frequency.
Signals received by a remote sensor contain additional vibrational modes,=
 which are
structural resonances in the engine excited by the shock wave as it hits the=
 cylinder
wall........

Adaptation Requirements
An engine-knock detection algorithm must be able to adapt to a number of=
 variables to
enable the controller to generate optimum spark timing so that the engine=
 can run at
the knock threshold. As mentioned previously, the structural design of an=
 engine and
the mounting location of the knock sensor(s) affect which frequency modes=
 are
detectable by the sensor. Usually, the transfer function between the=
 cylinder and
the sensor is different for each cylinder. This causes both the relative and=
 absolute
magnitudes of the vibrational modes to be different for each cylinder. A=
 good
detection scheme should allow different calibrations for each cylinder.

Another variable that must be accounted for is changes in nonknocking=
 (reference)
signal amplitude due to the mechanical vibration of the engine at different=
 RPMs.
As the engine speed increases, the background vibration level increases.=
 When a
fixed reference is used, a compromise in performance must be made because=
 signal
magnitudes that would indicate knock at lower engine RPMs are equal to or=
 less than
the background level at higher engine RPMs........ For this reason, some=
 knock
detection systems are shut off above 4000 RPM, and very conservative spark=
 timings
are used to guarantee that knock will not occur. A good detection strategy=
 should
adapt to .............

Signal Conditioning
Knock detection systems must perform some type of signal conditioning prior=
 to
executing the detection strategy. Information about the signal strength in=
 the
frequency range(s) excited by knock must be extracted from the measurement.=
 If a
tuned sensor with a very narrow resonant peak about the fundamental=
 frequency
is being used, no further signal conditioning is required. In all other=
 situations,
either a filtering technique (analog or digital) or a spectral estimation=
 technique
must be used.

Analog filtering is the predominant method used today, due to its low cost,=
 ease of
implementation, and lack of computational power of the engine controller=
 CPU. The
output of a simple analog filter tuned to the fundamental knock frequency is
integrated and sent to the engine control unit (ECU) to execute the=
 detection
strategy. However, now that higher precision and/or multiple frequency=
 ranges are
desired, an analog implementation is becoming cost prohibitive.

Digital filters are starting to become practical as the computational=
 performance of
the ECU increases..............

....... Today, at engine speeds above approximately 4000 RPM, a combination=
 of very
conservative spark timing maps and shutting off the control strategy is used=
 to
guarantee knock-free operation. This results in less than optimal=
 performance and fuel
efficiency at higher engine speeds, particularly for systems using only=
 fundamental
frequency detection.......

The tradeoffs between using only the fundamental frequency or the=
 combination of
fundamental and vibration mode frequencies concern the issues of false=
 triggers
vs. complexity, available CPU time, and cost. When multiple frequencies are=
 used,
a better signature is available to determine if knock is present. This=
 effectively
increases the signal-to-noise ratio of the system. As a result, either the=
 RPM range
for reliable detection can be extended or the baseline spark timing at lower=
 engine
speeds can be advanced to allow the engine to continuously run closer to the=
 knock
threshold.

Control Strategies
Knock control strategies today adjust spark timing to let an engine run at=
 the knock
threshold. Look-up tables are used to obtain a baseline setting for a given=
 speed, load,
and temperature. Based on the level of knock detected, timing can be=
 advanced (no knock)
or retarded (knock). The rate of advance or retardation can also be modified=
 based on
knock magnitude and/or offset from the baseline spark timing setting.

The strategies fall into two categories. The simplest strategy, global=
 control,
retards the spark timing of all cylinders by the same amount when any knock=
 is detected.
This approach has the advantage that only one knock value has to be tracked=
 ....
However, engine performance can be compromised if only one or two cylinders=
 are more
likely to knock at a given operating condition......

A more sophisticated strategy is to control each cylinder individually so=
 that all
cylinders are running at the knock threshold and provide the best power and=
 fuel
efficiency. To implement this type of control .....

Tom Cloud