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"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. Damage to pistons, rings, and exhaust valves can result if sustained heavy knock occurs. Additionally, most automotive customers find the sound of heavy engine knock objectionable.
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.
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. Pressure sensor usage will increase in the future as sensor costs are reduced and automotive companies develop more sophisticated engine control strategies that monitor the combustion process.
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. One advantage of using remote sensors is that, with careful placement, only one or two sensors are required to monitor all cylinders. In addition, the sensors are less expensive, primarily due to a less harsh operating environment.
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. The resulting part number proliferation
increases overall system costs for the manufacturer. To eliminate the cost penalty, sensor bandwidth can be made wide enough to encompass all expected variations in the fundamental frequency. However, doing so can possibly decrease system performance.
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. Since variations in the fundamental frequency can be expected for different engine configurations, a programmable solution provides the flexibility to easily modify the frequency range being monitored with minimal impact on system cost.
Knock Detection Overview.
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–7 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 ± 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. Typically, two to four additional frequency peaks are evident between the fundamental frequency and 20 kHz. Each engine structure can have different higher vibrational modes. Sensor mounting location can affect which modes are detectable and the amplitude of each with respect to the background mechanical noise.
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. The reference must be set low enough that knock can be detected at lower RPMs, which limits the algorithm’s ability to function at higher speeds. 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 varying levels of background vibration levels to allow trace knock to be detected at all engine speeds.
Finally, an engine’s operating characteristics change with time. As an engine wears, tolerances between components change, which could change the magnitudes of the vibrational modes detected by a remote sensor. Normal background vibrational levels could be higher for a given engine speed. The signal-to-noise ratio could decrease at higher engine speeds. A good detection strategy should adjust to changes in daily
operating characteristics to allow reliable identification of trace knock without false triggers...."