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High performance unleaded vehicles like these Holden Special Vehicles Commodores use knock sensing feedback loops to prevent potential engine damage caused through detonation. Knock Sensing Many cars with engine management systems have knock sensors. These are used to retard the ignition timing if knocking is occurring. As a result, most cars with knock sensors will provide increased performance if premium unleaded fuel is used because they can then employ optimum ignition advance. By JULIAN EDGAR 4  Silicon Chip Sensing when engine knocking is occurring has become in­ creasingly important in recent years. This is so for two reasons: (1) in older cars, the ongoing reduction in the lead content of fuel has meant that knocking is more likely; and (2) in cars using electronic engine management, knock sensing is used to allow the engine to run almost constantly on the threshold of knock. But while sensing knock initially looks to be a straight­forward task, it becomes much more complex when the subject is examined in depth. Engine knock occurs when the air/ fuel mixture ignites within the combustion chamber in an uncontrolled manner, rather than by the progressive action of a moving flame front. The Knock resonant frequency = 900/(πr) where the resonant frequency is measured in Hertz and “r” is the cylinder radius in metres. Electronic sensors Engine knocking can be sensed by any of the following means: (1) a pressure sensor installed flush with the combustion chamber; (2) a pressure sensor connected to the spark plug; (3) temperature measurement at the cylinder wall; (4) acceleration sensor, frequency tuned; (5) acceleration sensor, not frequency tuned; (6) force measurement at the cylinder head bolt by the use of a special washer; NUT CONNECTOR WEIGHT RESISTOR HOUSING PIEZO ELEMENT Fig.1: a typical knock sensor uses a piezoelectric element to generate a voltage output. This sensor is fitted to a turbocharged Subaru Liberty RS. Knock sensing is of particular importance in turbo cars. (Subaru). WITHOUT KNOCK AMPLITUDE terms “ping­ing” (a light, barely observable knock) and “pre-detonation” (knock caused by the ignition of the charge slightly before the full ignition of the flame front by the spark plug) are also commonly used. One definition of knock is “an undesirable mode of combus­ t ion that originates spontaneously and sporadically in the en­gine, producing sharp pressure pulses associated with a vibratory movement of the charge and the characteristic sound from which the phenomenon derives its name”. If allowed to occur in an unchecked manner, the very sudden pressure change within the cylinder can damage the engine. At worse, pistons, rings and even the head itself can suffer catastrophic damage. Obviously, heavy knocking is something to be avoided! In everyday driving, knock is most likely to be heard when using too high a gear for the engine speed and load conditions – like labouring up a steep hill with your foot flat to the floor, in third gear and travelling at 40km/h. Depending on the engine, knock can sound like a ‘ting, ting’ noise, or even a little like coins rattling in a coin tray. In some engines, the audible note is much deeper. In turbocharged cars, or cars where the compression ratio has been substantially increased, knocking can occur at high engine speed and high loads, making it very difficult for the driver to hear it above the general noise level. The frequencies generated by knock generally lie between 2kHz and 12kHz. The following equation can be used to estimate the knock resonant frequency for a specific engine: FK Y NC QUE FK = KNOCKING FREQUENCY IN THE COMBUSION CHAMBER FRE CRANK ANGLE KNOCKING FK 45° AFTER TDC Fig.2: The output of a knock sensor with and without knocking. Note that the amplitude of the knock frequency (FK) is substan­tially less than that of other frequencies also being transmitted by the block, making knock detection difficult. (Bosch). ENGINE SPEED = 2500 RPM (7) deformation measurement of the cylinder head bolt; (8) using a spark plug with a ring made of piezo ceramic material; and (9) the ionic current measuring method. The most commonly used are the acceleration sensors, which make use of piezo ceramics. These sensors consist of a piezoelec­tric disc and an associated seismic mass, with the latter either cast in plastic or formed by the body of the sensor itself. When a piezoelectric material is subjected to deformation, a propor­ tional voltage is generated. The sensor is mounted directly on the engine and so ‘lis­tens’ for sounds transmitted through the head and block. The fact that numerous components other than typical knock frequencies are contained within this noise signal is the major disadvantage of this technique. However, it has proven to be the most practical December 1995  5 FR EQ U EN CY AMPLITUDE CRANK ANGLE FK FK FK = KNOCKING FREQUENCY IN THE COMBUSION CHAMBER TDC ENGINE SPEED = 4500 RPM FK 90° AFTER TDC method of detecting knock. Fig.1 shows the components inside a typical knock sensor. Signal analysis Separating the sound of engine knock from the noise of valves opening and closing, pistons rising and falling, cam chains clanking and the general under-bonnet hubbub has proved to be the hardest part of detecting when knock is occurring. One way to reduce the problem has Fig.3: the structure-borne noise generated by knocking in the same cylinder for three successive combustions can be seen here. While the amplitudes of the knocking frequency are almost the same in all three cases, their positions change radically with respect to the frequency and time of occurrence. (Bosch). been to decrease the time for which the sensor is actually “listening”. The major compon­ents of knocking for a specific cylinder occur during a time “window” which extends from shortly after the piston reaches top dead centre to between 60-90 crankshaft degrees later. If the knock signal is averaged only when the engine is in these time windows, then the task is made slightly easier. Crankshaft position sensing is therefore required for this technique. INTERPRETIVE CIRCUIT Signal processing CONTROL CIRCUIT IGNITION MODULE KNOCK SENSOR GATE REFERENCE ACTUATOR ELECTRONIC CONTROL UNIT Fig.4: in this knock control system the analog sensor signal is processed by a 10kHz wide bandpass filter. The signal is then split, with one branch becoming the conditioned reference signal, against which the other signal is compared. A gate relates the test signal to crankshaft position, to determine whether or not it is in fact indicative of engine knock. If it is, the ignition advance is reduced. (Bosch). 6  Silicon Chip However, even examining the knock signal only within rela­tively narrow time windows doesn’t greatly ease the task! The upper part of Fig.2 shows the frequency spectrum of the struc­tureborne noise in the crankshaft angle range between TDC and 45° after TDC for one combustion, while the lower part of the diagram shows a knocking combustion over the same time period. The dark line, “FK”, is the knocking frequency and it is notable that other frequencies appear with substantially higher am­ plitudes than that of the knocking. In other words, it’s not enough to listen for the loudest noises. Instead, the specific frequencies within that noise must be pinpointed. Furthermore, when successive com­ bust­ ions are examined for the same cylinder, the patterns of noise, frequency and crank angle can vary substantially. Fig.3 shows the noise generated by knocking in the same cylinder for three successive combust­ ions. While the amplitudes are almost the same in all three cases, their positions change radically each time with respect to the frequency and time of occurrence during the combustion. In addi­tion, large differences occur between individual cylinders and from engine to engine in the same series! With a sensor tuned to a specific frequency, it can be difficult to always sense the largest amplitude in the frequency spectrum produced through knocking. Wideband sensors are there­fore more generally used, although extensive signal processing is then required to achieve good results. Unless the vehicle driver is to be an active participant in the engine management process, it is pointless letting him or her know that knocking is occurring. This means that all but one (aftermarket) knock sensing system is part of a wider automatic engine control strategy, with ignition timing retard and/or turbocharger boost reduction occurring as a result of knock detection. Fig.4 shows an example of a knock sensor control system. The analog sensor signal is processed so that signals irrelevant to knocking are filtered out; this is achieved by means of an ap­proximately 10kHz wide bandpass filter. Beyond the bandpass filter, the To reduce or eliminate these problems, some manufacturers have adopt­ed a self-learning system. Typically, this consists of five elements: a Pre-programmed Spark Advance Memory (PAM); a Gener­ ated Spark Retard Memory (GRM); an Updating History Memory (UHM); a Gain Function; and a Learning Function. PAM is a programmed spark advance map which gives the best fuel economy within the constraints of legal exhaust gas emis­sions. It has three dimensions: spark advance, engine speed and engine load, and the data is stored in read-only memory (ROM). GRM holds the spark retard map, which is updated every engine cycle. This data is held in random access memory (RAM) which is smaller than the PAM ROM because knock occurs only in a limited area of engine operating conditions. UHM holds the number of updating times of each combination of engine speed and load in GRM. The data in this memory repre­ sents the control history of GRM and has the same construction as the GRM. The Gain Function determines the retard or re-advance value of the ignition timing in proportion to the knock intensi­ty. Because direct measurement of the severity of knocking is difficult, the time between successive knock signals is used as an alternative value of knock intensity. Finally, the learning function defines the learning coefficient as a function of the UHM data. Results Using an adaptive approach such as that discussed above can give impressive results. Fig.5 shows the test results of the knock level during 10-40km/h wide-open throttle acceleration runs in third gear, with and without the Learning Control System (LCS). It can be seen that the LCS approach reduced the number of times the engine knocked in each acceleration test from eight times to only twice. Note also that without LCS, the propensity of the engine to knock actually increased over time – probably as a result of the engine increasing in temperature. Acceleration, though, was better with some knocking present – although probably at the expense of SC engine longevity! WITHOUT LCS x10-2G WITHOUT LCS 5 WITH LCS ACCELERATION Self-learning systems Turbocharged engines like this Subaru Liberty RS unit have very high cylinder combustion pressures, and so knocking can easily occur. Knock sensing in this car is used to retard only ignition timing but in some turbo cars, the boost is also reduced. NUMBER OF KNOCK TIMES signal is split, with one branch being conditioned to become the reference signal which is compared with the ‘useful’ signal. Further comparison with a test window related to crankshaft position determines whether or not the signal is in fact indica­tive of engine knock. As already stated, a “knocking” outcome leads to a retard in ignition timing in most engine management systems. However, depending on how well-developed the system is, the following problems can occur with this approach: (1) Harsh knock sounds in a steady control condition, caused by the difficulty in detecting engine knock; (2) Initial hard knock during abrupt acceleration, caused by the poor response time of the knock sensing system; (3) Unstable operation of the engine, caused by fluctuations in spark timing; (4) False alarming of the knock sensing system, causing the “limp home” engine mode to be adopted. 11 10 WITH LCS 9 0 1 2 3 4 1 5 ACCELERATION RUN NUMBER 2 3 4 5 Fig.5: the reduction in knock level that can be achieved by self-learning control systems can be seen here. The knock level during 10-40km/h wide-open throttle acceleration runs in third gear was reduced by about 80% with the implementation of the Learning Control System (LCS), although acceleration was slowed somewhat. (Toyota). December 1995  7