Engine Knock Detection and Evaluation: a Review
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PROCEEDINGS OF THE INSTITUTE OF VEHICLES 5(109)/2016 Jakub Lasocki1 ENGINE KNOCK DETECTION AND EVALUATION: A REVIEW 1. Introduction Proper combustion process in spark-ignition (SI) engine occurs when a mixture of fuel and air is ignited by a spark plug and burns in a uniform manner, generating a flame kernel that grows and propagates through a combustion chamber, from the point of ignition to cylinder walls. The flame front moves across the cylinder volume at a velocity much below the velocity of sound. Therefore the pressure in the cylinder can be considered constant. In the certain operating states of the engine deviations from this normal course of combustion can be observed. A particular example of such abnormal combustion is engine knock, which causes a significant increase in cylinder pressure and the propagation of pressure waves across the combustion chamber. Some possible negative consequences of knock include the following [1–4]: – damage of individual engine parts (erosion of cylinder head, piston crown and piston top land, breaking of piston rings and spark plug electrode, melting of piston and valves), – serious damage to the engine structure over a long period of time, – decrease in engine efficiency (and consequently increase in fuel consumption), – increase in pollutants emission, – increase of engine noise and vibrations. The examples of engine components damaged as a direct result of intensive knock have been shown in fig. 1. a) b) c) Fig. 1. Engine components damaged as a direct result of intensive knock: (a) broken spark plug electrode, (b) melted exhaust valve, (c) broken piston ring land [1] Engine knock is widely recognized as a major obstacle for the further improvement of both natural aspirated and turbocharged SI engines [1, 2]. It also restricts the 1 Jakub Lasocki, PhD. Eng.; Institute of Vehicles, Warsaw University of Technology 41 Pobrano z http://repo.pw.edu.pl / Downloaded from Repository of Warsaw University of Technology 2021-09-27 development of dual fuel compression-ignition (CI) engines supplied with gaseous fuels (e.g. methane, biogas, liquefied petroleum gas) and a pilot dose of diesel oil used to initiate combustion [5, 6]. Currently, there is a strong demand to design engines which work closer to the allowable knock limit, have higher compression ratio, boosted thermal efficiency and increased power output [7]. The difficulty of the prediction of knock occurrence relates to the large number of parameters, which have to be taken into account, from fuel properties through engine geometry to engine operating conditions. This paper aims at providing a concise theoretical foundation for the terms of engine knock by revision of previously published works. It discusses mechanism of knock formation, explains difference between knock in SI and CI engines, reviews methods of knock detection and presents the indices for the evaluation of knock intensity. 2. Engine knock fundamentals Heywood [8] defines engine knock as an abnormal combustion phenomenon involving auto-ignition of the end-gas (unburned mixture of fuel, air and residual gas) ahead of the advancing regular flame front, accompanied by extremely rapid release of energy, which results in high frequency pressure oscillations inside the cylinder. This pressure oscillations produce vibrations of substantial amplitude, propagated through the engine structure, causing sharp metallic noise. The example of such pressure variations occurring during knocking combustion in an engine cylinder has been shown in fig. 2. p p TDC V TDC α Fig. 2. Engine indicator diagrams depicting cylinder pressure during knocking combustion: p – pressure, V – volume, α – crank angle, TDC – Top Dead Centre The frequency of cylinder pressure variations (so-called fundamental frequency) caused by knocking combustion in SI engines has a typical value of about (5 ÷ 7) kHz [9]. However, some researchers, e.g. [10, 11], extend this range up to (5 ÷ 20) kHz. Particular engine configurations can further change the frequency value by about 400 Hz [9]. The main variables affecting knock frequency are primarily cylinder bore diameter and combustion chamber temperature (the latter influences the sound velocity) [8, 9, 12]. In general, knock frequency decreases with the increase in cylinder bore diameter and the decrease in temperature. The value of knock amplitude varies, depending on the engine operating conditions, and usually does not exceed 1 MPa for SI engines [1]. The term mega knock or super knock refers to a phenomenon characterized by extremely large knock amplitude that can reach up to (6 ÷ 10) MPa [1, 13, 14] or more and occurs very rarely. Random 42 Pobrano z http://repo.pw.edu.pl / Downloaded from Repository of Warsaw University of Technology 2021-09-27 variation of the value of knock amplitude can be expected due to cycle-to-cycle variability of combustion characteristics, changing from light knock to heavy knock [8]. There are two generally accepted theories postulated to explain the origin of knock: auto-ignition and detonation [2, 8, 12]. The first theory assumes the existence of so-called hot spots in the end-gas region. These hot spots are formed as a consequence of non-uniform conditions, i.e. fuel concentration, temperature and pressure, prevailing in the combustion chamber. When fuel-air mixture is ignited by the spark plug, the end-gas is subjected to compression by the expanding products of combustion and heating by radiation from the flame front. If the local pressure and temperature of the end gas exceed its auto-ignition point, one or more hot spots would ignite spontaneously. Rapid sequence of chemical reactions releases chemical energy causing pressure waves that propagate at high velocity [2, 8, 12, 15]. The detonation2 theory holds that there is a possibility of the acceleration of the advancing flame front to sonic (or supersonic) velocity. The flame front propagates uniformly from spark plug to cylinder walls, but consumes the end-gas at a rate much faster than during normal combustion. This generates intense shock waves which reflect from the walls of the combustion chamber, causing pressure oscillations characteristic to knock [2, 8, 12]. Although neither of the above theories have been proved conclusively yet, there is much more evidence to support the auto-ignition theory [8, 12, 14]. For this reason, it becomes the most widely accepted explanation for knock. As for the engine damage caused by knock, it is assumed to be mainly related to overheating [12]. Under knocking conditions, engine components in the combustion chamber are exposed to a large dose of additional heat. This negative impact is further intensified by transient character of this thermal load. Eventually, after long-term exposure to knock, thermal boundary layer of metal components is irreversibly damaged through melting, formation of cracks and erosion [3, 4] 3. Knock in compression-ignition engines Auto-ignition is the normal working principle in CI engines supplied with diesel oil. Therefore knock, understood as the symptom of abnormal combustion initiated by auto- ignition of the end-gas, does not apply in this context. On the other hand, CI engines may sometimes experience abrupt cylinder pressure raise due to the rapid increase of the fuel combustion rate (fig. 3). In consequence, a peculiar ‘knocking’ sound and vibrations occur, having much larger intensity than under regular combustion conditions. This phenomenon is known as ‘hard work’ of an engine, so-called ‘diesel knock’ [8, 16], and may lead to engine damage. Hard work of the CI engine takes place when the ignition delay (i.e. the time between the injection of fuel and its ignition) is too long. If the burning of first droplets of fuel is delayed, a great quantity of fuel get accumulated in the combustion chamber, and – when ignition finally initiates – begins to combust at once violently [8]. Generated pressure oscillations have various frequency, which depends i.a. on the position of the piston in the cylinder (or combustion chamber geometry) [16]. Unlike knocking 2 The term ‘detonation combustion’ is sometimes incorrectly used to encompass the phenomenon of knock, regardless of the explanation of knock mechanism. 43 Pobrano z http://repo.pw.edu.pl / Downloaded from Repository of Warsaw University of Technology 2021-09-27 combustion in SI engines, hard work of CI engines is usually difficult to distinguish from normal operation, because of the relatively small amplitude of pressure waves. p CI engine Start of injection SI engine TDC α Fig. 3. Comparison of knock in SI engine and hard work in CI engine: p – pressure, α – crank angle, TDC – Top Dead Centre (modified from [17]) As opposed to conventional CI engines, knocking combustion is one of the major challenges in the development of dual fuel CI engines that run primarily on gaseous fuels (e.g. methane, biogas, liquefied petroleum gas) with a pilot dose of diesel oil [5, 18]. The combustion process in these engines is much more complex. The gaseous fuel is mixed with air in the intake manifold or in the cylinder (if directly injected) and compressed, but does not ignite. Then, near the end of the compression stroke, a small amount of diesel oil is injected into the cylinder and auto-ignites, initiating the combustion of the gaseous fuel-air mixture. The emerging flame has thus a dual character: diesel oil burns just like in conventional CI engines, while gaseous fuel forms a visible flame front in a similar way as it happens in SI engines. In these conditions, knock can occur as a result of rapid pressure rise in the region of unburned gaseous fuel-air mixture, beyond the zone of burning diesel oil stream. 4. Methods of engine knock detection Knock detection methods provide a quantifiable indication for determining knock onset, which is usually referenced to crank angle. To serve this purpose, several different approaches have been developed [2, 8–10, 13, 19].