PROCEEDINGS OF THE INSTITUTE OF VEHICLES 5(109)/2016

Jakub Lasocki1

ENGINE KNOCK DETECTION AND EVALUATION: A REVIEW

1. Introduction Proper process in spark-ignition (SI) engine occurs when a mixture of fuel and air is ignited by a and burns in a uniform manner, generating a flame kernel that grows and propagates through a , from the point of ignition to 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 , 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 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

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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 , 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 – 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 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

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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.

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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 manifold or in the cylinder (if directly injected) and compressed, but does not ignite. Then, near the end of the compression , 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]. They can be classified into two general groups, i.e. direct and indirect methods. The first group involves direct measurement and analysis of in-cylinder combustion parameters, such as pressure, temperature, heat transfer, intermediate radicals and species concentration. The second group utilizes measurement and analysis of indirect parameters, which can be influenced by knock, for example engine vibration and acoustic emission. Indirect methods are primarily used in industrial applications, whereas direct methods usually support research works. Measurement of in-cylinder pressure is the most widely investigated way to detect knock occurrence, since pressure oscillations are straightly connected to knock and are not affected by other mechanical sources of vibration. Pressure sensor installed directly in engine combustion chamber (often incorporated in a spark plug) records signal, which can be band-pass filtered and analysed in the time domain. Typical indicators of knock obtained in this way have been summarized in table 1 [20].

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Pobrano z http://repo.pw.edu.pl / Downloaded from Repository of Warsaw University of Technology 2021-09-27 Table 1. Knock detection methods based on in-cylinder pressure measurement

Object of analysis Knock onset indicator Raw pressure signal Crank angle at peak pressure Band-pass filtered pressure signal Crank angle at max. amplitude or amplitude exceeding a threshold value 1st derivative of filtered pressure signal Crank angle at max. 3rd derivative of filtered pressure signal Crank angle at max. negative amplitude Heat release rate Crank angle at abrupt rise of heat release (calculated from in-cylinder pressure) or at change to high frequent signal components

The above mentioned methods have some notable disadvantages. Firstly, the cost of pressure sensors and software used for signal post processing is high, especially for systems with one sensor per cylinder. Besides, since sensors are in contact with hot and high-pressure media inside the combustion chamber, their durability is limited, and their accuracy can diminish within time [9]. Secondly, pressure signal obtained at certain location of the sensor cannot be considered as representative for the entire volume of the combustion chamber. Knocking pressure oscillations have local character due to non- homogeneity of end-gas [8]. Knock detection can also be achieved through the analysis of heat release to the walls of the combustion chamber [21, 22]. Generally, heat transfer rate increases when knock occurs. According to Grandin and Denbratt [21], if knock amplitude exceeds 0.2 MPa, increased heat flux can be observed. With further raise of the amplitude above 0.6 MPa, the peak heat flux can be up to 2.5 times larger than in the normal combustion. On the other hand, thermal effects are usually negligible under light knock conditions. Therefore the scope of application of this method for engine knock control in non- laboratory conditions should be assessed as narrow. Another method for knock onset detection involves monitoring of exhaust gas temperature. It has been found that in the case of knocking combustion the temperature of exhaust gas is reduced [23]. The viability of this method stems from its simplicity, convenience, resistance to engine noise, which does not interfere with the signal, and easy application for all types of engines [2]. There is also a possibility of knock detection by means of spectroscopic and chemiluminescence techniques. They can be adopted to analyse chemical reactions caused by the end-gas auto-ignition [12]. Free radicals (atoms, molecules or ions with unpaired valence electrons) and intermediate reactants, such as CH, HCO, HCHO and OH, became useful markers of combustion phases [2]. For example, HCHO concentration allows detection of hot spots, because it increases in the end-gas before the advancing flame front. Combustion at high temperatures is dominated by the chain of reactions producing OH radicals [15], while CH radicals can be markers of normal combustion [2]. The use of this technique, however, is infeasible in industrial applications. Knock detection in mass-produced motor vehicles is largely dominated by indirect, nonintrusive methods based on engine vibration analysis. They employ specific knock sensors directly attached to the , cylinder head or intake manifold [2, 8]. Typical knock control system based on engine vibration measurement consists of one piezoelectric accelerometer, tuned to engine knock frequency (typically (6 ÷ 8) kHz),

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Pobrano z http://repo.pw.edu.pl / Downloaded from Repository of Warsaw University of Technology 2021-09-27 which generates voltage signal proportional to the acceleration. This solution has two major advantages: excellent durability and low cost [2]. However, these convenience and cost-effectiveness come with lower accuracy of the measured signal, because engine vibrations may have multiple sources, other than combustion events.

5. Evaluation of knock intensity The randomness and complexity of knock phenomenon make it difficult to recommend a universal evaluation metric to its intensity. The most commonly accepted approach is to determine the value of one of the following knock indices: – Maximum Amplitude of Pressure Oscillations (MAPO), – Integral of Modulus of Pressure Oscillations (IMPO), – Integral of Modulus of Pressure Gradient (IMPG), – Rate of Heat Release (ROHR), – Net Cumulative Heat Release (CHRNET). Other, less frequently used indices include: – Dimensionless Knock Indicator (DKI), – KI20, – MAHLE Knock Intensity (KI). Many researchers [11, 19, 24] studied or used MAPO, IMPO and IMPG. Those indices are based on the high frequency analysis of filtered in-cylinder pressure data:

θ ζ MAPO  max( pˆ 0 ) (1) θ0

θ ζ 1 N 0 IMPO  pˆ dθ (2) N   1 θ0

θ ζ 1 N 0 dpˆ IMPG  dθ (3) N   d 1 θ0 where: p – filtered in-cylinder pressure, N – number of computed cycles, θ0 – crank angle corresponding to the beginning of the window of calculation, ζ – value of the window of calculation. Among those three indices, MAPO and IMPO (fig. 4) are likely the most employed. The difference between the two is that MAPO aims at evaluating the maximum peak of the pressure oscillations due to knock, which makes it considerably resistant to noise effects, while IMPO is related to the energy of high frequency pressure oscillations and includes noise [2, 11].

Comparison MAPO

Band-pass filter Absolute e.g.(4÷20)kHz value

Integration IMPO

Fig. 4. Determination of MAPO and IMPO from in-cylinder pressure signal (modified from [11])

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Pobrano z http://repo.pw.edu.pl / Downloaded from Repository of Warsaw University of Technology 2021-09-27 ROHR and CHRNET evaluate knock in terms of heat release in the engine cylinder. Knocking combustion usually negatively influences cycle efficiency, due to the increase in heat losses, thus heat release decreases [2]. ROHR represents the rate of energy release from the combustion processes minus wall heat transfer and crevice flow losses [8]. The value of ROHR is usually estimated based on the in-cylinder pressure signal, as given by

γ dV 1 dp ROHR  p  V (4) γ 1 dθ γ 1 dθ where: θ – crank angle, p – in-cylinder pressure, v – cylinder volume, γ – specific heat ratio (cp/cv). Integration of (4) with respect to the crank angle yields CHRNET:

θe CHR  ROHRdθ (5) NET  θs where θs, θe – starting and ending angles of combustion. The CHRNET value for knocking cycles is consistently lower than that of non- knocking cycles, due to the increase in heat losses. The advantage of this parameter is a strong relation to knock intensity and lack of dependency on the pressure sensor position [2]. DKI is an example of a special index, developed to facilitate detection of the start of knock, evaluation of its intensity and determination of the Knock Limited Spark Advance (KLSA) [25]. The DKI value is derived as the ratio between IMPO and MAPO, including the width of the computational window:

IMPO DKI  (6) MAPO ζ

The greater the knock intensity, the lower the DKI value. Graphic interpretation of DKI is associated with the two surfaces. IMPO is the surface under the pressure signal and product of MAPO and ζ is the surface of the computational window. Therefore, DKI represents the “weight” of IMPO within the computational window [25]. This approach was developed mainly for scientific purposes and has not been widely used in practical applications. Another special knock index – KI20 [10] is calculated from high-pass filtered in- cylinder pressure and mean pressure data. A crank angle window stretches from first pressure pulse to subsequent 20 crank angle degrees. The exact formula is as follows:

1 n ˆ 2 KI20  (p(i)  pmean) (7) n i1 where: p (i) – high-pass filtered in-cylinder pressure, pmean – high-pass filtered mean pressure value, n – the number of pressure pulse within 20 crank angle degrees. Some indices for the evaluation of knock intensity consider stochastic character of engine cycle variation. KI index, developed by MAHLE [13], requires measurement of

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Pobrano z http://repo.pw.edu.pl / Downloaded from Repository of Warsaw University of Technology 2021-09-27 in-cylinder pressure over a predetermined number of four-stroke cycles. The amplitudes of pressure oscillations are divided into several classes having different weighting factors. KI calculation uses the following formula:

m n k f k KI  S k0 (8) c where: S – standardization constant, k – class number, m – maximum number of classes, nk – number of cycles with pressure amplitude in a class, fk – weighting factor of a class, c – number of cycles measured. The main difficulty associated with the evaluation of knock intensity, regardless of the index used, is in defining a threshold value suitable for knock determination. Examples found in literature are not always consistent [12]. Hence, it should always be clearly defined when pressure oscillations can be regarded as the sign of engine knocking occurrence.

6. Summary Despite over a century of documented research into the phenomenon of engine knock, the underlying combustion mechanisms remain not fully understood because of their complexity. On the other hand, the demand to design highly efficient engines has significantly grown in recent years as automobile manufacturers seek ways of reducing fuel consumption. One possible solution is to develop engines operating substantially at the edge of allowable knock limit. This requires the use of advanced methods of knock detection and control in engine management systems. This review paper summarizes current knowledge of engine knocking combustion. Firstly, physical processes which define knock were presented along with its possible causes and negative results. Among all the existing theories, the auto-ignition theory is the most well-substantiated. Secondly, the differences between knock in SI engines and hard work of CI engines, or so-called ‘diesel knock’, were discussed. In general, the term ‘knock’, understood as a symptom of abnormal combustion initiated by auto-ignition of the end-gas, does not apply in the case of conventional CI engines. However, even those engines occasionally experience cylinder pressure oscillations of much larger intensity than under regular combustion conditions. Next, an overview of direct and indirect methods of knock occurrence detection was given. The most widely investigated and accurate method is based on in-cylinder pressure measurement, but because of its high cost and complexity it is mainly used in research works. On the contrary, industrial applications usually employ vibration sensors attached directly to an engine. The final paragraph was a more in-depth view on the various knock indices, which are useful for quantification of the intensity of knock. It was stated that MAPO and IMPO, based on a high frequency analysis of filtered cylinder pressure data, are likely the most employed knock indices.

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Pobrano z http://repo.pw.edu.pl / Downloaded from Repository of Warsaw University of Technology 2021-09-27 References: [1] Wang Z., Liu H., Song T., Qi Y., He X., Shuai S., Wang J.X.: Relationship between super-knock and pre-ignition, International Journal of Engine Research, 2015, Vol. 16(2), pp. 166–180, [2] Zhen X.D., Wang Y., Xu S.Q., Zhu Y.S., Tao C.J., Xu T., Song M.Z.: The engine knock analysis – an overview, Applied Energy, 2012, Vol. 92, pp. 628–36, [3] Fitton J., Nates R.: Knock erosion in spark-ignition engines, SAE Technical Paper no. 962102, 1996, [4] Konig G., Maly R.R., Bradley D., Lau A.K.C., Sheppard C.G.W.: Role of exothermic centres on knock initiation and knock damage, SAE Technical Paper no. 902136, 1990, [5] Kruczyński S.W., Orliński P., Wojs M., Owczuk M., Matuszewska A.: Ocena zjawiska spalania stukowego w dwupaliwowym silniku ciągnika rolniczego zasilanego dodatkowo biogazem, Combustion Engines, 2015, Vol. 162(3), pp. 639–646, [6] Różycki A.: Knock combustion in dual fuel turbocharged compression ignition engines, Journal of KONES Powertrain and Transport, 2009, Vol. 16(4), pp. 393–400, [7] Chmielewski A., Lubikowski K., Mączak J., Szczurowski K.: Geometrical model of cogeneration system based on a 1 MW gas engine, Combustion Engines, 2015, Vol. 162(3), pp. 570–577, [8] Heywood J.B.: Internal combustion engine fundamentals, New York, 1988, McGraw-Hill, [9] Horner T.G.: Knock detection using spectral analysis techniques on a Texas Instrument TMS320 DSP, SAE Technical Paper no. 960614, 1996, [10] Laurent D., van Gilles C.B.T., Pierre L.: Noise robust spark ignition engine knock detection with redundant wavelet transform, International Conference on Noise and Vibration Engineering (ISMA), September 16–18, 2002, Leuven, Belgium, [11] Brecq G., Le Corre O.: Modeling of in-cylinder pressure oscillations under knocking conditions: Introduction to pressure envelope curve, SAE Technical Paper no. 2005-01-1126, 2005, [12] Trijselaar A.: Knock prediction in gas-fired reciprocating engines: Development of a zero-dimensional two zone model including detailed chemical kinetics, Master thesis, University of Twente, Enschede, 2012, [13] MAHLE: and engine testing, 2nd ed. Wiesbaden, 2016, Springer, [14] Rothe M., Heidenreich T., Spicher U, Schubert A.: Knock behavior of SI-engines: thermodynamic analysis of knock onset locations and knock intensities, SAE Technical Paper no. 2006-01-0225, 2006, [15] Warnatz J., Maas U., Dibble R.W.: Combustion: physical and chemical fundamentals, modeling and simulation, experiments, pollutant formation, 4th ed., Berlin, 2006, Springer, [16] Schmillen K., Schneider M.: Combustion chamber pressure oscillations as a source of noise, Proceedings of the International Symposium on Diagnostics and Modeling of Combustion in Internal Combustion Engines COMODIA, September 4–6, 1985, Tokyo, Japan, [17] Bohacz R.T.: Knock, Knock. The cause of combustion detonation, {Available 15.11.2016: https://www.hemmings.com/magazine/hcc/2008/11/Knock-- Knock/1718386.html},

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Pobrano z http://repo.pw.edu.pl / Downloaded from Repository of Warsaw University of Technology 2021-09-27 [18] Ashok B., Ashok S.D., Kumar C.R.: LPG diesel dual fuel engine – a critical review, Alexandria Engineering Journal, 2015, Vol. 54, pp. 105–126, [19] Shu G., Pan J., Wei H.: Analysis of onset and severity of knock in SI engine based on in-cylinder pressure oscillations, Applied Thermal Engineering 2013, Vol. 51, pp. 1297–1306, [20] Hettinger A., Kulzer A.: A new method to detect knocking zones, SAE Technical Paper no. 2009-01-0698, 2009, [21] Grandin B., Denbratt I.: The effect of knock on heat transfer in SI engines, SAE Technical Paper no. 2002-01-0238, 2002, [22] Ollivier E., Bellettre J., Tazerout M., Roy G.C.: Detection of knock occurrence in a gas SI engine from a heat transfer analysis, Energy Conversion and Management, 2006, Vol. 47, pp. 879–893, [23] Mohamad A.Q.: Exhaust gas temperature for knock detection and control in spark ignition engine, Energy Conversion and Management, 1996, Vol. 37, pp. 1383–1392, [24] Cavina N., Corti E., Minelli G., Moro D., Solieri L.: Knock indexes normalization methodologies, SAE Technical Paper no. 2006-01-2998, 2006, [25] Brecq G., Bellettre J., Tazerout M.: A new indicator for knock detection in gas SI engines, International Journal of Thermal Sciences, 2003, Vol. 42, pp. 523–532.

Abstract The paper aims at providing a concise revision of current knowledge of engine knock phenomenon based on previously published works. Theories behind the origin of knock are discussed. Differences between knocking combustion in spark-ignition and hard work of compression-ignition engines are explained. An overview of direct and indirect methods of knock occurrence detection is given. The most commonly used indices for the evaluation of knock intensity are presented.

Keywords: knock, combustion, knock detection, knock indices

WYKRYWANIE I OCENA SPALANIA STUKOWEGO: PRZEGLĄD LITERATURY

Streszczenie Artykuł ma na celu zwięzłe podsumowanie obecnego stanu wiedzy w zakresie zjawiska spalania stukowego na podstawie dotychczas opublikowanych prac. Omówiono teorie dotyczące powstawania spalania stukowego. Nakreślono różnice między spalaniem stukowym w silnikach o zapłonie iskrowym i twardą pracą silników o zapłonie samoczynnym. Scharakteryzowano bezpośrednie i pośrednie metody wykrywania spalania stukowego, a także przedstawiono najczęściej stosowane wskaźniki do oceny intensywności spalania stukowego.

Słowa kluczowe: spalanie stukowe, spalanie, wykrywanie spalania stukowego, indeksy spalania stukowego

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