Role of Exothermic Centres on Knock Initiation and Knock Damage

König, Gerhard; Maly, R.; Bradley, D.; Lau, A. K. C.; Sheppard, C. G. W.

doi:10.4271/902136

Cited by 97 publications

(31 citation statements)

References 22 publications

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“…Such gradients of reactivity on the threshold of autoignition are characterized by gradients of t i . At a point, the localized autoignitive propagation velocity, relative to the unburned mixture, u a , is given by [7].…”

Section: Autoignition Velocitiesmentioning

confidence: 99%

“…Observation of the autoignition and propagation of an autoignitive flamefront at hot spots requires good spatial and temporal resolution. Schlieren images of a turbulent flame in an engine and the onset of autoignition in the end gas ahead of it, at several hot spots [7], is shown in figure 2. The present paper examines, in a generalized way, the nature of autoignition at a hot spot and how this depends upon the characteristics of the fuel and the gradient of reactivity in the mixture.…”

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confidence: 99%

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Autoignitions and detonations in engines and ducts

Bradley

2012

Phil. Trans. R. Soc. A.

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The origins of autoignition at hot spots are analysed and the pressure pulses that arise from them are related to knock in gasoline engines and to developing detonations in ducts. In controlled autoignition engines, autoignition is benign with little knock. There are several modes of autoignition and the existence of an operational peninsula, within which detonations can develop at a hot spot, helps to explain the performance of various engines. Earlier studies by Urtiew and Oppenheim of the development of autoignitions and detonations ahead of a deflagration in ducts are interpreted further, using a simple one-dimensional theory of the generation of shock waves ahead of a turbulent flame. The theory is able to indicate entry into the domain of autoignition in an 'explosion in the explosion'. Importantly, it shows the influence of the turbulent burning velocity, and particularly its maximum attainable value, upon autoignition. This value is governed by localized flame extinctions for both turbulent and laminar flames. The theory cannot show any details of the transition to a detonation, but regimes of eventually stable or unstable detonations can be identified on the operational peninsula. Both regimes exhibit transverse waves, triple points and a cellular structure. In the case of unstable detonations, transverse waves are essential to the continuing propagation. For hazard assessment, more needs to be known about the survival, or otherwise, of detonations that emerge from a duct into the same mixture at atmospheric pressure.

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Section: Autoignition Velocitiesmentioning

confidence: 99%

mentioning

confidence: 99%

Autoignitions and detonations in engines and ducts

Bradley

2012

Phil. Trans. R. Soc. A.

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“…The end gas is non-uniform in temperature and composition. Because of this non-uniformity, three modes of postknock combustion have been identified [13] …”

Section: Modes Of Knock Combustionmentioning

confidence: 99%

“…This mode is termed "thermal explosion" in Ref. [13] because there is fast heat release from a sizable region. These waves could be of minor annoyance to the driver, or be quite intense and cause damage to the engine via the repeated pounding on the combustion chamber surfaces by the local high pressure and high temperature.…”

Section: (B) Acoustic Knockmentioning

confidence: 99%

“…The Livengood and Wu integral [10] is used to relate the fuel air mixture ignition delay behavior to the knock on set in an engine. The latter category includes the visualization of the knock phenomenon [11,12], and the numerical simulation of knock in a fuel air mixture [13]. This paper addresses the combustion phenomena after the onset of knock.…”

Section: Introductionmentioning

confidence: 99%

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The Anatomy of Knock

McKenzie¹,

Cheng²

2016

SAE Technical Paper Series

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The combustion process after auto-ignition is investigated. Depending on the non-uniformity of the end gas, auto-ignition could initiate a flame, produce pressure waves that excite the engine structure (acoustic knock), or result in detonation (normal or developing). For the "acoustic knock" mode, a knock intensity (KI) is defined as the pressure oscillation amplitude. The KI values over different cycles under a fixed operating condition are observed to have a log-normal distribution. When the operating condition is changed (over different values of , EGR, and spark timing), the mean () of log (KI/GIMEP) decreases linearly with the correlationbased ignition delay calculated using the knock-point end gas condition of the mean cycle. The standard deviation  of log(KI/GIMEP) is approximately a constant, at 0.63. The values of  and  thus allow a statistical description of knock from the deterministic calculation of the ignition delay using the mean cycle properties

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Investigations on a Spark‐Ignited Engine Using Two‐Dimensional Laser‐Induced Fluorescence

Bäuerle

Hoffmann

Behrendt

et al. 1993

Ber Bunsenges Phys Chem

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The method of laser-induced fluorescence (LIF) was applied to a modified industrial two-stroke engine with a single cylinder and spark ignition. Together, the disc-shaped combustion chamber and the low clearance height of the piston ensure mainly two-dimensional flame propagation. Formaldehyde is used as a natural tracer species for detecting and studying exothermic centres (so-called "hot spots") in the end gas. The latter contain a larger amount of formaldehyde than the enclosing end gas due to the slightly higher temperatures. To obtain two-dimensional excitation of the formaldehyde a laser light sheet enters from sideways through a ring-shaped quartz window. The fluorescence is detected via a full-size top window by means of an ultrashort-exposure ICCD-camera, and the frames are stored in a PC. Here, frames are presented which show 2D-LIF of formaldehyde in non-knocking as well as in knocking cycles. Simultaneous time and space resolved measurements of CO,, O,, CO, N,, H,O mole fractions and temperature are performed by using Raman and Rayleigh scattering. The investigated system is a turbulent jet of CO injected into the coflowing hot exhaust of a lean CH,/N,/O,-flame. Mean values, joint one-point 6D-pdfs and characteristic statistic quantities are determined. The results for mean values agree well with measurements from independent methods. Determined joint one-point pdfs resemble more-dimensional normal distributions. The pdfsare non-symmetric in the reaction zone with positive or negative correlation coefficients for the reactants. This indicates that the low Damk6hler number system under investigation is not diffusion controlled. A quantification for this is given by the calculation of correlation coefficients and turbulence intensities. The measurements are suitable to verify models for transport equations of pdfs in turbulent reacting flows at low DamkOhler numbers,

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Role of Exothermic Centres on Knock Initiation and Knock Damage

Cited by 97 publications

References 22 publications

Autoignitions and detonations in engines and ducts

Autoignitions and detonations in engines and ducts

The Anatomy of Knock

Investigations on a Spark‐Ignited Engine Using Two‐Dimensional Laser‐Induced Fluorescence

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