Autoignitions and detonations in engines and ducts

Abstract

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.

Engine hot spots: Modes of auto-ignition and reaction propagation

“…The necessary localised temperature elevations at hot spots that are compatible with auto-ignition are unlikely to arise solely from the dissipation of turbulent energy.  , E , and hot spot temperature gradients [6]. These interrelationships are expressed by Eq.…”

“…Following the earlier employment of the detonation peninsula [5][6][7][8][9][10], it has been used in a combined experimental engine and LES study to demonstrate how increasing spark advance, and the associated increasingly severe engine knock, cause the corresponding engine cycle loci to cross the threshold into the detonation peninsula [30].…”

“…(11), through the driving hot spot temperature gradient [6,20]. Higher values of E/R make oblique detonations more unstable, creating an irregular cellular structure [21].…”

“…In these, and all other cases considered here for the derived data on Fig. 4, it is assumed that  varying as P -1.7 [6,32]. Values of e  were taken from [5] with the same allowances for  and P. These enabled  ,  , and E to be evaluated [32].…”

“…The boundaries were defined by the dimensionless groups, a/u a , and This approach has been employed in studies of engine knock and super-knock in gasoline engines [5][6][7][8][9][10]. The phenomena leading to super-knock are a rather complex sequence of events comprising the primary formation of hot spots that pre-ignite and initiate premature flame propagation, with earlier increases in pressure and temperature in the unburned mixture.…”