Engine knock detonation

This type of explosion is important in engines where detonation can vary from an incipient stage, often referred to as pinking, to a more severe condition known as knocking. ... 

C.D. Miller, Roles of Detonation Waves and Autoignitions in SI Engines, Knock as Shown by Photographs Taken at 40,000 and 200,000 Frames per Second, SAE Quarterly Transactions 1, January (1947). 

The octane number (rating) of a gasoline indicates how smoothly it burns and how much engine knock it produces. (Engine knock is caused by premamre detonation of fuel in the combustion chamber.) 2,2,4-trimethylpentane, isooctane, has excellent combustion properties and was arbitrarily assigned an octane number of 100. Heptane, CH3(CH2)5CH3, has very poor combustion properties and was assigned an octane number of zero. 

In the first case (Fig. 8.14a), the DDT occurs near the closed-end, the process follows the slow compression of the initial mixture, no noticeable motion of the mixture ahead of the flame front is observed. At the average pressure rise, heat perturbations (or hot spots) are anticipated in the compressed gas. These hot spots interact and form the center of a secondary fuel-air explosion/detonation. Such a phenomenon is the characteristic of ICE strong mode, which is known as engine knock . 

Knock can occur over the entire range of engine speed. During acceleration, a large volume of fuel enters the intake manifold and mixes with air. Vaporization and evaporation of the more volatile, low-octane compounds occurs readily. These compounds enter the combustion chamber and detonate quickly to initiate knock. The higher-octane, less-volatile compounds do not vaporize as rapidly from the fuel volume. See FIGURE 5-5 for fuel combustion chamber abnormalities. 

A sharp emission peak produced when the shock wave hits the end of the reaction vessel lasts 1-2 fisec. Work on knock and antiknocks has shown, with amyl nitrite etc used as sensitizer, that knock in the internal combustion engine is identical with ho mo-geneous detonation and that iwhen an antiknock, such as PbEt4, is added to the mixture, the sharp peak of emission is smoothed out to a curve characteristic of slower burning and the suppression of detonation... 

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