Knock occurrence

Fig. 7.18. Comparison of measured knock occurrence with that predicted using the Hu and Keck model for various coolant temperatures. 90 primary reference fuel 1900 rpm. (a) clean engine, showing good agreement, (b) engine with deposits knock occurs earlier than predicted, (c) engine with deposits, as (b) but modelled with fitted amounts of hydrogen peroxide (1-5 ppm) in the end gas agreement restored, providing evidence for chemical octane requirement increase. From [76].

In 1985 Leppard [156] reported engine measurements, for stoichiometric ethane-air, of pressure and end gas temperature, the latter derived from the energy equation. The occurrence of autoignition agreed closely with prediction based on an earlier chemical model of Westbrook and Dryer [52]. From their engine experiments, Cowart et al. [59] also compared, for iso-octane and -pentane, the predictions of the simplified models of Hu and Keck [75] and Chun et al. [157], and the more detailed kinetic predictions of Westbrook et al. [158]. These were found to simulate the time of knock occurrence if the kinetic data were re-calibrated. This, and the subsequent work of Brussovansky et al. [76], showed the need for accurate allowances for heat transfer and piston blow-by, because of their important effect on the derived end gas temperature. Where end gas temperature can be measured directly this problem is circumvented. 

K.M. Chun, J.B. Heywood and J.C. Keck, Prediction of Knock Occurrence in a Spark-Ignition Engine, 22nd Symp. (Int.) Comb. (The Combustion Institute, Pittsburgh, 1989) p. 455. 

Domino effect An incident which starts in one piece of equipment and affects other nearby items, such as vessels containing hazardous materials, by thermal blast or fragment impact. This can lead to escalation of consequences or frequency of occurrence. This is also known as a knock-on effect. 

The preflame reactions include slow oxidation and cool flame reactions (110). Slow oxidation threshold temperatures and reaction rates have been considered important factors in controlling knock resistance (43, 75, 133). Knock ratings have been related to cool flame intensities and temperature limits (36, 43, 153). Recently, Barusch and Payne (9) have found striking correlations between octane number and the position of the cool flame in a tube (a parameter which should be a function of Ti). The heat evolved during cool flame reaction may also be a vital factor in determining the occurrence of knock (106,156,179). 

The earliest method of detecting surface ignition was based on the occurrence of abnormal noise. Ricardo mentions that in 1904 preignition was accompanied by a dull thud, but no trace of the ringing knock was found (72). Pressure-time records of combustion also have been used since the early days to detect the occurrence of surface ignition (8). A sudden increase in temperature of the cylinder head also has been taken... 

Compared with petrol and diesel fuels, the combustion of jet fuel takes place, not in a batch reactor, but in an open reactor, thus avoiding the occurrence of knock. Therefore, the problems connected with RON or cetane numbers, such as autoignition, do not impose stringent conditions on the chemical composition of jet fuels. 

---