Knock intensity

Adapted in each case to obtain maximum knocking intensity, it is usually between 1.05 and 1.10. 

The octane value of an unknown fuel sample is determined by comparing its knocking tendency to various primary reference fuels. Its measured octane is equal to the octane of the PRE which has the same knocking intensity. Knock intensity is controlled to an average value by varying the compression ratio of the CER engine. In practice, the exact value of a fuel s octane number is determined to the nearest 0.1 octane number by interpolation from two PREs that are no more than two octane numbers apart. 

Octane. An octane number is a quantitative measure of a fuel mixture s resistance to knocking. The octane number of a particular sample is measured against a standard blend of n-heptane, which has zero octane, and iso-octane, which has 100 octane. The percent of isooctane that produces the same knock intensity as the sample is reported as the octane number. 

A knock which is recurrent and repeatable in terms of audibility. It is controllable by the spark advance advancing the spark increases the knock intensity and retarding the spark reduces the intensity. This definition does not include surface-ignition induced knock. 

By subtracting the heat release rate in a non-knocking cycle from that in a knocking cycle it is possible to estimate the heat released by autoignition as a function of crank angle [106]. The variations in heat release rates for three different knock intensities are shown in Fig. 7.13. The onset of knock is indicated by an asterisk and occurs at the maximum heat release rate, which is appreciable at the highest knock intensity. The unburned mass fraction is shown by the axis at the left and it shows that... 

Fig. 7.13. Net heat release rate, Q, and unburned mass fractions at different crank angles for knock intensities of (1) 1490, (2) 111 and (3) 6.5 kPa. Asterisks denote onset of knock. Fuel PRF, stoichiometric mixture. From [106].

Pressure-time records, for a knocking and non-knocking cycle are shown in Fig. 7.14, together with the digitally filtered pressure signal for the former, filtered between 5 and 25 kHz [92]. The minimum pressure for the autoignition to lead to knock, or knock threshold pressure, also was measured. Knock was defined, arbitrarily, as a knock intensity greater than 0.2 bar, and the knock onset pressure as the pressure at which the filtered pressure first reached 0.1 bar. 

Bradley et al. [80] report elevations of CARS temperatures, with a 90% iso-octane/10% heptane fuel prior to autoignition, of about 100 K above such values, with mean values of about 900 K in non-knocking cycles. The greater the end gas pressure, the greater was the CARS temperature and the knock intensity. The temperature elevations were in line with the computed predictions of the simplified five-reaction parrot scheme, de-... 

So far, explanations have neglected any compression of the unburned gas. Any such compression reduces the effective values of t,. Curve (c) shows values of t,, reduced below those of curve (d) by compression, to increase dxIdTj. A feedback mechanism exists by which an increase in the rate of propagation of the autoignition front further compresses the unburned gas. This reduces the effective values of t, and increases the rate of propagation still further. As a result, with a sufficiently reactive mixture, dxIdTj in the deflagration mode can increase towards the sonic velocity, with an associated increase in knock intensity [114]. 

C.V. Ferraro, M.R. Marzano, G. Millo and N. Bochicchio, Comparison Between Heat-Transfer and Knock-Intensity on a Statistical Basis, SAE Technical Paper 962101 (1996). 

The octane number of a gasoline is determined from a calibration curve relating knock intensity to the % iso-octane in a mixture of iso-octane and heptane. One way to calibrate the octane number is to place a transducer on the side of the cylinder to measure the knock intensity (K.l.) (pressure pulse) for various mixtures of heptane and iso-octane. The octane number is the percentage of iso-octane in this mixture. That is. pure iso-octane has an octane number of 100. 80% iso-octane/20% heptane has an octane number of 80, and so on. The knock intensity is measured for this... 

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