Rapid compression machines

Images of spray and combustion in the rapid compression machine obtained for conditions representative of typical HSCI-engine operation. The sequence of four images covers the period immediately after injection—far left, and until the full development of a reacting jet—far right. (From Lu, P.-H., Han, J.-S., Lai, M.-C., Henein, N., and Bryzik, W., Combustion Visualization of DI Diesel Spray Combustion inside a Small-Bore Cylinder under Different EGR and Swirl Ratios, SAE, 2001-01-2005, 2001. With permission.)... 

Bombs, tube apparatus, rapid compression machines, and engines have been used extensively in knock studies. In recent years refinement of the engine as a research instrument has made possible detailed and quantitative investigations of the knock... 

There is no direct experimental evidence for this complex decomposition and it may well occur by several steps [107]. However, substantial yields of unsaturated carbonyl compounds are formed particularly at high pressures [78] under initial reaction conditions where cool flames propagate. For example, the cool-flame oxidation of 2-methylpentane at 525 °C and 19.7 atm in a rapid compression machine [78] yields no less than 14 unsaturated carbonyl compounds viz acrolein, methacrolein, but-l-en-3-one, pent-2-enal, pent-l-en-3-one, pent-l-en-4-one, trans-pent-2-en-4r one, 2-methylbut-l-en-3-one, 2-methylpent-l-en-3-one, 4-methylpent-l-en-3-one, 2-methylpent-l-en-4-one, 2-methylpent-2-en-4-one, 2-methyl-pent-2-enal and 4-methylpent-2-enal. Spectroscopic studies of the preflame reactions [78] have shown that the unsaturated ketones account for ca. 90 % of the absorption which, occurs at 2600 A. At lower initial temperatures and pressures acrolein and crotonaldehyde are formed from n-pentane [69, 70] and n-heptane [82], and acrolein is also formed from isobutane [68]. 

CSTR = Continuously-stirred tank reactor, PER = Pressurized flow reactor, RCM = Rapid compression machine, JSR = Jet-stirred reactor, JSFR Jet-stirred flow reactor. 

Fig. 6,7. Pressure and light output records typical of those observed from the two-stage ignition of hydrocarbons. These results were obtained by the authors during the combustion of n-pentane in a rapid compression machine. The duration of the compression stroke was 22 ms, as shown in the lower pressure record. The first- and second-stage time intervals, measured from the end of compression, are marked as ti and T2 respectively. Compressed gas temperature derived from (6.16) was 740 K [49].

If ignition delays measured in different devices under the same conditions are brought together it becomes clear that there are incompatibilities that cannot be attributed to differences in composition or pressure. In rapid compression machines the discrepancies between different sets of results may arise from different rates of compression [142], which can affect the rate of heat transfer in the early stage of the post-compression interval as a result of the extent of gas motion that is created by the piston motion [50,102]. Ignition delays become longer as the heat loss rates are... 

Fig. 6. 21. Ignition delays as a function of compressed gas temperature for the normal alkanes, n-butane to n-heptane, i-octane (2,2,4-trimethylpentane) and toluene in stoichiometric proportions in air. The results were obtained in a rapid compression machine at a compressed gas density of 128 mol m (0.65-0.75 MPa). Ignition in the ntc and lower temperature range was not observed under these experimental conditions [50].

Fig. 6.26. Concentration - time profiles for intermediate products measured during the two-stage ignition of n-butane in a rapid compression machine. The concentrations are based on the carbon balance. For the purposes of numerical modelling the abscissa is normalized to the duration of the ignition delay. Experimental data are marked as points, numerical results are shown as solid lines. (After Minetti et al. [22]).

The numerical model for n-butane oxidation, by Pitz et al. [228], was used also by Carlier et al. [21] to simulate experimental studies of the two-stage combustion of n-butane at 0.18 MPa on a flat-flame burner and, following this validation, to simulate the ignition delays of n-butane in a rapid compression machine. The numerical studies of the burner experiments were extended by Corre et al. [233]. For simulations of the behaviour on a flat-flame burner the chemical model was computed in an isothermal mode, the experimental one-dimensional temperature profile being introduced as an input parameter. Among the important aims of the tests by Corre et al. [233] was the rationalization of the predicted extent of n-butane consumption throughout the development of the first (cool-flame) and second stages of combustion, with that observed experimentally. The experimental study by Minetti et al. [22, 116] included the detection and measurement of RO2 and HO2 radicals by esr, the one-dimensional spatial profiles of which were simulated by Corre et al. [233],... 

A detailed kinetic model for n-butane combustion has also been reported by Kojima [234] comprising 700 reversible reactions. Although this may prove to be a useful development, it has not been the subject of chemical validation, having been used only to simulate ignition delays for n-butane in a shock tube and in a rapid compression machine. In the absence of complementary chemical tests, the prediction of ignition delay really constitutes an application rather than a test of a comprehensive kinetic scheme. 

Fig. 7.8. The effect of two types of anti-knock on the characteristics of two-stage ignition measured in a rapid compression machine. Ti is the time from the end of compression to the cool-flame is the subsequent time to the true ignition AT is the temperature rise over the cool-flame. Conditions 0.5 stoichiometric mixture of 2-methyl pentane and air at 720 K and total concentration 3.2 x 10 mol cm . TEL (tetra-ethyl lead) affects the second stage, but not ti, NMA (N-methyl aniline) affects both ti and t2. From [42],...

Comprehensive models aim to employ all the relevant reactions, while reduced and simplified models attempt to select the reactions of critical importance. Almost all modelling is based on, and validated by, experiments in model combustion or chemical kinetic systems. Measurements include product distributions in various types of reactor, ignition delaytimes in static reactors, rapid compression machines and shock tubes, and various types of explosion limits. These are discussed in detail in Chapter 6. 

Fig. 7.10. Comparison of ignition delay-times measured in a rapid compression machine (points) with Shell model predictions (lines) [71]. Fuels are all RON 90 with different sensitivities PRF, primary reference fuel, 10% n-heptane, 90% isooctane, MON = 90 TRF toluene reference fuel, 30% heptane, 70% toluene MON = 77.9 2-methyl-2-hexene, MON = 78.9. Compression ratio 9.6, 0.9 stoichiometric mixtures, wall temperatures 373 K. (a) Effect of temperature at end of compression charge density 3.20 x 10" mol cm . (b) Effect of charge density end of compression temperature 690 K. (Note all end of compression temperatures are averages over whole charge.) From [71].

Griffiths [85] and Schrieber et al. [86] have shown how the Muller scheme can be modified to incorporate these features by slightly expanding the low temperature part of the mechanisms and adding a further intermediate. While mass and energy balances were intrinsic in the formulation, an empirical approach was adopted both for the form of the additional reaction and for all the rate constants. The latter, for instance, included pressure dependent terms. Good fits were obtained to rapid compression machine and shock-tube autoignition delay-times for heptane, iso-octane, and their mixtures. 

Fig. 7.17. Influence of NO2 on ignition delay-times in a rapid compression machine [141], Fractional ignition delays are shown for various post-compression temperatures. Note that at the higher temperatures NO2 enhances autoignition at small concentrations while hindering it at greater concentrations. Stoichiometric mixture of 90% iso-octane/10% n-hep-tane, total concentration 3.2 x 10 mol cm . ...

J.E. Elsworth, W.W. Haskell and I.A. Read, Non-Uniform Ignition Processes in Rapid Compression Machines, Comb, and Flame 13 (1969) 437. 

When a reaction mixture is under the same conditions of temperature and pressure as in Figure 2, where an autoignition is likely to develop, there is a certain time lapse, called the autoignition delay, before this becomes effective. This delay is important in the Diesel engine but also in the safety of industrial combustion reactions. The measurement of these delays is carried out mainly using batch reactors, rapid compression machines and shock tubes. 

Figure 6 describes schematically an example of a rapid compression machine. It is composed of the following elements ... 

---