Primary Reference Fuels

A motor fuel has an octane number X if it behaves under tightly defined experimental conditions the same as a mixture of X volume % of isooctane and (100 - X)% of n-heptane. The isooctane-heptane binary mixtures are called primary reference fuels. Octane numbers higher than 100 can also be defined the reference material is isooctane with small quantities of tetraethyl lead added the way in which this additive acts will be discussed later. 

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. 

The difference between RON and MON for a particular fuel is called the sensitivity. By definition, the RON and MON of the primary reference fuels are the same and the sensitivity is 2ero. For all other fuels, the sensitivity is almost always greater than 2ero. Generally, paraffins have low sensitivities whereas olefins and aromatics have sensitivities ranging up to 10 and higher. 

Huang, Y, Sung, C.J., and Eng, J.A., Laminar flame speeds of primary reference fuels and reformer gas mixtures, Combust. Flame, 139, 239, 2004. 

Isooctane and heptane are utilized as primary reference fuels for octane number determinations. Isooctane has an octane rating of 100, and n-heptane has an octane... 

Either primary or secondary reference fuels can be utilized in the cetane engine when determining the cetane number of distillate fuel. Primary reference fuels are n-cetane and heptamethylnonane. Secondary reference fuels are identified as T Fuel and U Fuel. The characteristics of each of these fuels are outlined in TABLE 4-7. 

Caprio et al. [18] measured heat release rates from i-butane oxidation at 100 kPa in the reactant mixture [i-C4Hio] [O2] [N2] = 1 2 1 (Fig. 6.6). A considerably higher heat release rate than that from propane occurred in this system despite the lower partial pressure of reactant. It seems likely that the differences in residence times in the respective experiments is contributory since variations in the heat release rate from i-butane oxidation were obtained when the mean residence time was changed [18]. However, Lignola et al. [47] showed that, at constant tres, the heat release rate from primary reference fuel mixtures, comprising i-CgHig and n-C7H16, depended on the proportions of the fuel components (Fig. 6.6). 

Although there had been earlier attention [201], towards the end of the 1980s interests became focused more strongly on the study of the behaviour of the primary reference fuels for gasoline (petrol), i.e., n-heptane and i-... 

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].

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].

W. Leppard, The Autoignition Chemistries of Primary Reference Fuels, Olefin/Par-affin Binary Mixtures, and Non-Linear Octane Blending, SAE Technical Paper 922325 (1992). 

H. Li, D.H. Miller and N.P. Cernansky, Development of a Reduced Chemical Kinetic Model for Prediction of Preignition Reactivity and Autoignition of Primary Reference fuels, SAE Technical Paper 960498 (1996). 

S. Kowalski, T.J. Held, Y.S. Stein and F.L. Dryer, Reactivity of an 87 Octane Number Blend of the Primary Reference Fuels with Added Nitric Oxide, Paper No. WSSCI 92-91, Presented at the Fall Meeting of the Western States Section of The Combustion Institute (Berkeley, California, October 1992). 

The behaviour of a real fuel is compared to that of a mixture of Primary Reference Fuels, denoted by the abbreviation PRF. The two PRFs of petrols are n-heptane CH3-CH2-CH2-CH2-CH2-CH2-CH3 of RON = 0 and iso-octane or 2-2-4-trimethylpentene C(CH3)3-CH2-CH(CH3)2 of RON = 100. These mixtures are referred to as PRF... 

The capacity of diesel fuels for autoignition is characterized by the cetane number. Commercial diesel fuels have cetane numbers of between 48 and 55. This number is measured with the aid of a CFR diesel engine by comparison with mixtures of primary reference fuels cetane or n-hexadecane has a cetane number of 100 (by definition), and heptamethylnonane has a cetane number of 15. A binary mixture of these two species containing X % by volume of cetane has therefore a cetane number equal to X + 0.15 (100-X). 

Callahan, C., Held, T., Dryer, R, Minetti, R., Ribaucour, M., Sochet, L., Faravelli, T., Gaffuri, R, Ranzi, E., (1996). Experimental Data and Kinetic Modeling of Primary Reference Fuel Mixtures, Proceedings of the Combustion Institute Vol. 26, Issue 1, pp 739-746... 

Instantaneous maps of the mixture fraction, temperature, and main combustion products (H2O, CO2, CO) are shown in Figure 7.4 for the n-heptane. N-heptane is a fuel commonly used in engines. Its cetane number is approximately 56, which is typical for diesel fuel, because its properties of ignition and combustion are similar to those of diesel fuel [7]. The n-heptane has received substantial interest because it is a major component of the primary reference fuel (PRF) in internal combustion engine studies [6] and is considered a surrogate for liquid hydrocarbon fuels used in many propulsion and power generation systems [8]. 

The DRG method was first applied to a model system of ethylene combustion (Lu and Law 2005 Luo et al. 2011) with a full scheme of 70 species. A value of e of 0.16 gave a skeleton scheme of 33 species, i.e. quite a substantial degree of reduction. In application to n-heptane and iso-octane combustion using full schemes of 561 and 857 species (Lu and Law 2006c), e values of 0.19 and 0.17 resulted in reduced schemes of 188 and 233 species, respectively. DRG methods have since been widely applied for the reduction of large combustion schemes including for methane (Sankaran et al. 2007), primary reference fuel (Lu and Law... 

Luong, M.B., Luo, Z., Lu, T.F., Chung, S.H., Yoo, C.S. Direct numerical simulations of the ignition of lean primary reference fuel/atr mixtures under HCCI condition. Combust. Flame 160, 2038-2047 (2013)... 

Wang, H., Yao, M., Reitz, R.D. Development of a reduced primary reference fuel mechanism for internal combustion engine combustion simulations. Energy Fuels 27, 7843-7853 (2013) Wamatz, J. Resolution of gas phase and surface combustion chemistry into elementary reactions. Proc. Combust. Inst. 24, 553-579 (1992)... 

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