1 DIESEL FUEL CONTAINING LONG-CHAIN GLYCEROL ACETALS The present invention relates to a diesel fuel composition containing long-chain cyclic acetals of glycerol. Energy control and the savings it implies, entail for the global automotive 5 industry an evolution towards economic diesel-type engines and also diesel engines functioning with replacement fuels. However, the inconveniences of these diesel engines are well-known especially in the sphere of pollution which is also another priority of all governments. Therefore, it is necessary to have petrol-saving fuels, which allow engines to function satisfactorily in a wide range 10 of conditions in terms of temperatures, current pollution norms, and their foreseeable evolution, especially in terms of emissions, particularly uncombusted waste. The use of glycerol acetals as diesel fuel additives has already been the subject of various studies. 15 The study of Richard Wessendorf that appeared in "Erdol und Kohle Erdgas - Petrochemie vereinigt mit Brennstoff-Chemie Bd. 48, Heft 3, Marz 1995, pages 138 to 143, makes a strong comprehensive point on the use of various glycerol derivatives as components of a fuel. The fourth chapter of this study is dedicated to cyclic acetals. Its table XI illustrates the performances 20 obtained with glycerol acetals in fuel compositions for diesel engines. One observes in this regard that the various illustrated compositions - GAC 4 to GAC 10 - provide particularly disappointing results in terms of cetane indices. Indeed, an addition of just 10% in weight of these acetals makes the fuel pass well-below permissible norms in terms of cetane index for diesel fuels. 25 European patent application 1 321 502, submitted by the French Petroleum Institute, also relates to diesel fuel compositions containing glycerol acetals and/or glycerol acetal ethers. The examples of this patent illustrate the use of some of these acetals and glycerol acetal ethers in diesel-fuel compositions, and this at 5% content in weight, measuring its incidence on 30 particle emissions. Irrespective of the scientific interest of these results, it is evident that they do not make it possible to contemplate the use of these molecules in a 2 substantial quantity in commercial fuels due to their incidence on the cetane index of the fuels. Indeed, in the article by R. Wessendorf, table Xl shows that the highest cetane index of the acetal in mixture (with the base diesel fuel) is no more than 25 for the glycerol acetal plus butyraldehyde (GAC 6), and for the 5 glycerol acetal plus 2-ethylbutyraldehyde (GAC 10). The other described acetals have cetane indices of less than 6. The cetane index data for the acetals described by the French Petroleum Institute in their European patent application are not mentioned. Considering the similarities of the structures these do not differ from those appearing in the publication by R. Wessendorf. 10 The applicant has discovered unexpectedly in respect of the prior art that it is possible to propose glycerol acetals presenting a high cetane index in mixture, and therefore able to enter diesel fuel compositions in substantial proportions making it possible to obtain benefits in environmental terms, related to the natural origin of the added components, and energetic performances 15 compatible with the final application. The present invention relates to a diesel fuel composition containing long-chain glycerol acetals. The invention targets a diesel fuel composition comprising a diesel fuel base of either petroleum origin, or vegetable origin and 1 to 60% in weight of 20 glycerol acetals, characterised in that the glycerol acetal corresponds to the following formula OHO He HO O 2 and/or 0 in which R 1 is an alkyl radical comprising 5 to 13 carbon atoms at least 5 of which form a linear chain, R 2 is either H, or an alkyl radical containing from 1 25 to 7 carbon atoms, wherein the total number of carbon atoms of R 1 + R 2 is less than 20, and preferably less than 12. The preferred compositions of the invention are those in which R 1 is an 3 n-alkyl radical of 6 carbon atoms or an iso-alkyl radical of 7 carbon atoms, R 2 being H. These two acetals come from in the first case the reaction of glycerol on n-heptanal (also called heptanaldehyde), and in the second, glycerol on 2 ethylhexanal. These two aldehydes are preferred due to their availability on the 5 market and the vegetable origin of the first. The glycerol acetals of the invention are prepared by the reaction of an aldehyde or of a cetone on the glycerol, for example following the procedures described in the article of F. A. J. Meskens entitled "Methods for the Preparation of Acetals from Alcohols or Oxiranes and Carbonyl Compounds" pages 501 to 10 522 of the "Synthesis" magazine of July 1981 (Georg Thieme Verlag - Stuttgart - New York). Where the synthesis of long chain cyclic acetals is concerned, one may cite the article of G. Stefanovic and G. Petrovic, Bulletin Liv. de l'Acad6mie Serbe des Sciences et des Arts, 54(14) (1976) 53-73, which describes the 15 synthesis of several long chain glycerol acetals including heptylidine glycerol (reaction with heptanaldehyde), octylidine glycerol (reaction with octanaldehyde), decylidine glycerol (reaction with decanaldehyde), and dodecylidine glycerol (reaction with dodecanaldehyde). The glycerol acetals of the invention can be prepared following various 20 other methods: by reactive extraction between the aldehyde and a glycerol solution, the produced acetal thus being less soluble in the aqueous phase can be easily extracted and separated; by reaction between the glycerol and the aldehyde in a simulated mobile bed in order to strongly displace the balances of the acetalisation reactions. 25 In view of their use in diesel fuels, the acetals of the invention must present physical properties compatible with the norms such as for example French norm NF EN 590. The main properties are the volumetric mass, the boiling temperature, flash point, solubility in water, and cetane index. If one considers the (2-hexyl-1,3-dioxolane-4-yle) methanol and the 2 30 hexyl-1,3-dioxane-5-ol, which are the two isomers of the acetalisation reaction of heptanaldehyde on the glycerol, one can observe that they have a boiling temperature in the region of 270-290 0 C, and a weak solubility in water suitable 4 for the application. Additionally, they have a flash point of about 130*C and a cetane index in mixture that is particularly high, in the region of 50, near to that required for the diesel fuel base itself. The long chain glycerol acetals are particularly interesting because they 5 have a high cetane index as well as a considerable oxygen content (14 to 25% in weight) which favours the reduction of particle emissions leading to a reduction in NOx. Also, they can be produced from renewable raw materials, both in terms of glycerol which is a co-product of the biodiesel obtained through transesterification of triglycerides, as well as heptanaldehyde which can be 10 obtained through thermal cracking on the basis of natural sterified castor oils. Cetane boosters can be added to the typical gasoil doses, such as used in the present invention, preferably at contents of at most 1% in weight. Cetane boosters may be selected from nitro compositions and dialkylperoxydes. The diesel fuel compositions of the invention are illustrated by the 15 following examples. Example 1 Preparation of a mixture between a gasoil and the acetal. The cyclic acetal is prepared by reaction of the glycerol on the heptanaldehyde. An aqueous solution of glycerol (at 60% in weight of glycerol, 40% in weight in water) containing 10 moles of glycerol (i.e. 920 g) is mixed with 20 10 moles of heptanaldehyde (i.e. 114 g), in other words a heptanaldehyde/glycerol molar ratio of 1. A solution of hydrochloric acid at 35% is added to the aqueous solution of glycerol + heptanaldehyde, in a proportion of 15% in relation to the glycerol. The mixture is then heated and kept at 40*C for 5 hours under agitation. 25 Then, the two phases are separated by decantation, and the organic phase is washed with water, until obtaining a neutral pH. The solution obtained is then dried on MgSO4 anhydride then concentrated by vacuum evaporation. The obtained product has been analysed by RMN and mass spectrometry and quantified through chromatographic analysis. 30 Then the obtained acetal is mixed with a diesel fuel base representative of Euro 2000 formulations: density in the region of 0.830 at 15*C, sulphur content of about 290 ppm, cetane index of 51, distillation interval 170-360 0 C. The 5 acetal/fuel base ratio is 10% in weight. Then the cetane index of the mixture is measured following norm ISO 5165. This is slightly above 51 allowing us to deduce that the cetane index of the acetal in the mixture is in the region of 52. Example 2 Comparative tests of the mixture with the diesel fuel base. 5 Tests were carried out aimed at evaluating the performances in terms of particle emissions of diesel fuel compositions containing 10% in weight of the glycerol acetal and heptanaldehyde in comparison with those obtained with the reference diesel fuel. The tests were carried out on a diesel vehicle equipped with a direct 10 injection engine. These tests were carried out on the cycle described in European directive 70/220/EC, amended by directive 98/69/CE (cycle called MVEG-1 Is EURO 2000). This cycle consists of an urban phase (EUDC cycle with a length of 4,052 km), and an extra-urban phase (ECE cycle with a length of 6,995 km). The results of the tests, expressed in milligrams of particles per 15 kilometre are as follows: Diesel fuel only: ECE Cycle 65 mg/km, EUDC Cycle 52.5 mg/km, MVEG Cycle 57 mg/km. . Diesel fuel at 90% + 10 % acetal of glycerol and heptanaldehyde: ECE Cycle 47 mg/km, EUDC Cycle 41 mg/km, MVEG Cycle 43 mg/km. 20 The reduction in particle emissions with the fuel according to the invention is in the region of 25%.