US2241492A - Compression-ignition engine fuel - Google Patents

Description

Patented May 13, 1941 COMPRESSION-IGNITIQN ENGINE FUEL Byron M. Vanderbilt, Roselle Park, N. J., assignor to Standard Oil Development Company, a corporation of Delaware No Drawing. Application April 4, 1939,

. Serial No. 265,938

6 Claims.

now found to be very effective for improving the ignition qualities of Diesel fuels, even though corresponding nitroalcohols and nitroparaflins have been entirely or relatively ineffective.

It has already been shown that in operating high speed compression-ignition engines, commonly termed Diesel engines, the ease in starting and smoothness of running depends largely on the ignition quality of the fuel. While this characteristic of a fuel is fundamentally determined by the ignition lag measured in delay of crank angle between injection and combustion of the fuel, it is appropriately and conveniently determined by a value termed the cetane number, which is the per cent by volume of normal cetane in a mixture consisting of normal cetane and alpha-methyl naphthalene having an ignition lag equivalent to that of the fuel given the cetane number rating.

A number of attempts have been made to find a substance for accelerating ignition of Diesel fuels and these have led to proposals of addition compounds called dopes and primers, generally recommended on the basis that these compounds are thermally unstable, explosive, or have low spontaneous ignition temperatures. For example, peroxides, nitrates of glycerols, nitrates of glycols, and several low molecular weight alkyl nitrates, are among the proposed compounds. Of these, the alkyl nitrates and nitrites have been found somewhat effective in increasing the cetane number of Diesel fuels, but they have to be used in relatively high concentrations for desired improvement. Furthermore, such dopes tend to effect a substantial reduction in the flash point of the fuel, are

costly and hazardous to prepare, and generally do not have satisfactory stability.

To meet the need of a more satisfactory addition compound for improving the ignition quali ties of Diesel fuels, the present invention provides a type of compound more effective than the lower alkyl nitrates which have held precedence hitherto over others for this purpose.

For the purpose of determining ignition promoting and other desired properties of nitroalkyl nitrates, a variety of these compounds were prepared according to the general scheme involving first a condensation of a nitroparaflin with an aldehyde in the presence of a base to form a nitroalcohol, which, in turn, is esterified to the nitroa-lkyl nitrate. Nitroparaffins from the commercially developed vapor phase reaction of lower paraffin hydrocarbons and nitric acid may be used as an initialmaterial for the condensation. nitroalcohols by reaction with formaldehyde goes readily with the formation .of almost quantitative yields and the esterification of the nitroalcohols is also a smoothly and safely performed reaction resulting in close to 90% to 100% of theoretical yields. The following example illustrates the general procedure followed for preparing nitroalcohols and for converting the nitroe alcohol products to their corresponding nitrates:

309 g. (3 moles) of 2'-nitrobutane and 30 cc.

-of 0.92 N sodium hydroxide were placed in a one liter 3-neek flask equipped with stirrer, thermometer, and dropping funnel. 255 g. (3 moles) of approximately 37% of formalin solution was run in over a period of twenty minutes with good mixing. The reaction was slightly exothermic and the temperature was kept at 30 C. to C. by cooling with tap water. After the addition of the formaldehyde was complete, the mixture all in one phase was heated to 40 C. and allowed to stand at this temperature for two hours. The solution was cooled to room temperature and neutralized with hydrochloric acid. It was then placed in a Claisen flask and the methanol and water removed at about 200mm. of mercury absolute. The pressure was then further reduced to 5 mm. of mercury absolute and 2-methyl-2- nitro-l-butanol was distilled. This distillate was aerated at C. to polymerize color bodies, and then redistilled to obtain the nitroalcohol as a colorless liquid boiling at 98? C. at 10 mm. mercury.

In a similar manner, a number of other nitroalcohols were prepared by reacting various primary and secondary nitroparaffins with aliphatic aldehydes.

It is important to note that a primary nitroparaifin molecule having 2 or more carbon atoms may be condensed efficiently with two molecules of formaldehyde or with one molecule of formaldehyde and one molecule of a higher aliphatic The conversion of the nitroparafilns to secondary nitroparaflin molecule can be condensed with only one molecule of an aliphatic aldehyde to form only a monohydric nitroalcohol. The reactions and products may be represented by the following general equations and formulae:

HzC-OH RCHzNOz+lICIIO R(|JNO Primary Primary monoliydric nitronitroalcohol paraflin 1.1 HzC-O H RCNOa+HCHO --v RCN:

it i Secondary Primary monohydric nitroparnitroalco aflin III! /II lit-('3 OII RCHzNO1+IVC=O R(|1'NO;

H Primary Aliphatic Secondary monohydrie nitroaldehyde nitroalcohol parafiin l I /H HC0 H RCNOz+R"C=O -v R-CN0z 1'. R Secondary Secondary monohydric nitroparafiin nitroalcchol H1O OH RCHzNOz+2i1ClIO r R( 1--N0z H; O H Primary Diliydric nitroalcohol nitroparatiin H R C 0 H nonmoi+ncno+rvc =o R-(B-NO:

H; -OH Primary nitro- Dihydric nitroalcohol paraflin In each of these equations, R, R and R" represent alkyl radicals. Formaldehyde is the least expensive aliphatic aldehyde and,the only one which may be used in preparing primary nitroalcohols. Moreover, it is the most reactive and gives the best yields. To obtain good yields of monohydric, nitroalcohols the use of formaldehyde in about a mole to mole proportion with the nitroparafiln was found sufficient. Nitromethane reacts somewhat exceptionally as it condenses with higher aldehydes, as well as with formaldehyde, to form monohydric dihydric, and even trihydric nitroalcohols.

Nitroalkyl mononitrates were prepared from the monohydric nitroalcohols and nitroalkylene dinitrates from the dihydric nitroalcohols. The procedure which may be employed is similar to that used in esterifying aliphatic alcohols. The following example is typical of the method of preparation used and results obtained:

A solution of 35.7 g. (0.3 mole) of 2-methyl-2- nitro-l-propanol in 25 cc. of chloroform was added dropwise above the surface of a mixture of 67.5 g". of 70% nitric acid (150% excess) and 118 g. of 96% sulfuric acid. Agitation was maintained at all times, and external cooling maintained the reaction mixture at to 5 C. Addition of the alcohol was made in about 2,241,492 aldehyde, such as acetaldehyde, and that a minutes and the mixture was agitated an additional fifteen minutes. The reaction mixture was then poured onto crushed ice and was separated into layers. The lower layer washed with water, then with 2% sodium bicarbonate, followed by a further washing with water, was heated under a partial vacuum to distill of! chloroform and any adhering water and the residue in the distillation flask was finally heated to C. at 10 mm. The residue consisted of 42.4 g. of practically colorless liquid nitrate amounting to substantially a theoretical yield.

Although this nitration was carried out at a maximum temperature of -5 C., it was found that higher temperatures could be used for the primary nitroalcohols. Somewhat lower temperatures are required in the nitration of secondary nitroalcohols. Also, the proportions of the reactants may be varied over a wide range. One cc. samples of some of the nitrates were heated with 5 cc. samples of sulfuric acid to 45 C. and showed no decomposition. Even the nitrate of Z-methyl-z-nitro-l-propanol which appeared more unstable formed no insoluble carbonaceous material or violent gas evolution in this test which undoubtedly demonstrates that the nitrates of the nitroalcohols have a stability highly advantageous in their preparation. All of the nitroalkyl nitrates prepared are liquids at room tem perature except that of 2-methyl-2-nitro-1,3- propanediol which is a solid melting at 39" C. All of the nitro alkyl nitrates were found to be insoluble in water but soluble in most organic solvents. They are stable at 0., none showing any discoloration on being heated at this temperature for three to five minutes. All burn spontaneously when dropped on a hot plate at a temperature of 400 to 500 C.

For the sake of clarity and simplicity, classes of nitrates of nitroalcohols which come into consideration are formulated as follows:

1. Nitroalkyl mononitrates:

wherein R is an alkyl radical and R' and R" are selected from the class consisting of hydrogen and alkyl radicals, thepreferred branched nitroalkyl mononitrates being represented by this formula when R" is an alkyl radical and when R is hydrogen.

v2. Nitroalkylene dinitrates:

H-CR

wherein R and R are selected from the class consisting of hydrogen and alkyl radicals, the preferred dinitrates being represented by this formula when the sum of the carbon atoms in R and R is at least 2 so that there is a total of at least 5 carbon atoms per molecule of the compound.

of each was used. A 2% solution of the nitrate of 2-methyl-2-nitro-l-propanol 1n Diesel fuel showed no separation on being cooled to C.

For testing the effectiveness of the nitroalkyl nitrates as ignition promoters, the method described in the S. A. E. Journal of 1936, page 225, was used. This test consists in matching the pose by splitting off nitric acid to form an olefin according to the following reaction:

Thedinitrates of 4 carbon atoms per molecule are definitely inferior to the mononitrates, It is interesting to note that the dinitrate of Z-ethyl- 2-nitro-1,3-propanediol is much more effective than that of 2-methyl-2-nitro-1,3-propanediol,

performance of a test sample with blends conalthough the latter contains more nitrate per sisting of cetane and alpha-methyl naphthalene; unit weight. This is strong evidence in support the per cents by volume of cetane in the blend of the view that the dinitrates and polynitrates giving the same ignition quality test performance in general tend to be too unstable to be very efas the fuel in question was taken as the cetane fective. The dinitrates of higher hydrogen and number of that fuel. The Diesel fuel employed carbon content show possibilities in low concenwas a petroleum gas oil fraction having a cetane trations.

number of 38. Tests were made first on the Further analysis of the foregoing table brings nitroalcohols themselves, and it was found that out some very interesting facts. Although nitroless than 1% by volume of these compounds were g c pou ds ontain ng nitrogen only in the soluble-in th fuel and that their saturated soluform of & nitro group Such as op aflins and ti h w d no higher ta rating tha the nitroalcohois have been found to be entirely or unmodified h d a l, T following relatively ineffective as ignition promoters for table summarizes results obtained when using Diesel fuels, it is clear from the data that the amyl nitrate and various nitroalkyl mononitrates nitro group in combination with a nitrate group and dinitrates for comparison: of the same molecule makes a very effective com- Increase of cetane number when using g gg iii; Percent rz se et ar i s G of .of 1 liter of gz g zg gg nitrate VIK uIIJHyG ignite/r fuel 10 points 10 points Nitrate of Sec-amyl alcohol l0 6. 5 6 10 i0. 5 1.05 1,00

2-methyl-2-nitro-1-butanol. 17 l3 l2 3. 2 15. 7 0. l. 22

2-methyl-2-nitro-l-propanol 20 13 i1 3. 5 17. 1 0. 60 l. 28

2-nitro-ltanol 14 9 9 6.5 15.7 1.02 1.24

2-nitro-lutanol 14 11 11 3.5 17.1 0.60 1. 24 Dinitrate of- 2-ethyl-2-nitro-1, 3-prepanediol. 9 8 7 T10. 0 17. 6 Ti. 76 l. 44

2-rnethy1-21nitro-1,3-propanediol 4 e 18. 7 M. P. 39 C.

tValue extrapolated Value not obtained due to insufficient solubility of the promoter.

In addition to the above data, a mixture of 2 g. pound. Ihis requirement is emphasized by other of dinitrate of 2-methyl-2-nitro-1,3-propanediol data which shows that a nitro-paraffin does not and 1.3 g. of the nitrate of 2-methyl-2-nitro-limprove the ignition promoting effectiveness of propanol per one liter of the fuel raised the cetane an alkyl nitrate, when the two types of comnumber of the fuel by 6 points; when using 1 g. 5 pounds are used together with a Diesel fuel. each of the two nitrates, the cetane number was It is to be noted from the above data that it u d 5 points, takes about one-half as much nitrogen in the The results indicatethat the nitroalkyl monoform of the nitrate of a nitroalkyl mononitrate nitrates are more effective and more suitable for to be as effective as nitrogen in the alkyl nitrate use as ignition promoters than the alykl nitrates although one-half of the nitrogen in the former and even better than the nitroalkylene dinitrates. type of compound is supposedly bound in an in- They further show that the nitroalkylene dinieffective nitro group while all the latter is prestrates tend to approach the nitroalkyl mononient in a nitrate group. Evidently a combination trates in efiectiveness as they increase in carbon of nitro and nitrate'groups in a molecule is betcontent or when they are used in a mixture with ter than either singly for ignition promoting efthe mononitrate. f ectiveness and various other desirable properties.

From among the mononitrates, the branched In accordance with experimental data obtained, chain isomers of 4 and 5 carbon atoms per a nitrate of an aliphatic nitroalcohol may be molecule are more effective than the straight modified to contain other functional groups and chain compounds, especially in the higher constill be properly constituted for use in promoting centrations. This is believed due to the fact that ignition qualities of Diesel fuels, provided the hythe branched chain compounds having the fordrogen and carbon content of the compound is l adequate. For example, a nitro group, a chloro group, or a thio group, may be present as an addi-- l v tional substituent with or in place of a second R C CH*I nitrate group when the compound contains at N01 least five carbon atoms per molecule. This is where R and F. represent hydrocarbon radicals, illustrated very effectively by the dinitrates of are more thermally stable than those of the nitroalcohols. The upper limit for the number straight chain type which might tend to decomof carbon atoms per molecule in these compounds is mainly governed by their availability or feasibilityof their-preparation. At present, the preferred nitroalkyl mononitrates are most conveniently and economically prepared when they contain about three to eight carbon atoms per molecule, but it is possible to prepare suitable promoters of this claw having as many, as 20 or more tend to replace halogen substituents by nitro.

groups; and in some instances, the .nitro group may be introduced by direct nitration.

An inspection of the formula belonging to each nitrate of nitroalcohols having satisfactory ignition promoting properties establishes that a number of these compounds are characterized by containing what may, be called the active grouping, designated as follows:

but even when containing this group, the compound lacks the desired promoter properties unless it contains at least flve carbon atoms per molecule, if other functional substituents are present, such as a second nitrate group. On the other hand the mononitroalkyl mononitrates, are suitable if they contain less than 5 carbon atoms per molecule; although for both technical and economic reasons, the mononitroalkyl mononitrates should contain preferably from about 3 to 8 carbon atoms per molecule.

The novel ignition promoters herein described may be used with hydrocarbon fuels in amounts varying with the amount of improvement desired in the fuel. They may be used in proportions varying from a fraction of 1% to the limit of their solubility in the fuel, but from about 0.25% to about 3% by volume are the preferred concentrations of these compounds in the fuel. Av gas oil fraction boiling in the range of about 400 F. to 700 F. or, in general, having a suitable boiling range for use as a fuel in Diesel-type engines may be used as the hydrocarbon fuel to be improved. A narrowly cut fraction such as one boiling from about 450 F. to 600 F. or from 500 F. to 600 F. may be used. Ordinarily, the hydrocarbon fuel may be said to have a boiling range above that of gasoline and an end point below 750 F.

This invention makes commercially feasible the preparation of Diesel fuels giving satisfactory engine performance from various crude oils, gas oils, and residual fuels having low pour points, low A. P. I. gravities, and high heat values such as those prepared from naphthenlc base or mixed base stocks. The combustion characteristics of even highly paramnic hydrocarbon fractions are also enhanced by these novel ignition promoters. Small quantities of these highly'eflfective ignition promoters may be used for adopting various mixtures of hydrocarbons for use asDlesel fuels including recycle stocks from cracking operations, non-paraflinic extracts, oils from destructive distillations of pyrobitumens orasphaltic substances, or mixtures of these with one another or with crude petroleum fractions. v

Other agents for enhancing various other qualities of the fuels may be simultaneously added without detracting substantially from ignition qualities of the fuel, such as oiliness agents, sludge dispersers, dyes, corrosion inhibitors, antioxidants, viscosity improvers, pour point depressants, and gum solvents. Also, other promoters of ignition qualities may be admixed, for example, alkyl nitrates, alkyl nitrites, and other ni-' tro-type compounds. I

Besides the increased efllciency of automotive operation gained through the use of the herein described novel ignition promoters, many other advantages are obtained through the fact that they are economically and safely prepared and handled. Also, they have the desired physical and chemical properties which makes their use satisfactory at low temperatures as well as high temperatures, especially because they do not tend to precipitate out in cold fuel, and because they do not adversely effect the flash point of the fuel.

It is to be understood that this invention is not to be limited by specific embodiments which are merely given by way of illustration. The appended claims define the novel features of my invention.

Iclaim:

1. An improved Diesel fuel comprising a hydrocarbon fuel and dissolved therein a minor proportion of a nirtoalkyl mononitrate in a sumcient quantity to impart improved ignition qualities to the fuel.

2. A fuel for compression-ignition engines of the Diesel type comprising a hydrocarbon fuel and admixed therewith a small amount of mononitroalkyl mononitrate in sufllcient quantity to improve ignition qualities of the fuel.

3. An improved Diesel fuel comprising a hydrocarbon fuel and admixed therewith a minor proportion of a nitroalkyl mononitrate in sufficient quantity to improve ignition qualities of the fuel, said nitroalkyl mononitrate containing 3 to 8 carbon atoms per molecule and having the formula:

wherein R, R and R" represent hydrogen atoms or alkyl substituents, at least one aikyl substituent being present.

4. An improved Diesel fuel comprising a hydrocarbon fuel and admixed therewith a minorproportion of a nitroalkyl mononitrate in sufllcient'quantity to improve ignition qualities of the fuel, said alkyl mononitrate having the formula: