Fuel compositions

Abstract

Coking in and around the injector nozzles of indirect injection compression ignition engines is reduced by means of distillate fuel with which has been blended suitable concentrations of (i) organic nitrate ignition accelerator, and (ii) a boronated phenolic compound.Also described are additive mixtures of (i) and (ii) for use in distillate fuels in amounts sufficient to reduce the coking tendencies of such fuels when used in the operation of indirect injection compression ignition engines.

Description

FIELD

This invention relates to compression ignition fuel compositions and additive mixtures of organic nitrate ignition accelerator and boronated phenolics in amounts sufficient to resist the coking tendencies of compression ignition fuel compositions when used in the operation of indirect injection diesel engines.

BACKGROUND

Throttling diesel nozzles have recently come into widespread use in indirect injection automotive and light-duty diesel truck engines, i.e., compression ignition engines in which the fuel is injected into and ignited in a prechamber or swirl chamber. In this way, the flame front proceeds from the prechamber into the larger compression chamber where the combustion is completed . Engines designed in this manner allow for quieter and smoother operation. The FIGURE of the Drawing illustrates the geometry of the typical throttling diesel nozzle (often referred to as the "pintle nozzle").

Unfortunately, the advent of such engines has given rise to a new problem, that of excessive coking on the critical surfaces of the injectors that inject fuel into the prechamber or swirl chamber of the engine. In particular and with reference to the FIGURE, the carbon tends to fill in all of the available corners and surfaces of the obturator 10 and the form 12 until a smooth profile is achieved. The carbon also tends to block the drilled orifice 14 in the injector body 16 and fill up to the seat 18. In severe cases, carbon builds up on the form 12 and the obturator 10 to such an extent that it interferes with the spray pattern of the fuel issuing from around the perimeter of orifice 14. Such carbon build-up or coking often results in such undesirable consequences as delayed fuel ignition, decreased rate of fuel injection, increased rate of combustion chamber pressure rise, increased engine noise, and can also result in an excessive increase in emission from the engine of unburned hydrocarbons.

While the composition of the low cetane number fuel is believed to be a major contributing factor to the coking problem, it is not the only relevant factor. Thermal and oxidative stability (lacquering tendencies), fuel aromaticity, and such fuel characteristics as viscosity, surface tension and relative density have also been indicated to play a role in the coking problem.

Thus, an important contribution to the art would be a fuel composition which has enhanced resistance to coking tendencies when employed in the operation of indirect injection diesel engines.

THE INVENTION

We have now discovered that the coking problem can be ameliorated by the addition to the fuel of an organic nitrate and a boronated phenolic compound. The boronated phenolics contemplated for use in the invention are diverse and can be any boronated phenolic compound which, when added to distillate fuel in combination with an organic nitrate ignition accelerator, reduces, minimizes or inhibits coking in the prechamber or swirl chamber of indirect injection compression ignition engines operated on such a fuel.

Thus, broadly stated, the present invention is directed to distillate fuel for indirect injection compression ignition engines containing, in an amount sufficient to minimize coking, especially throttling nozzle coking, in the prechambers or swirl chambers of indirect injection compression ignition engines operated on such fuel, at least the combination of (i) organic nitrate ignition accelerator and (ii) a boronated phenolic compound which, when added to said fuel in combination with said organic nitrate ignition accelerator minimizes said coking.

Since the invention also embodies the operation of an indirect injection compression ignition engine in a manner which results in reduced coking, a still further embodiment of the present invention is a method of inhibiting coking, especially throttling nozzle coking, in the prechambers or swirl chambers of an indirect injection compression ignition engine, which method comprises supplying said engine with a distillate fuel containing at least the combination of (i) organic nitrate ignition accelerator and (ii) a boronated phenolic compound capable of inhibiting said coking when added to said fuel in combination with said organic nitrate ignition accelerator, said combination being present in an amount sufficient to inhibit such coking in an indirect injection compression ignition engine operated on such fuel.

A feature of this invention is that the combination of additives utilized in its practice is capable of suppressing coking tendencies of fuels used to operate indirect injection compression ignition engines.

A wide variety of organic nitrate ignition accelerators may be employed in the fuels of this invention. Preferred nitrate esters are the aliphatic or cycloaliphatic nitrates in which the aliphatic or cycloaliphatic group is saturated, contains up to about 12 carbons and, optionally, may be substituted with one or more oxygen atoms.

Typical organic nitrates that may be used are methyl nitrate, ethyl nitrate, propyl nitrate, isopropyl nitrate, allyl nitrate, butyl nitrate, isobutyl nitrate, sec-butyl nitrate, tert-butyl nitrate, amyl nitrate, isoamyl nitrate, 2-amyl nitrate, 3-amyl nitrate, hexyl nitrate, heptyl nitrate, 2-heptyl nitrate, octyl nitrate, isooctyl nitrate, 2-ethylhexyl nitrate, nonyl nitrate, decyl nitrate, undecyl nitrate, dodecyl nitrate, cyclopentyl nitrate, cyclohexyl nitrate, methylcyclohexyl nitrate, cyclododecyl nitrate, 2-ethoxyethyl nitrate, 2-(2-ethoxy-ethoxy)ethyl nitrate, tetrahydrofufuryl nitrate, and the like. Mixtures of such materials may also be used. The preferred ignition accelerator for use in the fuels of this invention is a mixture of octyl nitrates available as an article of commerce from Ethyl Corporation under the designation DII-3 Ignition Improver.

As previously mentioned, the boronated phenolics of the invention are diverse. They include any boronated phenolic compound or mixture of boronated phenolic compounds which, when combined with an organic nitrate ignition accelerator or mixtures of organic nitrate ignition accelerators, in a distillate fuel, minimizes and/or reduces coking in the prechambers or swirl chambers of indirect injection compression ignition engines operated on such fuel. They include, but are not limited to, products obtained by reaction between a hindered phenolic compound and a completely esterified symmetrical oxy acid of boron whose esterifying radicals are derived from a monohydric alcohol and having discrete monovalent hydrocarbon ester groups. The dinuclear phenolics used to obtain these reaction products include 1,1-bis-(3,5-di-tert-butyl-4-hydroxyphenyl)methane; 1,1-bis-(2-hydroxy-3-tert-butyl-5-methylphenyl)methane; 1,1-bis-(3,5-di-tert-butyl-4-hydroxyphenyl)phenylmethane; 3,3'-di-(1-ethylallyl)4,4'-di-hydroxydiphenyl; 1,10-di-(3-sec-amyl-2-hydroxyphenyl)octadecane; 2,2'-bis-[3-(1-phenyl-1-ethyl)-4-hydroxyphenyl]propane; 4,4'-isopropylidene-di-(2-isopropylphenyl); 3,5,5'-tridodecyl-3'-ethyl-4,4'-di-hydroxydiphenyl; 3,3',5,5'-tetra-tert-butyl-4,4'-di-hydroxyphenyl; 2,2-bis-(2-hydroxy-3-tert-butyl-5 -methoxyphenyl)propane; 1,1-bis-(4-hydroxy-3,5-di-tert-butylphenyl)methane; 3,3'-di-cyclohexyl-4,4'-di-hydroxydiphenyl; 2,15-di-(4-isopropyl-3-hydroxyphenyl)hexadecane; 3-isopropyl-4,4'-hydroxydiphenyl; 4,4'-benzylidene-di-(2-isopropylphenyl); and numerous similar others. The boron esters used to obtain these products include isopropyl metaborate, hexyl metaborate trimer, 2-ethoxyethyl metaborate trimer, tri-sec-butyl orthoborate, tri-n-hexyl orthoborate, diethylphenyl boronate, n-butyl-di-p-tolylborinate, tetra-n-hexyl pyroborate, methyl metaborate trimer, n-butyl polyborate, isopropyl metaborate trimer, n-butyl metaborate, methyl di-n-butyl metaborate, methyl di-n-butylborinate, dibutyl dodecyl orthoborate, and many others.

The boronated phenolics of the invention also include the boron esters derived from boron acids selected from the group pyroboric, boronic, and borinic acids wherein an esterifying group is an alkylphenol. These include tri-(2-6-di-tert-butylphenyl)orthoborate; tri-(2,6-di-tert-amylphenyl)orthoborate; di-(2,6-di-tert-amylphenyl)monoether orthoborate; mono-(2,6-di-tert-butylphenyl)dibutyl orthoborate; mono-(2,6-tert-butylphenyl)ethylene glycol orthoborate; tri-(2,6-di-tert-butylphenyl)trimeric metaborate; tri-(2,6-di-tert-heptylphenyl)trimeric metaborate; tetra-(2,6-di-tert-decylphenyl)pyroborate; tetra-(2,6-di-tert-heptylphenyl)pyroborate; di-(2,6-di-tert-butylphenyl)propylboronate; (2,6-di-tert-octylphenyl)di-phenylborinate; as well as 2,6-di-tert-alkylphenyl glycol orthoborates; 4,4'-methylenebis(2,6-di-tert-alkylphenyl)dialkyl orthoborate; 4,4'-methylenebis(2,6-di-tert-alkylphenyl)glycol orthoborates; 2,6-di-tert-alkylphenyl dialkyl orthoborates; 2,6-di-tert-alkylphenyl metaborates; 2,6-di-tert-alkylphenyl metaborates; di-(2,6-di-tert-alkylphenyl)alkyl orthoborates; and the like. These compounds may be prepared from (1) phenolics including 2,6-di-tert-butylphenol; methylenebis(2,6-di-tert-butylphenol); 2-tert-butyl-6-tert-amylphenol; 2,6-diamylphenol; 1,1-bis-(3,5-di-tert-butyl-4-hydroxyphenyl)methane; and the like; and (2) boron compounds including tri-n-butyl orthoborate; orthoboric acid; bis-(1,1,3-trimethyltrimethylene)pyroborate; isopropyl-(2-methyl-2,4-pentylene)orthoborate; metaboric acid; isopropyl metaborate; butyl borinic acid; dibutyl monochloroborinate; tetraalkylpyroborates; isopropylmetaborate trimer, and the like.

The boronated products of the invention include borate esters of wax alkylated phenol, wax alkylated naphthol, wax alkylated cresol, cetylphenol, octadecylphenol, di-tert-octylphenol, isohexadecylphenol, and C.sub.16-20 branched chain alkylphenols. These esters are usually obtained by reaction with boric acid.

Also included in the invention are the 2,6-dialkyl alkylphenyl dialkyl borates such as 2,6-di-tert-butylphenyl-di-n-butyl orthoborate.

The most highly preferred boronated phenolic component or additive of the present invention is a mixture of about 0-50 weight percent 4,4'-methylenebis(2,6-di-tert-butylphenol), about 25-75 weight percent 4,4'-methylenebis(2,6-di-tert-butylphenol)-mono-(di-sec-butyl orthoborate), and about 10-75 weight percent 4,4'-methylenebis(2,6-di-tert-butylphenol)-di-(di-sec-butyl orthoborate). The most highly preferred boronated phenolic component of the invention may be prepared by a process comprising reacting at elevated temperature (a) one mole part of 4,4'-methylenebis(2,6-di-tert-butylphenol) and (b) 0.5-5.0 mole parts of a tri-sec-alkyl orthoborate wherein the sec-alkyl group contains 4-12 carbon atoms.

The mole ratio of tri-sec-alkyl orthoborate to 4,4'-methylenebis(2,6-di-tert-butylphenol) can vary over a wide range. A useful range is about 0.5-6 moles of tri-sec-alkyl orthoborate per mole of 4,4'-methylenebis(2,6-di-tert-butylphenol). A more preferred range is about 1-3 to 1.

The additives are made by forming a reaction mixture of the tri-sec-alkyl orthoborate and 4,4'-methylenebis(2,6-di-tert-butylphenol) and heating this mixture while distilling out displaced sec-alkanol.

The reaction should be conducted at a temperature high enough to cause the phenol hydroxyl to displace a sec-alkyl group from the orthoborate ester and to cause the displaced sec-alcohol to distill out but not so high as to cause decomposition of the reactants or products. A useful temperature range in which to experiment is from about 150.degree.-300.degree. C. When using tri-sec-butyl orthoborate, the preferred temperature range is about 200.degree.-275.degree. C.

The transesterification reaction can be catalyzed by a small amount of an acidic material. This includes sulfuric acid, phosphoric acid, methane sulfonic acid, p-toluene sulfonic acid, and the like. The preferred catalysts are the lower fatty acids such as formic acid, acetic acid, propionic acid, butyric acid, and the like. The most preferred catalyst is acetic acid.

Only a small catalytic amount of catalyst is required. A useful catalyst range is about 0.02-0.2 weight percent based upon total reaction mass.

The principal components in the reaction mixture are 4,4'-methylenebis(2,6-di-tert-butylphenol) mono-(di-sec-C.sub.4-12 alkyl orthoborate) ester and 4,4'-methylenebis(2,6-di-tert-butylphenol)-di-(di-sec-C.sub.4-12 alkyl orthoborate) ester together with varied amounts of unreacted 4,4'-methylenebis(2,6-di-tert-butylphenol). Analysis of products made from tri-sec-butyl orthoborate show them to contain about 0-50 weight percent 4,4'-methylenebis(2,6-di-tert-butylphenol), 25-75 weight percent 4,4'methylenebis(2,6-di-tert-butylphenol)-mono-(di-sec-butyl orthoborate) ester and 10-75 weight percent 4,4'-methylenebis(2,6-di-tert-butylphenol)-di-(di-sec-butyl orthoborate) ester.

Thus, in a more preferred embodiment of the present invention there is provided distillate fuel for indirect injection compression ignition engines containing, in an amount sufficient to minimize coking, especially throttling nozzle coking, in the prechambers or swirl chambers of indirect injection compression ignition engines operated on such fuel, at least the combination of (i) organic nitrate ignition accelerator, and (ii) a boronated phenolic mixture of about 0-50 weight percent 4,4'-methylenebis(2,6-di-tert-butylphenol), about 10-75 weight percent 4,4'-methylenebis(2,6-di-tert-butylphenol)-di-(di-sec-butyl orthoborate) ester and about 25-75 weight percent 4,4'-methylenebis-(2,6-di-tert-butylphenol)mono-(di-sec-butyl orthoborate) ester.

In a still further embodiment of the present invention there is provided distillate fuel for indirect injection compression ignition engines containing, in an amount sufficient to minimize coking, especially throttling nozzle coking, in the prechambers or swirl chambers of indirect injection compression ignition engines operated on such fuel, at least the combination of (i) organic nitrate ignition accelerator, and (ii) a boronated phenolic reaction product made by reacting at elevated temperature:

(a) one mole part 4,4'-methylenebis(2,6-di-tert-butylphenol), and

(b) 0.5 to 5.0 mole parts of a tri-sec-alkyl orthoborate wherein said sec-alkyl group contains 4-12 carbon atoms.

The boronated phenolic components of the invention should be used at a concentration of at least about 20 PTB (pounds per thousand barrels) to insure that the finished blend contains an adequate quantity of the foregoing ingredient although smaller amounts may be successfully employed.

The nitrate ignition accelerator, component (i), should be present in an amount of at least 100 to 1000 PTB (pounds per thousand barrels) of the base fuel. Preferably, the concentration of the ignition accelerator is about 400 to 600 PTB.

It is not believed that there is anything critical as regards the maximum amount of components (i) and (ii) used in the fuel. Thus, the maximum amount of these components will probably be governed in any given situation by matters of choice and economics.

The coking-inhibiting components (i) and (ii) of the invention can be added to the fuels by any means known in the art for incorporating small quantities of additives into distillate fuels. Components (i) and (ii) can be added separately or they can be combined and added together. It is convenient to utilize additive fluid mixtures which consist of organic nitrate ignition accelerator and the boronated phenolic components of the invention. These additive fluid mixtures are added to distillate fuels. In other words, part of the present invention are coking inhibiting fluids which comprise organic nitrate ignition accelerator and boronated phenolic compounds.

Use of such fluids in addition to resulting in great convenience in storage, handling, transportation, blending with fuels, and so forth, also are potent concentrates which serve the function of inhibiting or minimizing the coking characteristics of compression ignition distillate fuels used to operate indirect compression ignition engines.

In these fluid compositions, the amount of components (i) and (ii) can vary widely. In general, the fluid compositions contain about 5 to 95% by weight of the organic nitrate ignition accelerator component and 5 to 95% by weight of the boronated phenolic component. Typically, from about 0.01% by weight up to about 1.0% by weight of the combination will be sufficient to provide good coking-inhibiting properties to the distillate fuel. A preferred distillate fuel composition contains from about 0.1 to about 0.5% by weight of the combination containing from about 25% to about 95% by weight of the organic nitrate ignition accelerator and from about 75% to about 5% by weight of the boronated phenolic component.

The additive fluids, as well as the distillate fuel compositions of the present invention may also contain other additives such as corrosion inhibitors, antioxidants, metal deactivators, detergents, cold flow improvers, inert solvents or diluents, and the like.

Accordingly, a further embodiment of the invention is a distillate fuel additive fluid composition comprising in proportions sufficient to minimize the coking characteristics of such fuel, especially throttling nozzle coking in the prechambers or swirl chambers of indirect injection compression ignition engines operated on such fuel, (i) organic nitrate ignition accelerator, and (ii) a boronated phenolic compound, which when added to said fuel in combination with said organic nitrate ignition accelerator minimizes said coking.

In a still further embodiment of this present invention there is provided a distillate fuel additive fluid composition comprising, in proportions sufficient to minimize the coking characteristics of such fuel, especially throttling nozzle coking in the prechambers or swirl chambers of indirect injection compression ignition engines operated or such fuel (i) organic nitrate ignition accelerator and (ii) a boronated phenolic reaction product made by reacting at elevated temperature:

(a) one mole part 4,4'-methylenebis(2,6-di-tert-butylphenol) and

(b) 0.5 to 5.0 mole parts of a tri-sec-alkyl orthoborate wherein said sec-alkyl group contains 4-12 carbon atoms.

EXAMPLE I

In order to determine the effect of the fuel compositions of the present invention on the coking tendencies of diesel injectors in indirect injection compression ignition engines, use was made of a diesel fuel injector test apparatus developed for the purpose of screening chemical agents for use as anticoking, antideposit and antivarnish agents. The design of the apparatus allows it to accommodate any type of conventional automotive diesel fuel injector used in diesel engines such as the Bosch injectors used in turbocharged XD2S engines and the Lucus penciltype or mini-fuel injectors used in 6.2 liter or 350 cu. in. diesel engines. The apparatus comprises a diesel fuel injector nozzle assembly attached to and extending into an aluminum cylinder 2.5 inches in width and 5.0 inches in diameter. Attached to and extending into the opposite side of the aluminum block is a 1-inch pipe assembly consisting of a connector nipple and tee which acts as a combustion chamber into which diesel fuel is injected oy the injector assembly. The chamber is coupled to a flash arrestor and exhaust-gas assembly. Also coupled to the combustion chamber is a serpentine-gas/air heater, 0.5 inches in diameter and 6.5 inches in length. The heater controls the temperature of the air entering the combustion chamber. If desired, air temperatures up to 750.degree. C. can be produced. Under normal testing conditions, air temperature is maintained at a range between about 470.degree. C. and 525.degree. C.

Air flow rate, which is critical to the operation and replication of the test, is maintained by a mass flow controller to within 0.1 liter per minute at flow volumes of 20 to 50 liters per minute. A standard single cylinder diesel engine Bosch fuel pump is used to develop pressure and fuel volume passing into the injector. A 1-horsepower motor directly connected to the fuel pump is operated at 1750 RPM providing approximately 875 injections of fuel per minute. The fuel pump can be adjusted to provide fuel flow rates ranging from 35 milliliters to 3000 milliliters per hour. Standard operating fuel flow rates used for testing generally range between about 80 and 120 milliliters per hour. Under the standard operating conditions of air flow and fuel flow, incipient combustion of injected fuel occurs. Tests are carried out using 1-quart samples of fuel, with or without additives. The length of each test is four hours. After the test operation, the injectors are carefully removed from the apparatus so as not to disturb the deposits formed thereon.

After the test, the amount of deposit, coke or varnish on various areas of the injector external or internal parts are rated. Visual differences in amounts of deposits between a nonadditive test and one with an additive are used to distinguish and establish the effect of the chemical agent being tested as an anticoking additive. The areas of the injector parts which are rated for deposits include (i) the external area of the nozzle face, (ii) an area around the injector orifice extending one millimeter in diameter from the center of the orifice, (iii) the rim of the nozzle orifice, (iv) the exterior pintle tip, (v) the pintle obturator, and (vi) the nozzle face.

To demonstrate the anticoking effects of the present additives, a base fuel was prepared consisting of a commercially available diesel fuel having a nominal cetane rating of 37. FIA analysis indicated that the fuel was composed by volume of 41% aromatics, 2.0% olefins and 57% saturates. The base fuel also contained 140 pounds per thousand barrels (PTB) of mixed octyl nitrates (a commercial product available from Ethyl Corporation under the designation DII-3 Ignition Improver).

A test blend was prepared from this base fuel and was designated Fuel A. Fuel A contained, in addition to 140 PTB of mixed octyl nitrates, 50 PTB of a boronated Phenolic mixture containing 4,4'-methylenebis(2,6-di-tert-butylphenol), 4,4'-methylenebis(2,6-di-tert-butylphenol)-mono-di-sec-butyl orthoborate and 4,4'-methylenebis(2,6-di-tert-butylphenol)-di-(di-sec-butyl orthoborate. The diesel fuel injection test apparatus was operated for four hours on the base fuel followed by operation for four hours on the test blend (1-quart samples of each). Operating conditions for all tests were as follows:

______________________________________Air Temperature 510.degree. C. to 520.degree. C.Air Flow Rate 32.5 liters per minuteRPM 1750Fuel Flow Rate 135 cubic centimeter/hour______________________________________

Before each test, a new Bosch DNOSD-251 nozzle was installed in the apparatus.

After the tests, the injectors were carefully removed from the apparatus so as not to distrub the deposits formed thereon. Visual ratings of injector deposits were made with a deposit rating system in which 1=clean and 5=extreme deposit build-up.

The test results are given in Table I below:

TABLE I__________________________________________________________________________ Deposits within Deposits on ext. area 1 mm. area of injector in dia. from center Deposits on rim Deposits on Deposits on Deposits onFuel nozzle face of nozzle orifice of nozzle orifice external pintle tip pintle obturator nozzle face__________________________________________________________________________Base 3.5 3.5 2.5 3.5 2.5 4.0A 2.5 3.0 2.0 2.0 1.3 2.8__________________________________________________________________________ The results presented in Table I indicate less coking deposits with Fuel A as compared to the Base Fuel.

Claims

1. Distillate fuel for indirect injection compression ignition engines containing, in an amount sufficient to minimize coking, especially throttling nozzle coking in the prechambers or swirl chambers of indirect injection compression ignition engines operated on such fuel, at least the combination of (i) organic nitrate ignition accelerator and (ii) a boronated phenolic compound which, when added to said fuel in combination with said organic nitrate ignition accelerator minimizes said coking.

2. A composition of claim 1 wherein said ignition accelerator is a mixture of octyl nitrates.

3. Distillate fuel for indirect injection compression ignition engines containing, in an amount sufficient to minimize coking, especially throttling nozzle coking, in the prechambers or swirl chambers of indirect injection compression ignition engines operated on such fuel, at least the combination of (i) organic nitrate ignition accelerator and (ii) a boronated phenolic reaction product made by reacting at elevated temperature:

(a) one mole part 4,4'-methylenebis(2,6-di-tert-butylphenol), and

(b) 0.5 to 5.0 mole parts of a tri-sec-alkyl orthoborate wherein said sec-alkyl group contains 4-12 carbon atoms.

4. A composition of claim 3 wherein said process is conducted in the presence of a catalytic amount of an acidic catalyst.

5. A composition of claim 4 wherein said catalyst is acetic acid.

6. A composition of claim 5 wherein said tri-sec-alkyl orthoborate is tri-sec-butyl orthoborate.

7. A composition of claim 6 wherein said process is conducted in the presence of an acidic catalyst.

8. A composition of claim 7 wherein said catalyst is acetic acid.

9. A composition of claim 3 wherein said ignition accelerator is a mixture of octyl nitrates.

10. Distillate fuel for indirect injection compression ignition engines containing, in an amount sufficient to minimize coking, especially throttling nozzle coking in prechambers or swirl chambers of indirect injection compression ignition engines operated on such fuel, at least the combination of (i) organic nitrate ignition accelerator and (ii) a boronated phenolic mixture of about 0-50 weight percent 4,4'-methylenebis(2,6-di-tert-butylphenol), about 10-75 weight percent 4,4'-methylenebis-(2,6-di-tert-butylphenol)-di-(di-sec-butyl orthoborate) and about 25-75 weight percent 4,4'-methylenebis(2,6-di-tert-butylphenol)-mono-(di-sec-butyl orthoborate).

11. A composition of claim 10 wherein said ignition accelerator is a mixture of octyl nitrates.

12. A method of inhibiting coking, especially throttling nozzle coking, in the prechambers or swirl chambers of an indirect injection compression ignition engine, which method comprises supplying said engine with a distillate fuel containing at least the combination of (i) organic nitrate ignition accelerator and (ii) a boronated phenolic compound capable of inhibiting said coking when added to said fuel in combination with said organic nitrate ignition accelerator, said combination being present in an amount sufficient to inhibit such coking in an indirect injection compression ignition engine operated on such fuel.

13. A method of claim 12 wherein said organic nitrate ignition accelerator is a mixture of octyl nitrates.

14. A method of inhibiting coking, especially throttling nozzle coking in the prechambers or swirl chambers of an indirect injection compression ignition engine, which method comprises supplying said engine with a distillate fuel containing at least the combination of (i) organic nitrate ignition accelerator and (ii) a boronated phenolic mixture capable of inhibiting said coking when added to said fuel in combination with said organic nitrate ignition accelerator said boronated phenolic mixture comprising about 0-50 weight percent 4,4'-methylenebis(2,6-di-tert-butylphenol), about 10-75 weight percent 4,4'-methylenebis(2,6-di-tert-butylphenol)-di-(di-sec-butyl orthoborate) and about 25-75 weight percent 4,4'-methylenebis(2,6-di-tert-butylphenol)-mono-(di-sec-butyl orthoborate), said combination being present in an amount sufficient to inhibit such coking in an indirect injection compression ignition engine operated on such fuel.

15. A method of claim 14 wherein said ignition accelerator is a mixture of octyl nitrates.

16. An additive fluid concentrate for use in distillate fuels comprising, in proportions sufficient to minimize the coking characteristics of such fuel, especially throttling nozzle coking in the prechambers or swirl chambers of indirect injection compression ignition engines operated on such fuel, (i) organic nitrate ignition accelerator and (ii) a boronated phenolic compound, which when added to said fuel in combination with said organic nitrate ignition accelerator minimizes said coking.

17. A concentrate of claim 16 wherein said ignition accelerator is a mixture of octyl nitrates.

18. A concentrate of claim 16 comprising about 5-95 percent by weight of said organic nitrate ignition accelerator and about 5-95 percent by weight of said boronated phenolic compound.

19. An additive fluid concentrate for use in distillate fuels comprising, in proportions sufficient to minimize the coking characteristics of such fuels, especially throttling nozzle coking in the prechambers or swirl chambers of indirect injection compression ignition engines operated on such fuels, (i) organic nitrate ignition accelerator and (ii) a boronated phenolic reaction product made by the process of reacting at elevated temperature:

(a) one mole part 4,4'-methylenebis(2,6-di-tert-butylphenol, and

(b) 0.5 to 5.0 mole parts of a tri-sec-alkyl orthoborate wherein said sec-alkyl group contains 4-12 carbon atoms.

20. A concentrate of claim 19 wherein said process is conducted in the presence of a catalytic amount of an acidic catalyst.

21. A concentrate of claim 20 wherein said catalyst is acetic acid.

22. A concentrate of claim 21 wherein said tri-sec-alkyl orthoborate is tri-sec-butyl orthoborate.

23. A concentrate of claim 22 wherein said process is conducted in the presence of an acidic catalyst.

24. A concentrate of claim 23 wherein said catalyst is acetic acid.

25. A concentrate of claim 19 wherein said ignition accelerator is a mixture of octyl nitrates.

26. An additive fluid concentrate for use in distillate fuels comprising, in proportions sufficient to minimize the coking characteristics of such fuels, especially throttling nozzle coking in the prechambers or swirl chambers of indirect injection compression ignition engines operated on such fuels, (i) organic nitrate ignition accelerator and (ii) a boronated phenolic mixture of about 0-50 weight percent 4,4'-methylenebis(2,6-di-tert-butylphenol), about 10-75 weight percent 4,4'-methylenebis(2,6-di-tert-butylphenol)-di-(di-sec-butyl orthoborate) and about 25-75 weight percent 4,4'-methylenebis(2,6-di-tert-butylphenol)-mono-(di-sec-butyl orthoborate) which, when added to said fuel in combination with said organic nitrate ignition accelerator minimizes said coking.

27. A concentrate of claim 26 wherein said ignition accelerator is a mixture of octyl nitrates.