Fuels for diesel engines

The invention relates to high energy, oxygenated fuel compositions suitable for use in compression ignition internal combustion engines which fuel compositions contain a synergistic combination of dimethyl ether, methanol, and water, the combination providing a single liquid phase with good ignition characteristics. More particularly fuels comprising from about 72 to about 95 weight percent of dimethyl ether, from about 0.1 to about 20 weight percent of methanol, and from about 0.1 to about 20 weight percent of water are disclosed.

FIELD OF THE INVENTION 
This invention relates to improved fuels for compression ignition internal 
combustion engines. More particularly, this invention relates to high 
energy, oxygenated diesel fuel compositions which contain a synergistic 
combination of dimethyl ether, methanol, and water. 
BACKGROUND OF THE INVENTION 
It is well known that internal combustion engines have revolutionized 
transportation following their invention during the last decades of the 
19th century. While others, including Benz and Dailmer, invented and 
developed engines using electric ignition of fuel such as gasoline, Rudolf 
C. K. Diesel invented and built an engine named for him which employs 
compression for autoignition of the fuel in order to utilize low-cost 
organic fuels. Development of improved diesel engines for use in 
automobiles has proceeded hand-in-hand with improvements in diesel fuel 
compositions which are today typically derived from petroleum. Modern high 
performance diesel engines demand ever more advanced specification of fuel 
compositions, but cost remains an important consideration. 
As alternatives to conventional hydrocarbon diesel fuel produced by 
refining petroleum, other liquid fuels obtained by the conversion of 
methane or coal have been under consideration since the 1920's. Methanol 
has been proposed as one such alternative fuel for internal combustion 
engines. Methanol is usually manufactured from carbon monoxide and 
hydrogen, which have historically been obtained in large volume from 
either natural gas or coal. Carbon monoxide can also be obtained from 
almost any carbon-containing substance, including agricultural and forest 
products and many waste materials. The large supply and wide distribution 
of raw materials for manufacturing methanol is responsible to a large 
degree for its growing use as a fuel for internal combustion engines. 
However, methanol has a very low heating or BTU value. Thus, the 
performance of an internal combustion engine declines considerably when 
methanol is employed as the fuel. By contrast, relative to methanol, 
dimethyl ether has a higher BTU value and is non-toxic. In addition, 
dimethyl ether is a clean-burning fuel whose combustion gases are 
essentially free of solid particles. 
German Patent Number 654,470 (Dec. 20, 1937) to Otto Gross Wanne-Eickel 
describes mixtures of methanol and dimethyl ether containing from 5 
percent to 45 percent methanol (and hence from 95 percent to 55 percent 
dimethyl ether) as being a suitable fuel to meet increasing fuel needs for 
internal combustion engines. Existing amounts of fuel available from 
petroleum were not sufficient at that time to meet such increasing fuel 
needs. Selection of dimethyl ether with a limited methanol content, 
instead of pure methanol, is said to be desirable because the specified 
mixtures can be used, generally, to fuel internal combustion motors 
without causing the considerable loss in performance of such motors using 
pure methanol for fuel. The patent also states, however, dimethyl ether 
itself has such a strong tendency to cause knocking in spark ignition 
motors that is not possible to achieve normal operation. No working 
examples or other supporting data are provided in this 1937 German patent. 
Numerous methods have been disclosed for the production of dimethyl ether 
in combination with methanol and water from synthesis gas obtained from 
various sources, such as natural gas, coal or essentially any 
carbon-containing substance. Bell et al., U.S. Pat. No. 4,341,069; Van 
Dijk et al., U.S. Pat. No. 5,177,114; and published European Patent 
Applications Numbers 0324475 and 0409086 A1 are examples of such 
disclosures. In particular, European Patent Applications Numbers 0324475 
and 0409086 A1 disclose how process conditions can be controlled in one 
such method in order to produce various mixtures of dimethyl ether and 
methanol having a wide range of mole ratios of dimethyl ether to methanol. 
In numerous methods for the manufacture of dimethyl ether, dimethyl ether 
is produced in a product mixture that also contains methanol and/or water. 
Furthermore, removal of methanol and water from dimethyl ether in such a 
product mixture requires additional processing steps. Thus, it would be 
highly desirable to be able to employ mixtures of dimethyl ether, methanol 
and water--or, in other words, crude or unpurified dimethyl 
ether--directly as diesel fuels in order to avoid the aforesaid additional 
processing steps associated with purifying crude dimethyl ether and, 
ideally, so that process conditions could be employed in order to produce 
such mixtures directly from synthesis gas. In that way it would be 
possible to avoid or at least minimize the need for additional processing 
steps, such as purification steps, and still produce a highly effective 
and economical alternative diesel fuel. 
U.S. Pat. No. 4,422,412 to John H. R. Norton describes modification of a 
compression ignition engine (diesel) to pass a portion of a methanol fuel 
stream directly into a cylinder of the engine and divert a portion of the 
methanol fuel stream to catalytic conversion of the diverted methanol to 
dimethyl ether and water in a reactor whose outlet is in communication 
with the same cylinder. The combined gaseous mixture in the cylinder is up 
to about 50 percent by weight, preferably from about 5 to 30 percent by 
weight dimethyl ether. Stoichiometry of this methanol conversion requires 
molar amounts of dimethyl ether and water formed to be equal. Therefore, a 
balance of the combined gaseous mixture in the cylinder is at least about 
31.6 percent methanol (50 percent dimethyl ether, 18.4 percent water), and 
preferably about 58.9 to 93.2 percent methanol. 
U.S. Pat. No. 4,603,662 to John H. R. Norton and Peter R. Rebello describes 
a fuel composition that contains a mixture of at least one ether and at 
least one alcohol, and optionally may contain water, normal diesel fuel, 
and cetane improvers. For use in a compression ignition engine the patent 
states that, generally, from 5 to 80 percent by volume, more usually from 
5 to 20 percent by volume of fuel may be ethers. The fuel may contain 
small amounts of lubricants, e.g. up to about 2 percent by volume (more 
generally about 1 percent by volume) of an oil such as castor oil. 
Dimethyl ether and methanol is described as a convenient fuel because 
dimethyl ether is soluble in methanol (without water) at room temperature 
and pressures. U.S. Pat. No. 4,603,662 specifically illustrates fuels 
containing: (a) 5 percent of dimethyl ether by volume and 95 percent of 
methanol with and without an additional 1 percent of castor oil in 
Examples 1 and 3; and (b) 20 percent of dimethyl ether and 80 percent of 
methanol or 78 percent of dimethyl ether and 2 percent castor oil and as 
in Example 5.1. 
Levine, U.S. Pat. No. 4,892,561, describes a first diesel fuel composition 
free of methanol that contains 95-99.9 percent by weight of dimethyl ether 
and 0.1-5 percent by weight of a cetane number-improving additive such as 
water. Addition of water to dimethyl ether, forming a mixture rather than 
a compound, is said to avoid formation of toxic methanol. This patent also 
discloses a second diesel fuel composition that contains at least 50 
percent by weight of the aforesaid first diesel fuel and the remainder 
conventional hydrocarbon diesel fuel. 
However, thus far, there has not been a disclosure of the compositions of 
mixtures of dimethyl ether, methanol and water that contain the balance of 
concentration levels of dimethyl ether, methanol and water necessary for 
the resulting diesel fuel to afford both environmental benefits and good 
ignition characteristics, that can be produced economically without the 
need for costly purification steps, and that can be maintained as a stable 
single liquid phase both in use and during storage. 
It is therefore a general object of the present invention to provide an 
improved alternative diesel fuel composition which overcomes the aforesaid 
problems and affords the aforesaid benefits. 
More specifically, it is an object of the present invention to provide an 
improved alternative diesel fuel composition that has a high BTU value. 
It is another object of the present invention to provide an improved 
alternative diesel fuel composition that is a clean burning material whose 
overall emissions are lower and whose combustion gases are essentially 
free of solid particles. 
It is a further object of the present invention to provide an improved 
alternative diesel fuel composition that affords good ignition 
characteristics. 
It is another object of the present invention to provide an improved 
alternative diesel fuel composition that can be produced economically 
without the need for costly purification steps. 
It is an additional object of the present invention to provide an improved 
alternative diesel fuel composition that is maintained in a stable single 
liquid phase both in use and during storage. 
Other objects and advantages of the present invention will become apparent 
upon reading the following detailed description and appended claims. 
SUMMARY OF THE INVENTION 
These objects are achieved by an improved, high energy, oxygenated fuel 
composition suitable for use in compression ignition internal combustion 
engines comprising dimethyl ether, methanol, and water, wherein the 
percentages thereof, based upon the total weight of dimethyl ether, 
methanol, and water present, fall within the trapezoidal region delineated 
on the attached triangular phase diagram of FIG. 1. 
For a more complete understanding of the present invention, reference 
should now be made to the embodiments illustrated in greater detail in the 
accompanying drawings and described below by way of examples of the 
invention.

BRIEF DESCRIPTION OF THE INVENTION 
Some of the preferred, high energy, oxygenated fuel compositions which 
combine the above mentioned outstanding properties are plotted in the 
triangular composition diagram FIG. 1. The trapezoidal region delineated 
in the diagram corresponds to fuel compositions of the present invention. 
Coordinates for each point of this region define a fuel in terms of 
percentages of dimethyl ether (e), methanol (m), and water (w), based upon 
the total weight of dimethyl ether, methanol, and water present. The 
region is substantially bounded by the shortest line circumscribing 
coordinate points--(80, 18), (72, 8), (94, 3), and (92, 4)--where the 
coordinates are e percent of dimethyl ether, and m percent of methanol. 
Preferred percentages e, m, and w are depicted in the trapezoidal region 
delineated in the diagram and have the following approximate ranges: about 
72 to about 95 weight percent of dimethyl ether, from about 0.1 to about 
20 weight percent of methanol, and from about 0.5 to about 20 weight 
percent of water. While it is difficult to calculate a numerical range for 
the percentages, for convenience the following limits are given. 
Particularly, when w is in the range upward from about 5.2 to about 20 in 
the present invention, the lowest concentration of methanol in weight 
percent (min. meth. conc.) that is permitted in the diesel fuel 
composition containing a given water concentration in weight percent 
(water conc. w) is defined by the relationship, 
min. meth. conc=0.5 w-2.6, 
and the largest concentration of methanol in weight percent (max. meth. 
conc.) that is permitted in the diesel fuel composition containing the 
given water concentration in weight percent (w) is defined by the 
relationship, 
max. meth. conc.=20-0.6 w. 
If the alternative diesel fuel composition of the present invention 
contains less than about 70 weight percent of dimethyl ether, the problems 
of poor ignition characteristics and of separation of the diesel fuel into 
two liquid phases result and prohibit the composition from being used 
effectively as a diesel fuel. 
Water is present in the alternative diesel fuel composition of the present 
invention at a level of from about 0.1 weight percent, preferably from 
about 1 weight percent, more preferably from about 2 weight percent, up to 
about 20 weight percent, preferably up to about 10 weight percent. If the 
composition of the present invention contains more than about 5.2 weight 
percent of water in admixture with dimethyl ether alone, it will separate 
to form two liquid phases unless methanol is also present. 
Within limits to be described hereinbelow, the presence of certain amounts 
of methanol in the mixture of dimethyl ether and water stabilizes the 
mixture against separation into two liquid phases. The amount of methanol 
that is necessary to provide this stabilization increases as the 
concentration of water in the mixture with dimethyl ether increases. 
However, if too much methanol is present in the mixture containing a 
particular concentration of water, the ignition characteristics of the 
mixture are adversely affected. Thus, for a mixture of dimethyl ether with 
a given concentration of water, the concentration of methanol in such 
mixture must be at least a certain minimum level in order to prevent phase 
separation and must be below a certain maximum level in order to avoid 
poor ignition characteristics. 
The specific maximum and minimum concentrations of methanol in the mixture 
depend on and vary with the particular water concentration in the mixture. 
The minimum concentration of methanol in weight percent (min. meth. conc.) 
in the diesel fuel depends on the water concentration therein in weight 
percent (w) and is determined by the approximate relationship 
min. meth. conc=0.5 w-2.6. 
The maximum concentration of methanol in weight percent (max. meth. conc.) 
in the diesel fuel containing a given water concentration is determined by 
the approximate relationship 
max. meth. conc.=20-0.6 w. 
Both of these approximate relationships were determined empirically based 
on actual measurements of ignition characteristics and phase separations 
using a significant number of different mixtures of dimethyl ether, 
methanol and water. 
As these relationships indicate, it is not necessary that methanol be 
present in the alternative diesel fuel composition of the present 
invention unless the fuel composition contains at least 5.2 weight percent 
of water. In addition, the maximum concentration of methanol that can be 
present in the alternative diesel fuel composition of the present 
invention under any circumstance is about 20 weight percent. Furthermore, 
when the alternative diesel fuel composition of the present invention 
contains about 20 weight percent of water, the fuel composition must also 
contain at least 7.4 weight percent of methanol in order to prevent phase 
separation but must not contain more than 8 weight percent of methanol, 
otherwise poor ignition characteristics result. This range of effective 
methanol concentration is so narrow that for all practical purposes, the 
upper limit of the concentration of water that may be present in the 
alternative diesel fuel composition of the present invention is about 20 
weight percent. 
In another preferred embodiment, if it is desirable to improve the ignition 
characteristics of the alternative diesel fuel composition of this 
invention, any convenient conventional cetane number-improving additive 
can be added to the diesel fuel composition in cetane number-improving 
amounts. Examples of suitable cetane number-improving additives include 
inorganic peroxides such as hydrogen peroxide, organic peroxides such as 
ethyl t-butyl peroxide and di-t-butylperoxide, and alkyl nitrates such as 
ethyl hexyl nitrate, amyl nitrate, and nitromethane. More specifically, 
the cetane number-improving additive is employed at a concentration in the 
diesel fuel composition in the range of preferably from about 0.01, more 
preferably from about 0.05, preferably to about 3 weight percent, more 
preferably to about 1 weight percent. 
In an additional preferred embodiment, the alternative diesel fuel 
composition of this invention can additionally comprise up to 50 weight 
percent of either a conventional hydrocarbon diesel fuel, preferably 
derived from petroleum, or a biodiesel fuel derived from plants and 
vegetables. 
EXAMPLES OF THE INVENTION 
The following Examples will serve to illustrate certain specific 
embodiments of the herein disclosed invention. These Examples should not, 
however, be construed as limiting the scope of the novel invention as 
there are many variations which may be made thereon without departing from 
the spirit of the disclosed invention, as those of skill in the art will 
recognize. 
A QWF thick walled glass cylinder with a volume of 50 mL and a first blind 
end and a second open end was used in these tests. The open end was 
coupled to a stainless steel plate using an O-ring as sealing material. 
The removable stainless steel plate was supplied with an on/off valve and 
connected to a 1/8 inch flexible tube. The weight of this equipment was 
determined by an electronic balance. 
EXAMPLE 1 
Water and Dimethyl Ether 
In this series of experiments solubility of water in neat dimethyl ether 
(DME, 99.99 percent by weight) was determined. Water was first added to 
the glass cylinder. Weight of water added was determined by electronic 
balance. The vapor space in the glass cylinder was then purged at ambient 
pressure with gas phase DME at ambient temperature (about 22.degree. C.). 
The purge was approximately 5 times the volume of the cylinder and purge 
time was approximately 20 seconds. Weight of the cylinder's contents was 
recorded. 
Liquid DME was added by opening the bottom of the DME supply cylinder and 
the on/off valve at the stainless steel plate on the glass cylinder. The 
amount of DME entering the cylinder was selected manually. The flexible 
tube was removed and the specific amount of DME added was determined using 
the balance and subtracting the amount of water. 
The physical state of the contents was noticed visually. The physical 
behavior of the liquid was either one liquid phase or two liquid phases. 
More neat DME was added to the glass cylinder and the actual ratio of 
water and DME was determined by the balance. The number of phases were 
visually determined and noted. It was determined that the maximum measured 
amount of water soluble in DME and maintaining one homogenous phase at 
ambient temperature was 5.5 percent by weight. 
EXAMPLE 2 
Water, Dimethyl Ether and Methanol 
In this series of experiments physical phase conditions of mixtures of 
water, dimethyl ether and methanol were determined using the same 
equipment and procedures as in Example 1. A known mixture of water and 
methanol was introduced into the cylinder. Amount of water and methanol 
was determined by electronic balance. The DME purge was subsequently 
performed. Addition of DME was performed step by step and, between 
additions of DME, the number of phases noted. 
FIG. 2 is a rectangular phase diagram of the data in which a deleterious 
two phase region is delineated -for mixtures of dimethyl ether, methanol, 
and water. Squares denote experimental mixtures with two separate liquid 
phases plus a vapor phase. Circles denote experimental mixtures containing 
only one liquid phase plus a vapor phase. Solid lines indicating 
compositions at the boundary between the one and two phase regions were 
determined by statistical analysis of the data. 
EXAMPLE 3 
Ignition Quality 
Several of the above described mixtures containing various amounts of 
water, DME, and methanol were tested as fuels in a diesel engine. Fuel 
mixtures were prepared in 1 liter batches and tested in a Yanmar single 
cylinder 0.273 diesel engine (YDG 3000). Injector opening pressure was 100 
bar, and the engine was started at ambient temperature (about 22.degree. 
C.) at each experiment. The engine was in operation at idle and loads of 
50 percent and 100 percent. The engine was tested with the fuels 
identified in Table I. 
TABLE I 
______________________________________ 
Test No. DME MeOH Water Ignition 
______________________________________ 
1 100.0 0.0 0.0 OK 
2 94.5 0.0 5.5 OK 
3 92.0 4.0 4.0 OK 
4 90.0 8.0 2.0 OK 
5 82.0 8.0 10.0 OK 
6 81.0 19.0 0.0 Limit 
7 72.0 8.0 20.0 OK 
8 47.0 53.0 0.0 NO 
9 34.0 33.0 33.0 NO 
10 10.0 90.0 0.0 NO 
______________________________________ 
Data in Table I illustrate that fuel compositions set forth in test numbers 
1 to 5, inclusive, and 7, experienced ignition after a few revolutions 
without use of any starting aids. These fuels also achieved smooth 
operation at idle and loads of 50 percent, and 100 percent. No ignition 
was achieved using the fuel compositions set forth in test numbers 8 to 
10, inclusive. FIG. 3 is a rectangular phase diagram in which a critical 
region of compositions of methanol and water in DME having good ignition 
characteristics is delineated outside the deleterious two phase region. 
EXAMPLE 4 
The present invention will be more clearly understood from the following 
specific example. A diesel fuel composition containing 94 weight percent 
of dimethyl ether, 3 weight percent of water, and 3 weight percent of 
methanol was tested in a Navistar T 444E diesel engine having a 90 degree 
V-8 with a displacement of 444 cubic inches, a bore diameter of 4.11 
inches, and a stroke of 4.18 inches. The diesel engine was a turbocharger 
equipped with an air-to-air intercooler and an electronically controlled 
direct injection fuel system and was fitted with an exhaust gas 
recirculation system. For this testing, since more DME has to be injected 
to achieve the same power output as conventional hydrocarbon diesel fuel, 
slightly oversized injectors were used. In addition, due to the higher 
volatility of the dimethyl ether-containing composition tested, a modified 
feed pump was employed in order to prevent fuel cavitation in the 
injector. The engine test was performed using an 8-mode steady-state test 
cycle that simulates the U.S. EPA transient test cycle. The following 
exhaust emissions were measured: hydrocarbons, carbon monoxide; nitrogen 
oxides, smoke and particulates. 
Test results indicate that the consumption of the dimethyl ether-containing 
composition was substantially equal to that of conventional diesel fuel 
when the emission level was 5 gm/bhp-hr of nitrogen oxides and was 
significantly lower than that of conventional diesel fuel when the 
emission level was less than 3.64 gm/bhp-hr of nitrogen oxides. The level 
of nitrogen oxides emissions was only about 1.7 gm/bhp-hr which is a 
significant improvement over the level of nitrogen oxides emissions of 
pure dimethyl ether alone. The soot content of the emissions was only 
about 0.03 gm/bhp-hr, and the level of hydrocarbon emissions was about 0.3 
gm/bhp-hr which is only slightly above that of pure dimethyl ether alone. 
The combination of (1) the sum of the levels of nitrogen oxide and 
hydrocarbon emissions of about 2.1 gm/bhp-hr, and (2) the level of 
particulates in the emissions of about 0.034 gm/bhp-hr measured in this 
test is already within the upper limits therefor of 2.5 gm/bhp-hr and 0.05 
gm/bhp-hr, respectively, mandated by the California ULEV, which will not 
go into effect until 1998. 
From the above description, it is apparent that the objects of the present 
invention have been achieved. While only certain embodiments have been set 
forth, alternative embodiments and various modifications will be apparent 
from the above description to those skilled in the art and are considered 
equivalent and within the spirit and scope of the invention. 
For the purposes of the present invention, "predominantly" is defined as 
more than about fifty percent. "Substantially" is defined as occurring 
with sufficient frequency or being present in such proportions as to 
measurably affect macroscopic properties of an associated compound or 
system. Where the frequency or proportion for such impact is not clear, 
substantially is to be regarded as about twenty per cent or more. The term 
"essentially" is defined as absolutely except that small variations which 
have no more than a negligible effect on macroscopic qualities and final 
outcome are permitted, typically up to about one percent. 
Examples have been presented and hypotheses advanced herein in order to 
better communicate certain facets of the invention. The scope of the 
invention is determined solely by the scope of the appended claims.