Technical Report Number 187-45
German Diesel Fuels
Review of the work done in Germany during the past few years to produce a satisfactory diesel fuel for high-speed engines, and the research conducted by scientists to investigate possibilities of further improvement. Study of additives such as: nitrates, nitrites, peroxides, alcohols, etc., and of various processes such as "nitration" and "ozonisation" to raise the cetane rating of fuels.
Table of Contents
Sources of Information and Key Persons Interviewed
German Diesel Fuels
1. German Diesel Fuel Specifications
Slow speed stationary diesel engines engines were operated on such liquid fuels as were available. These fuels included petroleum residues, petroleum crude, coal and lignite tars and crude shale oil. No specifications could be maintained as the fuels for these types of engines were inadequate at all times and became increasingly scarcer as the war progressed. So much so that attempts, only partly successful, were made to use these local supplies, alone or mixed with other diesel fuels, in automotive engines. The results are reported in the 1944 quarterly reports on the "Reichs Minister fur Rustung und Kriegs-produktion" (Department of Planning and War Production), Fuels and Lubricants Section, under direction of Dr. Bokemuller of the Daimler Benz Company in Gaggenau (Baden).
Diesel fuel specifications were prepared for the various armed forces. A comparison of the requirements of the "Wehrmacht", the "Luftwaffe" and the "Kriegsmarine" are tabulated on Table I. Small differences are apparent in this tabulation between the requirements of the various branches.
The German Navy (Kriegsmarine) in particular was definitely opposed to the use of any additive in diesel fuels for fear or corrosion, and never did use specification KI of the Luftwaffe for fear of vaporlock.
In addition to these diesel fuels most generally used and recognized there came out, from time to time, certain fuels known as "Sonder Diesel Kraftstoff", "Leicht Diesel Kraftstoff", "Speziall Diesel Kraftstoff" prepared to order for a definite purpose or for certain experiments. Some were blends of various types of diesel fuels, others were blends of diesel fuels and gasolines. The latter could be classified in the United States as "tractor fuels".
2. Sources of Diesel Fuel in Germany
Note: Whenever a production process is mentioned, such as Fischer-Tropsch synthesis, or hydrogenation, the reader is referred to the reports on these various processes prepared by the U.S. Naval Technical Mission in Europe.
(a) Natural Petroleum Distillates
Crude oil was obtained from the Polish, Rumanian, Austrian and German Fields, to serve as a base for the German fuels and lubricants industry. Although the quantity of these oils was continually decreasing under the impact of Allied air and land offensives, a surprisingly large amount was still on hand almost up to the end of hostilities.
(b) Synthetic Distillate known as Kogasin II
The high boiling fractions distilled from the Fischer-Tropsch low pressure, low temperature, catalytic process for obtaining hydrocarbons from solid fuels has the following typical analysis (1945) and was used as blending product for the preparation of Diesel fuels:
(c) Synthetic Distillate known as Kogasin I.
The lighter fractions distilled in the Fischer Tropsch process, with boiling range below 225 degrees centigrade, known as Kogasin I, and with a cetane number from 35 to 60, according to the cut, have also been used as one ingredient in various mixtures, for the production of Diesel fuels. Some work, mentioned below has been done to improve their suitability.
(d) Distillates obtained from the Low-Temperature Carbonisation of Coal and Lignite (Schwelter)
Low-temperature carbonization of lignite produces about 3 percent of an oil containing as much as 20 percent of "creasote" and, according to the refining process, varying in cetane rating from 38 to 48. Its composition makes it a rather poor diesel fuel, and it is not generally used without further treatment.
(e) Distillates from the Hydrogenation Process
The oil obtained from the hydrogenation of coal, coal tar and lignite constitutes a good diesel fuel, whether it is the middle from the sump phase, or the residue of distillation from the gas phase. Typical analysis of these hydrogenation diesel fuels cover a wide variety according to their boiling range:
(f) Oils from High-Temperature Carbonized of Coal
In the Ruhr where large quantities of coal are coked for the metallurgical industry, a tar-oil is available, of high gravity and low ignitability which is not suitable as diesel fuel for high-speed engines. It can however, be processed and mixed with other products.
(g) Shale Oil
The distillation of shale yields about 3.5 percent of an oil suitable as a Diesel fuel after further treatment and mixed with other products, in a manner similar to the treatment of coal tar mentioned above. This will be described further. A typical analysis of shale oil, such as produced at Dotternhausen (near Rottwell) is:
3. Preparation of the Diesel Fuel Blends
It is of interest to relate the manner in which all fuels, for aircraft as well as for ground forces, were prepared for consumption in war-time Europe.
The utmost secrecy was maintained at all times inside and outside Germany regarding sources of fuels and preparation of blends. Producers were ordered to ship certain quantities of products to locations designated by a number and were not informed of the disposition or ultimate destination of the products. Blending was accomplished upon the orders of certain organizations, without any knowledge of the identity of the products blended. People who had complete knowledge of the identity of the products blended. People who had complete knowledge of the fuels and lubricants situation were, for automotive engines, the personnel of the 'Zentral Buro fur Mineral Oel" in Berlin and for aviation engines, the personnel of the "Oberkommando der Luftwaffe." Under these, the "Wirtschaftliche Forschung Gesellschaft" or WIFO, assisted by the "Reichs Amt fur Wirtschafts Ausbau" in Berlin, took care, at strategic points throughout Europe, of the storage, blending and distribution of fuels and lubricants.
WIFO Depots, such as the huge WIFO No.1 at Hizacker, on the Elbe south of Hamburg, were equipped not only to store fuels and lubricants in underground tanks and in barrels, but also to make control analysis in a well-equipped laboratory, and to carry on a certain amount of research independently from the manufacturers. This set-up explains some of the difficulties encountered in the attempt to get a complete picture of the liquid fuel situation in Central Europe.
For diesel fuels the usual types of blends were prepared in WIFO depots. For the more elaborate mixtures only, necessitating preliminary treatment, did the manufacturer take a hand on the process.
From the point of view of ignitability alone, Kogasin II from the Fischer-Tropsch process is an ideal product. It has been demonstrated however that, used alone, it is far from being an advantageous diesel fuel. Tests have proven that, compared with a diesel fuel of petroleum origin having a cetane rating of 47 and a specific gravity of .856, Kogasin II with 86 cetane rating and .770 specific gravity, when used in engines adapted to low cetane fuel, the only engines available at this time, showed:
This was attributed to the fact that Kogasin II does not have enough "body", as it is referred to. It ignites too fast but burns too slowly and must be blended with some of the other products listed above. Therefore Kogasin II was used mostly to upgrade the ignitability of other fuels.
Typical blends used as Diesel Fuels, with the specifications indicated above, and abtained by simple mixing, were for instance:
When the mixture involves tar oils from the carbonization of coal, or shale oil, serious complications arise from the fact that these oils contain high percentages of asphalt, gum and carbon-forming elements which rapidly clog he injection nozzles, especially in small high-speed engines. Furthermore these components have a tendency to segregate out of the liquid, when Kogasin II is added and to settle in storage tanks. Originally inhibitors were used to prevent this condition, either 2 cubic centimeters of cresol for 100 centimeters of fuel, or 0.02 grams of mono-benzyl-amido-phenol in weak alcohol solution per 100 centimeters of fuel.
It became therefore necessary to refine the mixtures, and Dr. H. Kolbel of the "Rhein-Preussen Company" in Homberg, Ruhr, developed several processes for that purpose. The initial process consisted of a 20 percent liquid sulfuric acid was at atmospheric temperature followed by a caustic was with fullers earth filtration. Later aluminum chloride was substituted for the sulfuric acid wash, followed by neutralization and filtration. The latest method, covered by Deutsche Reichs Patent No. 730853 dated 28 January 1943 for the "cleaning of mixtures of tar oils and aliphatic hydrocarbons" consists of a treatment by sulfur dioxide SO2, in gas form, at atmospheric pressure. On the following tabulation can be seen a typical picture of the changes taking place in the fuel by the use of this process.
Proportions of carbon and hydrogen in these mixed Diesel fuels are:
87-90 percent C, to 9-12 percent H with a fraction of one percent of sulfur compared with following proportions in the Kogasin:
C - 85 percent
The proportions of Kogasin II in the mixtures vary according to the results desired. It has been found that the cetane rating increases practically in proportion to the quantity of Kogasin in the mixture; thus 35 percent tar oil and 65 percent Kogasin has a cetane rating of 57 while a 50-50 mixture has a cetane rating of 52.
The refining process is said to be inexpensive, as the loss of SO2 is extremely small, the bulk being used over and over again. The amount of heavy tar, asphalt, and carbon precipitated out of the fuel during the treatment is about 10 percent of the total in weight. The phenolic products eliminated can be recovered and marketed.
The refined mixture, known as diesel fuel "R" is clear, stable, mixes readily with all other diesel fuels, and is even less corrosive (zinc test) than petroleum gas oil. Diesel motor tests have shown that it compares advantageously with petroleum diesel fuel as to exhaust gas temperature, low co content of these gases, and consumption per horsepower.
For production of a low pour point diesel fuel with high centane rating, distillates of paraffinic nature can be treated by liquid SO2 followed by a butane extraction, to remove most of the waxes that cause a high pour point. This process is very similar to the Edeleanu process, but was not used extensively in Germany an account of the low yield. For example 200 kilograms of distillate from the liquefaction of brown coal, treated as above, will yield only 60 kilograms of diesel fuel of -30°F pour, 53 cetane; or 88 kilograms of diesel fuel of -5°F pour, 44 cetane.
(a)Use of Gasoline in Diesel Engines
At various times during the war the Germans experienced a serious shortage of adequate diesel fuel. These shortages were partly due to the necessity of concentrating production efforts upon the preparation of high-test aviation gasoline. It was necessary at these times to operate diesel engines on gasoline and gasoline blends which were or could be made available locally. Regardless of the type of gasoline blend used, an addition of 5 percent of motor oil to the fuel was prescribed, to protect the fuel injection pumps and not to improve the ignitability. There is a basic difference between diesel injection pumps, lubricated and sealed by the fuel itself and gasoline injection pumps, used currently on aircraft engines, lubricated by a supply of motor oil. Naturally the situation being temporary, no change over of the injection system was made. The only difficulties mentioned in connection with the use of gasoline in diesel engines were the excessive overheating of the engine and some tendency to vapor-lock in hot weather. In certain cases additives were needed to reduce the octane rating and bring the fuel more in line with a diesel fuel. Chlorpicrin was used as one of these additives. A cetane rating of 35 was aimed at. It can be obtained either by fractionating properly the Fischer-Trospch gasoline, or by blending a gasoline of as high as 60 octane with a high cetane diesel fuel. Naturally low aromatic gasolines such as "Ruhrbenzin" were selected in preference to higher grades.
An attempt was made to use benzol. Ignition was possible only by mixing ethyl nitrate vapour with the air used of vapour was cut off. The ethyl nitrate was kept in a brass container in a water bath, as its boiling point is 194°C. The vapour circuit had to be absolutely tight as ethyl nitrate vapours are toxic. Approximately .3 cubic centimeters of nitrate was used per one liter cylinder. An attempt to spray liquid ethyl nitrate in very small quantity into the cylinder resulted in a serious explosion.
During the diesel fuel shortage of 1941-42 tests were conducted at the Leuna plant of I.G. Farben for the use of a 50-50 mixture of diesel fuel and Leuna gasoline produced from the hydrogenation of brown coal tar, and having a cetane rating of 35. No difficulties were noted except a slight reduction in power in trucks and locomotive diesels or in small construction engines. In large, slow speed diesels vapor-lock difficulties developed.
4. Diesel Fuel Additives
In Germany as well as in the United States considerable research work was carried forward to raise the "ignitability", the cetane rating, of diesel fuels. Additives were used, either without further treatment, in various proportions, or were added to the fuel in conjunction with some treatment such as "nitration", "ozonisation".
For convenience, the work done with various additives and the results obtained are summarized in table form.
A further limitation is found in the solubility of some of these additives.
(a) Solubility of Peroxides in Various Fuels
An important aspect of the addition of chemical into diesel fuels is their solubility in the various types of oils at different temperatures. This in several cases, limits their use. In general it can be said that the solubility of peroxide additives increases as the percentage of un-saturated and aromatic hydrocarbons in the oil increases, while their solubility decreases proportionally to the percentage of paraffinic components.
(b) Effect of Peroxides on Ignitablity
Further tests were conducted at the Technische Hochschule of Munich, in a test engine where the compression ratio could be modified from 10:1 to 18:1, for the purpose of determining, with several diesel fuels and various peroxide additives, how low the compression ratio could be brought, in each case, before ignition would fail to occur.
The characteristics of the test were as follows:
Below are the results, indicated in "Degrees of ignition delay, measured on the indicator diagrams", for various fuels at various compression ratios. The sign = means that no measurement was available, the sign - means "ignition fails to occur".
The reference fuel in all these tests is a petroleum gas-oil from Persian crude with a cetane rating of 45.
1st Series. The fuel selected for the tests was a widely used type of brown coal tar oil referred to as "diesel fuel B" from the low-temperature distillation of lignite from Middle Germany (probably Saxony) with 2 percent of the additives indicated:
Tetraline, Dibenzoyl, Dioxy-Diethyl, etc. Show practically no difference in ignition delay.
2nd Series Same diesel fuel "B" but with only 1 percent of peroxide additives:
These two series of tests show the effect of even small percentages of certain peroxide additives in bringing back the ignition delay of a Diesel fuel towards the ignition delay of the reference fuel of petroleum origin. In other words it can be seen how certain additives permit the use of a synthetic diesel fuel which could not be adequately consumed without these additives at the compression ration available in certain diesel engines.
The next series of tests shows the influence of peroxide additives on the stability of diesel fuels. For every compression ration the ignition delay is shorter when peroxides were added to the fuel before its prolonged storage.
3rd Series A similar diesel fuel (brown coal tar) from low-temperature distillation of lignites, with 2 percent additives except as noted, and after ten months of storage.
The fourth series of tests reported on the next tabulation shows that some "ignition accelerator" must be added to the diesel fuel obtained from low temperature coal tar before it can be used as diesel fuel.
4th Series A Diesel fuel from low-temperature distillation of coal, which does not ignite satisfactorily in the engine at compression ratio of 18:1 or below, and even at 18:1 has a 27° ignition delay, with 2 percent of various peroxide additives, or 1 percent as noted.
A final set of tests was made to see the action of the peroxide additives on a diesel fuel of petroleum origin. The same "reference fuel" is tabulated as for the first series of tests. Results show that the additives improve the ignitability of the diesel fuel beyond the quality of the reference fuel.
5th Series. Same additives in 2 percent concentration in a petroleum gas-oil (origin not known):
In addition to the better ignitability of diesel fuels containing peroxide additives, a much smoother operation and a much cleaner exhaust could be noticed.
(c) Effect of Other Type Additives on Ignitability
Two additives, tried at the Technische Hochschule of Munich are worth mentioning. One is "chlorpicrin" which is "trichlor nitro methane"; added in proportion up to 4 percent it increases the cetane value materially. For instance a Kogasin II, of 92 cetane, goes up to 116 cetane by nitration and to 170 with addition of chlorpicrin after nitration. The other additive is "Lupanol", a "tera nitro methane" which in concentrations up to 3 percent has a marked influence on ignitability.
In 1942 the Technische Hochschule in Munich conducted experiments with other organic additives. It was found that the following chemicals reduce the ignitability:
The following chemicals, on the other hand, increase the ignitability:
For alcohols in particular the following tabulation illustrates the results, and shows that, from n-Octyl alcohol up an appreciable increase in cetane rating can be noticed.
(d) Effect of Nitration on Ignitability
Best results with high molecular alcohols were obtained when a "nitration" treatment was given the mixture, in the following manner. First a Fischer-Tropsch Kogasin I was selected and the fractions boiling below 212°F were removed. The balance had a cetane rating of 39. This was mixed with 50 percent of various alcohols, and the mixture subjected to a nitration process by bubbling gaseous concentrated nitric acid through the liquid. The results were as follows:
In most of these experiments a precipitation of pitch took place, and the oil had to be filtered after reaction.
These fuels are not corrosive unless they are from coal origin and contain phenols.
(e) Effect of Ozonisation on Ignitability
The Technische Hochschule of Munich conducted a considerable number of tests in 1940-42 along the line of ozonisation of Diesel fuels for the purpose of raising the cetane rating. It was found that the length of contact of the diesel fuel with the ozone was important, and that the ignitability was raised regardless of the oil, for instance Ruhrchemie (gasoline) was as susceptible to the ozonisation treatment as Kogasin. Taking a Ruhr gasoline of 47 cetane the following increases were noted:
The above tabulation is a self-explanatory. A comparison of the results with those obtained by the mere addition of a chemical permits an evaluation of the effect of a treatment such as nitration, ozonisation, or a combination of the two processes.
(f) Object of German Research with Additives
It is to be noted that the study of "diesel fuel additives", conducted in various German scientific activities was concentrated upon single feature; an increase in ignitability. Extensive studies of the performance of these additives in various oils, their stability, their corrosive action, before and after combustion, were apparently not made. Neither have the investigators come in contact with any studies of real value about additives that would improve the entire process of combustion regardless of the ignitability. The reason seems to be that, up to this date, additives have not been considered as the proper method to improve a diesel fuel. The German efforts were rather to concentrate upon the preparation of the proper blends of oils, natural or synthetic, which would operate satisfactorily without the use of additives.
5. The Significance of Cetane Increase
At the Technische Hochschule of Stuttgart, considerable work was done to determine, at various temperatures, the "critical compression ratio" of a diesel engine; that is, the compression ratio below which ignition of the diesel fuel cannot be entertained. The two charts show some of the results. In Figure 1 critical compression ratios are plotted against air temperatures for fuels of various ignitabilities and for ignition accelerating vapours in the air used for combustion. Figure 2 shows the correlation between cetane ratings and critical compression ratios at normal temperature (65°C) for various additions of ethyl nitrate.
Thus it would appear that an increase in cetane rating by permitting a reduction of the "critical compression ratio" at every temperature makes it possible to build an engine that develops more power per pound or conversely, to obtain a greater output from an engine built for a given compression ratio.
In reviewing these laboratory efforts to raise the cetane rating of oils used as diesel fuels the following questions have been justifiable asked. "How high should the cetane rating of a fuel be raised? What can be gained? What seems to be the desirable cetane limit in the light of today's knowledge? What is the diesel engine manufacture's position with regards to extremely high cetane ratings?
The German scientists and manufactures interviewed seem to agree on the answer summarized below. For the present high speed diesel engine 50 cetane rating is satisfactory. For the engine of tomorrow the discussion revolved around the question of "efficiency versus compression ratio".
An examination of Figure 3 shows, in the range of the Otto gasoline engine, an increase in efficiency with an increase in compression ratio, the latter being made possible by an increase in the "octane rating" of the fuel. A further examination shows, in the range of the diesel engine, a somewhat smaller increase in efficiency with decreasing compression ratio. Such decrease, as shown above, is made possible by use of a fuel having a high ignition quality.
Therefore an increase in cetane rating, in ignitability instead of being used to further an "increase in compression ratio" is used in an entirely opposite direction, namely to make a smoother running with a "lower" compression ratio, and all the simplification in weight, material, lubrication friction losses, that such a reduction entails. The principles appears to be perfectly sound especially when friction losses are considered and will undoubtedly guide manufacturers on both sides of the ocean towards the construction of a lighter more economical engine that may eventually permit an engine intermeliate between Otto and diesel types.
During the war it can be said that the Germans had no high quality diesel fuel as such, but blends of synthetic and natural products, each playing a definite part in the performance of the fuel. They explored the field of chemicals additives but did not use them in practice. They developed certain treatments that resulted in cetane ratings of fantastic proportions. Gasolines had to be used at times when other diesel fuels were scarse. No innovations were found in the way of producing, storing or handling diesel fuels. the processes of nitration and ozonisation of diesel fuels and blends and the treatment of tar oil blends with gaseous SO2 for purification are of major importance and may well contribute to an improvement in, or extension of American diesel fuel supplies, with their further exploration and adoption.
The FKFS Testing Stand
The Technische Hochschule of Stuttgart had a department, at times 450 employees strong, for technical developments in the science of engines. It was called the "Forschungs Institut fur Kraftfahrwesen und Fahrzeugmotoren an der Technische Hochschule Stattart: located at Unter Turckheim near Stuttgart in a group of buildings of the Daimler Benz Factory. There a testing motor was developed, used for gasoline a well as for Diesel research, the FKFS Motor, having the following characteristics.
For diesel tests, in particular four (4) types of combustion chamber are available: