Union Pacific and Distillate Fuel

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This page was last updated on August 18, 2026.

Overview

Union Pacific and its subsidiary companies used gasoline in their motor cars until the mid 1920s. After that time, distillate fuel replaced gasoline as the fuel of choice due to much lower costs.

Union Pacific and other railroads were always looking to reduce costs, and "distillate" fuel was much cheaper than gasoline. Industry standards in the 1920s were not anything close to what we have now, so distillate fuel was essentially whatever the refiners said it was, and the quality of it varied considerably. The major motivation for the use of distillate fuel was cost. The price of distillate fuel was one-third to one-fourth the price of gasoline, which was about 15 cents per gallon in those days.

When Union Pacific began using distillate fuel in about 1926-1927, they used what was known as a Duff carburetor, using the design patented by Ralph Duff in 1918. Prior to that time, UP used gasoline in its motor cars.

What Is Distillate Fuel?

Distillate fuel was a type of fuel used for internal-combustion motor cars with either mechanical transmissions (McKeen cars), or electric transmissions (EMC and Brill cars). Distillate fuel came into use on Union Pacific in the time period between 1923, when the last Mckeen 200hp gasoline engine was placed into service in car M-27, and 1927, when the first EMC Winton-engined, distillate-fueled cars were delivered in cars M-31 through M-35. These early EMC motor cars were delivered with distillate engines equipped with Dilworth carburetors. They were started with gasoline, and ran until the engine warmed up, then were changed over to distillate. The use of two fuels meant that they needed two fuel tanks, 500 gallons for distillate (in two 250 gallon tanks), and 100 gallons for gasoline.

Research suggests that the term "distillate" fuel, was a generic name applied any low-cost substitute for gasoline. McKeen's own 200hp engine design was a gasoline engine, and Union Pacific and other railroads were always looking to reduce costs, and "distillate" fuel was much cheaper than gasoline.

Research also suggests that the most common and credible modern equivalent today for distillate fuel is called No. 2 fuel oil, also known as home heating oil, or furnace oil.

Distillate was a viable commercial product before modern gasoline and diesel engines were available, and was an appropriate fuel for slow-moving gasoline motor cars on branch lines. Like its cousin kerosene, distillate was less volatile and less explosive than gasoline, so there were several engines were designed around its properties. It was cheap and safe.

In the 1954 biography of Dick Dilworth, EMC's chief engineer, the author wrote about Dilworth's five-year search for an economical fuel that began in 1927.

First of all, Dilworth had to discover what a distillate was. About the best definition he could arrive at was that it was anything that didn't classify as heavy fuel oil. It might range from a low-grade gasoline, to painter's naphtha, to gas oil. In fact it was anything the refinery didn't happen to want at that particular time. The most uniform product Dilworth came across was something known as Dubbs oil, the heavy half of the pressure benzene taken off during the Dubbs cracking process. Attempting to burn this stuff in a carburetor engine, according to Dilworth, was grim business.

After five years of work in the lab, and in test installations, in 1932 Dilworth was finally able to figure it out, and patented a carburetor that allowed distillate to be used in the Winton engines used in the EMC motor cars.

Distillate was an early product of the refiners, and was a viable commercial product before modern gasoline and diesel engines became available. But the oil refineries and auto manufacturers saw the future in gasoline, which offered the potential of more horsepower and longer range per gallon, they switched to gasoline as their primary product. This was especially true after catalytic cracking became available in the 1936-1937 period, and even more so during World War II. As better refinery processes became more efficient at obtaining more gasoline and less distillate, the cost of distillate increased and became less attractive as a substitute to gasoline.

Dilworth Carburetor

Distillate was more difficult to burn than gas, and required two significant changes to the gasoline-burning engine. One was a different carburetor system and the other was a higher-powered ignition system. Working with Winton, EMC developed a combination intake and exhaust manifold which included a separate carburetor for each cylinder. The purpose was to heat the carburetor and provide adequate vaporization of the fuel. The previous two spark plugs per cylinder of the gasoline engine, were replaced by four spark plugs per cylinder, and fired at an electrical current more than twice that of the gasoline engine.

While distillate was about one-third to one-fourth the cost of gasoline, it also carried an increase in maintenance costs, with the distillate engine requiring more frequent service due the the distillate engine's reduced reliability.

Dick Dilworth was chief engineer for Electro-Motive Corporation. In his biography, "The Dilworth Story," published in 1954, tells of his search for a cheaper fuel bringing him to distillate fuel, which is noted as costing three cents per gallon, compared to gasoline's fifteen cents per gallon. The following comes from "The Dilworth Story," pages 38-40.

The railroads kept asking for power, but unfortunately Electro-Motive had just about reached a ceiling. It was not only difficult to crowd more than 800 horsepower into a car, but the cost of gasoline was rising to a point where the rail car's margin of economy was disappearing. To avoid this dilemma, Hamilton went hunting for a power plant burning cheaper fuel. He asked Dilworth to work with the Winton Engine Company on an engine that would burn distillates at three cents a gallon instead of gasoline at fifteen.

Dilworth remembers with a wry grimace the 5-year struggle to create a distillate burner. He doesn't consider the campaign a success, though out of it arose certain benefits that have been of permanent value to the railroad business.

The most uniform product Dilworth came across was something known as Dubbs oil, the heavy half of the pressure benzene taken off during the Dubbs cracking process. Attempting to burn this stuff in a carburetor engine, according to Dilworth, was grim business.

"In order to mix Dubbs oil with air and get it safely into the cylinder, we had to have a carburetor on each pair of cylinders," Dilworth recalls, "and these carburetors were fearful and wonderful things. On our largest model we even converted the intake valve of each cylinder into a carburetor, so that the mixture could be introduced into the cylinder practically at once."

Burning the stuff after it got into the cylinder was like "trying to set fire to a wet haystack." The designers had to put four spark plugs in each cylinder head. Where a gasoline engine would fire with one spark plug carrying about 35 milliamperes at 10,000 volts, the four spark plugs of the distillate engine each delivered 70 milliamperes at 20,000 volts.

A number of distillate burners were produced, climaxed by a twelve-cylinder V-engine with a 9-inch bore and 12-inch stroke. It was a heavy and somewhat clumsy engine weighing about 35,000 pounds and rated at 900 horsepower. It was installed on a Santa Fe train and was still running in 1950, though most of its life it burned gasoline instead of Dubbs oil. This happened because in the early years of the depression, when the engine was put into service, the price of gasoline dropped.

Though the entire distillate experiment was a kind of comedy of errors and compromises, some by-product advantages occurred when the big engine was installed in its power car, and this story furnishes an interesting side light on the way engineering developments happen.

The Dilworth carburetor was invented by Richard M. Dilworth, and the patents was assigned to his employer Electro-Motive Company in January and December 1932.

Duff Carburetor

Kratville and Ranks, in their "Motive Power of the Union Pacific," first published in 1958, note that Union Pacific was instrumental in the development of the use of distillate as a substitute for gasoline as a fuel for the railroad's motor cars. The following comes from page 132 of that book.

Most all McKeen motor cars and the Gas-Electric cars used a special "Duff" carburetor for burning distillate fuel rather than gasoline. This item was an invention of a Nebraska man and was employed on the original Streamliner, City of Salina, also in conjunction with its Winton distillate engine.

To take advantage of the Duff carburetor, and the low cost distillate fuel, the engines were started with a novel air-actuated "starter" which got the main engine going and then the distillate was fed into it and the motor ran from then on from this fuel..

The speed of the electric transmission cars was controlled by the speed of the engine as on the mechanical drive McKeen cars. The engine burned regular furnace fuel, made possible by the use of the Duff carburetors. The UP, though not generally known, was a pioneer in the use of distillate fuel.

The Duff carburetor, invented by Ralph A. Duff of Nebraska City, Nebraska, was largely responsible for the successful design that allowed the use of distillate fuel on railroad motor cars.

(Duff patent, U. S. Patent 1,285,648, dated November 26, 1918)

(Duff patent, U. S. Patent 1,753,235, dated April 8, 1930)

Replaced By Diesel

The Winton Engine Company had begun building small Diesel engines in the 1922-1923 time period, but these were small engines used in boats and yachts. These small designs had their start in Alexander Winton's own interest in 1916 to design a Diesel engine for his own yacht.

Beginning in the 1928-1929 period, EMC worked closely with Winton to develop an economical and reliable diesel engine. In a three-month period in early 1934, EMC delivered two small streamlined and articulated passenger trains. Union Pacific's M-10000 Streamliner was delivered in February 1934 and used distillate as its fuel. CB&Q's Pioneer Zephyr was delivered in April 1934 and used diesel fuel, as did UP's second Streamliner, M-10001, delivered in October 1934.

The Diesel design used in the CB&Q Zephyr proved to have the better engine design, and Union Pacific's M-10000 Streamliner was the last use of distillate on the main line.

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