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Using Bunker 'C' As Locomotive Fuel

This page was last updated on March 3, 2013.

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Union Pacific did not use "Bunker C" fuel in either its gas turbine locomotives, or its diesel locomotives. The railroad actually used what was generically known as "Heavy Fuel," which technically is known as No. 5, or Bunker B.

A review of "Operation of 8500-Hp Gas Turbines in Locomotive Service" by Harold Rees, Chief Mechanical Engineer, Union Pacific Railroad, October 1960, shows that the original specification for turbine fuel was for a residual fuel that would flow at a viscosity of 95 SUS at 210 degrees Fahrenheit. The specification matches the American Society for Testing and Materials (ATSM) standard known as Number 5 fuel oil, also known as Bunker B. (Read the Wikipedia article about the definition of SUS, used as a measure of viscosity)

So, Union Pacific used No. 5 heavy fuel in its gas turbine locomotives, not Bunker C. Number 5 heavy fuel (Bunker B) was a slightly higher grade of residual fuel than the Bunker C used in steam locomotives. As mentioned in Rees' report, burning residual fuel in a gas turbine was different than burning residual fuel in a steam locomotive, and the higher grade fuel was needed to control the ratio of non-combustible, destructive by-products in the fuel itself.

Why "Bunker C"?

Bunker C is simply a generic name for one of several heavy residual fuels. Railroaders were not chemists. Many were veterans of the U. S. Navy, where the term "Bunker C" was very common. The black heavy fuel used for the Gas Turbines looked and acted like the black fuel used in ships and in steam locomotives. Right or wrong, they continued to use Bunker C as the name of the nasty black, residual fuel that made their daily lives so difficult. And railfans tend to use whatever names railroaders use.

(Read more about the Bunker C name, below)

Using Heavy Fuel in the Gas Turbines

Union Pacific turbine number 50, the demonstrator, used solely diesel fuel during its tour. In 1952-1954, with the delivery of turbines 51 to 75, General Electric worked with petroleum suppliers to develop a specification of a low-cost, residual fuel that would minimize the amount of ash in the gas flow through the turbine blades, resulting in deposits of foreign material on the internal components of the gas turbine, but not contain corrosive chemicals present in the fuel used in early stationary tests.

In his 1975 book "Turbines Westward", Tom Lee wrote, "Diesel fuel was used to start the turbine and to bring it up to 80 percent of its 6900 RPM maximum, at which time modified No. 6 heavy black fuel was cut in. The black turbine fuel was generally referred to as Bunker C oil, but the fuel was actually different from regular Bunker C."

(Note that Tom Lee used the term "Bunker C" here based on the early documents he had available at the time. Later research has found that the fuel specification was changed very soon after the earliest gas turbines went into service, with the change being driven by the presence of excess fly ash damaging the internal components of the turbine itself.)

Steve Lee wrote on April 17, 2000:

When the turbines were envisioned, it was intended that they use Bunker C as the primary fuel. In this way, the high fuel consumption rate of the turbines would be offset by the low price. However, the turbine could not be started on heavy fuel, due to its sludge-like consistency. Once the turbine had been started and was up to speed and temperature, it could be successfully switched to heavy fuel. Thus all of the turbines carried small diesel fuel tanks on the A unit. The diesel fuel was used to start the turbine and bring it up to speed. It was also used in the 1-30 class to power the Cooper-Bessemer "pony" engine (also in the A unit) that was used to hostle the locomotives around the shops and in the yard, and was initially supposed to be used instead of the turbine itself on long downhill grades, to provide excitation for the dynamic braking, rather than running the turbine for that purpose. I should point out here that the turbines themselves (as opposed to the turbine locomotives) only had two speeds; full speed and slightly less than full speed; fuel consumption was high at either speed, hence the pony engine.

Early on in the program, it was found that using Bunker C as the initial heavy fuel of choice increased the maintenance costs and reduced the availability of the turbines. This was due to erosion of the compressor blades and flame buckets, corrosion, imbalances caused by deposits on the blades, etc. Some tests were run using Bunker B (No. 5) fuel instead of Bunker C (No. 6), and most of these problems either went away completely or were drastically reduced through the use of No. 5 fuel oil, which is produced several ways; today, it's usually a product of the refining process, but it can be produced by running Bunker C through several steps of the normal refining process, and it can be produced by re-refining used lube oils. No. 5 is thick and tarry and has to be heated like Bunker C, but the corrosive trace elements have been removed by the refining process, most of the asphaltic solids (which left baked-on deposits) are removed, as well. The drawback is that No. 5 cost (in those days) a cent or two more per gallon than did Bunker C. Keep in mind that this was when diesel fuel was costing the railroads about 10-12 cents per gallon. Even though a turbine sucked down far more fuel than did diesels of comparable horsepower, the turbine's advantage was that it's fuel cost only 2-2.5 cents per gallon, so even if it used twice or three times as much fuel as comparable diesels (it did, and more) , it's total fuel cost was lower than the diesels.

An ongoing problem throughout the service lives of the turbines was the fact that as refining processes and technologies improved, the amount of heavy fuels left over from the refining of a barrel of crude oil got smaller and smaller. That, in turn, caused the price of both No. 6, known as Bunker C, and No. 5 to go slowly upward. Every time the price of No. 5 went up, the turbines lost a little of their fuel cost advantage over diesels. By the late 1960's, turbine fuel costs (per horsepower) surpassed diesel fuel costs, and that was the end for the turbines.

Once it was found that the turbines lasted longer and stayed out of the shops a little longer on No. 5 fuel, UP quietly began using only No. 5 fuel in the turbines. This was never advertised, but it is fact. (Steve Lee, April 17, 2000, via email to The Streamliner discussion group)

Steve Lee wrote on April 18, 2000:

Bunker C, known in marine circles as simply bunker, or bunker fuel, is a waste byproduct of the crude oil refining process. It is not crude oil, but it is not very far from it. Forty to fifty years ago, the refining of a barrel of crude oil left quite a bit of Bunker C. This was considered a waste product and was sold to railroads for locomotive fuel and to marine operators as ship fuel, for just a couple of cents a gallon; at times it sold for only a fraction of a cent per gallon.

Bunker C is a thick, tarry substance with a high asphalt content; it also contains the same trace minerals that were in the original crude oil, but have been concentrated as a result of the refining process. It is so thick it has to be heated in order to flow reliably; if it isn't heated, when the ambient temperature is below freezing, Bunker C solidifies to the point that you can walk on top of it and not leave footprints. It's a good fuel for open-flame boilers such as locomotive boilers and ship boilers.

Most oil-fired steam locomotives used Bunker C, although in some parts of the country they actually burned raw crude oil. As noted before, Bunker C was readily available and dirt cheap. (Steve Lee, April 18, 2000, via email to The Streamliner discussion group)

Tony Thompson wrote on November 27, 2007:

Bunker C is NOT crude oil, but a residual after the "good stuff" like kerosene and gasoline, etc. is removed from the crude. It was the lowest grade of residual, unless you count asphalt. There were also Bunker A and Bunker B fuels. Nowadays oils like that are termed heavy fuel oils; Bunker A is now classified as about No. 2 oil, Bunker B is No. 4 or No. 5 oil, and Bunker C would be No. 6 fuel oil.

Crude oil in different fields, sometimes only yards apart on the surface, could be quite different, from very runny or watery light oil, to very heavy oil like the Kern River or many Sunset oils. It is a poor idea to generalize about the characteristics of "crude" oil -- it only means "the stuff right out of the hole." Even in the Midway-Sunset field, there was a wide range of crude oil produced.

At one time SP did own a lot of oil land in the southern San Joaquin Valley, though refining was mostly done by Associated Oil, which SP controlled from 1909 to 1920. But even by 1920, SP was buying fuel oil from the lowest bidder, and there is evidence that the oil companies involved changed from time to time. Thus it didn't all come from Associated, and certainly not from any one field or region. SP did burn unrefined crude in the first few years of oil-burning locomotives (maybe 1902 to 1905) but soon the oil companies as well as SP realized it was a waste of the lighter fractions in the oil. The oil burned in locomotives got heavier and heavier as fractions like gasoline became more important; I think the change to Bunker C was in the late 1920s.

To restate, crude oil is subject to refining to remove everything valuable from it, starting with the light fractions like kerosene (before automobiles were so prevalent), and gasoline. At the end you have residuals like Bunker C. (Tony Thompson, November 27, 2007, via email to the Espee discussion group)

What Was Bunker M Fuel?

In Cinthia Priest's book about Union Pacific diesel locomotives (The Union Pacific Diesel, Volume One, By Dr. Cinthia Priest, 1999, ISBN 0-9651896-3-5), on page 181, the text claims that the 1-30 class burned bunker M fuel oil - a lighter grade than Bunker C. Tom Lee's book (Turbines Westward, By Thomas R. Lee, 1975, ISBN 0-916244-01-6) makes no such statement indicating that they used the heavier fuel up to the end. Who is right?

There was a dissusion back in April 2000 of UP's use of "Bunker M" as a heavy fuel for its turbines and other locomotives. This was on the Union Pacific discussion group at Yahoo Groups. Ms. Priest stated that the term came from internal Union Pacific documents, but it turns out that the notation to "Bunker M" was a simple typographical error, and should have read as "Bunker C."

Using Heavy Fuel in the GP9s and SD24s

In an move to reduce operating costs, many of UP's 300 class GP9s were equipped to burn low grade heavy residual fuel. This fuel was similar to the fuel that UP was using in its Gas Turbine locomotives. Those GP9 units that were modified to burn heavy fuel received a large 2,400 gallon fuel tank which contained the electrical heating coils needed to heat the fuel and keep it flowing.

The use of heated heavy fuel as a fuel meant that heat built up under the walkways, so the modified GP9 units were equipped with open metal grating applied as walkways to dissipate excess heat. Also included was a two-stage fuel filter, located between the air compressor and the equipment rack in the rear of the carbody interior. The larger fuel tank forced one of the twin, GP9 air reservoirs to be mounted cross-wise in the area just ahead of the battery boxes at the front of the locomotive.

The larger fuel tank forced the air cooling coils from the normal GP9 location under the walkway, out to the right side of the modified locomotive. The large fuel return line was located on the left side of the new fuel tank. Later modifications to the 300 class included the same 2400-gallon fuel tank as a simple increase in fuel tank size. This modification used the same design as the heavy fuel tank, but lacked the heating coils and large return line. All of the units originally equipped to burn heavy fuel were later changed to burn diesel fuel, but retained the larger fuel tank.

The SD24s were purchased as heavy fuel locomotives in 1959, to also make use of this inexpensive fuel. The SD24s were delivered with the same electric heaters and two-stage fuel filters, with these features either being removed, or retired-in-place when the units were changed to using normal diesel fuel.

By the late 1960s the oil refiners were finding other markets for heavy fuel, with a subsequent raise in price. With the retirement of all the gas turbines at the same time, UP decided to end the use of heavy fuel. During the mid 1970s all of the 300-class GP9 units were returned to using diesel fuel, allowing the removal of the large pipes from the left side of the fuel tanks. (The SD24s were converted during the early 1960s.)

An exact list of units modified to burn heavy fuel is not available. Using photographs to identify the units is difficult because at the same time as the heavy fuel feature was removed, UP decided to increase the fuel capacity on many of the other GP9s by applying the same 2,400 gallon fuel tanks to GP9s that had never burned the heavy fuel. A company roster dated September 1, 1968 shows 86 GP9 and GP9B locomotives with the larger 2,400 gallon fuel tank, including six units in the 130-299 series. Twenty of the 300-class GP9s and GP9Bs never received the larger fuel tank, either as heavy fuel units or as regular fuel units. Also, fifteen 300-class B-units equipped with steam generators for passenger service were equipped with the same 2,400 gallon tank, split with 1,300 gallons of fuel and 1,100 gallons of water. When the steam generators were retired in place on these units, the plumbing was changed to allow the water tank to be used as a fuel tank. Of the 100 units in the 300-class, all but five A-units received either turbochargers or larger fuel tanks. These last five units, UP 312, 333, 341, 345, and 346, retained their original, as-built appearance throughout their careers on UP.

Using Bunker C in Steam Locomotives

While the terminology is different depending on individual railroads, Southern Pacific, Western Pacific, Santa Fe, and Union Pacific all used what was known as Bunker C in their steam locomotives. However, detailed research through available published sources reveals that each company had its own specification for what was used.

Persons with first-hand experience, who worked as locomotive firemen on oil-fired steam locomotives, differ greatly as to their recollections of what Bunker C was, and how well it flowed when unheated. All agree that it was dirty, nasty, black stuff that was hard to get off your skin, and almost impossible to get off your work boots and clothing. When spilled during refueling operations, or during normal handling, it solidified to a black stain on equipment that was seldom, if ever, removed.

The "Bunker C" Name

The name for Bunker C fuel comes from the name for certain heavy fuels used in marine vessels, including those of the U.S. Navy. There were thousands of navy veterans working for the railroads in the late 1940s through the early 1960s, and to them, any heavy, black fuel was "Bunker C," whether it really was or not.

The following comes from GlobalSecurity.org, and discusses the fuels used by the U.S. Navy:

Residual Fuel Oil is a topped crude oil or viscous residuum that, as obtained in refining or after blending with other fuel oil, meets or is the equivalent of Military Specification Mil-F-859 for Navy Special Fuel Oil and any more viscous fuel oil, such as No. 5 [Bunker B] or No. 6 [Bunker C].

When steamships were coal-fired,"bunkers" was the home for the bins used to hold the coal. As marine diesel engines became prevalent, the term was carried over to include the liquid fuel tanks. The United States Navy refers to Bunker C as "Navy Heavy". At one time, the lighter fuel oils Bunker A and Bunker B were also available. Bunker A oil is Bunker C cut with 20 percent diesel fuel. Fuel oil numbers 4, 5, and 6 are commonly known as "residual oils" since they are manufactured in whole or in part from distillation residues from refinery processing.

Fuel Oil No. 5 [Bunker B] consists of straight-run and cracked distillates and residuals, and contains aliphatics and aromatics. ASTM specifications list two grades of No. 5: light and heavy. Light no. 5 is lighter and under some climatic conditions may be handled and burned without preheating. This residual oil of intermediate viscosity is used in burners capable of handling fuel more viscous than fuel oil no. 4 without preheating. In some cases, preheating may be necessary in some types of equipment and in colder climates for handling. Heavy no. 5: This residual fuel oil is more viscous than grade no. 5 (light), but is intended for similar use. Preheating to 170-220 F is recommended before handling or use.

Fuel Oil No. 6 [Bunker C] is a petroleum distillate fraction with a boiling point > 400 degrees F. Fuel oil No. 6 is the highest boiling fraction of the heavy distillates from petroleum. Bunker "C" fuel oil is a sticky, black liquid similar in appearance and smell to asphalt sealing compounds. At 10° C it has a consistency of liquid honey or corn syrup. At 0° C it barely flows. No. 6 oils represent approximately 5 to 8% of the original crude petroleum, but the exact yield depends on the source, refinery design and operations, and product requirements. Fuel oil No. 6 contains about 15% paraffins, 45% naphthenes, 25% aromatics, and 15% non-hydrocarbon compounds.

More Information

Wikipedia article about fuel oil and residual fuel -- According to this article, Bunker C and Number 6 (No. 6) fuel oil are different names for the same product. What we know today as "diesel" fuel used for highway and railroad (off-road) applications, matches the name "Number 2 fuel oil" in this article.