Rail operators are increasing the lengths of freight trains to improve shipping efficiency. In 2018, 5 percent of freight trains were considered superlong at over 10,000 feet or two miles in length. As many as eight locomotives-four at the front, two in the middle and two at the end-are used to control the movement of one of these behemoths, according to a June 15 article in the Wall Street Journal.

Modern locomotives are designed to be fuel efficient and reduce emissions. Tier 4 locomotives meet the United States Environmental Protection Agencys Emissions Standards for Non-road Engines and Vehicles, which regulate particulate levels and nitrogen oxide found in exhaust.

Tier 4 locomotive technology reduces particulate emissions from diesel locomotives by as much as 90 percent and nitrogen oxide emissions by as much as 80 percent. As additional locomotives are needed, or older units replaced, Tier 4 locomotives are being phased into rail fleets nationwide, the American Association of Railroads noted on its website.

While fuel consumption varies by train size and terrain, locomotives consume about 100,000 gallons of fuel per year. Thus, fuel economy interests operators and oil formulators, said Ashwin Bharadwaj, a research scientist at Dow Chemical Co., during an industry meeting in May. Bharadwajs research has shed some light on a novel type of additive that could tug fuel economy further down the tracks.

Locomotive Performance

A typical $25,000 to $35,000 passenger car has a gasoline-powered engine that produces between 100 and 200 horsepower to apply several hundred pound-feet of torque to rotate the axles of the vehicle, while a $40,000 to $70,000 heavy-duty pickup truck with a diesel engine produces 300 to 400 horsepower and as much as 1,000 pound-feet of torque-enough to tow 20,000 pounds of livestock or landscaping equipment.

By comparison, a $3 million Evolution ET44AC locomotive from General Electric Co. has a diesel engine that produces 4,500 horsepower and drives electric motors that generate a total of 180,000 pound-feet or more of torque. That is, 30,000 pound-feet of torque is applied to rotate each of the six axles. A single GE ET44AC is powerful enough to pull 170 Boeing 747s, according to a 2014 article published in CNET.

Diesel-electric motors are the most widely used power generators in locomotives for freight trains. How much lubricant is needed to keep them chugging? Quite a lot. A typical diesel locomotive carries between 200 and 300 gallons of engine oil, and a GE ET44AC carries 430 gallons of lube.

Samples of in-service locomotive engine oils are collected regularly for routine oil analysis to determine total acid number and total base number, as well as concentrations of soot and wear metals.

TAN tests measure levels of acidic chemicals such as by-products of lubricant degradation, and TBN tests analyze the presence of basic or alkaline chemicals, including detergents and additives used to neutralize harmful acids. The results are used to determine drain intervals, according to Infineum Insight.

Railroad operators expect engine oils to provide reliable locomotive performance over a 184-day period, with top-ups as needed. This six-month interval corresponds to the schedule of biannual locomotive safety inspections mandated by the Federal Railroad Administration. More frequent drain intervals add to operating costs, remove locomotives from active service, disrupt schedules and can force an operator to increase the size of their fleet, Infineum noted.

Engine Oil Formulations

Lubricants for diesel engines in locomotives are predominantly heavy-duty SAE 40 monograde and SAE 20W-40 multigrade crankcase oils. They are blended from API Group II or Group III mineral oils or polyalphaolefin base stocks. Viscosities range from 14 to 16 centistokes at 100 degrees Celsius and 130 to 160 cSt at 40 C. Viscosity index is between 100 and 120.

Most pour points are between -18 and -30 C, and most flash points between 230 and 260 C. These values are consistent with the specification for GE ET44AC locomotives that allow for engine operating temperatures between -40 and 42 C.

Commercial locomotive engine oils are approved by two major original equipment manufacturers, General Electric and Electro-Motive Diesel, and a nonprofit organization, the Lubricant Maintenance Officers Association, which is part of the Coordinated Mechanical Associations.

Similar to gasoline-powered automotive engine oils, locomotive diesel engine lubricants are distinguished by their ability to neutralize acids produced by combustion and oxidation reactions and for dispersing contaminants including fuel, water and fine particles.

But locomotive engines are one of a small number of applications that require zinc-free lubricants that contain less than 10 parts per million of the element, according to ExxonMobils website, because they contain silver bearings.

Silver bearings are manufactured by using electroplating techniques to apply silver coatings on steel bearings. The silver coating reduces friction, enhances engine performance and increases bearing service life. If a bearing becomes starved (lacks lubricating oil), its silver coating prevents seizure and allows the engine to function until it is shut down safely.

However, silver is vulnerable to attack by zinc. This means that zinc dialkyldithiophosphate, widely used as an antiwear additive in automotive motor oils, cannot be used in applications with silver bearings. Additionally, locomotive engine oils can contain only minute traces of chlorine and SAPS (sulfated ash, phosphorus and sulfur) in order to comply with EPAs Tier 4 standards, ExxonMobil stated.

Evaluating Oil Soluble Polyethers

Bharadwaj presented his study evaluating novel oil soluble polyethers as additives for locomotive engine oils at the annual meeting of the Society of Tribologists and Lubrication Engineers in Minneapolis.

Polyethers are synthesized by reacting alkylene oxides with organic alcohols to make polymers. The compounds are related chemically to polyalkylene glycols, which are polymers and copolymers of alkylene oxides used in lubricating oils and metalworking fluids.

Oil soluble polyethers are used as primary base stocks or blended with API Group I-IV base stocks to make compressor and refrigeration lubricants, hydraulic fluids, gear oils and engine oils. Additionally, they are used as deposit control additives, friction modifiers and viscosity builders in mineral oils.

First, Bharadwaj used a Mini Traction Machine from Precision Instruments to compare friction between a ball and disc for six base stocks. Test conditions were 0.9 megapascals of applied pressure at 1,000 millimeters per second and slide-to-roll ratio between 0 and 150.

Two oil soluble polyethers and a PAO produced the lowest friction, while friction was significantly higher for API Group II and III mineral oils at 40 and 100 C (See graph on Page 28.) These two oil soluble polyethers showed potential to improve lubricating performance of locomotive engine oils formulated with Group II and III base stocks, Bharadwaj said.

Additive Blends

Next, based on prior work conducted by Dow, Bharadwaj blended oil soluble polyethers and antioxidants at a total of 10 percent by weight in a commercial locomotive engine oil with viscosity of 136.4 cSt at 40 C and 15.1 cSt at 100 C, and V.I. of 118.

Using the MTM at 50 percent and 150 percent slide-to-roll ratios, 50 Newtons of applied force and speeds from 20 to 2,000 mm/s, he observed that OSP-AO blends reduced the friction of commercial lubricant by 10 to 30 percent in the boundary lubrication regime at 80 C. Friction decreases were smaller (10 to 20 percent) at 150 C under boundary lubrication conditions.

Bharadwaj used ASTM D4172 four-ball wear tests to compare these formulations at 100 and 150 C. All wear scars on steel balls were between 0.36 and 0.43 mm, but the oil soluble polyether and antioxidant blends had no significant effect on wear scar diameters. Bharadwaj noted that additional tests with silver bearings would be needed to validate that such OSP-additive blends do not affect wear for locomotive engine oil applications.

Two additional tests were used to investigate the effects of oil soluble polyethers on commercial locomotive engine oil. First, Bharadwaj used the Panel Coker test (FTM-791B method 3462) to measure the tendency of lubricants to undergo oxidation and thermal breakdown at high temperatures and form a solid deposit residue called coke. Layers of coke form on interior surfaces of engines and oil distribution systems; these deposits can shed particles that block filters, cause wear and damage contacts.

In the Panel Coker test, lubricant in a sump was heated to 125 C and splashed onto an aluminum panel at 315 C. Bharadwaj reported that deposits were heavier with oil soluble polyether and antioxidant additive blends than the as-supplied commercial locomotive engine oil. He noted that the Panel Coker was a screening test, and that field tests with locomotives would be a more accurate means to evaluate coking tendency.

Dow developed an in-house test for thermo-oxidative stability based on ASTM D5704 for evaluation of lubricants for manual transmissions and final drive axles. In the Dow test procedure, lubricant is heated at 150 C and aerated for 300 hours in a round-bottom flask containing two spur gears and a coil of steel and copper. Oil samples are collected, and the TAN is measured to monitor the accumulation of acids as the lubricant undergoes thermal and oxidative breakdown.

TAN values increased strongly for the commercial locomotive engine oil modified with one oil soluble polyether and antioxidant blend (See graph on Page 28.). TAN increased more slowly for commercial oil modified with two other oil soluble polyether and antioxidant blends with higher antioxidant levels.

Bharadwaj concluded that using oil soluble polyether to modify locomotive engine oils can reduce friction in the boundary lubrication regime with no adverse effects on wear, but that it is important to use appropriate levels of antioxidants. One locomotive lubricant formulator is currently evaluating the additives for use in its oils, he noted. z

Mary Moon, Ph.D., is a professional chemist, consultant and technical writer and is technical editor of The NLGI Spokesman. Contact her at mmmoon@ix.netcom.com or (+1) 267-567-7234.