RAILWAYS have experimented with natural gas powered locomotives over many decades, but recent advances in drilling have led to rapid, sustainable growth in the supply of gas and resulted in the price of gas disconnecting from that of oil.
The economics of powering locomotives by natural gas are now very attractive given the significantly lower cost per unit of energy, especially for applications with high diesel fuel usage. In addition to fuel cost savings, emissions including NOx, particulates and CO2 are typically lower with natural gas compared with diesel engines.
Given the increased supply, fuel cost savings and lower emissions, railways are very interested in using natural gas to power their locomotive fleets, and manufacturers are making substantial investments to develop and validate natural gas solutions for new and existing locomotives.
Caterpillar and Electro-Motive Diesel (EMD) have several technologies available for combustion of natural gas in locomotive diesel engines: spark ignited, dynamic gas blending and high-pressure direct injection.
Engines designed to operate with 100% natural gas use spark ignition like a petrol engine. This technology is well proven and uses significantly lower compression ratios to prevent pre-ignition of the fuel before the piston reaches the top of its stroke. Caterpillar produces hundreds of spark-ignited engines each year.
Spark-ignited natural-gas engines typically have a lower power rating compared with diesel engines of the same size. These engines provide a good solution for yard locomotives and other specialised applications but present a challenge for heavy-haul operations.
Dual-fuel engines burn a mixture of natural gas and diesel fuel. One approach introduces natural gas into the engine early in the combustion cycle. Since the pressure in the engine air intake system is relatively low, high pressure is not required for the gas to flow into the engine.
Early cycle introduction of natural gas presents a challenge due to the tendency for the mixture to pre-ignite as a result of its temperature in the cylinder as it compresses, limiting the amount of gas that can be substituted for diesel fuel. Typically, dual-fuel engines using this method provide 50-60% substitution of gas for diesel fuel on a duty-cycle basis.
Engine modifications such as reducing the compression ratio may improve operation with natural gas and increase the substitution rate. However, such changes may reduce the efficiency of the engine when operating on 100% diesel fuel and the engine may be more difficult to start when cold.
Advantage
An advantage of early cycle gas introduction is the dual-fuel capability, enabling operation on 100% diesel fuel. This provides the flexibility for railways to use diesel fuel or a mixture of natural gas and diesel.
EMD is launching Caterpillar's dynamic gas blending (DGB) dual-fuel technology for its series 710 engines. With DGB, a small computer-controlled valve opens adjacent to the lower liner air intake ports, feeding a mixture of natural gas and air into the cylinder, which is then compressed. Near the top of the stroke, the injector provides diesel fuel, and the ignition of the diesel fuel causes the natural gas to burn.
EMD, which began running DGB locomotives last year and has completed 8000km of track testing, is currently conducting field tests on locomotives equipped with DGB retrofit kits.
Another technology under development called high-pressure direct injection (HPDI) injects natural gas into the engine much later in the compression cycle, eliminating issues with pre-ignition and enabling the engine to run like a diesel.
Since the natural gas must overcome the pressure in the cylinder and requires injection over a short duration, the gas is introduced at high pressure using a special injector developed jointly with Caterpillar and Westport Innovations. This injector provides a small amount of diesel fuel (5%) to ignite the mixture.
Engine performance with HPDI is very encouraging. EMD's series 710 engine develops full power with 95% natural gas and demonstrates significantly lower emission levels compared with existing diesel models. HPDI maximises fuel savings while providing a responsive performance.
Liquefied natural gas (LNG) is preferred over compressed natural gas (CNG) for long distance rail applications as it is easier to store and has more than twice the energy density of 200 Bar CNG. In addition, the process of liquefying the gas produces nearly 100% methane (CH4), resulting in predictable engine combustion.
A LNG tender can be located between two locomotives and provide gas to both units, with equivalent or increased energy storage compared with existing diesel fuel tanks.
Special safety conditions must be considered when locomotives are modified to operate on natural gas.
A small amount of gas is contained onboard the locomotive, with most stored in the tender. Methane detectors are used to identify a gas leak on the locomotive and alert the control system to shut off the gas supply.
Natural gas can provide a sustainable alternative to diesel fuel for heavy-haul locomotives. Technologies such as DGB can substitute up to 60% natural gas for diesel fuel, and larger cost savings will come from HPDI with up to 95% substitution. Applications with the most severe duty cycles and high diesel fuel usage are well-suited for EMD's DGB and HPDI natural gas solutions.
(Visit EMD at InnoTrans in Hall 6.2, stand 220)
Photos: BNSF
