General Electric ES44AC

The ES44AC (successor of the AC4400CW) is a member of the family of GE's latest and greatest diesel-electric freight locomotives, the Evolution Series.  Other locomotives in the Evolution Series include ES40DC, ES44DC, ES44C4 for the domestic market and ES44ACi (Kazakhstan), ES59ACi (China), and ES44DCi (Australia) for export.  The direct competitor of the ES44AC is the SD70ACe developed by Electro Motive Diesel (EMD).  The name ES44AC can be simply decoded as Evolution Series 4,400 horsepower AC traction (this naming scheme does not apply to EMD products or the ES59ACi which actually outputs 6,250 horsepower).  However, unlike electric locomotives, the rated power of diesel-electric locomotives are measured at the prime mover (i.e. the diesel engine), not at the traction motors.

The Evolution Series were developed to meet the stricter EPA Tier 2 emission standards.  These locomotives use the GEVO prime movers instead of the FDL prime movers used on the older Dash 9 and AC units.  A 12-cylinder GEVO engine can produce the same amount of power as a 16-cylinder FDL engine with much lower emissions.  All variants of ES units use the 12-cylinder GEVO engines except for the more powerful ES59ACi exported to China, which is equipped with a 16-cylinder version.  The ES44AC weighs 145,000 lbm (pound-mass, approximately 188 metric tonnes) provides up to 198,000 lbf (pound-force, product of pound-mass and gravitational acceleration) of start-up tractive effort and 166,000 lbf of continuous tractive effort.  The top speed of the ES44AC is 75 mph.


ES44AC leading an AC4400CW.  Note the larger radiator on the Evo.  (Minot, ND)


A robotised ES44AC at mid-train.  (Train 103, Vaughan, ON)

All US Class 1 railroads operate the ES44AC except for Canadian National, which only operates the ES44DC.  The ES44AC fleet sizes of US Class 1 roads are: Burlington Northern Santa Fe 134 units, Canadian Pacific 200 units, CSX Transportation 250 units, Kansas City Southern 110 units, Norfolk Southern 24 units, and Union Pacific 897 units.

If you are interested, click on "read more" to see some background information on diesel-electric locomotives and tractive effort.  Next week, we will take a look at the Montreal Metro (they are trains on rubber wheels!).


Diesel, but why diesel-electric?  Locomotives are required to haul heavy loads.  However, diesel engines (similarly, petrol engines) have the property such that they produce high torque (tendency of a force to rotate an object about an axis, defined as the vector product of a force and a radius of rotation) but low power at low rpm (revolutions per minute) and high power but low torque at high rpm (car magazines often provide horsepower and torque curves for certain cars they review, for example, the petrol engine used in a Volkswagen Golf City produces a maximum power of 85 kW at 5,200 rpm and a maximum torque of 165 Nm at 2,600 rpm).  This is not ideal for heavy haul, therefore a different kind of traction equipment is required.  Electric motors happened to be a such device, they have constant torque across the power spectrum.  In diesel-electric locomotives, the diesel engine (prime mover) drives an alternator that generates the electricity to power the traction motors.  The other type of diesel locomotives is diesel-hydraulic locomotives.

Tractive effort.  Unlike passenger equipment, freight locomotives are purposely made heavy.  This is to increase tractive effort, a force that results in the ability to haul heavy load and / or to accelerate without wheel slip which causes damages to both the tractive equipment and rails.  Theoretical maximum tractive effort (a force) of a locomotive is the vector component of the product of the coefficient of friction (static or kinetic) between the wheels and rails (a dimensionless number) and its weight (a force which is the product of mass and gravitational acceleration) orthogonal (i.e. perpendicular) to the rails.  Under realistic, good ambient conditions, the static coefficient of friction of steel on steel (dry, not lubricated) is approximately 0.5, compared to 1.0 for rubber on dry concrete (this value drops to approximately 0.3 with wet concrete).  Therefore, greater mass of the locomotive results in greater weight and greater tractive effort.  Under identical environmental conditions (e.g. grade, conditions of rails and equipment) and train weight, more tractive effort means faster acceleration without wheel slip.  Under identical environmental conditions and rate of acceleration, more tractive effort means the ability to pull heavier loads without wheel slip.  Most of the time, static friction is greater than kinetic coefficient (i.e. it is harder to get a box to start dragging across the floor than to keep it going once it has started going), which explains why maximum start-up tractive effort of a locomotive is greater than its maximum continuous tractive effort.

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