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AC versus DC locomotives

National Rail Corporation Ltd

Moderators: Basil Benjamin (University of South Australia) and Philip Laird (Wollongong University)

Preamble

National Rail current train operations utilise traditional diesel electric DC locomotive equipment. With recent development in diesel electric AC technology rail operators around the world have been considering the option between AC and DC equipped diesel electric locomotives. Rail operators are now grappling with the question of which balance of technology provides the best fit and the required commercial returns from any significant locomotive investment.

National Rail would like to understand the strengths, weaknesses and opportunities that AC diesel electric locomotives may offer in the Melbourne to Brisbane corridor.

Both major US locomotive manufacturers (EMD and GE) market and sell AC diesel electric locomotives of various horsepower and configuration. US Class 1 railroads now operate an estimated 1,000 AC diesel electric locomotives in a fleet of 20,000 locomotives. These AC locomotives have been delivered predominantly in the last 3 years.

Outline of National Rail operation

National Rail operates approximately 200 trains a week across the standard gauge network. Long distance trains between Melbourne-Brisbane, Sydney-Perth and Melbourne-Perth have priority in the overall business given their competitiveness with other modes of transport.

Refer attachments outlining the types and number of trains operated on a weekly basis by National Rail. The 3 major business groups in National Rail are Intermodal, SteelLink, and Specialised Services.

Intermodal (which includes Shipping) and NR Express Freight transports containerised freight for numerous customers generally between Capital cities. The NR Express business also operates TrailerRail (road and rail capable trailers) and high priority intermodal freight between Melbourne-Perth and Sydney-Perth. The maximum speed of these trains varies between 110 and 115 km/h.

SteelLink operates general freight trains dominated by freight and service for BHP steel products. These trains generally operate at 80 km/hr maximum speed due to rollingstock specifications.

Specialised Services operates trains such as interstate passenger (Sydney-Adelaide-Perth, Melbourne-Adelaide and Adelaide-Alice Springs), coal in the Hunter valley NSW, bulk products (Broken Hill-Port Pirie 400 km, Shellharbour NSW-Sydney 130 km, Medway NSW- Port Kembla 105 km)

Problem definition

National Rail is seeking a comparison between using NR class DC locomotives and AC locomotives on the Melbourne-Brisbane corridor for the Intermodal part of the business. The comparison will assist National Rail ascertain the suitability of converting the current NR class from DC to AC traction.

Infrastructure constraints and DC locomotive performance leads to National Rail currently planning to operate 1,500 metre Intermodal trains at 3,500 tonne between Melbourne and Brisbane. The logic behind this plan is based on the hauling ability of the NR class locomotives. That is, 2 DC locomotives ( 1,750 tonne/loco hauling capacity in this section) will operate Melbourne to Junee where there is a driver change and an additional locomotive attached to operate through to Taree (1,300 tonne/loco hauling capacity in this section). A driver change occurs at Taree along with detaching the locomotive.

Infrastructure constraints currently have Track Access Authorities limiting the maximum axle load of locomotives to 22 tonne for an equivalent total gross weight of 132 tonne. Train length is also restricted to 1,500 metre maximum between Melbourne and Brisbane. The ruling gradient on the corridor can be taken as 1:40. Full track profile gradients and curvature details are available if required. Maximum train speed varies throughout the corridor with the absolute maximum speed being 115km/hr in some areas.

Intermodal load characteristics required by the market is considered to be between 2.4 to 2.5 gross tonne per metre of train.

NR class conversion consideration

Modifying the existing NR class locomotives with AC technology has been given preliminary consideration. It appears that conversion is technically feasible with current power thyristor technology however it will add to the gross weight of the locomotive by approximately 2 to 3 tonne. This may mean an equivalent fuel load reduction or revised approvals from Track Access Authorities to allow a 22.5 tonne axle load, ie, 0.5 tonne per axle increase.

National Rail investigation revealed the AC versus DC performance difference as listed in the table below.

Some details about AC and DC locomotives

Both AC and DC drive systems can provide high torque at low speeds, but for railway locomotives with steel wheels on steel rails, the torque needs to be limited at low speeds to prevent wheel slip.

The table below indicates that AC locomotives have some advantage over DC locomotives in starting adhesion.

Also AC locomotives can continue to supply high power over long periods at low speeds, whereas DC locomotives seriously overheat over long periods of high power at low speeds; the table lists 23 km/h as the lowest speed for continuous working. AC locomotives can continue working at much lower speeds.

The adhesion percentages in rows 2 and 3 of the table refer to the percentage of a locomotive's weight on driving wheels that is converted into tractive effort.

With both AC and DC drive systems there is available electromagnetic braking as well as the normal air brakes. The electromagnetic braking is available by using the energy of the train to drive the electric motors which then generate electrical energy that is usually dissipated as heat through banks of resistors. Here again AC locomotives have an advantage over DC locomotives, as illustrated in the diagram.

1. Based on 128 tonne loco, 36% adhesion and 2.6 kg/tonne resistance
2. Based on 128 tonne loco, 30% adhesion and 2.6 kg/tonne resistance

Fuel consumption needs to be considered given the NR class locomotive in its current DC configuration has a fuel range Melbourne to Brisbane. Tests and operational experience indicates that a fully loaded NR at 1,300 tonnes departing Melbourne with a full fuel tank arrives in Brisbane with approximately 1,000 litres of fuel remaining.

Engine fuel consumption can be considered constant between the AC and DC options for each notch position.

Outcomes

In order to provide an achievable task for MISG National Rail has limited the consideration of AC locomotives to a part of its business, that is, Intermodal business between Melbourne and Brisbane. National Rail would like to understand the strengths, weaknesses and opportunities that AC diesel electric locomotives may offer in the Melbourne to Brisbane corridor. The AC locomotives will operate at different speed and acceleration resulting in different transit times. Additional benefits AC locomotives are expected to provide are reduced specific fuel consumption (litres/'000 Gross Tonne Kilometre), reduced locomotive investment and reduced locomotive maintenance costs.

It is expected that AC locomotives, with their higher hauling capacity, will avoid the need for attaching a third locomotive at Junee and thereby reduce the fleet size for a given freight task. In all circumstances it is assumed that trains will occasionally operate at maximum load for the specified locomotives and on average achieve 85% of the maximum load