November 28, 2012

Four into One: Bombardier's muti-engine Traxx DE ME

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Bombardier's multi-engined Traxx DE ME was one of the star exhibits at InnoTrans in September, with visitors waiting up to 40 minutes for the chance to see inside this innovative locomotive. With the need for lower emissions and greater fuel economy becoming ever more pressing, Keith Barrow asks whether four engines might be better than one.

COMPARED with the heady days of the late-2000s, these are certainly tough times for Europe's locomotive manufacturers, and with the prolonged downturn in freight volumes in the wake of the global economic crisis, the recovery of the locomotive market has been frustratingly slow.

Yet there are grounds for optimism. A significant chunk of Europe's diesel locomotive fleet is well over 20 years old, and despite the spread of electrification it is clear that there is an imminent need to replace life-expired, polluting equipment.

The German Railway Industry Association (VDB) says that more than 600 diesel locomotives are over 30-years old and still operating in Germany with their original engines. The association has suggested the federal government offers operators environmental grants of 20% off the price of new locomotives to both kick-start production and rid the network of its dirtiest machines.

In the meantime, suppliers have focused their efforts on pushing forward diesel locomotive technology to meet the stringent EU Stage IIIB emissions standards, introduced in January this year, while continuing to address fuel consumption and maintenance costs.

At the InnoTrans exhibition in Berlin in September, Bombardier revealed its answer to the twin challenges of greater efficiency and lower emissions, the Traxx DE ME. This locomotive has the now-familiar look of the Traxx family, but under the skin it is a very different animal. Instead of the MTU 16V 4000 power unit at the heart of the standard Traxx DE, the Traxx DE ME is built around four Caterpillar C18 engines, generating a total output of 2252kW.

On the face of it, replacing one engine with four might seem illogical but this arrangement gives the Traxx DE ME a number of important advantages over its single-engined cousin. Firstly, the C18 is a standard six-cylinder four-stroke engine mass-produced primarily for the marine market and in widespread use around the world. This brings all the benefits of a mass-market engine design, with commonality of components and a global technical support network already in place. It also means the manufacturer is no longer tied to a single engine supplier and power units can be selected according to the needs of the customer.

Secondly, Bombardier claims the Traxx DE ME can offer near 100% availability, because even with one engine out of service, three engines provide sufficient power to generate the maximum tractive effort of 300kN.

Thirdly, because the four engines work together to optimise fuel consumption, the Traxx DE ME is 10% more fuel efficient than the equivalent single-engined locomotive. As a rule, diesel locomotives use their installed power only during acceleration. This means that while cruising with lighter trains, as well as coasting and braking, the power of the diesel engine is hardly utilised.

traxx-de-meThe Traxx DE ME overcomes this limitation because at every load change, the locomotive determines how many engines will be needed to deliver the required output, and at what load. It also ensures that the number of operating hours and the average load remains consistent between all four engines. In addition, Bombardier says the ability to shut down selected engines reduces ambient noise, a particular advantage in environmentally- sensitive locations such as depots.

In order to reduce nitrogen oxide emissions to the level required by Stage IIIB, the diesel engines are fitted with exhaust gas recirculation, together with particulate filters which capture up to 97% of the particulate matter from the exhaust.

Each powerpack comprises a C18 engine; a flanged synchronous alternator mounted onto a common support frame; and exhaust systems and air filters, which are fitted inside the roof of the locomotive. Bombardier says that under even the most challenging operational conditions there is no interaction between air flows at the intake points and the exhaust gas outlet points. Each powerpack is equipped with its own cooling tower, which incorporates heat exchangers for the diesel engine and the alternator.

Extensive testing was necessary to validate the performance of this engine arrangement, and Bombardier set up a test bench in a specially-adapted container at its Powerlab in Zurich which comprised a "half locomotive" with two engines and associated cooling units and exhaust systems.

Tests closely simulated real operating conditions, focusing on the interaction between the diesel engines, as well as the behaviour of the alternators and the traction converter. This also allowed engineers to corroborate the temperature behaviour of the engines and demonstrate that the overall system was easy to control.

Bombardier says arranging four engines in a bodyshell designed for a single power unit was particularly challenging, and the development process focused on optimising weight balance, engine mounting, distribution of cooling air and accessibility for maintenance.

The use of Bombardier's proven TraxxControl control system means the driver sees little difference between the Traxx DE ME and a conventional Traxx diesel. The most obvious distinction in the driver interface is that there are four temperature indicators and rev counters instead of one, and the driver has the option of shutting down or starting engines according to the prevailing load, track conditions, and acceleration/ braking requirements.

The debut customer for Traxx DE ME is German Rail (DB), which signed a framework agreement with Bombardier in 2011 for up to 200 locomotives, with an initial order for 20 160km/h passenger units, which are designated class 245. The first locomotives will enter service in the fourth quarter of next year, and will be used on regional trains around Frankfurt and Kempten in southwest Germany.

While the class 245 undergoes certification in Germany, Bombardier says it will continue to operate the test bench in Zurich with the aim of further optimising fuel consumption and interaction with mechanical equipment.

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