The electrification infrastructure that enables electric trains to draw their power from the national grid offers many advantages, most of which are due to trains not requiring diesel engines. For the foreseeable future, diesel remains the only credible alternative traction power to electrification. For the same weight, diesel fuel stores fifty times the energy of a modern battery. Hence battery-powered vehicles can only be suitable for short distance services.
Diesel engines have obvious problems. They are expensive to buy and maintain, as well as being heavy, and so require additional track maintenance, especially at high speeds. The power output of a diesel engine is limited by its rating. Traction power is further reduced as a diesel engine also has to supply the train’s hotel load.
Electric traction power is limited by its thermal loading and so can operate for short periods at peak power. Partly for this reason, an electric multiple unit has typically twice the acceleration of a diesel multiple unit.
A diesel train operating at variable power settings is less efficient than a train that has its power generated by highly efficient power-station steam turbines at almost constant load. An RSSB report on the efficiency of traction energy use (T618) considers that power stations operate at 40 per cent efficiency compared with 32 per cent for diesel traction, but showed that transmission losses account for 1.4 per cent of the power supplied to electric trains.
Fuel and wastage
Diesel fuel is also significantly more expensive than electric traction. A recent ORR report revealed that diesel fuel accounts for 40 per cent of Virgin West Coast’s traction cost, yet only 15 per cent of its fleet is diesel powered.
As electric trains can be powered by any source of power, they are not susceptible to oil price rises and shortages. With electricity being increasingly generated by renewables, the carbon footprint of electric trains is being reduced accordingly. Indeed, all Dutch electric trains are now powered by wind energy.
When braking, the enormous kinetic energy of a train, which is proportional to the square of its speed, cannot be stored on-board, so on a diesel train it is dissipated in heat from its brake discs or from roof-mounted rheostats, if it is a diesel-electric train using the traction motors as generators for braking. However, on electric trains, this braking energy can be regenerated and fed back into the grid, offering energy savings of up to 20 per cent and reduced brake wear.
Of course, electric traction also eliminates harmful diesel engine emissions and particulates which are a particular issue at stations.
The one major disadvantage of electrification is its high initial capital cost. For this reason, it is not appropriate to electrify lightly trafficked lines.
Many countries understand these benefits and have a large percentage of their rail network electrified. These include Netherlands (76 per cent), Italy (71 per cent), Austria (70 per cent), Spain (61 per cent), Germany (52 per cent) and France (51 per cent). In the UK, 42 per cent of the network is electrified.
This article was written by David Shirres.
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