The driving pattern, e.i. the speed over time diagram, has a considerable influence on the energy consumed by a train on a given trip. For given restrictions (time table, stops, speed restrictions on the way and installed traction power) a shortest time driving strategy can be determined, which is basically given by

• Full acceleration up to maximum speed given either by speed limit or by maximum traction power
• Speed holding at maximum speed until train has to start braking. The phase of speed holding may be more complicated due to varying speed limits. In this case, the shortest time driving strategy implies full exploitation of speed limits using maximum acceleration and braking power.
• Braking at the latest possible point in order to come to a stop when reaching the station

This driving strategy is illustrated in Figure 1 for a hypothetical service between two stations.

Figure 1: Shortest time driving strategy (hypothetical example)

Source: IZT

Energy efficient driving strategies

Time tables usually include a recovery time added to the minimal running time to allow for short delays. This recovery time is normally between 5 and 12% of the minimal running time.

This time buffer allows to apply a driving strategy which saves energy in comparison with the shortest time driving strategy. There are several possible driving strategies:

• Reduced maximum speed: Train accelerates to a speed inferior to speed limit.
• Reduced acceleration rate: Train accelerates to maximum speed using less acceleration power.
• Coasting: Train shuts off traction as early as possible before station in order to reach station without braking.

These strategies are illustrated in Figure 2 for a simple service (constant speed limit between station 1 and 2). Of course any combination of these strategies can be used as well.

Each of these strategies increases running time. This does not pose any problem as long as time buffers provided by timetable are exploited.

Figure 2: Energy efficient driving strategies

Source: IZT

Realisation of energy efficient driving strategies

For a given timetable efficient driving strategies can be realised in two ways:

1. Instruction and training of drivers and/or use of special internal timetables indicating to the driver when to shut off traction or what maximum speed to use (cf. Energy efficient driving by low-tech measures).
2. Driving advice systems (cf. DAS for main line operation, DAS for suburban operation and DAS for freight operation)

Acceptance

One of the main tasks of train drivers is punctuality. Therefore any measure that could compromise punctuality is met with scepticism.

Actual energy efficiency potential primarily depends on time buffer (provided by timetable) and driving strategy chosen.

A simulation study made at the National Cheng Kung University in Taiwan compared the energy saving effect of different driving strategies (using the train characteristics of German ICE), namely:

1. Reduction of maximum speed, e.g. running the train with the maximum speed of 280 Km/h instead of 300 Km/h;
2. Reduction of maximum acceleration rate, e.g. running the train with 90% of the maximum acceleration when it is in the state of acceleration;
3. Coasting, e.g. starting the train to the coasting state at the place 50% earlier than its original initial place of deceleration;
4. Saw-tooth coasting, e.g. running the train in the coasting state during a speed range between 300 km/h and 275 km/h

Various stopping services of a main line service were considered. Among other the study yielded the following result:

• For the case of a train stopping at all stations, the effect of the acceleration reduction is smaller than that of reduced maximum speed: Strategy 1 leads to an energy consumption reduced by 11% for a running time increased by 3.9%. Strategy 2 (with 80% acceleration rate) saves 4.7% energy for an increase of running time of 3,5 %. For a train stopping at all stations strategy 1 therefore has a better cost/benefit ratio.
• For an express train stopping only at three stops, coasting (strategy 3) is better than reduced maximum speed as far as cost-benefit ratio is concerned.