CONTRATOS DE ESTABILIDAD JURIDICA

The model presented above can now be used to investigate the effect of driving strategies on conventional and hybrid regional trains. There are two simple driver control strategies considered for the conventional vehicle. These are:

i) Limiting the maximum allowable speed of the vehicle (subject to the line speed limit) ii) Applying a degree of coasting prior to braking on the approach to speed reductions

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These measures lead to reductions in both the energy dissipated during braking and the work done against resistance at the expense of increased journey time. Figures 3-8 and 3-9 show the calculated fuel consumption (in litres/car-km) against the calculated total journey time normalised by the scheduled journey time for the route. Constant station dwell times of 60s and 30s have been assumed for the main-line and branch-line routes respectively.

Figure 3-8 – Effect of increasing coasting and reducing maximum vehicle speed on relative journey time and fuel consumption for main-line operation with station dwell time of 60s

Figure 3-9 – Effect of increasing coasting and reducing maximum vehicle speed on relative journey time and fuel consumption for branch-line operation with station dwell time of 30s

Figures 3-8 and 3-9 show the effect of applying coasting and speed limits; each data point is calculated for a particular value of Dcoast and maximum vehicle speed. The findings fall into

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1. There is considerable scope to look at energy efficient driving techniques for these routes, as simulation with a flat out driving strategy takes around 85% of the scheduled journey time.

2. For a given journey time the maximum fuel saving is achieved by accelerating the vehicle to the full line speed limits and applying the required amount of coasting during deceleration.

3. There is a different effect on the two routes when applying the strategy of limited maximum vehicle speed. On the main-line route this results in a significant reduction in fuel consumption regardless of how much coasting is used. On the branch-line route however significant reductions in fuel consumption are only possible through coasting.

In summary, coasting is the most fuel efficient of the simple driving strategies considered. The vehicle control algorithm used here presents a simple method of identifying appropriate coasting points for drivers.

3.2.1. Effect of timetabling constraints on fuel consumption

The analysis presented in Section 3.2 assumes overall journey time is the only time constraint on the train services considered. A constant value of Dcoast has therefore been used

throughout the journey, defined here as a ’uniform coasting’ strategy. However, the impact of driving strategies is also affected by timetabling constraints. When using coasting, these restrictions make it necessary to apply appropriate values of Dcoast to meet the scheduled

arrival times at each intermediate station, defined here as ’timetable-limited coasting’ strategy. The calculated fuel consumption for three different driving techniques are shown in Table 3-2; flat out driving with scheduled intermediate departure times (the worst case driving style as shown in Figure 3-7), uniform coasting to achieve overall journey time (i.e. uniformly applied Dcoast value) and timetable-limited coasting to achieve overall journey time

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Driving Strategy Main-line Branch-line

litres/car-km saving* litres/car-km saving* Flat out with scheduled intermediate departure times (worst case) 0.475 - 0.481 - Uniform coasting to achieve overall journey time 0.302 36% 0.340 29% Timetable-limited coasting to achieve overall journey time 0.334 30% 0.369 23%

* percentage saving relative to the worst case fuel consumption

Table 3-2 – Effect of coasting strategy and timetable restrictions on fuel consumption

These results show that the fuel consumption is lowest when the uniform coasting strategy is used. Compared to timetable-limited coasting, uniform coasting achieves significant fuel savings of 10% and 8% on main-line and branch-line routes resepctively. This can be explained by considering the general form of the fuel consumption (fc) vs. journey time curve when coasting is applied for a journey between two stations, as shown in Figure 3-10.

Figure 3-10 – Illustration of the cause of fuel consumption increase when timetable-limited coasting is used to achieve overall journey time

The general shape of fc-time curve means that the increase in fc for a time reduction, -ΔT, is more than the decrease in fc for a time extension, +ΔT. Hence using less coasting in one section of the route and more in another tends to increase the overall fc for the same overall journey time. Comparing the results of the uniform coasting analysis with the route timetable shows that shorter inter-station distances have less margin in the schedule and so less coasting can be used. The extra time available for coasting on longer sections makes little difference to the fuel consumption as significant coasting is already being used, and ∆(fc)/∆T is therefore small.     _{is ve} dt fc d ve is dt fc d   2 2 T T+∆T T-∆T Time Fuel ∆(fc)1 ∆(fc)2  t F fc   fc 1   fc 2  

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In practice this means that time-table restrictions should be relaxed where possible. This would be advantageous in reducing fuel use and encouraging drivers to take a consistent approach to energy efficient vehicle control. The issue of time-tabling also has a direct impact on the performance of hybrid trains due to the different braking strategies required. This is considered in Section 3.3.3.