Higher value of starting train resistance reduces the starting acceleration of train. Train's tractive effort should be capable to overcome the starting train resistance for train operation. Higher value of running train resistance increases the consumption of electrical energy, SEC and reduces the residual acceleration, balancing speed & % regeneration of train. Trains having higher running resistance take more time & cover long distance to achieve maximum service speed. Energy required to overcome the train resistance is dissipative and can not be recovered at all. Air resistance experienced by a travelling train is influenced by cross-sectional area of the vehicle, external geometry, train length, surface roughness & zones of turbulence related to train shape. Air resistance (subset of train resistance) can be reduced by adopting aerodynamic design of train. Studies indicate that due to air resistance, a locomotive pulling open empty cars consumes more energy than one travelling with filled heavy load.
Higher train weight decreases the starting as well as residual acceleration for a given tractive effort graph and train performance is affected accordingly.
To achieve the same acceleration, tractive effort graph of train has to be improved accordingly which result into higher power / current drawn from the supply system to achieve maximum service speed.
Higher weight of train also increases the train running resistance slightly and train performance is affected accordingly. Heavy trains draw more energy from the supply system to reach its maximum service speed as compared to lighter train.
On braking, for the given regenerative braking effort graph, regenerated energy in both case will remain same, however, % regeneration of electric energy will be more for light weighted train.
It is concluded that the light weighted trains are more beneficial from energy consumption point of view.
Effect of train weight on its performance also depends on the type of section (i.e. suburban, local or mainline). This may be perceived by evaluating the following simulation results of a train:
Common Train parameters:
Number of Motor Coach | 3 |
Number of Trailer Coach | 6 |
Length of coach | 25m |
Rotating Mass | 7% |
Maximum service speed | 100 kmph |
Constant deceleration value | 1 m/s^{2} |
Combined propulsion equipment efficiency | 87.5% |
Auxiliary load of a complete train | 360 KW |
Auxiliary converter efficiency | 92% |
Maximum tractive effort of a train | 375 KN |
Start speed of power zone in traction mode | 35 kmph |
End speed of power zone in traction mode | 45 kmph |
Maximum regenerative braking effort of a train | 360 KN |
Start speed of power zone in braking mode | 40 kmph |
End speed of power zone in braking mode | 50 kmph |
OHE system voltage | 22.5 KV |
Power factor of train load current | 0.99 |
Train resistance at 100 Kmph | 25.6 KN |
Stop time at each station | 0 sec (stop & start again) |
Wt. of MC (ton) | Wt. of TC (ton) | Wt. of Train | No. of stop | Max. Adhesion value % | St. Accel. m/s2 | Max. Speed kmph | Avg. Speed kmph | Journey Time | Consume Eng. KWh | Reg. Energy KWh | % Regen. | SEC KWh/1000GTKm |
---|---|---|---|---|---|---|---|---|---|---|---|---|
(60+8) | (50+8) | 552 | 40 | 18.75 | 0.6245 | 88.36 | 59.85 | 80 min. 10 sec. | 3214.73 | 542.91 | 16.88 | 61.47 |
(55+8) | (45+8) | 507 | 40 | 20.24 | 0.6818 | 92.52 | 61.08 | 78 min. 34 sec. | 3079.28 | 551.07 | 17.89 | 63.33 |
Table No. 1
Wt. of MC (ton) | Wt. of TC (ton) | Wt. of Train | No. of stop | Max. Adhesion value % | St. Accel. m/s2 | Max. Speed kmph | Avg. Speed kmph | Journey Time | Consume Eng. KWh | Reg. Energy KWh | % Regen. | SEC KWh/1000GTKm |
---|---|---|---|---|---|---|---|---|---|---|---|---|
(60+8) | (50+8) | 552 | 10 | 18.75 | 0.6245 | 100 | 85.30 | 56 min. 15 sec. | 1650.69 | 145.84 | 8.834 | 34.625 |
(55+8) | (45+8) | 507 | 10 | 20.24 | 0.6818 | 100 | 86.13 | 55 min. 43 sec. | 1572.51 | 145.84 | 9.273 | 35.74 |
Table no. 2
Wt. of MC (ton) | Wt. of TC (ton) | Wt. of Train | No. of stop | Max. Adhesion value % | St. Accel. m/s2 | Max. Speed kmph | Avg. Speed kmph | Journey Time | Consume Eng. KWh | Reg. Energy KWh | % Regen. | SEC KWh/1000GTKm |
---|---|---|---|---|---|---|---|---|---|---|---|---|
(60+8) | (50+8) | 552 | 1 | 18.75 | 0.6245 | 100 | 98.51 | 48 min. 43 sec. | 1045.94 | 14.58 | 1.394 | 23.73 |
(55+8) | (45+8) | 507 | 1 | 20.24 | 0.6818 | 100 | 98.62 | 48 min. 39 sec. | 1019.35 | 14.58 | 1.430 | 25.17 |
Table no. 3
Basically, adhesion available at a power axle is the ratio of tractive effort per power axle & axle load and also depends upon environmental conditions. Hence, for a given axle load, lower value of allowed adhesion limits the value of maximum tractive effort which can be developed on a power axle. Therefore, lower value of allowed adhesion reduces the maximum starting acceleration of a train for given number of power axle. Higher value of allowed adhesion may create wheel slip/slide state during bad environmental conditions and will increase the wear & tear of wheel & rail.
Value of maximum tractive effort is decided based on the adhesion limit and requirement of train starting acceleration.
Starting point of traction power zone is adjusted based on the supply system current limitation, if applicable.
End point of power zone is decided based on the requirement of residual acceleration, propulsion equipment rating and other performance requirements.
It is always tried to minimise the power requirement in power zone & span of power zone, to reduce the rating of traction motor, converter-inverter
& traction transformer.
In power zone of tractive effort graph, generally tractive effort is kept inversely proportional to the speed. Hence, power drawn from the supply system
remains constant in this zone.
Power in KW = Tractive effort (in KN) * Train speed (in m/s) / Combined propulsion equipment efficiency
When a train is accelerated upto maximum service speed, most of the energy supplied to the train is stored in the form of kinetic energy.
When train run at its maximum service speed continuously, it takes comparatively less energy to fulfil the requirement of train resistance and train auxiliary load.
Hence, the values of regenerative braking effort at higher speed directly control the regenerated
electrical energy during service brake. Other controlling factors are service brake deceleration, efficiency of propulsion equipment, auxiliary load of metro train & driving pattern etc.
Wide power zone spreading upto maximum service speed provides higher regenerated energy, but increases the propulsion equipment rating.
Effect of regenerative braking effort graph on regenerated electrical energy may be perceived by evaluating the following simulation results:
Common Train parameters:
Number of Motor Coach | 4 |
Weight of Motor Coach in ton | (60+8) |
Number of Trailer Coach | 4 |
Weight of Trailer Coach in ton | (50+8) |
Weight of Train in ton | 504 |
Rotating Mass | 7% |
Maximum service speed | 100 kmph |
Combined propulsion equipment efficiency | 87.5% |
Maximum tractive effort of train | 500 KN |
Start speed of power zone in traction mode | 40 kmph |
End speed of power zone in traction mode | 60 kmph |
Starting acceleration of train | 0.9175 m/s^{2} |
Auxiliary load of complete train | 360 KW |
Auxiliary converter efficiency | 92% |
Train resistance at 100 Kmph | 26.5 KN |
Max. Reg. BE in KN | Start speed of power zone kmph | End speed of power zone kmph | Service Brake Dece. (m/s^{2}) | Max. % Adhesion due to Reg. BE | Journey Time | Consume Eng. KWh | Reg. Energy KWh | % Regeneration | SEC KWh/1000GTKm |
---|---|---|---|---|---|---|---|---|---|
480 | 40 | 50 | 1.0 | 18.00 | 2 min. 21 sec. | 98.92 | 20.37 | 20.59 | 52.78 |
520 | 40 | 50 | 1.0 | 19.50 | 2 min. 21 sec. | 98.92 | 22.30 | 22.54 | 51.48 |
480 | 50 | 90 | 1.0 | 18.00 | 2 min. 21 sec. | 98.92 | 30.66 | 30.99 | 45.86 |
Table no. 4
Lower value of service brake deceleration provides higher regenerated energy for a given regenerative braking effort graph.
Effect of service brake deceleration on regenerated electrical energy may be perceived by evaluating the following simulation results:
Common Train parameters:
Number of Motor Coach | 4 |
Weight of Motor Coach in ton | (60+8) |
Number of Trailer Coach | 4 |
Weight of Trailer Coach in ton | (50+8) |
Weight of Train in ton | 504 |
Rotating Mass | 7% |
Maximum service speed | 100 kmph |
Combined propulsion equipment efficiency | 87.5% |
Maximum tractive effort of train | 500 KN |
Start speed of power zone in traction mode | 40 kmph |
End speed of power zone in traction mode | 60 kmph |
Starting acceleration of train | 0.9175 m/s^{2} |
Auxiliary load of complete train | 360 KW |
Auxiliary converter efficiency | 92% |
Train resistance at 100 Kmph | 26.5 KN |
Max. Reg. BE in KN | Start speed of power zone kmph | End speed of power zone kmph | Service Brake Dece. (m/s^{2}) | Max. % Adhesion due to Reg. BE | Journey Time | Consume Eng. KWh | Reg. Energy KWh | % Regeneration | SEC KWh/1000GTKm |
---|---|---|---|---|---|---|---|---|---|
480 | 50 | 90 | 1.0 | 18.00 | 2 min. 21 sec. | 98.92 | 30.66 | 30.99 | 45.86 |
480 | 50 | 90 | 0.9 | 18.00 | 2 min. 23 sec. | 98.40 | 33.81 | 34.35 | 43.40 |
480 | 50 | 90 | 0.8 | 18.00 | 2 min. 25 sec. | 97.69 | 36.19 | 37.04 | 41.32 |
Table no. 5
It is unrealistic to evaluate the performance of two different trains running in dissimilar sections by comparing their SEC value only.
Results mentioned in table 1, 2 & 3 indicate that a 552 ton train have SEC value of 61.47 KWh/1000GTKm,
34.625 KWh/1000GTKm, 23.73 KWh/1000GTKm,
when it is operated for metro services, local EMU services and express services respectively (i.e. different station distance).
Results mentioned in table 1, 2 & 3 also indicate that light weighted train consumed less energy but its SEC value is higher than that of heavier train because the effect of low weight on SEC is dominant compared to less energy consumption.
Results mentioned in table no. 5 & 6 indicate the effect of regenerative braking effort graph & service brake deceleration on SEC value respectively.
Effect of propulsion equipment efficiency & train auxiliary load on SEC may be perceived by evaluating the following simulation results:
Common Train parameters:
Number of Motor Coach | 4 |
Weight of Motor Coach in ton | (60+8) |
Number of Trailer Coach | 4 |
Weight of Trailer Coach in ton | (50+8) |
Weight of Train in ton | 504 |
Rotating Mass | 7% |
Maximum service speed | 100 kmph |
Service Brake Deceleration | 0.9 m/s^{2} |
Maximum tractive effort of train | 500 KN |
Start speed of power zone in traction mode | 40 kmph |
End speed of power zone in traction mode | 60 kmph |
Starting acceleration of train | 0.9175 m/s^{2} |
Maximum regenerative braking effort of train | 480 KN |
Start speed of power zone in braking mode | 50 kmph |
End speed of power zone in braking mode | 60 kmph |
Auxiliary converter efficiency | 92% |
Train resistance at 100 Kmph | 26.5 KN |
Efficiency of traction transformer | Efficiency of traction converter | Efficiency of traction inverter | Efficiency of traction motor & gear | Combined propulsion equipment efficiency | Auxiliary load of train in KW | Journey Time | Consume Eng. KWh | Reg. Energy KWh | % Regeneration | SEC KWh/1000GTKm |
---|---|---|---|---|---|---|---|---|---|---|
96 % | 99 % | 99 % | 91.41 % | 86.00 % | 360 | 2 min. 23 sec. | 99.88 | 33.18 | 33.21 | 44.81 |
96 % | 99 % | 99 % | 93 % | 87.50 % | 360 | 2 min. 23 sec. | 98.40 | 33.81 | 34.35 | 43.40 |
97 % | 99 % | 99 % | 93.62 % | 89.00 % | 360 | 2 min. 23 sec. | 96.82 | 34.41 | 35.54 | 41.93 |
96 % | 99 % | 99 % | 93 % | 87.50 % | 120 | 2 min. 23 sec. | 89.79 | 35.85 | 39.92 | 36.24 |
Table no. 6
Residual acceleration of train is equivalent to train acceleration corresponding to maximum service speed.
At a given service speed, residual acceleration is developed by the difference of available tractive effort and the tractive effort necessary to maintain that speed.
Metro trains should have adequate residual acceleration to reach its maximum service speed in short distance, which result into higher average speed.
Effect of residual acceleration on train performance may be perceived by evaluating the following simulation results:
Number of Motor Coach | 4 |
Weight of Motor Coach in ton | (60+8) |
Number of Trailer Coach | 4 |
Weight of Trailer Coach in ton | (50+8) |
Weight of Train in ton | 504 |
Rotating Mass | 7% |
Maximum service speed | 100 kmph |
Train resistance at 100 Kmph | 26.5 KN |
Auxiliary load of complete train | 360 KW |
Maximum tractive effort of train in KN | Start speed of power zone in kmph | End speed of power zone in kmph | Starting acceleration of train in m/s^{2} | Distance covered to reach 50 kmph (meter) | Time required to reach 50 kmph (sec.) | Acceleration on 50 kmph (m/s^{2}) | Distance covered to reach 100 kmph (meter) | Time required to reach 100 kmph (sec.) | Residual acceleration on MSS /100 kmph (m/s^{2}) | Balancing speed of train in kmph | Max. Power in KW |
---|---|---|---|---|---|---|---|---|---|---|---|
500 | 30 | 40 | 0.9175 | 139 m | 18 sec | 0.424 | 2370 | 115.5 | 0.0625 | 125 | 5170 |
600 | 30 | 40 | 1.10 | 115 | 15 | 0.514 | 1856 | 91.5 | .085 | 130 | 6120 |
500 | 30 | 90 | 0.9175 | 131 | 17.5 | 0.536 | 1060 | 60 | 0.2028 | 157 | 5170 |
600 | 30 | 90 | 1.10 | 109 | 14.5 | 0.6483 | 865 | 49 | 0.2533 | 165 | 6120 |
450 | 40 | 90 | 0.82 | 124 | 17.5 | 0.6483 | 880 | 52 | 0.2533 | 165 | 6120 |
Table no. 7
The energy can neither be generated nor destroyed,
it only changes the form from one type to another during a given process. Based on this principle,
when the train accelerates, electrical energy drawn from traction installation gets converted into the kinetic
energy. On each speed restriction, energy is wasted in proportion to weight of train * (train speed^{2} - speed restriction limit^{2}).
Effect of speed restrictions on energy consumption of train may be perceived by evaluating the following simulation
results of a train:
Common Train parameters:
Number of Motor Coach | 4 |
Weight of Motor Coach in ton | (60+8) |
Number of Trailer Coach | 4 |
Weight of Trailer Coach in ton | (50+8) |
Weight of Train in ton | 504 |
Length of speed restriction including train length | 250m |
Rotating Mass | 7% |
Maximum service speed | 100 kmph |
Constant deceleration value | 1 m/s^{2} |
Combined propulsion equipment efficiency | 87.5% |
Auxiliary load of complete train | 360 KW |
Auxiliary converter efficiency | 92% |
Maximum tractive effort of train | 500 KN |
Start speed of power zone in traction mode | 40 kmph |
End speed of power zone in traction mode | 60 kmph |
Maximum Regenerative Braking effort of train | 480 KN |
Start speed of power zone in braking mode | 40 kmph |
End speed of power zone in braking mode | 50 kmph |
Train resistance at 100 Kmph | 26.5 KN |
Speed Restriction | No. of stop | Max. Adhesion value % | St. Accel. m/s2 | Max. Speed kmph | Avg. Speed kmph | Journey Time | Consume Eng. KWh | Reg. Energy KWh | % Regen. | SEC KWh/1000GTKm |
---|---|---|---|---|---|---|---|---|---|---|
No SR | 1 | 18.75 | 0.9175 | 100 | 76.05 | 2 min. 21 sec. | 98.92 | 20.37 | 20.59 | 52.78 |
SR of 80 kmph | 1 | 18.75 | 0.9175 | 100 | 73.59 | 2 min. 26 sec. | 120.09 | 23.81 | 19.82 | 64.69 |
SR of 60 kmph | 1 | 18.75 | 0.9175 | 100 | 69.24 | 2 min. 36 sec. | 136.92 | 28.40 | 20.74 | 72.92 |
Table no. 8
Time tables usually include a recovery time added to the allout running time to allow for short delays. This recovery time is normally between 8 and 10% of the allout time and depends upon type of traffic, traffic density and design parameters of trains.
This time cushion gives opportunity to apply a driving strategy which saves energy in comparison with the shortest time driving strategy i.e. allout run. In allout mode operation (i.e. full traction upto maximum service speed, operation at maximum service speed followed by full service brake), train take minimum time to complete journey but in this mode, propulsion equipment (ignore the auxiliary load of the train) consumed maximum energy. Energy consumption of a train can be minimised by following two ways :
1. Energy saving strategies : reduce energy consumption by optimising train speed profile parameters,
2. Energy recovery strategies : use of regenerative braking systems on electric train
Optimal speed profile consists of traction (acceleration), cruising (constant speed operation), coasting (traction motor power cut mode) and braking (deceleration). Since, timetable specifies the time that a train has to travel between stations, hence time and distance are considered as input parameters for minimum energy operation model. Slight allowances in the time-table has the potential to increase the energy efficiency when the train operates according to a strategically positioned coasting point or prefer to dynamic brake or decreased constant speed operation or combination of above. Hence, optimisation of train speed profile parameters is a trade off between the running time
and energy cost. In terms of energy efficiency, the operation of a train should have included following features :
1. Train should capable to reach its required speed / maximum service speed as soon as possible, since the energy required to achieve maximum service speed & cover same distance is almost same in both case :
(i) Train have low acceleration and take more time / covered long distance to reach maximum service speed. In this case maximum power requirement of the train will be less but its duration will be more.
(ii) Train have high acceleration and take less time / covered short distance to reach maximum service speed. In this case maximum power requirement of the train will be more but its duration will be less.
2. Operation at maximum service speed or lees based on buffer given in timetable
3. Coasting from a pre-defined location
4. Optimum utilization of regenerative braking capacity during braking
Energy efficient driving strategies for railway vehicles are becoming even more important because lot of energy can be saved by optimisation of journey time-table only. By varying the journey time allocation, the regenerative power utilisation ratio
can be further improved. Effect of driving pattern on energy consumption may be perceived by evaluating the following simulation results:
Common Train parameters:
Number of Motor Coach | 4 |
Weight of Motor Coach in ton | (60+8) |
Number of Trailer Coach | 4 |
Weight of Trailer Coach in ton | (50+8) |
Weight of Train in ton | 504 |
Rotating Mass | 7% |
Maximum service speed | 90 kmph |
Constant deceleration value | 1 m/s^{2} |
Combined propulsion equipment efficiency | 87.5% |
Auxiliary load of complete train | 360 KW |
Auxiliary converter efficiency | 92% |
Maximum Regenerative Braking effort of train | 520 KN |
Start speed of power zone in braking mode | 50 kmph |
End speed of power zone in braking mode | 60 kmph |
Maximum power drawn in traction power zone | 5650 KW |
Train resistance at 90 Kmph | 23.26 KN |
Operation mode | Maximum tractive effort of train in KN | Start speed of power zone in kmph | End speed of power zone in kmph | St. Accel. of train in m/s^{2} | Max. Adhesion value % | Max. achieved speed in kmph | Journey time | Consume Energy in KWh | Reg. Energy in KWh | Net Energy in KWh | % Regen. | SEC KWh/1000GTKm |
---|---|---|---|---|---|---|---|---|---|---|---|---|
Allout mode | 550 | 30 | 35 | 1.01 | 20.63 | 90 | 114.23 sec. | 73.98 | 26.28 | 47.70 | 35.52 | 48.07 |
Min. energy run for same time | 550 | 30 | 90 | 1.01 | 20.63 | 84.6 | 114.23 sec. | 60.15 | 22.11 | 38.04 | 36.75 | 38.34 |
Table no. 9
Operation mode | Maximum tractive effort of train in KN | Start speed of power zone in kmph | End speed of power zone in kmph | St. Accel. of train in m/s^{2} | Max. Adhesion value % | Max. achieved speed in kmph | Journey time | Consume Energy in KWh | Reg. Energy in KWh | Net Energy in KWh | % Regen. | SEC KWh/1000GTKm |
---|---|---|---|---|---|---|---|---|---|---|---|---|
Allout mode | 550 | 30 | 90 | 1.01 | 20.63 | 90 | 109 sec. | 74.01 | 26.37 | 43.64 | 35.63 | 48.01 |
Min. energy run for given time | 550 | 30 | 90 | 1.01 | 20.63 | 84.6 | 114.23 sec. | 60.15 | 22.11 | 38.04 | 36.75 | 38.34 |
Min. energy run for given time | 550 | 30 | 90 | 1.01 | 20.63 | 77.6 | 120 sec. | 53.23 | 19.30 | 33.93 | 36.25 | 34.20 |
Table no. 10
Average distance between stations and traffic density are main important factors to decide the maximum operation speed of a train transport system. Journey time in a train transport system can be reduced by increasing the maximum operation speed but on other side, the net energy consumption of a train transport system also increases. For metro train system, where average stations intermediate distance is around one kilometre, higher operation speed is not cost-effective and its effect on journey time saving is also insignificant, hence the maximum operation speed of 70 kmph or less may be selected based on the requirement of traffic density in peak hours. For average stations intermediate distance of two kilometres, higher operation speed may be selected based on the requirement of traffic density. This may be perceived by evaluating the following simulation results of a metro train :
Number of Motor Coach | 2 |
Weight of Motor Coach in ton | (56+12) |
Number of Trailer Coach | 2 |
Weight of Trailer Coach in ton | (46+12) |
Weight of Train in ton | 252 |
Rotating Mass | 7% |
Auxiliary load of complete train | 180 KW |
Combined propulsion equipment efficiency | 87.5% |
Auxiliary converter efficiency | 92% |
Maximum tractive effort of train | 276 KN |
Start speed of power zone in traction mode | 30 kmph |
End speed of power zone in traction mode | 90 kmph |
Maximum power of train in traction power zone | 2834 KW |
Starting acceleration of train | 1.02 m/s^{2} |
Value of maximum adhesion used during traction | 20.7 % |
Maximum Regenerative Braking effort of train | 260 KN |
Start speed of power zone in braking mode | 50 kmph |
End speed of power zone in braking mode | 60 kmph |
Maximum power of train in braking power zone | 2974 KW |
Constant deceleration value (pneumatic + electric brake ) | 1 m/s^{2} |
Value of maximum adhesion used during braking | 19.5 % |
Case 1 :
Operation of a train in metro services having average stations intermediate distance of 1 km :
Distance between stations in meters | Max. Service Speed Limit in kmph | Journey Time in seconds | Average Speed in kmph | Consumed Energy in KWh | Regenerated Energy in KWh | Net Consumed Energy in KWh | SEC KWh/1000GTKm | % Regeneration |
---|---|---|---|---|---|---|---|---|
1000 | 70 | 72.68 | 49.53 | 21.22 | 9.80 | 11.42 | 46.06 | 46.16 |
1000 | 75 | 71.12 | 50.61 | 23.43 | 10.73 | 12.69 | 51.19 | 45.81 |
1000 | 80 | 70.08 | 51.37 | 25.80 | 11.60 | 14.20 | 57.25 | 44.97 |
1000 | 85 | 69.47 | 51.82 | 28.32 | 12.42 | 15.90 | 64.13 | 43.84 |
1000 | 90 | 69.24 | 51.99 | 30.98 | 13.18 | 17.80 | 71.77 | 42.54 |
Table no. 11
Operation of a train in metro services having average stations intermediate distance of 2 km :
Distance between stations in meters | Max. Operation speed in kmph | Journey Time in seconds | Average Speed in kmph | Consumed Energy during traction in KWh | Regenerated Energy during braking in KWh | Net Consumed Energy in KWh | SEC KWh/1000GTKm | % Regeneration |
---|---|---|---|---|---|---|---|---|
2000 | 70 | 124.11 | 58.01 | 27.04 | 9.79 | 17.25 | 34.76 | 36.22 |
2000 | 75 | 119.12 | 60.44 | 29.26 | 10.73 | 18.53 | 37.35 | 36.67 |
2000 | 80 | 115.08 | 62.56 | 31.69 | 11.60 | 20.09 | 40.49 | 36.61 |
2000 | 85 | 111.83 | 64.38 | 34.30 | 12.41 | 21.89 | 44.11 | 36.20 |
2000 | 90 | 109.24 | 65.90 | 37.07 | 13.18 | 23.89 | 48.17 | 35.55 |
Table no. 12
In addition of maximum service speed, train weight is the key parameter for metro train services and train aerodynamic resistance is the key parameter for long distance high speed train, which control the energy consumption of train.
Net energy consumption (E_{net}) of the train during the journey in allout mode may be classified in following areas:
1. E_{accel} : Energy required from supply system to accelerate the train upto maximum service speed. Most of the energy taken during this period is stored in the
form of kinetic energy of the train and a share of this stored energy may be returned to the supply system during the regenerative braking.
E_{accel} ≈ (Effective mass of train * maximum achieved train speed^{2}) / (2 * η )
Where η = Combined propulsion equipment efficiency
2. E_{run} : Energy required from the supply system to overcome running resistance (mechanical friction + air drag) during the journey. The energy needed
to overcome the friction and air drag is converted finally into heat and can not be recovered.
E_{run} ≈ [ ∫ ( train resistance * train speed ) dt ] / η
3. E_{comfort} : Energy needed from supply system for passenger comfort functions
E_{comfort} ≈ ( train auxilliary load * jounery time ) / η
4. E_{reg} : Energy returned to the supply system during the regenerative braking
E_{reg} ≈ η * [ ∫ (regenerative braking effort * vehicle speed) dt ] ... during braking period only
E_{net} = E_{accel} + E_{run} + E_{comfort} - E_{reg}
When the service speed of the train is increased, the value of E_{accel} increases in square of the speed. The value of E_{run} also increases because the value of train aerodynamic resistance increases in square of the speed and the term "vehicle speed * journey time" remains constant for given section. The value of E_{comfort} reduces in the inverse of the speed. The value of E_{reg} increases from its previous value but the ratio of E_{reg}/E_{accel} reduces with increase in the maximum service speed. It is concluded that when the service speed is increased, journey time is reduced but energy consumption of the train is increased. Increment in energy consumption mainly depend upon the maximum achieved speed, distance between stations, aerodynamic resistance of the train, weight of the train, regenerative braking capacity of the train, combined propulsion equipment efficiency, number of speed restrictions in the section and root profile of the section. Hence, rail operator may choose an appropriate maximum service speed of the train based on trade-off between consumed energy cost and journey time.
Some technical papers explicate that high speed trains consume less energy per passenger-Km as compared to conventional trains. It may be possible due to the following reasons:
1. Route profile (i.e. Gradient, curve and speed restriction details) and number of train halt stations may differ for both types of the train.
2. High speed trains may be designed with improve aerodynamic profile to reduce train running resistance as compared to conventional running trains.
3. Generally high speed trains have tilting facility and thus can negotiate sharp curves without any speed reduction. However conventional trains have no tilting facility
and thus apply frequent braking to reduce the speed for negotiating sharp curves. Frequent braking followed by acceleration in the conventional train increase the energy
consumption.
4. Load factor, combined propulsion equipment efficiency, space utilization factor, auxiliary load and gross weight of both the trains may differ.