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Consumption and autonomy tests

It is essential for any operator of vehicles, "conventional" or electric, to have objective information about the energy consumption of their vehicles. Knowing the real energy consumption affects not only the autonomy but also the costs associated with the operation of a vehicle.

The implementation of standards and standardized tests that allow vehicle users to objectively and impartially compare the performance and efficiency of different models is a feasible and interesting solution. If the comparison of energy consumption of different models is relevant when choosing a vehicle, by allowing to estimate the TCO (Total Cost of Ownership) of the multiple offers on the market, a tool of this type is indispensable for operators of heavy passenger vehicles during the planning of a public service passenger transport operation.

Based on this premise, an initiative of the UITP Bus Committee, within the scope of the European project SORT - "Standardised On-Road Test Cycles", defined a set of 3 standardised tests that aim to allow an objective comparison of the energy consumption of different bus models, based on cycles representative of the operation of a bus in different contexts. Comparing the 3 test cycles:

 

  • SORT 1 (congested urban) is the shortest, with only 540m and a density of about 6 stops per km, during which the bus spends about 40% of its travel time at idle, being representative of the urban core of large metropolises such as London or Paris;

  • SORT 2, with a length of 920m, about 35% of idle time and 3 stops per kilometer, is a cycle quite representative of routes typical of major cities, but with lower congestion indexes, such as Madrid or Munich;

  • SORT 3 has a length of 1450m and an average speed of 25km/h, describing typical suburban traffic, with higher speeds and more dispersed stops located along the route, typical of smaller cities or outlying suburban areas.  

According to the ITU, the average urban traffic speed in large cities is between 10 and 14 km/h, while in smaller cities it is in the range of 16 to 20 km/h. By way of example, the commercial speed of Carris is 14.3 km/h while that of Rodoviária de Lisboa (RL) reaches almost 17 km/h, figures which clearly show the difference in the operational context of the urban core, where Carris operates, vis-à-vis the urban cycle in peripheral cities and suburban connections which characterize the RL network, in line with UITP observations. In suburban operations, commercial speeds rise to 24 to 30 km/h, values comparable to those recorded in the case of Transportes Urbanos Torrejanos (TUT), operated by Barraqueiro Oeste, and urban transport in the villages of the Municipality of Alenquer, operated by Boa Viagem. However, it is important to remember that in the case of Line 10, where the converted bus is expected to operate, the average speed of each circulation is around 16 km/h, in line with the SORT 2 cycle.

 

The SORT test cycles are characterized and built on a standard commercial speed, which is a relatively uniform intrinsic characteristic of bus passenger transport in the various countries of Europe, regardless of geography. However, taking into account the need to typify not only urban routes in peak period, during which traffic is more, intense, the SORT project defined 3 operating patterns representative of the main operations of a public bus service: urban with intense traffic, mixed urban without intense traffic, and suburban. 

The following figures summarize the basis of the methodology developed by the UTIP to define the different operation cycles, based on trapezoidal speed profiles. These speed profiles reflect the constant acceleration, deceleration and idling cycles to which public service buses are subjected. After analyzing data representative of the main European capitals, the UITP defined 3 reference operating cycles, which have as representative values the average speed and the density of stops for picking up and setting down passengers.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

The SORT test cycles, although showing a good representativeness of circuits with comparable commercial speeds, do not allow, in an isolated way, to estimate the consumptions in complex operations and routes, many of which contain sections well represented by the 3 base SORT cycles. Given the impossibility of individually modeling infinite cycles, it is possible to estimate consumption on complex routes by weighting consumption for the different cycles such that

 

Thus, depending on the local operational profile, in order to better estimate the energy consumption given local specificities, e.g. average speed, topography, average load, it is possible to make a linear combination of the results obtained for the standardized tests. In this way, after adjusting the coefficients to their operational reality, operators can easily compare the expected consumption for the different market offers.

With the appearance and dissemination of commercial electric solutions, UITP adapted its test standards to the specific reality of electric vehicles, UITP SORT brochure: Standardised On Road Test Cycles for electric buses, in which special attention is dedicated to procedures and equipment, although without changes to the operational cycles, and a test to determine the energy storage capacity of battery packs was also added.

The E-SORT cycle aims to measure, in a precise and systematic, i.e., reproducible way, the vehicle's primary energy consumption, namely the consumption of the traction system and primary auxiliary low-voltage systems (12V/24V), and therefore to make it possible to estimate the autonomy of an electric bus. The methodology used is quite simple, requiring only the measurement of the useful energy of the system/energy storage unit (RESS - Rechargeable Energy Storage System) and a set of energy consumption measurements during the performance of one or several well-defined SORT cycles.

The tests performed according to the E-SORT reference cycle do not contemplate the energy consumption related to the air conditioning required for passenger thermal comfort. However, considering the tremendous impact of climate control on energy consumption and autonomy of electric vehicles, a future revision of the conditions underlying the E-SORT cycles is planned, which will allow the resizing of the energy storage units according to different operational contexts, i.e. climates.

 

From a practical point of view, the recommended test methodology begins by identifying the key points of the route, coincident with the vertices of the trapezoids that mark the speed variation, using traffic signaling poles or cones. After marking the key points of the route, successive tests are performed until 3 measurements are found with a maximum relative deviation of 2%,

 

Given the reduced length of the SORT 1 cycle, the determination of consumption for this cycle is carried out by performing two consecutive cycles.

 

However, the tests must still observe a number of additional rules, in order to guarantee the repeatability of the results and the fair comparison between vehicles tested under different conditions. In particular, the tolerance for trapezoidal speed profiles is +/- 1 km/h from the target speed, and a deviation of +/- 3 km/h is admissible for brief moments and only in the transitions between acceleration and constant speed phases. In addition, the intervals between test cycles must not exceed 10 minutes, the wind speed must be less than 3 m/s (10.8km/h), with gusts up to 8 m/s, the outside temperature must be contained within the range between 0 and 30°C, and the relative humidity must be less than 95%.

 

The E-SORT, on the other hand, since it also gives an estimate of autonomy, in addition to the test requirements of the SORT cycle requires an additional test to gauge the energy storage capacity of the onboard battery pack. This test simply consists of integrating the energy provided by the ESS between the maximum SOCM state of charge and the minimum limit defined by the manufacturer as the minimum emergency reserve energy, SOCR.

Also in the case of E-SORT, it is defined that during testing all secondary auxiliaries, except the headlights, must be turned off during measurements. Additionally, the temperature of the driveline (engine, RESS and, if applicable, gearbox) shall be kept within the reference range and the energy exchanges with the RESS shall be monitored as close as possible to the RESS, with a minimum frequency of 20Hz and ensuring an accuracy of 2%.

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