ENERGY accounts for 15-20% of the operating costs of light rail and metro systems in western economies, and after staff is the second largest budget item for operators.

 Any effort to reduce energy consumption is therefore very welcome and the recently-completed European Research project, Osiris, attempted to do exactly this by analysing a number of technical and operational energy efficiency options.

Osiris OsloThis included a new auxiliary converter, lithium ion mobile energy storage batteries and deep water-cooled technical rooms. The three-year project brought together 17 partners including the International Public Transport Association (UITP) and was co-funded by the European Union's 7th Framework Programme.

One project presented during the final conference held in Brussels on March 31 was a proposal for an urban rail duty cycle methodology.

Irrespective of technical innovations, each train type possesses its own unique physical and dynamic characteristics. Selecting rolling stock that is energy-efficient is therefore of critical importance as the decision will bear consequences over the whole life cycle of a train.

In 2013, 40% of operators were making procurement decisions without any information on future energy consumption. However, 95% said that they were planning to include these requirements in their future tenders.

At the moment, there is no ready-to-use tool or method to do this effectively. Performing pre-sales consumption measurements on all potential suppliers would be a very tedious and prohibitively expensive process, as a result the idea came about to develop a set of operationally-representative duty cycles for metro and light rail rolling stock.

Adapted from similar methodology used in the automotive industry, these duty cycles would provide a standardised and unified way of measuring energy consumption in urban rail systems and provide a comparison of the respective energy consumption of different vehicles that could be used during the procurement process.

When looking for an effective methodology for urban rail transport, the Osiris partners have been exploring other domains. For years, car buyers have been able to compare the energy and emission-efficiency of different models because they have all been tested according to a pre-defined methodology. While the information does not provide an accurate consumption prediction tool for each customer, it does offer a relative ranking of energy-efficiency.

But with metro and light rail trains varying significantly from cars, how should this concept of pre-defined comparable tests develop? Current practices in urban buses and recommendations for main line railways were subsequently analysed to identify a suitable methodology.

In 2001-2003, in collaboration with bus operators, manufacturers and scientific institutes, UITP developed a process to compare the fuel consumption of city buses that could be used during tendering. The method was a success and has since become a routinely recognised reference in bus procurement.

The underlying principle proposes a limited number of cycles (Sort 1, 2 and 3) which reflect typical operating conditions and determine average commercial speed of 12, 17 and 25km/h. Operators can weigh the results of several cycles to come as close as possible to their specific conditions of operation.

The system was initially developed for 12m-long diesel buses, the largest market segment, but due to its success, it was adapted to CNG, hybrid and other vehicle sizes, including double-deckers.

After some initial verification, Osiris concluded that the approach of modular duty cycles defined as a combination of basic modules would be realistic, practical and reliable and decided to investigate the approach in more detail to adapt it to urban rail.

Modular approach

The "modular approach," which defines the duty cycles as basic modules, was selected with each module representing a small portion of a line between two stations, which is used to build the service profiles. The idea is that if duty cycles are representatives of real lines, the modules should be representative of a real section.

This approach reveals two key advantages:

  • with basic modules or "building blocks," a relatively short test track is sufficient to measure standard data, and
  • combining a limited number of modules offers the ability to build an unlimited number of lines, thus making it easier for operators or suppliers to learn and use the system.

This approach also provides a simple, easy-to-repeat and accurate comparison method of the energy consumption of different vehicles. The standardised typical duty cycles cover the following categories of passenger service profiles:

  • heavy metro
  • light metro and exclusive right-of-way light rail with at-grade crossings, and
  • mixed-traffic light rail.

These duty cycles are designed to represent a realistic, average and generically applicable methodology for European urban rail systems. As the modules have to be representative of real lines, Osiris decided to build the modules on the basis of robust statistical data of real lines, inter-station distance and speed data.

This data was collected thanks to the input of several UITP members, which met every six months at a UITP-managed Support and User Group (SUG). UITP often includes this type of working group in the EU projects in which it participates to enable its members to access the project's results as they become available and to contribute and validate them.

The data used to define the duty cycles covers more than 2000 sections of existing lines in 25 European cities with a wide range of dimensional, meteorological and geographical characteristics. Data has been collected for both peak and off-peak hours when available.

For a given distance between stations, several modules were defined, which differentiate from each other through their respective average speed. They also differ in their authorised maximum speed.

The main parameters of the duty cycles concering driving constraints and environmental and operational conditions are also shown in the table, along with the boundary conditions. This guarantees that duty cycles are used in a uniform way, and that results are comparable.

Stops at stations are important phases of train operation with passengers boarding and alighting. Therefore, each "module" will be concluded with the provision of a dwell time and door opening and closing. As station dwell durations vary a lot on different lines and stations, the duration will be defined as a parameter.

The Osiris duty cycle concept was designed and checked for consistency with reality using various simulation tools to analyse performance achievement. For example, the deviation delta between cycles and reality on a metro line in Milan and a tram line in Vitoria Gasteiz, Spain, is deemed acceptable with the following results: distance deviation of +0.55% and total time deviations of -6.8% in Milan, and distance deviation of +8.55% and total time deviations of -0.08% in Vitoria Gasteiz.

One recurrent criticism heard during the Osiris project when this concept was presented is that the Osiris duty cycle method considers only single train energy consumption in isolation, while real operation involves multiple trains. It is well understood that operation with multiple trains does have a significant impact on energy efficiency, particularly when braking and accelerating units interact through braking energy recovery.

However, the efficiency of braking energy recovery depends primarily on non-rolling stock features such as network characteristics, power supply architecture, operation model, and timetable design. It was subsequently concluded that the approach can offer a simple and fair estimation of intrinsic energy-consumption features of a train and that the relative energy-efficiency of the various types of train would translate in the same way when considering fleet interaction behaviour.

So where do we go from here? Due to Osiris' time restrictions, it was not possible to perform energy consumption measurement in the modules and compare them with real consumption over a period of time. Although the Osiris team is confident in the concept, this critical step is yet to be done.

In addition the procedure for energy measurements, result validation and test protocol has also not yet been fine tuned along with the procedure for verification and contractual arrangements. With 15 years experience, UITP's standardised on-road test cycle (Sort) testing will certainly help to find the right approach and identify the best compromise between reliability and test affordability.

In addition, UITP is willing to launch a large awareness and information campaign in order to explain, promote and convince stakeholders to finalise and adopt the Osiris duty cycle methodology and tool. UITP and Unife, with their diverse membership of operators and manufacturers, offers an ideal platform to advance this project, and an Osiris follow-up is proposed as part of the work plan for 2015-2017.