Sustainability: keeping the wheels of public transport turning

In the 1980s, the United Nations (UN) started to think about sustainability. In a report in 1987, the UN defined sustainable development as development that meets the needs of the present without compromising the ability of future generations to meet their own needs. In public transport, that means finding ways of using existing supplies of fuel more efficiently and ultimately finding sources of fuel that can be completely renewable.

Intercity and express coaches are likely to rely on diesel fuel far into the foreseeable future. At a recent launch of 20 new express coaches in Warsaw for his PolskiBus.com subsidiary, Sir Brian Souter said that fuel burn per seat was a crucial factor in the strategy for operating express services. In Poland, fuel accounted for 35% of operating costs, therefore it was very important to maximise fuel economy.

Urban buses are much riper for sustainable development. Normally, they return to the same depot each night, therefore it is relatively easy to install the infrastructure to support alternative fuels or drive systems.

If we start with standard diesel buses, Mercedes-Benz has supplied more vehicles to Euro VI standards than any of its competitors. The latest engines are extremely clean in terms of emissions with the added bonus that they are also more silent. Mercedes-Benz and its customers have found savings in fuel consumption of up to 7-8% compared with the previous generation. These benefits are so significant that, unlike its competitors, Mercedes-Benz is not offering a Euro VI hybrid option.

Cummins is the largest independent supplier of bus engines, having delivered more than 7,000 last year to customers in Europe (including Turkey). Earlier this year, Cummins looked beyond Euro VI, saying that there would be more stringent checking of the quality of urea and monitoring of consumption, more stringent on-board diagnostics (OBD) for NOx, and the need to demonstrate compliance to In Use Performance Ratio limits that require the OBD system to operate regularly.

The company predicted that, if the expected improvements in air quality are achieved with Euro VI, that should be the end of regulations on NOx and PM emissions. They expected the legislators to move to efficiency and to focus on CO₂. Cummins listed around 12 engine-based technology developments that, between them, could lead to savings in fuel consumption of more than 20%. Some were relatively minor, but others, like waste heat regeneration, could bring significant savings.

In recent years, hybrid drive systems have become a promising route towards more sustainable transport. Both series and parallel systems are now available. Prominent suppliers like Allison, BAE Systems, Eaton, Siemens, Voith, Vossloh Kiepe and ZF all offer solutions. Their systems are used by a number of manufacturers, principally Alexander Dennis, Iveco Bus, Solaris and Wrightbus with the New Routemaster. Volvo has chosen to go an independent route, with its own system, and is the market leader in hybrid registrations in Europe by some margin.

The best hybrid drive systems are achieving savings in fuel consumption of up to 40% compared to equivalent all-diesel buses running on the same routes. Some manufacturers are now able to offer the option of extending the percentage of all-electric operation through fast charging of buses for a few minutes at each end of their routes. This can be done either by charging plates that are set into the surface of the road at each terminal, connecting with plates on the underside of the vehicle, or by overhead gantries that can supply current to a pantograph or charging arm mounted on the roof of the vehicle.

Volvo has successfully demonstrated this technology in its home city of Gothenburg. Vehicles still have a small diesel engine, but it is normally only used in suburbs. Buses can travel through the city centre, including a route that goes inside a covered shopping arcade, in all-electric mode. Up to 70% of the mileage can be covered in all-electric mode.

Inductive charging, from underneath the vehicle, has a promising commercial future. Bombardier has developed the PRIMOVE system that can be used not only by buses, but by other suitably equipped vehicles. It has wide potential for other vehicles, such as taxis, local delivery trucks, postal and municipal vehicles. The technology already exists to charge them for the amount of electricity consumed at each stop.

Electricity is an emotional subject. If it is produced from coal, oil or gas, that has to be taken into the sustainability equation. The Swiss city of Zurich operates a large fleet of trams, trolleybuses and diesel buses. They reckon that 85% of their mileage is carried out by electric vehicles and they insist that, wherever possible, they use renewable hydro-electric energy.

Buses with full hybrid drive systems are considerably more expensive than standard diesel vehicles. Normally, the investment can only be justified if there is financial support for the difference in price. That is starting to change, because operators realise that the higher capital cost can be recovered through savings in fuel.

Much more recently, we have seen the introduction of mild hybrid systems, which are considerably less expensive, but still offer savings in diesel consumption in the region of 20%. Like full hybrid systems, they recover the energy that is normally lost whenever the brakes are applied, either descending hills or coming to a complete stop. The difference is that the mild hybrid system captures that energy in a flywheel which then returns the energy to the driveline whenever the bus restarts.

The system was originally developed by Williams for use in Formula 1 racing cars, but has been bought by GKN and put into volume production for other commercial applications. Following trials in retrofitted buses, Go-Ahead Group has signed for 500 of the systems to be installed in new buses. The first entered service in Oxford (UK) in September 2014 in a fleet of Alexander Dennis double-deck buses.

There are a number of manufacturers, notably Iveco and MAN, which have long experience of supplying engines that are fuelled by compressed natural gas. While it is still a fossil fuel, estimated global reserves are higher than those for oil and the emissions are even cleaner than those for diesel engines at Euro VI. Furthermore, gas engines run more silently and the latest units have performance characteristics similar to diesel in terms of acceleration.

There are down sides. Depots require gas refuelling facilities, normally including equipment that can take any impurities out of gas before it enters engines. Gas tanks, when fully-loaded, can add one tonne to the weight of a vehicle and that can limit the total number of passengers that can be carried within legal weight limits.

Liquefied petroleum gas is another alternative fuel, quite widely used in vehicles like taxis, but manufacturers like DAF and MAN stopped making LPG engines for buses and other heavy vehicles because their production was no longer economically viable. More recently, a couple of truck manufacturers have offered the option of liquefied natural gas that can be stored in tanks similar in volume to diesel but the fuel has had a troublesome history in buses in China.

There is also ethanol. Scania has stuck doggedly with this as an alternative fuel for more than 20 years. It can be produced in Sweden from the unwanted sap of felled trees. Over 700 buses in Stockholm are running on ethanol. Consumption per mile is quite a lot higher than diesel, so engines run hotter and also require more regular maintenance. However, ethanol can be produced from renewable resources. I have never been able to find the well-to-wheel equivalent cost to diesel, and its viability as a fuel for the bus industry must depend on comparative levels of taxation.

Still on the subject of Sweden, there is considerable demand for buses fuelled by biogas which is a renewable or sustainable resource. This is produced from sewage, waste from slaughter houses, surplus food from hotels and hospitals, etc. The Swedes – and the city of Oslo – see this as a win-win product, because there is a regular supply of raw materials and it is easier to dispose of the residual waste after the production process.

In the last few years, there has been a real surge in all-electric buses, propelled by on-board batteries, and therefore not reliant on overhead wiring like trolleybuses. The impetus has come mainly from China where the government has encouraged manufacturers, suppliers and universities to develop batteries and control systems. The best known manufacturer is probably BYD which has supplied all-electric vehicles, in small numbers, to several European customers.

The technology is still relatively in its infancy. If a full-size city bus is to have sufficient range for a full day’s operation, the weight of the batteries is around three tonnes. They not only have to propel the vehicle, but also provide power to heating, air conditioning, lighting, doors and all the other systems that consume power. That takes the unladen weight of a full-size all-electric bus to around 14 tonnes, and therefore restricts the number of passengers that can be carried.

The unladen weight can be reduced quite substantially, but that means that the vehicles have to be regularly recharged, with short boosts of current, throughout the working day. Going back to the definition of sustainability, it depends where and how that electricity is produced.

Last, but by no means least, looking towards the future, there is fuel cell technology. Almost 20 years ago, I sampled the first Mercedes-Benz prototype fuel cell bus, called NeBus. A heavily modified O405 city bus had a low-floor in the front part of the vehicle until a point just ahead of the rear axle. There were then three or four steps up and over the rear axle and all the equipment underneath. The performance was lively to say the least, with the only noise coming from auxiliaries like the power steering pump.

Mercedes-Benz later developed a second generation of fuel cell buses, putting them into trial operation with 10 European cities. They researched not only the performance of the vehicles but how hydrogen was produced.

A small number of third generation vehicles are now in service using hybrid technology. The fuel cells are far smaller than the earlier generation and much more efficient, so they have smaller on-board storage tanks. Daimler Buses will continue to work with this technology but readily admits it is only possible because they can share research and components with their colleagues in the car operations.

The cost of fuel cells has reduced dramatically in the last three to four years, but the price of hydrogen can be highly variable. In some cities it is almost a worthless by-product of other chemical processes, whereas in others it is expensive. It may well be a very desirable sustainable fuel but its widespread use in the bus industry is probably still 20 years ahead.

Biography

Doug Jack spent the first 17 years of his career in the commercial vehicle industry with British Leyland Truck & Bus in a number of senior management positions, including Company Secretary. He established Transport Resources International in 1986 as a specialised consultancy advising bus and coach manufacturers and suppliers. Doug publishes the comprehensive European Bus & Coach Guide every month and is the principal author of the World Bus & Coach Manufacturing Industry report that is published every two years. You can find out more about Doug’s activities on his website at www.dougjack.co.uk.