Buses

Benefits

Although experience with hybrid buses is still very limited they do seem to have some advantages over conventional motor buses.

• No exhaust cloud. Normal motor buses (and duo-buses when running in motor bus mode) create the most noticeable pollution, noise and sometimes even visible smoke when they are starting from rest - ie: pulling away from a bus stop, red traffic signals, etc. With hybrid buses the electric drive system takes the strain so therefore it is possible to sidestep this. The electric power also makes acceleration quicker and smoother, which are beneficial for city driving.
• Reduced air pollution Studies conducted in the USA between 2004 and 2006 found that hybrid buses give off less NOx and CO than diesel and compressed natural gas buses; lower particulate PM emissions than diesel buses; and comparable PM levels to CNG and to diesel buses equipped with particulate filters. However it must be remembered that whilst hybrid buses produce less air pollution they do still pollute their local environments, and therefore are simply incapable of being substitutes for quiet zero emission battery-electric buses and overhead wire electric trolleybuses. Anyone who thinks otherwise is being deluded.
• Lower fuel consumption Studies conducted in the USA between 2004 and 2006 indicate fuel savings typically in the range between 5% and 35%. It seems that the level of savings largely depends on factors such as the hybrid system configuration (series / parallel), the duty cycle, average speed, the level of stop-and-go driving; whether the hybrid system has been optimised for peak fuel economy / lower emissions / other performance characteristics and the level of 'drain' on the hybrid drive from the bus accessories (for instance, air-conditioning).
• Regenerative braking and extended brake life. By reducing wear on brake drums and brake pads the regenerative braking system extends the life of the brakes significantly, resulting in the need for less brake maintenance, which also translates into lower labour and materials costs.
• Same Infrastructure Diesel powered hybrid buses can use exactly the same fuelling facilities as is already being used by diesel mechanical buses. This means that there is no need for the transport operator to invest in new infrastructure. However, as experience with hybrid buses grows, and as the batteries begin to age, so some bus operators may find it advantageous to install battery charging equipment in the garages to provide overnight boosting. This way they could at least be sure that the batteries start the day fully charged. The financial implications charging overnight might be minimal, especially as off peak / night use of electricity is usually very advantageously priced.

It should be remembered however that there would be even less pollution if 'series' type hybrid (and diesel-electric) buses were to be fitted with trolleypoles and only use the fossil fuel engines to extend services beyond the wires.

Drawbacks.

Apart from still polluting their local environment hybrid buses have a few other disadvantages too...

• New hybrid buses carry a significant price premium over conventional diesel buses. Whilst large orders of new vehicles will result in some economies of scale by virtue of being more complicated vehicles with multiple propulsion systems they will always be more expensive to purchase brand new. That said, ever stiffening environmental regulations will partially erode the difference as the costs of new conventional diesel buses rise too.
• In addition to taking up space batteries are heavy, potentially reducing the total number of passengers the bus can carry whilst staying within the maximum weight limits allowed within local laws.

Will Battery Replacement Costs Create A Financial Disaster?

A potentially very major cost associated with hybrid buses is battery replacement.

Lead-acid batteries have an estimated life of approximately three years, during which time they will also benefit from periodic reconditioning. Being a mature technology replacement lead-acid batteries can be purchased 'off the shelf'. NiMH batteries are predicted to last longer than (between five and seven years) plus weigh less than lead acid batteries, which helps keep the bus within its legal weight limit without reducing the overall passenger capacity. However, NiMH batteries are more expensive to purchase 'upfront' and are expected to also be more expensive when replacement becomes due. Lithium-ion (Li-ion) batteries are much lighter than other types of battery and do not have a 'memory' effect - but a unique drawback is that they degrade (ie: decline slowly and predictably in "capacity") due solely to their age whether or not they are used.

The cost of replacement batteries may yet prove to be the 'Achilles heel' of hybrid buses. In the USA estimated costs of replacing the NiMH batteries are in the region of USD $260,000 per bus, which, if the cost is amortised over the full six year estimated life of the batteries works out a whopping $45,000 / year.

As yet (2009) hybrid buses are still too new to know whether the batteries will remain viable for as long as predicted. It may be that this will only be learnt the hard & expensive way.

The potential enormity of replacing batteries on buses fleetwide might yet make hybrids commercially unviable - that is, without government financial assistance. Dealing with the large number of waste batteries will also require some environmentally enlightened decisions. In the long term trolleybuses (or just removing the batteries and converting series hybrids to trolleybuses) might yet prove to be a cheaper option.

Information about hybrid buses has been sourced from the following websites and publications....
The Electric Tbus Group http://www.tbus.org.uk/hybrid.htm
The free online "Wikipedia" encyclopædia http://en.wikipedia.org/wiki/Hybrid_bus
The U.S. Department of Energy's National Renewable Energy Laboratory (NREL) page on Hybrid Electric Vehicles - http://www.afdc.energy.gov/afdc/vehicles/hybrid_electric.html
A report issued by the U.S. Department of Transportation Federal Transit Administration, which is titled "Analysis of Electric Drive Technologies For Transit Applications: Battery-Electric, Hybrid-Electric, and Fuel Cells". Note however that this report does not look at overhead wire powered electric trolleybuses, which means that it is deficient when trying to make a proper evaluation of available electric bus technologies. www.gobrt.org/Electric_Drive_Bus_Analysis.pdf

Note that to see some of these reports you will need the Adobe Acrobat Reader which is a free download from the Adobe website - although dial-up users will find the files too large to download so must source from elsewhere - such as a magazine cover disk (or try the 'Foxit' Reader which is a significantly smaller file that they can download from here... http://www.foxitsoftware.com/pdf/reader) If using Adobe's Acrobat Reader then once it has been installed it is very strongly advised to update the software for the latest security patches (go to the 'help' menu and then 'check for updates').
(All links to external sites open in new windows).

Ideal Services...

Despite their higher complexity, purchase costs and issues related to the extra weight - and ultimate disposal - of the batteries there are many advocates who see hybrid buses as offering some worthwhile advantages over traditional mechanical buses. Depending on the route served and the type of hybrid technology used ('series' / 'parallel' as detailed above) hybrid buses have the potential to be suitable for bus routes serving rural areas where air pollution is less of a severe issue, as well as infrequent urban services where fixed infrastructure simply cannot be justified (financially).

However, for busy urban services overhead wire electric trolleybuses remain the optimal solution, so that - as with trams / streetcars / light rail - there will be zero air pollution in the street. Experience in cities which operate trolleybuses and have investigated the costings have found that bus routes operating at frequencies of six vehicles per hour (ie: 10 minute headways) or greater are more economic to operate with trolleybuses. The advantages improve considerably when there are several routes which share some sections of overhead wiring.

The Electric Tbus Group has conducted a detailed study which suggests that for London the conversion of the busiest bus routes (such as those with a frequency of every 5 minutes or more) would offer significant financial and environmental benefits. Thanks to the network effect where multiple routes operate along the same roads the situation would soon arise whereby many subsequent conversions would entail less additional wiring - both increasing the cost effectiveness of existing wiring and reducing the cost of the electrification of additional routes.

Trolleybuses and other forms of electric buses are more fully explored on their own dedicated page.

In Britain the city of Newcastle-Upon-Tyne has a small fleet of hybrid buses which have been sourced from New Zealand. Introduced in 2005, there are 10 of these innovatively styled vehicles and they are dedicated to the high profile QuayLink services linking the waterfront with other parts of Newcastle and Gateshead. Costing £200,000 each, these are 'series' type hybrid buses which use an (approximately) 25kW gas turbine to charge solid gel, water-cooled batteries. In addition the batteries are also charged overnight. Each bus seats 30 passengers plus there is space for a wheelchair and 20-30 standing passengers.

The genesis of these buses was a partnership of Christchurch City Council, local bus company Redbus and New Zealand bus builder Designline, with the aim of creating an electric bus that would help reduce air and noise pollution in Christchurch. With batteries not being a viable option for a full day's service so hybrid technology was chosen as an alternative option which would at least partially meet their aspirations. Since 1998 these buses have been operating a high frequency inner-city centre shuttle service that is free to ride, has proven to be very popular and has even been credited as creating a positive image for buses that has spread fleetwide.

In Christchurch the buses are powered by a LPG fuelled gas turbine whilst (for convenience) the British versions burn diesel fuel. An advantage of gas turbines is that they can burn pretty well anything liquid or gaseous, and this they do extremely cleanly - without needing catalysts or particle traps or special fuel additives, etc. They are also reputed to be very quiet with minimal vibration, so that it can often require keen powers of observation to determine whether they are running, and need very little maintenance. Unfortunately on the day these buses were sampled and photographed they had their passenger compartment heaters switched on - and as these use very noisy fans it was not possible to sample just how quiet or smooth their gas turbine engines really are. A disadvantage of gas turbines is that especially for smaller automotive sizes they are much less fuel efficient than diesels at full load and in any size, large or small, gas turbines have simply appalling part load efficiencies. These reasons help explain why their use (for surface transport) remains relatively rare.

In addition to the innovative visual styling and innovative propulsion technology is a remote monitoring system which allows an engineer sitting at the office desk to interrogate the buses' on-board computers, monitor performance, carry out diagnostic checks, etc. A similar remote monitoring facility has also been fitted to the buses in Newcastle-Upon-Tyne.

There is a story circulating relating to an occasion when one the Newcastle buses was being trialed in Carlisle. Apparently one day the engine refused to start up and not being sure of how to resolve this the bus garage people telephoned New Zealand for support. The solution suggested by the Designline technician was to connect a laptop computer and mobile telephone to the buses' electronics so that an attempt could be made to start the engine remotely (ie: from 11,000 miles / 17700km away!). This having been successful the ensuing 'thankyous' included the technician in New Zealand pointing out that whilst he was happy to be able to help it would be better if 'next time' the support call was made at a time other than 2.45am (local time)!

Also in Britain, six UK-sourced 'series' hybrid buses are being trialed on a central London route which is shared with ordinary motor buses. Media reports suggest that each of these experimental buses cost £40.000 more than normal motor buses. The buses are midi sized, with a total passenger capacity of 57, of which 26 are seated. They are fitted with car sized 1.9 litre turbo diesel engines which are coupled directly to a generator. Energy storage comes in the form of 2 banks of 14 lead acid batteries which are mounted each side at the rear of the bus, and add 900kg to the vehicles' overall weight. Provision has been made at the bus garage for overnight booster charging of the batteries, although this will only be done once felt desirable. Testing on an off-road circuit prior to use in London found an 40% improvement in fuel consumption. So far no information has been released detailing actual experiences in London, other than that due to overheating during the hottest part of the summer they had to be withdrawn from service.

The travelling experience on these hybrid buses is different to regular motor buses in that the usual engine noise (which varies according to speed) is replaced by a flat droning background engine noise, which the bus driver described as being akin to that of the aircraft engine noise heard by passengers whilst flying. Depending on where a passenger is standing or sitting it is sometimes possible to feel vibration from the engine too. Noise apart the electric drive system makes this bus smoother to ride in than regular motor buses, plus there is considerably less "rattle" when stationery. If sitting at the very back of the bus there is a slight diesel smell and the warmth of the engine can also be felt.

Click the speaker symbol or here to download a 426kb mono soundclip (named inside-hybrid-bus.mp3) which was recorded using a mobile telephone and demonstrates the engine noise as heard whilst sitting at the back of the bus.

In 2007 more single deck hybrid buses were introduced on a second bus service, in the south-east of London.

Double-deck Hybrid Buses.

At the end of October 2006 the same British-based bus manufacturer which built the hybrid bus being trialed in London built a prototype double deck hybrid bus too. At around the same time London's Mayor publically stated his interest in the vehicle, and that once full-scale production begins he would like to see as many as 500 a year being introduced in London, replacing existing fossil fuel motor buses.

This prototype double deck hybrid bus entered passenger service in March 2007, and even before it had been extensively tested it was announced that another 10 similar vehicles were to be built, with one going to Dublin, the capital of the Irish Republic, and the rest going to London.

Although the double deck hybrids use the same 1.9 litre turbo diesel engines as the single deckers, they feature different battery technology. Instead they use 30 20volt lithium ion batteries which provide a nominal 600v output, have a longer life and both give / take charge much more quickly. In addition their 455kg weight is far less than lead acid batteries, which had they been used would have added another 1.5tonne to the double deck buses overall weight. Most of the time the batteries provide tractive force, however when accelerating hard the diesel engine provides power too. Although the diesel engine runs at a constant speed, this speed is increased if the batteries are becoming depleted. Following experience with the single deck hybrids the double deckers cannot be driven on battery power alone if the diesel engine runs out of fuel.

Perhaps unwittingly the creation of a series hybrid double bus means that the introduction of trolleybuses is one step easier; this is because trolleybuses and series hybrid buses both use an 'all electric' drive train. It is how the electricity is sourced (diesel engine or overhead wires) which differentiates the two.