There's still a long way to go towards eliminating Hong Kong's air pollution but two mainland-based makers of electric buses are about to take the city further along the road towards blue skies.
BYD, backed by US investment expert Warren Buffet, and fast-growing Great Dragon - a joint venture between mainland bus maker Yixing and Global Electric Vehicles, owned by locally-listed Dah Chong Hong - have shown keen interest in developing vehicles suitable for Hong Kong.
They believe the city is ideal for developing right-hand-drive vehicles for sale in larger markets such as Britain, Australia and India.
Both companies are set to test-drive pilot vehicles designed with Hong Kong's conditions in mind - more than 10 years after disappointing results from plans to introduce electrically powered vehicles in a city choking on its exhaust fumes.
'We are going to make a perfect bus for the Hong Kong market,' said Paul Lin for BYD, which is carrying out commissioning tests on a bus assembled at its Changsha production plant.
Raymond Lo Yuk-shun, Great Dragon's managing director, said tailoring electric buses to a specific locale was the key to capturing the Hong Kong bus market. 'You just can't have one design to meet all the needs. You have to fine-tune each bus for special needs.'
In 2001, some 14-seat electric minibuses were tested for six months, along with minibuses running on liquified petroleum gas. The electric buses were badly outperformed by the latter. They couldn't travel as far, were difficult to charge, spluttered going uphill and were expensive to maintain.
Two years later, an estate at Ma Wan introduced an electric single-decker bus for its shuttle service. Three of the buses were retired within two years, and the last one in 2010. The buses were said to run inefficiently and most of the time were sidelined for maintenance.
Not only must an electric bus meet legal requirements - technical and safety standards governed by road-traffic regulations - but it must suit Hong Kong's operating environment. And it must be reliable enough to transport millions of commuters every day.
One of the biggest concerns with electric buses is their range, which is often limited by the size and efficiency of batteries and the availability of recharging facilities.
With a fully charged battery an electric bus' usual range is between 200 and 300 kilometres, which approaches the average daily travelling range of each of the city's three franchised bus operators - Kowloon Motor Bus (KMB), New World First Bus (NWFB) and City Bus - who operate about 5,800 of the 13,000 buses in Hong Kong.
In 2010, the vehicles of the three bus firms travelled a total of 440 million kilometres. This means that, assuming 90 per cent of the fleet was used at all times, the daily average travelling range of each of these 5,800 buses was around 230 kilometres (KMB 246 kilometres, NWFB 205 kilometres City Bus 253 kilometres).
In reality, the terrain can vary tremendously depending on the routes, making the suitability of electric buses much more complicated. For example, with a driving range of about 200 kilometres, an electric bus can handle 87 trips without recharging on KMB's short 71B route, which runs about 2.3 kilometres within Tai Po new town. But a similar bus could only make four trips on KMB's longest route, a 50-kilometre trip from Fanling to Wan Chai.
Driving range is just one consideration. Climbing power is the weakest link for electric buses - making Hong Kong a steep challenge.
'Forget about getting up to The Peak on an electric bus for the moment,' said Lo. 'The battery can barely support the vehicle to finish the trip.
'Even if it can, the bus might need power replenishment right away at a terminus.'
A complete transition to electric buses in Hong Kong could take years. Until then, Lo predicts, the city's streets would see a mix of power sources for buses.
'It will still take us a long time to replace these buses, perhaps 10 to 20 years, I don't know,' he said. 'But this is going to happen sooner or later, when technology advances could lower the battery weight by as much as half.'
Apart from right-hand drive, Lo said, electric buses for Hong Kong would require larger batteries and motors than those for the mainland. They'll need greater power to handle the hills, high volume of passengers and need for air conditioning.
But a larger battery could mean fewer seats or a small luggage area, because the weight of the bus must not exceed the legal standard of 16 tonnes for a single decker - a rule that might have to be relaxed in the long term.
Where and how the battery cells are installed in a bus is another concern, because Hong Kong buses have to pass a tilt test - no flipping when tilted sideways at up to 35 degrees. Improper battery location might be dangerous.
Chinese University, located on top of a hill off Ma Liu Shui, is planning to introduce two electric buses under the city's pilot Green Transport Fund, set up to support the testing of green and innovative technologies for public transport and goods vehicles.
The university is getting a crash course in the vehicles' pluses and minuses.
'The ability to climb is a very crucial factor in selecting an electric bus,' said Joseph Chan Ping-tak, the university's environmental sustainability manager. But that strong climbing power and better driving range might come at the expense of passenger capacity and even the provision of air conditioning.
'We hope we don't have to make such a trade-off since we are not sure if the bus will be designed in a way that the windows can actually be opened,' Chan said, adding that none of the 14 diesel-run buses they have now were equipped with air conditioning. On average each bus can pack in as many as 60 students, mostly standing.
Great Dragon is now going full steam ahead to develop and build 16 electric single-decker buses early next year for trials by organisations awarded subsidies under the Hong Kong government's Green Transport Fund.
The procurement has yet to be determined after a public tender, but Lo is confident of winning its bid. 'We believe we are at least three to five years ahead of our rivals,' he said.
One likely rival is BYD, which signed a memorandum with KMB to supply an electric bus for trials that were to start late last year, but were then postponed to allow more time to develop the vehicles.
Lin denied that the early move was a public relations ploy for BYD, which has been plagued by plunging sales on the mainland for its electric cars. 'It makes no sense to develop a right-hand-drive bus in a left-hand-drive market, and that's why we think the Hong Kong market is very important to us. This is definitely not a public relations war,' Lin said.
KMB is also looking at another technology - a super-capacitor bus used at the World Expo in Shanghai. This bus doesn't need a huge battery and has a small travelling range of only 50 kilometres when fully charged. But, by using a pantograph - an extendable mechanical link - on the bus roof, it can be recharged every time it stops at a bus stop equipped with an overhanging power cable. A 30-second charge is said to provide power to go one kilometre.
The bus, nicknamed gBus, has been tested by KMB for internal use since 2010. While the bus firm said the test was satisfactory, it plans to introduce another prototype with a longer range within this year. And the company also plans to test battery-driven buses and hybrids.
KMB would check vehicles' power consumption over different terrain, in loading conditions and in different weather, a company spokeswoman said.
Lo said super-capacitor technology had some problems that need to be addressed. 'What if there is a serious traffic jam and the buses run out of power and the air conditioning stops?' he said. 'It might also need a larger charging station, and a strong voltage power cable might not be welcomed by residents living close to the bus stops.'
KMB said the gBus could have sufficient power to maintain 30 minutes of air conditioning if it was stuck in traffic.
Lo said the West Kowloon arts hub might be an ideal spot for these buses because of the flat spaces and short distances.
Electric-bus makers are not the only ones revving up for more business. Traditional bus makers are devising greener vehicles, too - including diesel-electric hybrids. Both Alexander Dennis and Volvo have rolled out these new-generation buses in Europe, especially in London, where the Olympic Games this year will boost green city transport.
These makers believe the hybrid buses, being so similar to current buses, offer plenty of advantages over electric powered vehicles.
'It is well recognised that given the limited road space and high passenger volumes, Hong Kong needs double-deck buses. This represents an even greater challenge in the context of an electric vehicle,' said a spokesman for bus maker Alexander Dennis in Hong Kong.
The number of electric buses that were used at the 2008 Beijing Olympics. They had a range of 130 kilometres