Vehicle technology improvements can reduce local and global pollutants from individual vehicles. A starting point would be to make more efficient combustion of existing petrol and diesel engines a key policy goal in developing nations.
Petrol and diesel driven vehicles
The introduction of more fuel-efficient catalytic converters has reduced levels of pollution but they are dependent on the availability of better fuel. Unleaded gasoline has entered the market for petrol-driven cars and lorries. A major innovation in the diesel market has been the use of direct injection turbo-charged engines, which use ultra-low sulphur diesel (ULSD) to reduce local particulate matter pollution. This diesel engine technology can provide fuel economy improvements in the order of 20-40% above those of its petrol driven equivalent but NOx levels remain higher than petrol-driven equivalent vehicles.
Natural Gas Vehicles
Natural gas vehicles (NGVs) run on Compressed Natural Gas (CNG), Liquified Petroleum Gas (LPG), and some NGV engines combine this with hydrogen. According to the International Association for Natural Gas Vehicles - IAGNV figures, there are over 5.7 million vehicles in the world today running on natural gas, with an annual growth rate of 30%.
NGV benefits include; lower emissions of local and global air pollutants, quieter engines, less odour, and reduced oil dependency. NGVs can reduce particulate matter by up to 85% compared to standard diesel engines. Carbon dioxide levels are 12% less than equivalent diesel engines when the whole life-cycle is considered.
Prompted by rising levels of local air pollution, New Delhi converted its public transport fleet to CNG in 2002. Levels of carbon monoxide fell in the city, but particulate matter only fell slightly. Levels of Nitrogen Oxides (NOx) actually increased.
This conversion also involved substantial investment in fuel supply infrastructure and more expensive vehicles. Some experts question if this investment is cost-effective given the modest environmental benefits of CNG.
Hybrid vehicles run partly on electricity supplied by an on-board battery, and partly on conventional fuel. Hybrids also save energy by capturing it from the car's braking system and going into idle mode when the vehicle stops. As petrol prices rise, hybrids are penetrating markets in the developed world with the likely consequence that the cost of new technology will fall as sales increase. The Toyota Prius passed the million sales mark in May 2008 but hybrids need to be made more easily available in the developing world. According to an IPCC Assessment Report, greater uptake of hybrids combined with forecast fuel efficiency technologies could lead to a 50% reduction in energy demand by 2030 amongst four-wheeled passenger vehicles.
Wholly electric vehicles (EV) could also play a strong role in reducing urban air pollution. There are three main types of EV; the first runs either on a battery, a supercapacitor or flywheel stored energy source; the second is hooked up directly to a mains transmission supply, the last generated on-board with a fuel cell. EVs release no emissions when driven and consume less energy than conventionally fuelled vehicles for each mile travelled. If supplied by a renewable energy source like solar energy or wind power, EVs can also impact on mitigating carbon dioxide emissions.
Since much of the vehicle fleet in Africa and Asia is composed of petrol-powered two-wheelers, there would be considerable benefit if electric models could replace them. There are, however, a number of technical and practical obstacles to securing greater uptake of electric vehicles. These range from their higher up-front cost, a limited mileage range and the difficulties of accessing points for safe, public electric re-charging.
Hydrogen fuelled vehicles
Hydrogen battery driven cars emit no carbon when they are driven. The most promising vehicle technology runs through an on-board fuel cell that reacts hydrogen with oxygen to generate electricity and water. Hydrogen fuel can be compressed significantly, so batteries are comparatively small, delivering a high level of on-board vehicle efficiency. Mercedes Benz Citaro hydrogen fuel cell buses currently operate in London and Reykjavik as part of the Fuel Cell Bus Club. The buses cost US$1.2M each and have a range of 300km.
Hydrogen vehicles are presently too expensive to be commercially viable. Hydrogen fuel cells use expensive raw materials such as platinum to separate hydrogen and oxygen, and to ensure that high-pressurised fuel is stored safely. High levels of investment would also be needed for a new fuel distribution infrastructure. Estimates of the cost in the US range from US$20 billion to half a trillion dollars. Essentially, this boils down to laying new pipelines and re-designing vehicle tankers and service stations. Hydrogen vehicles are also vulnerable to cold weather start-up problems and current types of battery have a short life compared to conventional batteries. Despite this, potentially zero carbon technologies like hydrogen fuel must be developed fast if we are to tackle the threat posed by climate change.
The rise in the number of vehicles being sold internationally is cancelling out vehicle efficiency improvements by a large magnitude. A 36% increase in demand for gasoline fuel is forecast to 2020 under present policy scenarios (Energy Information Administration, USA). This reflects an appetite for larger vehicles as populations become wealthier, and a preference for 'on-board' energy hungry devices such as A/C units in hotter countries. Vehicle technology will have an important role to play in reducing pollution but there is unlikely to be any 'technical fix' in the short to medium term to deal with transport's impact on climate change.