United Nations Environment Programme

There are several advanced new vehicle models that go beyond current legislation in terms of emission requirements and that use either less fuel or an alternative fuel. The three main options at the current time are clean diesel vehicles, compressed natural gas (CNG) vehicles, and hybrid electric vehicles (HEVs). The Action-new vehicle Sheet in the Inventory and Options (Tool 18) will give you estimate of emission and fuel reduction by choosing advanced vehicles when buying new.

Diesel engines are inherently more efficient than petrol engines, but historically have had problems with high emissions, especially NOx and particulate matter. A clean diesel vehicle is a conventional diesel vehicle equipped with advanced emissions control technologies, such as diesel particulate filters and selective catalytic reduction (SCR) systems (See Tool 11 ). Use of these technologies goes beyond what is currently legislated, but will soon be mandatory (e.g. Euro V standards in 2008).

Clean diesel technology is available for buses and trucks and increases in sales as a consequence of stricter tail-pipe emission regulations for heavy-duty vehicles.  Euro V trucks are today available on the market. These trucks meet the coming Euro V legislation by introducing a selective catalytic reduction system (SCR) together with a particulate filter.

Compressed natural gas (CNG) vehicles
CNGvehicles have engines that have been adjusted to run on natural gas (95% methane) that is stored under high pressure (around 200 to 240 bars) in a fuel tank in the car. Older engines, both diesel and petrol, can be retrofitted to run on CNG. New engines can be made run on both CNG and on petrol (bi-fuel).

The advantages of CNG include the use of lower sulphur fuel and cleaner combustion processes, resulting in less particulate matter and hydrocarbon emissions. Dedicated CNG vehicles typically have lower emissions of NOx compared with standard petrol vehicles. CNG vehicles can also use a 3-way catalyst, which also substantially reduces emissions.

Globally, there are over 5,000,000 CNG or biogas vehicles, with Argentina, Brazil and Pakistan leading with over 1,000,000 vehicles each. India and the US have replaced diesel buses with CNB buses for the express reason of reducing air pollution. For an example see UPS CNG strategy

Hybrid electric vehicles (HEVs)
HEVs are powered using a combination of an internal combustion engine and an electric motor. This design makes the HEV more energy efficient, potentially achieving almost twice the gas mileage and substantially reducing emissions. In Figure 1 below, the principal difference between a HEV and a conventional vehicle is illustrated.

Conventional and hybrid vehicles system. The direction of the arrows shows the flow of energy. For more details on this, see "The Hybrid Electric Vehicles" UNEP publication.

HEVs are energy efficient for several reasons:

• HEVs recovers energy lost when braking – by running the electric motor as a generator, this energy is used to charge the battery.
• The combustion engine is downsized to run at an optimal speed, thereby increasing fuel efficiency.
• The combustion engine turns off and restarts automatically when idling.

HEV passenger cars have been available in many parts of the world since 1997. Markets for HEV buses and trucks are also growing, with several manufacturers selling HEV delivery trucks. For an example see FedEx HEV Strategy

Eletra’s hybrid delivery truck Photo: Eletra

For more info on Natural gas >> www.iangv.org
and on Hybrids >> www.hybridexperience.ca

For a comparison between Clean Diesels, Hybrid trucks, and CNG trucks click here >>