Under the Nissan Green Program 2005, a mid-term environmental action plan, Nissan announced that ultra-low emission vehicles (U-LEVs) would account for 80% of its domestic unit sales by the end of March 2003. That goal was actually accomplished about two months ahead of schedule, as the U-LEV share of total unit sales exceeded 80% in January 2003. The effect of this high U-LEV ratio on improving air quality, in terms of reducing nitrogen oxide (NOx) and hydrocarbon (HC) emissions, is equivalent to selling approximately 400,000 zero-emission vehicles, such as fuel-cell vehicles, on an annual basis. Nissan is contributing substantially to environmental protection by incorporating practical technologies in products that are highly beneficial to the environment immediately and can be widely diffused at affordable prices.
| What are U-LEVs? |
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The Ministry of Transport (currently the Ministry of Land, Infrastructure and Transport) initiated a low-emission vehicle certification system in April 2000 to promote the diffusion of clean vehicles with low exhaust emission levels. Under this system, vehicles are ranked in three categories based on how much they reduce HC and NOx emissions relative to the levels mandated by the exhaust emission regulations enforced in October 2000. U-LEV certification is awarded only to vehicles that achieve the top ranking by reducing HC and NOx emissions by an additional 75% from the 2000 exhaust emission standards for passenger cars.
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Hydrocarbon (HC) discharge reduction level
Nitrogen oxide (NOx) discharge reduction level
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| Major Technologies Adopted on U-LEVs |
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Catalyst substrate with ultra-low heat mass |
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To quicken the temperature rise characteristic of the catalyst, the heat mass of the substrate was lowered by reducing the thickness of the substrate walls to an ultra-thin and unprecedented dimension of 2 mils. The lower heat mass allows the catalyst to warm up faster, which shortens the catalyst light-off time significantly. |
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Effect substrate wall thickness on catalyst light - off time
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High-accuracy air-fuel ratio control system |
The air-fuel ratio upstream of the catalysts is optimally controlled at all times to enable them to display their maximum conversion performance. This is accomplished by providing an air-fuel ratio sensor upstream of the close-coupled catalyst and an oxygen sensor in front of the high-efficiency HC trap catalysts located under the floor.
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High-velocity jet-type high-swirl combustion |
An electronically controlled continuously variable swirl control valve is used to achieve a high throttling rate, which increases the airflow velocity into the cylinder to facilitate high-swirl combustion. As a result, the combustion process is stabilized, especially right after engine start when combustion tends to be unstable, resulting in substantially cleaner engine-out emissions.
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Two-stage high-efficiency HC trap catalyst system |
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The HC trap system temporarily traps HCs on an adsorbed in the low-temperature range where the catalysts cannot function. Once the temperature rises to the catalyst light-off level, the adsorbed HCs are released for conversion. Moreover, HC trap catalysts are arranged in two stages. The second-stage catalyst again traps and converts HCs that were not removed by the first-stage catalyst, thereby enhancing HC conversion efficiency. |
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