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Diesel combustion engines are associated with pollutant emissions of which PM and NOx are of special concern. In this respect, conventional diesel buses contribute to the deterioration of urban air quality. In order to reduce these emissions of PM and NOx from diesel buses, there are a variety of different emission control technologies available.
Significantly reduced emissions can be obtained by re-powering older vehicles, presently equipped with old high emitting diesel engines, to use the latest low emissions diesel technology. Heavy duty engines are typically overhauled and rebuilt several times over the life-time of the vehicle (at a cost of $4000 to $8000 or more, depending on the size and the circumstances). However, in some cases it would be possible to replace the old engine it with a new one meeting current standards, provided that the engine fits into the engine compartment (which is normally easier to do if the engine is placed in the rear part of the bus) and it is adapted to the drive train.
The United States (US) and the European Union (EU) are introducing successively tighter emission standards for engines used in heavy duty vehicles. The following table lists the NOx and PM emission standards for the timeframe until 2010 (w1).
The tests are all performed by the use of engine dynamometers but since the test procedures differ between the US and the EU, the values given below can not be used for a direct comparison but only provides a general trend of successively tighter emission standards.
COST CONSIDERATIONS
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Nitrogen oxides
(NOx)g/kWh (g/bhp-hr) |
Particulate matter
(PM)g/kWh (g/bhp-hr) |
| Year |
United States |
European Union |
United States |
European Union |
| 1996 (EURO II) |
|
7.0 (5.3)
|
|
0.15 (0.11)
|
| 1998 (US 1998) |
5.3 (4.0) |
|
0.07 (0.05)** |
|
| 2000 (EURO III) |
|
5.0 (3.8)
|
|
0.1 (0.075)
|
| 2004 (US 2004)*** |
3.3 (2.5)* |
|
0.07 (0.05)** |
|
| 2005 (EURO IV) |
|
3.5 (2.9)
|
|
0.02 (0.015)
|
| 2007 (US 2007) |
0.27 (0.20) |
|
0.013 (0.01) |
|
| 2008 (EURO V) |
|
2.0 (1.5) |
|
0.02 (0.015) |
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*Including 0.67 (0.5) non-methane hydrocarbons (NMHC) - manufacturers have the flexibility to certify their engines to one of two options, the alternative being a combined limit of 3.2 (2.4) NOx+NMHC
** in-use PM standard 0.09 (0.07)·
*** As part of a consent agreement with the US government, most diesel manufacturers will comply with these standards in October 2002.
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Compliance with the Euro I standards generally required modest changes in engine design to minimize particulate emissions, as well as improvements to the fuel injection system. Compliance with the Euro II standards is somewhat more difficult, generally requiring the use of turbochargers and intercoolers as well as high-pressure fuel injection - generally with computer electronic control of the fuel injection timing. However, the turbocharger and intercooler should also help to reduce fuel consumption by about 10%. Meeting the U.S. 1998 and/or Euro III standards will require further improvements in fuel injection, and electronic engine control systems.
Further actions that have to be taken into account in order to meet lower emission standards include inspection and maintenance, fuel economy penalty (or gain), and the production of low sulfur fuel. The actual emerging costs will be a function of the individual applications and situations. The US EPA estimated the additional cost to reduce the sulfur content of the fuel from 500 ppm to 15 ppm to the range of 4 5 cents per gallon. (For further cost estimates of low sulfur fuels, see Cleaner Fuels.)
Engines meeting Euro and US standards are widely available around the world.
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