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Conventional diesel buses produce significant amounts of pollutant emissions - especially particulate matter (PM) and nitrogen oxides (NOx) - that cause a deterioration of air quality. In order to reduce these emissions, there are a variety of different emission control technologies available.

Substantial emissions benefits can be obtained by re-powering older vehicles that are now 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 of the vehicle, at a cost of $4000 to $8000 or more, depending on the size and the circumstances. Instead of rebuilding the existing engine, in some cases it would be possible to replace it with a new diesel engine meeting current standards, provided that the engine fits into the engine compartment and it is adapted to the drive train. Re-power of a bus is easier to do when the engine is placed in the rear part of the bus, giving more space in the engine compartment.

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 standards regarding the most important pollutants, NOx and PM, for the timeframe until 2010. All the tests are carried out by the use of an engine dynamometer.
It should be noted that since the conditions under which these standards must be achieved (test procedure) differ between the US and the EU, the values given below only provide a general impression of the legal demands, not an exact and direct comparison.

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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)

*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.

Diesel trucks in general can be considered to be very reliable, especially in comparison to newer technologies employing alternative power trains. Diesel engines look back on a comparatively long evolution of continuous development. In reliability tests of alternative fuel trucks, diesel trucks serve as the reference.

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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.
The incremental cost compared to the Euro II standard can be US$1,000 to $1,500. These numbers do not reflect the cost of producing diesel fuel with lower sulfur content, which would be a prerequisite for Euro III engines with catalytic converters.

Further costs that have to be taken into account include those for inspection and maintenance, fuel economy penalty (or gain) and low sulfur fuel. The actual emerging costs will be a function of the individual applications and situations. The US EPA estimated the additional cost for low sulfur fuel (15 ppm maximum compared to 500 ppm maximum) at 4 - 5 cents per gallon. (For further cost estimates of low sulfur fuels, see Cleaner Fuels.)

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Engines meeting Euro and US standards are widely available around the world.

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