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Conventional diesel bus

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 typical buses at present, these emissions are controlled primarily through improvements to the basic engine, rather than through the use of aftertreatment devices (other than diesel oxidation catalysts and particulate filters in limited applications.) These control techniques are usually limited by a NOx and PM tradeoff, where strategies to reduce one pollutant will result in an increase to the other.

NOx formation is directly dependent on temperature. Therefore, NOx control in an engine is accomplished by reducing peak combustion temperatures and the duration of these high temperatures in the combustion chamber. PM, on the other hand, is primarily the result of the incomplete combustion of diesel fuel. Control technologies to reduce PM generally focus on improving combustion of the fuel, which results in higher combustion temperatures and NOx. Some strategies currently used to control both diesel NOx and PM emissions include turbocharging, intercooling, combustion chamber design changes, injection timing retard, and high pressure fuel injection.

It is important to emphasize that control technologies are not added together one piece at a time but rather are integrated as a system by each engine manufacturer in a manner consistent with not only the emissions targets but also that manufacturers overall strategy considering costs, performance and fuel economy among other factors.


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 for the most important pollutants, NOx and PM, for the timeframe until 2010 (w1). All the tests are carried out by the use of a chassis 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.

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

7.0 (5.3)

0.15 (0.11)
1998 (US 98) 5.3 (4.0)

0.07 (0.05)**

2000 (EURO III)
5.0 (3.8)

0.1 (0.075)

2004 (US 04)*** 3.3 (2.5)*
0.07 (0.05)**

2005 (EURO IV)
3.5 (2.9)

0.02 (0.015)
2007 -2010 (US 07-10) 0.27 (0.20)

0.013 (0.01)


2.0 (1.5)


*including 0.w1w2 (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 (0.0w2)·
*** As part of a consent agreement with the US government, most diesel manufacturers will comply with these standards in October 2002.

Brazil and Argentina have introduced EURO II standards as reference standards (with slight modifications) from 1998 onwards (w2 ) and Chile is looking to introduce 50 ppm Sulfur in diesel fuel and Euro III standards in 2004. As part of a comprehensive package of measures, Chile has concluded that these measures will cost $127 million per year but have benefits of $260 million per year. (1)

At present, nearly all HDVs used and produced are diesel powered (se also the truck (introduction).


The turbocharger reduces both NOx and PM emissions by about 33%, compared to a naturally aspirated engine. The turbocharger boosts the pressure (and temperature) of the air entering the engine. This allows more fuel to be added to increase power output, while still inhibiting PM formation. The power to drive the turbocharger is extracted from the engine's exhaust stream. In other words the turbocharger increases engine power and efficiency, which may translate to lower emissions.

Intercooling of the turbocharged air results in even greater NOx and PM reductions, by decreasing the temperature of the charged air after it has been heated by the turbocharger during compression. Cool air is denser than hot air and, therefore, this approach complements the turbocharger by further improving cylinder filling.

Combustion chamber design includes modifications to the shape of the chamber, location of the injection swirl, crevice volumes, and compression ratios. These can result in significant NOx and PM reductions by changing the conditions that occur during fuel combustion.

Injection timing retard is used to reduce the peak flame temperature and, thus, NOx emissions. However, timing retard typically lowers efficiency, resulting in increased fuel consumption and PM emissions. High pressure fuel injection can regain some of the efficiency loss by improving the atomization of the fuel spray and air utilization, resulting in more complete combustion.
A thorough discussion of these technologies can be found in document (1) "Air pollution from Motor Vehicles - Standards and

A thorough discussion of these technologies can be found in document (1) "Air pollution from Motor Vehicles - Standards and Technologies for Controlling Emissions", Asif Faiz, Christopher S. Weaver, Michael P. Walsh, 1996, 266 pages.

In the 1990s, manufacturers introduced oxidation catalysts on some of their truck engines and the majority of the urban bus engines to reduce PM emissions. Flow-through oxidation catalysts effectively oxidize gaseous hydrocarbons, as well as the soluble organic fraction of PM. A recent test program showed that oxidation catalysts reduced transient FTP emissions of PM by 23 to 29% and hydrocarbons by 52 to 88%, using a typical D2 fuel (368 parts per million sulfur). Testing with a low sulfur diesel fuel (54 parts per million sulfur) resulted in an additional 13% reduction in PM. The 1990s were also the period when the diesel engine evolved into the electronic age.

The development and improvement of the emission control technologies outlined above were induced by tightened emission standards in North America, Europe and Japan. They led to a reduction of toxic pollutant emissions from vehicles over the last few decades. This trend is continuing through successively tighter emission standards in many parts of the world. The DieselNet lists such standards for the US, Europe, Japan and Latin America, as well as certain other countries worldwide (see also conventional diesel bus). These standards essentially reflect the emission reduction potential to be achieved by emission control technologies for light and heavy duty vehicles.

As noted earlier, engine manufacturers design engines with a variety of factors in mind including costs, performance, fuel economy and emissions as well as other factors. Different engines, therefore, while complying with the same emissions standards (e.g., Euro II) may utilize very different combinations of the above measures to achieve each engine's given market niche. Different manufacturers may also employ fundamentally different philosophies, believing for example that they have a competitive advantage with one technology versus another. Therefore in considering the discussion above, the important message is that there are a variety of technologies which have emerged and have been used by manufacturers to fulfill the emissions regulations of the market where the engine has been or will be sold.


Diesel vehicles in general can be considered to be very reliable, especially in comparison to more recent technologies employing alternative power trains. Diesel engines look back on a comparatively long evolution of continuous development.


So far heavy duty emission standards could largely be met through engine modifications and have not required exhaust aftertreatment - and therefore have not led to significant cost increases.


Buses meeting current and past EU and US standards are widely available around the world and are highly reliable.


Diesel technology is reliable and durable, it is important to assure that they receive the appropriate fuel and lubricants and necessary maintenance.

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