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Advanced diesel bus
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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 with resulting adverse impacts on public health. In order to reduce these emissions, advanced diesel buses are being developed with various emission control technologies. The term "advanced diesel bus" here refers to a bus meeting at least the 2004 US/2005 EU emission standards without the need of retrofitting (see ( w1 ) .

There are two types of system components that allow advanced diesel buses to produce lower pollutant emissions than regular diesel buses:

1. The fuel combustion system: advanced technologies refer to combustion optimization, improved fuel injection and variable geometry turbochargers ( w1 )

2. Aftertreatment emission control devices, including:

  • Diesel particulate filters (DPFs)
  • Diesel oxidation catalysts (DOCs)
  • Selective catalytic reduction (SCR)
  • NOx adsorbers
  • Exhaust gas recirculation (EGR) which extracts a portion of the exhaust gases and uses them to modify the combustion process itself.

Advanced diesel buses may employ different combinations of these. Most of the technologies listed in 2. can also be applied to conventional buses and are discussed in some detail in the retrofit bus section.

The technologies above require the use of low sulfur diesel (LSD) for optimal performance (for details see the retrofit bus or the cleaner fuels sections).

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The United States (US) and the European Union (EU) are introducing successively tighter emission standards for motor vehicles. The following table lists the standards regarding the most important pollutants, NOx and PM, for the timeframe until 2010 (w2). 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 procedures) 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
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.

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 V) 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 of different technologies below, the important message is that there are a variety of technologies which have emerged and have been used by manufacturers to achieve the emissions regulations which apply in the market where the engine has been or will be sold. Some of these technologies will be described below.

Injection rate shaping is one way of tailoring the fuel injection event to reduce peak flame temperatures without impacts on fuel consumption. Injection rate shaping is possible because of electronic control and advances to the fuel injectors that work with the electronics.

Approaches include using a pilot amount of fuel before the main injection event and splitting the main injection of fuel into two or more events. Injection rate shaping has been shown to simultaneously reduce NOx by 20 percent and PM by 50 percent, under certain operating conditions. Some fuel injection methods demonstrated to achieve effective rate shaping include the common rail, the mechanically actuated electronically controlled unit injector, and the hydraulically actuated, electronically controlled unit injector.

EGR has been researched extensively in recent years, especially as a means of complying with upcoming emission standards in the United States and Europe. EGR is the recirculation of a portion of the exhaust gas into the intake. It reduces NOx formation in the combustion chamber by diluting the air with inert mass (recirculated exhaust gas), which reduces peak flame temperatures when fuel is injected. Significant reductions of NOx emissions have been observed with the use of EGR. Laboratory studies have shown that EGR can reduce NOx by 40-50 percent at rated power, with minimal impact on PM, and by higher percentages at other loads, with some impacts on PM. These reductions rely on recirculating a large portion of the exhaust, with the amount depending on the speed and load of the engine. EGR systems have been used in Light Duty Diesels for many years.

One advanced combustion technique still under development for reducing engine-out emissions is homogeneous charge compression ignition (HCCI). Conventional diesel engines use a mixing, or turbulent diffusion, controlled combustion process, where fuel is injected late in the compression stroke into hot, compressed air, resulting in autoignition. The rate of combustion is controlled by the rate at which fuel can mix with air, because chemical reaction rates are much faster than mixing rates. NOx formation is high on the lean side of the flame, and PM formation is high on the rich side of the flame. In HCCI, fuel and air is premixed prior to introduction into the combustion chamber. Ignition occurs spontaneously throughout the mixture as a result of compression. This process produces ignition at a large number of sites throughout the combustion chamber, virtually eliminating both the locally lean and rich zones that cause high NOx and PM formation.

Under low and medium loads, preliminary data have demonstrated NOx reductions of 90 percent or more, compared to conventional diesel combustion. The thermal efficiency of HCCI has been shown comparable to that of conventional diesel combustion at part loads. Challenges associated with HCCI that are currently being researched include the control of combustion initiation and rate, effective fuel and air mixture preparation, and the achievement of stable HCCI under high loads and full power output. It is likely that further development in sensors and computer controls will be needed to achieve the necessary control of combustion for the broad range of operating conditions that occur in the field.

For compliance with the 2007 -10 US on-road emission standards, it was noted earlier that NOx and PM aftertreatment devices would be necessary to reduce emissions below levels achievable through engine modification strategies. A key reason for not previously using high efficiency aftertreatment devices is the lack of the in-use ultra-low sulfur fuel necessary to ensure the proper operation of aftertreatment devices and prevent sulfate formation. With the introduction of 15 parts per million sulfur diesel fuel in 2006, diesel engines equipped with aftertreatment devices and cooled EGR will be over 90 percent cleaner than today's engines. Diesel fuel with a maximum sulfur level of 10 ppm will be widely available across Europe by 2005 and will be used exclusively by 2009.

The NOx after treatment devices under development include the lean NOx catalyst, the NOx adsorber, and selective catalytic reduction (SCR). Lean NOx catalysts (active systems with diesel fuel as the reductant) have been shown to provide up to 30 percent NOx reduction under certain operating conditions, although an increase in fuel consumption, for supplying the reductant, results.

NOx adsorbers operate by storing NOx under typical diesel engine operations ("lean" conditions). Before the NOx adsorbent becomes fully saturated, engine operating conditions and fueling rates are adjusted to produce a fuel-rich exhaust, which reduces the stored NOx into harmless N2.

NOx adsorbers have been demonstrated to reduce NOx emissions by over 90 percent on ultra-low sulfur fuel, under many transient and steady-state conditions, with some fuel economy penalty.NOx adsorbers have a strong affinity for sulfur, which can deactivate the active catalyst sites and make the adsorbers less efficient over time. Improved NOx adsorber desulphurisation systems, more sulfur resistant active catalyst layers, and other methods are currently being developed to maintain the NOx adsorber's high efficiency over the long useful life of the engine.

SCR has been used in stationary source applications for many years. It works by injecting ammonia or urea into the exhaust upstream of a catalyst to reduce NOx emissions. Studies have shown SCR can reduce NOx emissions by 20 to 35 percent over the transient FTP and by 15 to 99 percent over off-cycle tests. The main issues surrounding SCR usage are controlling the rate of urea introduced to maximize NOx reductions, without any "ammonia slip" through the catalyst, and ensuring that the urea is properly replenished throughout the vehicle life to ensure emission reductions.

A well-demonstrated aftertreatment device for high efficiency-reduction of diesel PM is the diesel particulate filter (DPF). Over the last several years, test programs have focused on the emission reduction efficiency and durability of two types of DPFs, the catalyzed DPF and continuously regenerating DPF. In one program, using 54 parts per million sulfur fuel, the DPF reduced PM by 87 percent, to a level of 0.008 g/bhp-hr, sufficient to comply with the US 2007 on-road truck and ARB's 2002 transit bus PM standards. Another program showed that heavy-duty trucks retrofitted with DPFs and using fuel with 7 parts per million sulfur emitted 91 to 99 percent less PM, compared to trucks using diesel fuel with 121 parts per million sulfur and with no exhaust aftertreatment devices.

In recent years, there has been concern over the reduction of not only PM mass emissions but also the number of particles, especially those of small size. Several studies have now shown that DPFs reduce the PM number count by 1 to 2 orders of magnitude as well as substantially reducing mass emissions.

The 2004 US Heavy Duty standards lower NOx from present levels but retain the current PM standards. Therefore, manufacturers' approaches to complying with 2004 standards focus on improved fuel injection including rate shaping, combustion optimization, and in some cases exhaust gas recirculation (EGR) - possibly with variable rate turbocharging ( w1 ). According to the EPA, "engine manufacturers could meet the 2004 emission standards with engine control strategies" ( w3 ). While there are other possible technologies that might be employed in 2007-10, the two that appear most likely are NOX adsorbers and catalyzed particulate filter systems. Some Europe based manufacturers have indicated that they intend to use SCR technology.

According to the Clean Diesel Independent Review Panel, these technologies are making significant progress toward successful implementation in the 2007-2010 timeframe. The Panel found that Catalyzed Diesel Particulate Filters (CDPFs) are more mature than NOX adsorbers. According to the Panel, transit buses, school buses and other diesel vehicles are being retrofitted with CDPFs and other particulate filters throughout the US, and CDPFs are being used throughout Europe and elsewhere. Further, International Truck and Engine Company has already certified a CDPF-equipped medium-heavy-duty engine at the 2007 PM standard as well as the 2007 hydrocarbon standard. It should be noted, however, that these engines are limited to vehicle applications that fit the proper exhaust temperature profile and only use 15-ppm sulfur fuel.

In Europe on the other hand, many manufacturers expect to be able to achieve the EURO IV standards with oxidation catalysts and without the use of particle filters. Further NOx reductions will only be required in Europe in 2008.To achieve the Euro V standards, European manufacturers are focused on the use of SCR rather than NOx adsorbers and many are expected to use particle filters as well. One strategy being pursued would attempt to minimize engine out PM emissions and maximize fuel economy but with high engine out NOx; very efficient SCR would then be needed to meet the NOx standard. An alternative path would minimize engine out NOx and rely on very efficient particle filters to reduce PM. In the end it appears likely that some combination of these strategies will be needed.

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Diesel buses are very reliable. In 1996, the National Renewable Energy Laboratory (NREL) carried out a "Vehicle Evaluation Program" for the US Department of Energy (DOE), using the number of road calls to assess bus reliability of various alternative fuel buses ( 1 ). In this study, the diesel buses at almost all test sites had an average of less than 0.2 engine/fuel system related road calls per 1,000 miles, which in all cases was less than or (in one case) equal to the alternative fuel buses.

Experiences with the durability of the various aftertreatment devices can be found in the retrofit bus section. As advanced engine systems are only now entering the market, their reliability in long-term daily use remains to be assessed.

In retrofit applications, DPFs have demonstrated highly efficient PM control and durability in Europe, using ultra-low sulfur fuel.

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An approximate estimate of the incremental costs of complying with different engine standards is provided in the table below. It should be noted that the typical experience has been that over time these costs tend to come down as manufacturers become more proficient in designing their engines and as economies of scale take effect.

Emissions Standards
Estimated Approximate Costs
Euro 1
500$
Euro 2
2500$
Euro 3
3500$
Euro 4
4000$
Euro 5
7000$


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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Advanced diesel buses, i.e. new buses whose engines meet at least the US 2004 / EU2005 standards or equivalent, are currently entering the market - see ( w2 ). With regard to experiences with individual aftertreatment technologies, please visit the retrofit bus section.

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Apart from the extra costs involved, the main obstacle to the effective use of advanced diesel buses is likely to be the availability of low sulfur diesel required for the proper operation of many emission control devices.

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