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Buses and coaches are the backbone of public transportation in Latin American cities. Their specific energy demand and pollutant emissions per person transported are much lower than for private vehicles (provided they are utilized to a sufficient degree). In absolute terms, however, they may still contribute significantly to air pollution. Since buses tend to be used in the most densely populated urban corridors, their impact on public health can be disproportionate to their numbers. Therefore a transition to cleaner and more energy effective bus systems, in addition to other, non-technological measures, could have a significant positive impact on local air quality and public health and could also contribute to global climate protection.
All of the approaches listed in this section have different characteristics in terms of energy efficiency, life cycle and tailpipe emissions (see table below). Most have the potential to reduce emissions from existing buses but only if implemented carefully. It is important to note that there are several non-technical factors that affect the environmental impact from public transport buses. The organization of the public transport system, types of structure of ownership for buses, bus line commissions, design of line network, management of bus operations, renewal of the bus fleet and driver education will all affect environmental performance.
Which approach will achieve the greatest overall, or the most desirable specific environmental improvements compared to the currently used diesel buses, can only be determined by means of a thorough analysis which takes all the relevant local conditions into consideration.
In this section, four principle technology choices in order to improve the emissions from existing diesel buses are presented. These include: inspection and maintenance (I/M), retrofitting more advanced pollution controls, replacing existing engines with cleaner engines, and conversions of existing diesel engines to CNG or LPG fuels.
The choice of which option to use, or whether an entirely policy-based approach might be the most cost-effective option, depends on the specific situation.
All in use vehicles can benefit from the improved maintenance that follows a carefully designed inspection program. For example, a good smoke inspection program have the potential to identify many defective components which, if repaired, lowers particulate emissions substantially and could improve fuel economy as well.
Retrofit technologies have emerged in recent years and are increasingly being utilized. Such systems need to be carefully matched to the individual vehicle and may require very low level sulfur fuels but, when optimized, are capable of significant reductions of PM emissions and reduced NOx emissions. Some of the more advanced PM retrofit systems may improve fuel economy as well.
Replacement of existing diesel engines in buses with newer engines which achieve much lower emissions is also an option, capable of achieving not only significant emissions reductions but also improved fuel economy. However, some of these engines may also require lower sulfur fuel. For example, if the replacement engine is equipped with a catalytic converter, the maximum sulfur level in the fuel should usually be restricted to maximum 500 ppm. A common problem with this option is that the new engine needs more space in the engine compartments which may not be provided. The existing driveline (gearbox, rear axle, drive shafts, etc) of the vehicle must also be adjusted to the new engine
Conversion of existing diesel bus engines to alternative fuels such as CNG or LPG is another option in areas where such fuels are readily available. However, recent experience indicates that replacement ("Re-power") with a new alternative fuel engine may be a more reliable option.
There are many different arguments for and against the various bus systems described, in comparison to conventional diesel systems, which are brought forward by different experts and stakeholders. In many cases the published opinions and assessments are non-uniform and even contradictory.
The table below provides a first impression of the most important characteristics, in terms of advantages and disadvantages, of the various bus systems in comparison to conventional diesel buses. Buses and fuels are here treated as a single system.
NOTE: This table is not claimed to be exhaustive, as it is not possible to reflect all existing expert opinions - this is intended to be a tentative summary of the general findings consolidated in the individual bus and fuel pages of the Info Pool.
| System Type |
Main Advantages |
Main Disadvantages |
Retrofit systems
and low sulfur diesel |
- Depending on utilized
technology, lower emissions of
PM, CO and HC; more recently
NOx retrofits have been
successfully introduced
|
- Higher capital costs
- Usually requires more
expensive low
sulfur diesel
-Might delay fleet renewal
|
Conversions to CNG
or LPG installation of dedicated CNG engines or "Re-power"
|
- Potential for lower
emissions if conversions are
carried out properly
- Potentially lower fuel costs
- Uses inherently sulfur-free fuel |
- Converting existing diesel
engines is usually not very
successful
- Higher capital costs
- Higher Carbonyl (Aldehydes and Ketones) emissions
(can be reduced if equipped
with a catalyst)
- Frequently less developed
fueling infrastructure
- Less experience with operation
and maintenance
- Lower driving range
- Possible low catalyst durability (affected by methane)
- Questionable second hand market
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The Info Pool also provides expert statements on the comparison between CNG buses and advanced diesel buses, which are arguably the most relevant options under current technological as well as market conditions.
Two articles written by experts on this subject are available within the Info Pool:
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