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Ethanol bus
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Ethanol is produced primarily by the fermentation of starch from grains (mostly corn) or sugar from sugar cane. It is most commonly used as an oxygenate in reformulated gasoline and in a gasoline blend called "gasohol." These fuels can be burned in gasoline engines. Specialized engines are necessary in order to burn pure ethanol.

Ethanol would appear to be a good candidate for an alternative fuel for use in transit buses because it is a liquid fuel and has several physical and combustion properties similar to both diesel and gasoline fuels. For example, these properties are so similar that the same basic engine and fuel system technologies can be used for both ethanol and for diesel fuel ( 1 ). However, if used in its pure form rather than as a blend with conventional fuels, certain engine modifications are necessary ( 2 ). Ethanol can also be used as a fuel in spark-ignition engines. The term "ethanol bus" shall here refer to a bus using fuel that contains at least 85% ethanol and having an engine running according to the diesel principle.

Since ethanol has a lower energy density than diesel, ethanol buses have a lower driving range (on a test cycle used by the Swedish bus manufacturer Scania, diesel and ethanol buses used 45 and 80 liters of fuel respectively to go 100 km ( 3 ). This leads to the requirement of larger and heavier tanks, which in turn increases energy consumption. Other properties of pure ethanol fuel are high octane ratings and rather low cetane number.

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Ethanol buses have the potential to emit lower levels of certain pollutants than conventional diesel buses. According to manufacturer information, ethanol buses can meet EURO 4 standards and lead to emission reductions of 20-30% for PM and NOx ( 3 ), compared to a baseline diesel bus meeting the EURO 3 emissions limits of 5 g/kWh NOx and 0.10 g/kWh PM.

A study by NREL ( 4 ) likewise concludes that in general, the buses tested on ethanol appear to emit significantly lower PM levels than diesel buses without particulate traps, and levels similar to diesel buses equipped with particulate traps. Furthermore, most ethanol-powered buses emitted lower NOx levels - although they had significantly higher amounts of HC and CO, compared to the diesel buses. It should be noted that in this study results from the alcohol buses varied considerably from site to site and from bus to bus. Other studies confirm that ethanol buses have lower PM and NOx emission levels, but regarding CO and HC, there are contradicting claims (see ( 5 ).

According to ( 5 ), the exhaust emissions of volatile organic compounds (VOCs) from alcohol vehicles consist mainly of unburned ethanol, but they also contain a greater proportion of very reactive compounds such as aldehydes. It should be noted though that comparisons of exhaust emissions of VOCs from different vehicles, or the same vehicle in different tests, should be interpreted cautiously, as results can be influenced by a wide range of specific fuel and vehicle factors (see ( 5 ) for further details). To contain evaporative emissions from vehicles using alcohol fuel, measures may need to be implemented to control fuel vapor pressure ( 5 ).

Sulfur dioxide emissions from ethanol vehicles are very low ( 5 ).

Since ethanol is usually obtained from plants, which take up the same carbon during growth that is later released during the combustion process, greenhouse gas emissions can also be significantly lower than those from diesel buses. The extent of this depends on how the ethanol is produced (see for example ( 4 ) and ( 5 )). Generally, if ethanol is produced from cellulose, which is currently very expensive, greenhouse gas emissions are very low, if not zero. However, if produced from corn, emissions may actually increase.

Ethanol produced from corn has life cycle GHG emissions about 15 percent less than gasoline vehicles. Ethanol produced from woody biomass (E-100) has GHG emissions 60 to 75 percent below conventional gasoline.

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"One measure of the reliability of a bus is the average number of road calls. When the driver cannot complete his or her route and calls for a replacement bus, a road call (which encompasses events from engine failure to running out of fuel) is recorded" ( 4 ). 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 ( 4 ). Those calls involving engine/fuel system-related components, including "out of fuel" calls, were treated separately from those that involved other parts of the bus (e.g. doors).

For the alcohol (ethanol as well as methanol) fueled buses, "at every site, fuel-quality problems were a significant issue, and account for a major portion of the difference observed between the alcohol and diesel control buses.

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According to the US-Transit Cooperative Research Program, the actual incremental costs for ethanol buses were approximately $25,000 to $35,000 (1). In ( 4 ), an incremental cost of US$ 20,000 is estimated (in 1994 $ however). See ( 2 ) for the Swedish experience where incremental costs were reported to be rather less.

Because of the limited use of ethanol transit buses, no definitive estimate of the incremental maintenance costs exists. According to a 1996 DOE study, the maintenance costs of ethanol-powered bus engines and fuel systems were significantly higher than those of diesel buses ( 1 ). See ( 4 ) for case studies including some maintenance cost comparisons.

Ethanol prices are relatively high even when exempted from fuel taxation. That has effectively ruled out its use as a motor fuel except where, such as in Brazil and the U.S., it is heavily subsidized.

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While ethanol is used as a fuel to a significant extent in Brazil, and to a certain extent in the United States, mostly as a fuel additive, ethanol buses are not very widely used. In Sweden, there exists some experience with ethanol city buses (see ( 2 ) and ( 6 )), and in fact according to the Swedish company Scania, it is the largest ethanol bus manufacturer in the world ( 3 ).

In the United States the use of alcohol-based fuels (methanol and ethanol) has declined in recent years. According to FTA and industry officials, this decline has occurred because of the decreased performance and high operating cost of alcohol-fueled buses. The EIA has estimated that in 1999 there were 51 full-sized ethanol transit buses in the United States. There are no orders for ethanol buses currently. No manufacturer has produced alcohol-fueled engines since 1996 ( 1 ).

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Ethanol buses have various advantages in terms of emissions, but also disadvantages in terms of driving range, technical problems, possibly problems with CO and HC emissions, and higher prices. If technological advances are able to reduce these, the cost consideration is likely to remain a significant obstacle, especially since it is coupled with fuel prices and the development of an adequate infrastructure.

The high cost of producing ethanol (compared to hydrocarbon fuels) remains the primary barrier to widespread use.

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