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Fuel and technology alternatives for buses. Overall energy efficiency and emission performance
10 October 2012
Technical Research Centre of Finland (VTT)
In 2009–2011, a comprehensive project on urban buses was carried out in cooperation with IEA’s Implementing Agreements on Alternative Motor Fuels and Bioenergy, with input from additional IEA Implementing Agreements. The objective of the project was to generate unbiased and solid data for use by policy- and decision-makers responsible for public transport using buses. The project comprised four major parts: (1) a well-to-tank (WTT) assessment of alternative fuel pathways, (2) an assessment of bus end-use (tank-to-wheel, TTW) performance, (3) combining WTT and TTW data into well-to-wheel (WTW) data and (4) a cost assessment, including indirect as well as direct costs.
Experts at Argonne National Laboratory, Natural Resources Canada and VTT worked on the WTT part. The WTT emissions of various fossil fuels and biofuels were assessed by using GREET model from the United States, GHGenius model from Canada and RED methodology of the European Union. All these models follow the frame work of life cycle assessment.
In the TTW part Environment Canada and VTT generated emission and fuel consumption data by running 21 different buses on chassis dynamometers, generating data for some 180 combinations of vehicle, fuel and driving cycle. The fuels covered included diesel, synthetic diesel, various types of biodiesel fuels, additive treated ethanol, methane and DME. Six different hybrid vehicles were included in the vehicle matrix. The TTW work was topped up by on-road measurements (AVL MTC) as well as some engine dynamometer work (von Thünen Institute).
Based on the findings of the project it is possible to establish the effects of various parameters on bus performance. The largest variations and also uncertainties can be found for WTW CO2eqv emissions, or in fact the WTT part of the CO2eqv emissions. The variation is especially significant for biofuels. The WTT results vary due to the differences in the assessed biofuel chains, the regions of biofuel production, the raw materials used and the technology choices made. In addition, the results of any WTT assessment depend on the calculation assumptions made and are often vulnerable to uncertainties and sensitivities.
Over the last 15 years, tightening emission regulations and improved engine and exhaust after-treatment technology have reduced regulated emissions by a factor of 10:1 and particulate numbers with a factor of 100:1. The most effective way to reduce regulated emissions is to replace old vehicles with new ones. Hybridization or light-weighting reduce fuel consumption 20–30%, but otherwise the improvements in fuel efficiency have not been so spectacular. The driving cycle affects regulated emissions and fuel consumption by a factor of 5:1. The fuel effects are at maximum 2.5:1 for regulated emissions (particulates), but as high as 100:1 for WTW greenhouse emissions. Thus the most effective way to cut greenhouse gas (GHG) emissions is to switch from fossil fuels to efficient biofuels. WTW energy use varies a factor of 2.5:1.