Background  

Buses are the backbone of many public transport systems around the world. Until now, the baseline bus in most parts of the world has been a dieselpowered 12-m (40-ft) long bus. Now the spectrum of technology options for buses is increasing, both regarding vehicle technology (advanced diesel technology, hybridization, lightweight designs, etc.) and fuels (sulfur-free diesel, biofuels, synthetic fuels, gaseous fuels, etc.). CNG buses have been around for a while, and now several manufacturers are offering hybrid buses. The procurement or delivery of bus services is often handled by municipalities or the state in a centralized manner. As the service life of buses is as long as 20 years, solid data on the performance of new technology are needed.

Purpose and Objectives

It is obvious that the spectrum of vehicle and fuel technologies is widening, not closing in. This poses a challenge to decision makers at all levels of decision making: governments, local authorities, and fleet operators. Both when setting policies and when procuring new vehicles, the following questions must be confronted:

• Which technology or fuel/technology combination gives the best overall energy efficiency?
•  Which technology or combination yields the lowest overall greenhouse gas (GHG) emissions?
• Which technology or combination is best for reduced local emissions and improved urban air quality?
• Which option provides the best overall cost efficiency for reduction of GHG emissions as well as local emissions?
• Which clean fuel options can be implemented for existing vehicle fleets?

The objective of the task was to bring together the expertise of IEA’s transport-related implementing agreements to access reliable information on overall energy efficiency, emissions, and costs (both direct and indirect ) of various technology options for buses. The technology options vary with respect to engine technology, powertrain technology, and fuels. The outcome of the task will be unbiased and provide solid IEA-sanctioned data for use by policy- and decision-makers responsible for public transport using buses.

Activities

Two Implementing Agreements, namely, AMF and Bioenergy, were the lead partners in this exercise. These two Implementing Agreements formed projects (Annex of Tasks) to carry out the overall project:

• AMF: Annex 37
•  Bioenergy: Task 41/Project 3

In addition, all IEA transport-related Implementing Agreements were asked to submit outlook reports (timeline 2020) of their respective technologies.

The project comprised four major parts: well-to-tank (WTT) assessment of alternative fuel pathways, assessment of bus end-use (tank-to-wheel, TTW) performance, and combination of WTT and TTW data into well-to-wheel (WTW) data and cost assessment, including indirect as well as direct costs. The project was a combination of cost and task sharing.

Experts at Argonne National Laboratory, Natural Resources Canada, and VTT worked on the WTT part. Different fuel options were evaluated using RED (EU), GHGenius (Canada), and GREET (U.S.) methodology. 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 TTW work was topped up by on-road measurements (AVL MTC) as well as some engine dynamometer work (von Thünen Institute).

In vehicle and engine testing, the following fuels diesel and diesel substitutes were covered:

• conventional diesel fuel
• diesel fuels from unconventional fossil sources (natural gas, oil sandderived fuels)
•  biodiesel fuels (methyl esters as well as hydrotreated vegetable oils)

The alternative fuels requiring dedicated vehicles covered were:

• methane (biogas/natural gas)
• additive treated ethanol
• di-methyl-ether (DME)

To ensure real international significance, the vehicle matrix consisted of older as well as top-of-the-line new buses, and also some prototype vehicles. The driveline configurations included conventional as well as hybrid drivetrains. The emission certification of the vehicles varied from requirements of the late 1990s (U.S. 1998 and Euro II) to current regulations (U.S. 2010, Euro V/EEV). The U.S. 2010 requirements are roughly equivalent to Japan 2009 and the oncoming Euro VI regulation for Europe.

As for the WTT part, the spectrum of fuels evaluated was broader than the fuel matrix for actual vehicle and engine testing. The WTT part covered, e.g., several options for actual BTL (biomass-to-liquids) type fuels. GTL (gas-to-liquids) and HVO (hydrotreated vegetable oil) are already in the commercial phase, whereas actual BTL and DME are still in the development phase.

Results

A draft final report was completed in January 2011. 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 most effective way to reduce regulated emissions is to replace old vehicles with new ones. The most effective way to cut GHG emissions is to switch from fossil fuels to efficient biofuels.

The findings can be summarized and quantified as follows:

Vehicle Level

• Old vs. new diesel vehicles
– 10:1 and even more for regulated emissions
– 100:1 for particulate numbers
– close to neutral for fuel efficiency

• Hybridization and light-weighting
– 20-30% reduction in fuel consumption
– not automatically beneficial for regulated emissions
– energy consumption ratio between the least fuel efficient vehicle with conventional power train and the most efficient hybrid 2:1

• Effect of driving cycle
– 5:1 for fuel consumption and regulated emissions

• Fuel effects on tailpipe emissions (when replacing regular diesel)
– 2.5:1 at maximum (particulates)

• Alternative fuels (in dedicated vehicles)
– low PM emissions but not automatically low NOx emissions
– fuel efficiency depends on combustion system (compression or spark-ignition)
– diesel vs. spark-ignited CNG roughly equivalent for tailpipe CO2

Well-to-Wheel Level

• Conventional fossil diesel CO2eqv
– WTT some 20% and TTW some 80% of total WTW
– 2:1 for WTW for a given fuel (least fuel efficient vehicle with conventional power train and the most efficient hybrid)

• Coal-to-liquid (CTL) diesel CO2eqv
– WTT some 60% and TTW some 40% of total WTW

• CTL vs. conventional diesel for CO2eqv
– 2:1

• CNG, DME, and GTL vs. conventional diesel for CO2eqv (average)
– ~ +10 %
– CNG equivalent to diesel at its best (local gas)

• Biofuels vs. conventional diesel for CO2eqv
– relative reduction ~ 30…70% (biofuels from traditional feedstocks)
– relative reduction ~ 85…95% (biofuels from lignocellulosic feedstocks or waste in vehicles using diesel combustion)

• Conventional biogas vs. CNG for CO2eqv
– relative reduction ~ 65…90% CTL vs. best biofuel for CO2eqv
– 120:1 (fuel only)
– 240:1 (fuel and vehicle combined)

• Biofuels vs. conventional diesel for overall energy
– 2.5:1…1.75:1 • CNG and DME from remote natural gas and GTL vs. conventional diesel for overall energy
– ~1.5:1

Costs

• External costs for NOx and PM
– 12:1 variation in unit prices depending on country and region
– 200:1 in calculated external costs (including effects of country, region and vehicle, range 0.24…0.001 €/km)

• External costs for CO2eqv (at a price of 40 €/ton of CO2)
– 2:1 for vehicle (least fuel efficient vehicle with conventional power train and the most efficient hybrid)
– 120:1 for fuel (CTL vs. FAME from tallow)
– 240:1 (fuel and vehicle combined)

• Direct costs (investment, fuel and maintenance calculated as €/km), lowest vs. highest
– ~ +15% (baseline)
– ~ +20% (high diesel price)  

The project is in principle completed, and will be closed at ExCo 43 in May 2012.

Future Plans

After approval by the participants, the full final report will be brought into the public domain in the spring of 2012. In addition, the results of the project will be presented in workshops, conferences, and technical papers. The first workshop for the Finnish audience will be held already in February 2012.