The report focuses on a variety of promising vehicle and powertrain technologies including lightweighting; battery electric; and various hybrid architectures including stop-start, mild, diesel electric, mechanical flywheel, and hydraulic hybrids. The largest WTW CO₂ benefit, in terms of percentage change, from a single technology comes from a full series hybrid—40%.

The study also covered a range of alternative fuels including compressed natural gas, biomethane, dimethyl ether (DME), hydrotreated vegetable oil, second generation biodiesel and hydrogen. Payback times for the selected technologies for both single and double-deck vehicles were calculated both with and without current UK bus subsidies. The largest WTW CO₂ benefit, in terms of percentage change, from a single fuel comes from biomethane, with several pathways showing benefits of more than 140%.

The study confirmed a number of technologies (e.g. full electrical hybrid) with the potential to make very significant improvements of more than 30% to bus CO₂ emissions, but showed that the most technically effective technologies can have high investment costs with payback periods as long as 20 years—and hence intervention in the form of subsidies or regulation is required to encourage their implementation.

Payback times vs CO2 benefit. With subsidies (left) and without subsidies (right). Click to enlarge.

Technologies that were expected to have a payback time of less than 5 years without support were mild hybrid, flywheel hybrid, IVT (infinitely variable transmission), hydraulic series hybrid, PBS (pneumatic booster system) and smart ancillaries.

In the area of fuels, the study showed that the use of biofuels can give significant reductions in well-to-wheels CO₂ emissions. Biomethane could be particularly attractive as a drop-in fuel to replace fossil derived natural gas. However, available subsidies or incentives, the economics associated with developing an adequate infrastructure, and the price of the fuel itself will determine the rate of adoption.

The study generated sets of technology packages to give a range of benefits for UK buses. Selected packages were: stop start with smart ancillaries; mild hybrid with smart ancillaries; flywheel hybrid with stop start; and full series hybrid with biomethane-fueled engine.

The report also examined the suitability of the selected technologies for other commercial vehicle sectors to identify areas where economies of scale or other synergies may be achieved. Lightweighting, smart ancillaries, full hybrid, flywheel hybrid, pneumatic booster, biomethane/CNG and substitutional biofuels are expected to be applicable across a range of commercial vehicle sectors.

However, the major benefits currently being sought for the commercial vehicle industry are not likely to give the most significant CO₂ benefits for the bus industry, according to the report. Thus, specific action may be required to pull through bus specific technologies, the report suggests.

A short-term roadmap shows that many low carbon technologies require development for bus application. In the near term, while many biofuel and alternative fuel options exist for buses, only CNG or biomethane have the potential for mass market penetration, the report concluded. In the longer term, the development of advanced technologies for buses is needed, in parallel with improvements to internal combustion engines, to meet long term CO₂ targets.