In Search of a Better Bus
For decades now, transit agencies in some cities have been holding their breath waiting for a 'better bus' to come along. In the 1940's and 50's, many transit agencies were impressed by the notion of "flexibility" touted by the diesel bus industry and replaced electric transit with diesel buses only to find that the diesel engine's slow performance, noise and fumes actually drove riders away. Many cities about to build trolley systems have held off at the promise of a bus that would be 'better' than the trolley, only to find that their magic bus still hasn't arrived. Even cities with trolley systems have held off building much needed extensions.
One often hears statements like "the fuel cell bus is just around the corner" or "the hybrid bus holds a lot of promise". If we examine these technologies closely, we find that they might offer cleaner alternatives to diesel buses, but
they are clearly not a replacement for trolleybuses. Perhaps it's time we stopped holding our breath. An investment in proven trolleybus technology could allow us to
reduce our dependence on expensive petroleum fuels, increase the sustainability
of public transit, and reduce our environmental impacts.
state of the most commonly cited alternatives to diesel are summarized below:
- run on compressed natural gas, largelly made up of methane, which is an extremely strong greenhouse gas. Unburned methane released by these vehicles has a global warming potential some 21 times greater than the common greenhouse gas CO2.
- release toxic emissions into the streeets.
- produce less CO and NOx emissions than diesel engines, so they appear to have a cleaner emissions profile. Although the amount particulate produced by CNG engines is less when measured by weight, it is much finer than diesel particulate and therefore easily enters the bloodstream through the lungs. This particulate is believed to present health risks similar to those of diesel particulate. A thorough investigation of these risks needs to be completed before advocating any kind of large-scale switch to CNG.
- formaldehyde emissions from CNG engines are known to be strong carcinogens.
- emissions profile deteriorates rapidlly with age and wear and tear.
- may be similar in size to conventional diesel buses, but usually have less passenger carrying capacity due to the heavy weight of the CNG storage tanks.
- less energy efficient than diesels and much less efficient than electric trolleys.
- require more frequent maintenance than diesels or trolleys and thus are expensive to maintain; can often be less reliable than diesels or trolleys as well.
- require expensive infrastructure for refuelling.
- noise pollution is similar to that produced by diesel buses.
Diesel-Electric Hybrid buses
- a diesel engine charges a battery pack, which drives an electric motor.
- relatively new on the market, no extensive service record yet.
- better emissions profile than straight diesel or CNG, but definitely not superior to trolleys as emissions are still released into the streets.
- more energy/fuel efficient than straight diesel or CNG, not quite as good as trolleys; better fuel efficiency means lower fuel costs than for diesel or CNG.
- long-term maintenance record yet unknown; potential need for greater maintenance because two different power systems are involved.
- less noise on acceleration than straight diesel or CNG; more noisy than trolleys.
- potentially a viable alternative to straight diesel buses, likely less desirable than trolleys from an environmental or public perspective and definitely a poor trolley substitute if one already has trolley infrastructure in place.
claims of operating cost savings over diesel buses appear dubious - some
operators of hybrid fleets (i.e. New York City) are moving back to diesel buses
for new bus purchases.
Fuel Cell buses
- use hydrogen and a chemical reaction to generate electricity, which powers an electric motor.
- hydrogen is not a naturally occurring resource and must be manufactured. There are considerable costs associated with this in terms of the equipment needed and the processes used.
- no in-street emissions except water vapour, but emissions may be created in hydrogen production elsewhere. Most feasible sources of hydrogen are fossil fuels. Stripping hydrogen molecules from these fossil fuels yields considerable quantities of CO2.
- extremely inefficient in terms of energy use: no greater than about 11% overall energy efficiency from hydrogen source to wheels at present. Ten trolleybuses could be driven on the same amount of energy used to create the hydrogen for and propel one fuel cell bus.
- currently very expensive to operate---about three times the cost per km of a diesel or trolleybus. Vehicular fuel cell powerplants cost about three times as much as a conventional engine.
- expensive infrastructure required for hydrogen production and refuelling unless hydrogen can be manufactured from fossil fuels on board the vehicle.
- fuel cell technology known since 1830s, but still not truly proven as a technology for motive power. Greater potential for use in large stationary power plants.
- reliability problems with fuel cells---partly due to their sensitivity to impurities in the hydrogen--make their use to provide transit service problematic.
- fuel cell powerplants have five to ten times the weight of a conventional engine. Vehicle weight limitations mean that a 40 foot fuel cell bus cannot practically carry the same volume of passengers as a similar sized diesel or trolley.
- range may be a problem on routes that traverse great distances.
- low noise vehicles, similar to trolleybuses.
- need many years of development yet before they can be considered a serious competitor even to the diesel bus in many respects, let alone the trolleybus. Unlikely ever to better the energy efficiencies possible with direct electric traction (energy generated at power plants and fed to vehicles through overhead wires).
Drawing by Peter McLaughlin:
Developments in alternative technologies indicate that
there is unlikely to be a 'better' bus available than the trolleybus in the