An important recent development is that of the hybrid electric motor car that combines an internal combustion engine with an electric drive train and battery.25 The gains in effi ciency and therefore fuel economy achieved by hybrid vehicles are typically around 50%. They mainly arise from: (1) use of regenerative braking (with the motor used as a generator and captured electricity stored in the battery), (2) running on the battery and electric traction only when in slow-moving or congested traffic, (3) avoiding low-efficiency modes of the internal combustion engine and (4) downsizing the internal combustion engine through the use of the motor/battery as a power booster. Toyota and Honda were the first two to introduce hybrid vehicles and other manufacturers are following. An imminent development is of the plug-in hybrid which will enable the larger than normal car battery to be boosted by connecting with a commercial electricity supply. For shorter journeys the plug-in hybrid could run only on the battery in which case with fossil-fuel-free electricity, its carbon dioxide emissions would be eliminated.
Other significant efficiency improvements are coming from the use of lower weight structural materials, improvements in low-air-resistance design and the availability of direct injection diesel engines, long used in heavy trucks, for automobiles and light trucks.
Developments are also occurring in battery technology that soon should enable more extensive employment of electric vehicles which will use electricity from wholly carbon-free sources. During the next few years we will begin to see the introduction of vehicles driven by fuel cells (see Figure 11.24 below) based on hydrogen fuel that can potentially be produced from renewable sources (see page 377). This new technology has the potential eventually to revolutionise much of the transport sector.
Biofuels generated from crops can be employed to fuel motor vehicles thereby avoiding fossil fuel use. For instance, ethanol has been extensively produced from sugar cane in Brazil. Biodiesel is also becoming more widely available (see later section on biomass).
been proposed but more understanding of the mechanism is needed before serious work on its reduction can be carried out. Controlling the growing influence of aviation on the climate is probably the largest challenge to be solved in the overall mitigation of climate change.
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