The effectiveness of energy storage in hybrid vehicles

Public awareness of finite oil resources and concerns over climate change have spurred efforts to improve vehicle efficiency and reduce emissions by road transport. Hybrids have become an increasingly popular alternative to conventional powertrain vehicles. Large fuel savings are claimed (typically 70 + mpg) (Toyota, 2014), however, collective anecdotal evidence from owners of these vehicles suggests a more modest performance. A literature review yielded an abundance of literature relating to specific hybrid vehicle technologies, and control strategies, however the variation in energy savings over different journey types for different classes of vehicle has received less attention. A simulation tool was developed to compare the energy saving effectiveness of parallel hybrid powertrains with regenerative braking and energy storage across a broad range of vehicle and journey types. The realism of the simulation (in non-hybrid mode) was evaluated by comparison with practical trials. A range of validation methods showed that average fuel consumption could be calculated to within +/- 5-10% of measured consumption and, in cases where detailed data for a vehicle was available, this improved to within 3%. Simulated fuel consumption was around 15% greater that manufacturers’ claims – reasons for this were explored. Using the backward and forward looking simulation it was possible to calculate likely fuel savings in various scenarios. Results indicate a trend of improved potential savings with increased vehicle mass. Over urban journeys results ranged from around 16 to 23% energy savings for a small car and large coach respectively. On extra-urban journeys much more modest savings were calculated ranging from a maximum of 0 - 4 % across the same range of vehicles. The likely effects of vehicle mass and drag coefficient has also been explored along with the energy saving potential of start-stop engine technology, often used in hybrids and non-hybrids alike. The broad part of the study confirmed quantitatively that greatest fuel savings might be achieved on urban routes with public transport buses. The study then narrowed to consider this application, particularly with respect to exhaust emissions which are cause for growing concern. Possible reductions in exhaust NOx and PM emissions of up to 10 to 12% respectively were predicted through the application of parallel hybrid powertrains to existing bus designs and simulated on the MLTB cycle