EXAMINATION OF THE BRAKING ENERGY RECOVERY POTENTIALS OF A CITY BUS UNDER URBAN DRIVING CONDITIONS

Abstract

While operating under urban driving conditions, a city bus must have high enough traction power to overcome the rolling, aerodynamic, acceleration and positive road grade resistances. But, the bus must also have high enough braking power to decelerate the bus for a stop and to keep its speed at the required level when the road grade is negative. The rolling and aerodynamic losses are inevitable losses, but the braking energy losses are recoverable. At the present work, the operating characteristics affecting the braking energy of a 12 m long and 15 tons loaded weight city bus were analysed under real world urban driving conditions. Since the city bus driving under urban driving conditions involves frequent stop-and-go operations, recovery of the braking energy losses can provide a great potential to improve the fuel economy. The characteristics examined mainly include the speed and altitude profiles of the routes, micro-trip traction energies, and the cumulative traction and braking energies for the routes. Results of the analysis indicated that frequent stop-and-go operations of the urban routes increase the braking energy demands of the bus dramatically. Over the Campus route, which is a 22 km route with 88 micro-trips and an average speed of 19 km/h, the cumulative braking energy is 26 kW-h. It is accounted for 80% of the traction energy, and 14% of the micro-trips have the braking energies greater than 0.5 kW-h. But, over the route Karaman, which is a 41.5 km route with 58 micro-trips and an average speed of 26 km/h, the cumulative braking energy is 38 kW-h. It is accounted for 65% of the traction energy, and 50% of the micro-trips have the braking energies greater than 0.5 kW-h. If 50% of the braking energies are recovered, percentages of the energy savings for the Campus and Karaman routes will be approximately 40% and 30%, respectively.