Buses: ICE/Battery Hybrids docx

Buses: ICE/Battery Hybrids NN Clark and F Zhen, West Virginia University, Morgantown, WV, USA & 2009 Elsevier B.V. All rights reserved. Introduction Vehicles that utilize more than one energy source for propulsion are usually termed hybrid vehicles. Although the onboard energy storage system (ESS) may be a fly- wheel or a hydraulic accumulator, mos t hybrid vehicles use chemical batteries or capacitors and are termed hy- brid-electric vehicles. Fuel cells (FC s) may be used in hybrid-electric vehicles as primary power sources instead of conventional internal combustion engines (ICEs). In this article, ICE–battery hybrid transit buses are dis- cussed specifically and contrasted with conventional drive buses. Transit buses most typically are equipped with two axles and have a nominal length of 12 m (40 ft), although they may range in size from large vans to articulated units of 18 m (60 ft) nominal length. Conventional buses may be equipped with manual or automatically shifted transmissions, but most typically have conventional automatic transmissions with a torque converter. Modern automatic transmissions have improved bus powertrain efficiency by employing four or more speeds (gear ratios) with a lockup capability on the torque converter. Over- the-road coaches are similar to transit buses but are typically 14 m (45 ft) in length and may have three axles instead of two. Hybrid-electric buses typically have better fuel economy and lower emissions than comparable con- ventional buses. Instead of employing aerodynamic body design or weight reduction to improve vehicle fuel effi- ciency, the hybrid system focuses on recover ing energy loss during braking and deceleration and increasing powertrain efficiency over a cycle. Consider the road-load equation, used to describe the power demand at the drive wheels of a vehicle: vehicle power demand ¼ 1 2 rC d Av 3 þ C rr mgv þ mv dv dt þ mgv sina ½1 The road-load equation is composed of terms repre- senting aerodynamic drag, rolling resistance, and power required for changing inertia and potential energy. The aerodynamic term, 1 2 rC d Av 3 , represents the power re- quired to overcome aerodynamic drag where r is the air density, C d is the aerodynamic drag, coefficient, A is the equivalent frontal area of the vehicle, and v is the vehicle velocity. The rolling resistance term, C rr mgv, represents the power required to overcome the resistance of the tires to roll freely over the road where C rr is the rolling resistance coefficient, m is the mass of the vehicle, and g is the gravitational acceleration. The inertial term, mv(dv/ dt), represents the power required to accelerate and de- celerate the vehicle mass. Lastly, the potential energy term, mgv sina, represents the power demand when climbing or potential power source when descending a road with a gradient where a is the gradient angle of the road. Most modeling and chassis dynamometer-based bus testing is performed by assuming flat terrain (a ¼ 0), but the gradient term can be significant for modeling hybrid- electric bus performance in realistic on-road operation. Consider some typical urban bus operating speed schedules, exemplified by the Paris Cyc le or the Man- hattan Cycle. The Paris Cycle was set up to mimic a Paris bus route by France Agency of Environment and Energy Management (ADEME) and Autonom ous Operator of Parisian Transports (RATP). The Manhattan Cycle was developed to represent New York City bus operation by West Virginia University (WVU) and was a recom- mended drive cycle in Society of Automotive Engineers (SAE) Standard J2711. These cycles (Figure 1 ) show that buses undergo frequent acce leration and deceleration, owing to both scheduled stops and traffic constraints. Each acceleration event demands substantial propulsion energy, and this energy is wasted through braking during deceleration by conventional drivetrain buses. Consider the example of a 15 000 kg bus with a frontal area of 6 m 2 , drag coefficient of 0.79, and tire rolling loss coeffici ent of 0.008 being exercised over the Paris Cycle. Applying eqn [1] to this example yields only 810 kJ of energy to overcome aerodynamic drag (because operating speeds are low), but 6630 kJ to overcome rolling resistance and 21 200 kJ to accelerate the vehicle mass (increase inertial energy). In instances during the cycle where the required deceleration rate was low, that is less than that resulting from aerodynamic drag and rolling resistance, inertial energy was consumed or balanced by those losses (‘coasting’ or nonbraking deceleration). In instances where the required deceleration rate was high, or greater than the coasting deceleration rate, inertial energy had to be consumed by the friction brakes or by a retarder (if fitted). Equation [1] suggests that the drive wheels of the bus demand 26 140 kJ of energy, and the braking con- sumes 19 200 kJ of energy. The results of the preceding example are typical of a transit bus in stop-and-go low-speed operation, where more than 50% of the energy reaching the rear wheels is dissipated as braking energy. Recovering and reusing this energy, even with an efficiency of 60%, could provide for 203 . Buses: ICE/Battery Hybrids NN Clark and F Zhen, West Virginia University, Morgantown, WV, USA & 2009