Transit Systems Presently using Hybrids in North America
Hybrid Powertrain Generator Options
The hybrid electric vehicle combines electric propulsion with an auxiliary power unit, maximizing the advantages of each under varying operating conditions. The electrical drive draws energy from batteries and a combustion engine. In most applications, the hybrid transit vehicle uses a smaller diesel engine and a bank of batteries or ultracapacitors stored in the roof of the bus. In the "parallel " application, the vehicle's wheels can be driven mechanically via the diesel engine, or electrically via the batteries. In the "series " application, the diesel engine and the generator unit produce electricity, and vehicle's wheels are powered by electric traction motors which draw the electrical energy provided by the engine or the batteries. Both applications take advantage of regenerative braking to partially recapture energy that has been used to move the bus, but also acts as a secondary braking system, which complement the service brake in slowing the bus.
The regenerative braking system uses the electric motor component of the hybrid propulsion system to generate electricity when the bus is in movement. Using the electric motor, acting as a generator, to produce electricity also slows the bus down. The electricity generated by the slowing of the vehicle, is stored in the energy storage systems, batteries or capacitors, and can be re-used by the electric motor for acceleration.
In either system, the vehicles utilize battery power during times of high energy demand, such as acceleration. Therefore, the system utilizes electrical power and diesel power in the most energy efficient manner.
Transit Systems presently using Hybrids in North America
There are now over 900 hybrid buses in Canada, with a strong majority of these operating at the Toronto Transit Commission (TTC). New York City Transit tested a hybrid prototype in 1996 and 10 Orion hybrid buses in 1998. The initial results were very promising, with the buses meeting NYC Transit's standard performance specifications, and NYC Transit's emission targets. Brake life was extended by almost 100%, and customers and drivers were satisfied with the vehicles' performance. The TTC operates Canada's largest hybrid bus fleet with 650 vehicles in services. TransLink now operates over 140 hybrid buses in Vancouver's lower mainland. Ottawa's OC Transpo announced a purchase agreement of 202 hybrid buses.
General Benefits
Fuel Savings
By design, hybrid diesel electric buses consume less fuel than the conventional diesel bus, but fuel savings have a wide range and depend on a number of operating factors. The Société de transport de Montréal demonstrated fuel savings of 30%, while the Edmonton Transit System results ranged from 15% to 20%.
Green House Gas Emission Savings
Hybrid vehicles experience significantly lower emissions than conventional diesel powered buses, as the energy storage system supplies power during start-up and acceleration. At these times, the diesel engine is idling. Therefore, the mechanical engines are operated in their most optimal range. Green house gas emissions are reduced in direct proportion to the amount of fuel used.
Performance
Hybrid diesel-electric buses have proven to reduce specific maintenance costs, particularly brakes. In general, hybrid vehicles experience less wear on mechanical components. Since there is no transmission, there is no associated maintenance required. Existing fuelling systems can be used, so there are no fuel-related infrastructure costs. In regards to operators and customers, the ride is smoother and quieter, yet there is no loss of power on hills or during acceleration.
Commonalities with Conventional Diesel Transit Buses
With the exception of the new powertrain and the battery storage on the roof, hybrid diesel-electric buses are very similar to the conventional diesel transit buses.
Initial Cost Premium
Hybrid propulsion system buses generally command a premium of $200 000 to $270 000 over standard bus prices. The energy storage device (battery and/or capacitor) of the hybrid propulsion system is the most important contributor to the price differential. Within this cost category, battery type (ie Nickel Metal Hydride, Lithium-Ion) is an important factor. Other variables influencing price include options selected, timing and size of the order.
Life Cycle Costing
While the initial premium cost of 40 to 50% on the purchase of the hybrid diesel-electric transit bus is significant, it is noted that there will be savings in fuel and other maintenance costs over the life of the vehicle. Transit systems wishing to benefit from substantial cost savings must be prepared to change their operational plan by making specific allowances for the assignment of hybrid buses to routes that are best suited to their use (slow speed, high number of stops per km) and / or by redesigning their routes. Identifying routes and implementing such operational changes is required to maximize the benefits of hybrid propulsion systems.
Comparisons to Other Strategies
There are numerous other strategies designed to reduce fuel consumption and emissions. The Société de transport de Montréal demonstrated, through the Urban Transportation Showcase Program (UTSP), and other transit systems across North America, have successfully demonstrated that switching from a hydraulically driven to electrically driven cooling system could produce significant improvements to fuel consumption. This is a first step of reducing parasitic engine loads by converting engine driven accessories to electrically driven accessories. Optimization of the bus rear axle ratio can also make economies to fuel consumption, another initiative that can be implemented to a variety of power trains. Another strategy to improve fuel consumption if through operator training, such as the SmartDRIVER program, that teaches operators driving techniques to maximize fuel use.
Many Canadian transit systems are successfully operating their fleets with biodiesel, a combination of diesel and vegetable oils. Provided that there is a reliable supply of biodiesel fuel, this is an economical strategy to reduce fuel consumption and emissions.
Operations
In the parallel hybrid system, vehicle propulsion is provided by either the engine or the batteries, or, at times by both. The engine-generator combination works in parallel with the battery, providing electrical power to keep the battery charged. During start-up and acceleration, the electric transmission accelerates the vehicle, whereas, once the vehicle reaches cruising speed, the parallel system blends both the diesel engine and the electric motor power to maintain speed. At highway speeds, the diesel engine provides the power and charges the energy storage system.
Pros and Cons
The parallel hybrid system uses a continuously variable transmission. The top speed for the vehicle is over 105 km/hr. The incremental weight over a conventional diesel coach is approximately 1500 pounds. King County Metro (Seattle) chose the parallel system over the series system because parallel hybrids have advantages in stop and go and highway cruising applications. The parallel hybrid system functions best in the strictly urban environment, where buses travel in the main traffic stream, stopping and starting every block or two. Additionally, the parallel hybrid system offers a smaller diesel engine compared to a conventional drive train, as is also the case for the series hybrid. While the parallel system offers advantages in combined urban and highway operations, it cannot be converted to other engine options, such as turbines or fuel cells.
Operations
In the series hybrid application, propulsion is provided by an electric motor. The electricity required by the motor is generated by a small diesel engine and the batteries. The series system is most efficient in the urban, frequent stop and go environment because the electric motor has high torque at low speeds, resulting in smooth, fast acceleration.
Pros and Cons
The series hybrid is seen to be more effective for the urban transit environment, such as in New York City, where the Orion VII vehicles are equipped with the series hybrid system. The propulsion system for the series hybrid application is mechanically simple, with a minimum number of moving parts, and a single fixed gear reduction. The system eliminates traditional transmission components. The series hybrid system can also be converted to other drive trains, such as gasoline, natural gas, turbines or fuel cells. As a result, as other powertrain systems become more reliable and economical, the possibility of future conversion is viewed as a definite advantage. However, the series system does not provide as much power as the parallel system. As for the parallel series, a smaller diesel engine can be used to power the bus.
Hybrid Powertrain Generator Options
General
One of the key features of a hybrid bus that contributes to the reductions in fuel consumption and emissions is the smaller Internal Combustion Engine (ICE). In most of the current applications in transit coaches, the ICE is a diesel engine. However, there are options to the diesel engine, some of which are already in use, while others are still being developed or tested. Some of the optional powertrain generators will function satisfactorily in both the parallel hybrid systems, while others will only function in the series hybrid buses.
Diesel
Currently, the diesel engine is the most common powertrain generator in hybrid buses. This engine functions very satisfactorily, and technicians are already very skilled at working with the diesel engine. As a result, the training and transition to hybrid technology using the diesel engine is not complex.
Gasoline
Although not common in transit buses, at least one hybrid propulsion manufacturer in Canada offers the gasoline engine as an option in the hybrid application. This application, which uses the series hybrid system, including an ultra-low emission gasoline engine, is found mainly in California.
Natural Gas
A leading US research and development company, TIAX LLC, has concluded that, "A system that uses a "clean fuel" (e.g., natural gas or ultra low sulphur diesel) and advanced NOx exhaust after treatment, in conjunction with an optimized hybrid electric system, has the potential to achieve near-zero emissions". Therefore, while the natural gas infrastructure is costly, natural gas can be utilized to fuel the hybrid powertrain generator. A test bus is currently being operated in San Diego, California.
Turbine
Turbines could be used in hybrid buses, with either diesel or natural gas as the fuel. No manufacturer currently offers this option.
Fuel Cell
In the longer term (10 years hence and beyond), it is envisioned that fuel cell technology, particularly hydrogen fuel cells, will be used to power transit buses. Any current series hybrid system can be converted to fuel cell applications. TIAX LLC concludes, "Hybrid buses can be stepping stones to fuel cell propulsion systems, which show promise for zero or near-zero emission transit buses".
BC Transit is currently pilot testing a fleet of 20 hydrogen fuel cell series hybrid buses at its operations in Whistler, BC.
