hybrid electric bus

Abstract:-
The depletion of natural resources, air pollution, traffic congestion and the rising price of fossil fuels are all issues driving communities and individuals to search for alternative means of transportation. The rising concern for the environment has considerably fuelled the research and development activities in the field of Hybrid Electric Vehicles (HEVs). Heavy duty HEVs such as buses and trucks are now being produced commercially. Vehicular emissions could be reduced drastically if Hybrid Electric Buses were to be used for public transportation. Power Electronics plays an important role in the development of HEVs as these vehicles require an energy storage device to store the excess electrical energy. Therefore, it is necessary to carry out research in the field of energy storage in order to develop more efficient and reliable storage devices so as to enable the improvement of HEVs. This will go a long way in conserving our natural resources as well as protecting the environment.
Introduction:-
A hybrid-electric vehicle is one that combines an electric propulsion system with another power plant such as a conventional internal combustion engine (diesel, gasoline, propane, or natural gas), a turbine, or a fuel cell stack. In the case of hybrid-electric buses, the majority in service today use a diesel or gasoline engine with an electric motor and batteries, while a smaller number of hybrid buses in service use an ultracapacitor for energy storage. This combined system of an ICE engine, electric motor and energy storage device gives a transit operator the benefits of an electric drive system – better acceleration from a stop, quieter operation, greater energy efficiency – without the negatives of a pure battery-electric bus, like reduced range and reduced hill-climbing power. In addition, commercially-available hybrid buses today do not require recharging: the batteries are continuously recharged during driving. Thus, while transit operators will not receive the zero point-of-source emission benefit of a pure battery-electric bus, hybrids will lower emissions and fuel use, while providing the kind of performance that transit operations require.

Hybrid Electric Vehicle Concept:-
In recent years an evolution in power electronics technology has offered the possibility of revolutionary drive trains for passenger vehicles. Electric motors using efficient solid state power devices offer infinitely variable power and speed control. Several of the motors currently being offered by industry have very high power densities and can be controlled to also act as generators. When coupled with onboard energy storage systems such as chemical batteries, capacitors, or flywheels, this new drive train offers several advantages including:
• Elimination of multiple-gear transmissions
• Elimination of fluid coupling losses
• Near constant speed and load to the engine
• Recovery of energy during braking
• Reduced drive train and brake maintenance
Electrical power trains with energy storage uncouple the short-term power requirements of the vehicle from the load seen by the engine. An engine in such a power train can operate at its highest efficiency design point. In addition, the size of the engine can be reduced significantly in some vehicles and drive cycles to the long-term average value of power. Energy storage provides the short term power supplement needed during acceleration and for going up a grade. This reduced engine size, higher efficiency, and constant speed yields significantly improved emissions, fuel economy, and life for the engine.
Battery-Electric Buses:-
Battery-electric buses are often referred to as “pure” electric buses because the propulsion system is powered only by the electric energy stored in the battery. The battery pack is either recharged daily or “swapped out” when the batteries are depleted. Battery-electric propulsion systems are primarily targeted to smaller transit buses, such as those used for shuttle service or other vehicle routes that are short and low speed. This is due to the limited range and power of current commercial battery technologies. Because of the potential benefits of using zero emission buses in public fleets, there has been much R&D funding devoted to improving the battery technology over the last decade. In spite of this, battery-powered buses have not been able to achieve sufficient range at a commercially competitive cost. As a result, today there are only a handful of manufacturers offering battery-electric uses, primarily in the medium-duty shuttle bus market. Today there are approximately 90 to 120 battery-electric buses in transit operations. The drive system for a battery-electric bus consists of an electric motor, a battery pack to provide energy storage, and a control system that governs the vehicle operation. Electric motors offer greater efficiency and less noise than internal combustion engines (ICE). They provide their highest torque at low speeds, which results in better acceleration from a stop. Electric motors also increase energy efficiency by enabling regenerative braking: when the vehicle decelerates, the motor reverses field, becoming an electricity generator that can recharge the battery pack during braking events. In a conventional internal combustion engine system, braking energy is lost, as there is no mechanism to recover it. The ability to recover this energy is one of the key benefits of electric drive systems, adding to the overall higher efficiency of electric drive. The electric drive control system is quite complex, as it must receive input from the operation of the vehicle, and direct the response of the electric drive system. It is now common for conventional vehicles to have some level of complex electrical systems and controls, so this not a completely unfamiliar element for transit operators. Battery-electric buses must be recharged daily – or have the battery pack swapped out for a new one, an unlikely solution for most transit operations – so transit agencies must purchase expensive charging equipment to recharge the fleet. Recharging time varies, dependent on the battery type, capacity and voltage/current output of the charger. Most electric buses will be able to receive a full recharge in six hours, although “fast-charger” systems can reduce this to about two to three hours for certain battery types. Fast charger systems are a more expensive piece of equipment, however. One strategy for addressing the problem of insufficient range is opportunity charging, where a fleet will recharge the bus during its daily service, at charging stations placed at key spots on the bus fleets’ routes. This adds significant infrastructure costs, and presents challenges to delivering reliable, timely transit service which depends on adhering to tight schedules. As with all electric propulsion vehicles, support and training in understanding high voltage vehicle systems safety is required. Mechanic training in how to service and troubleshoot electric propulsion components is required.
Hybrid Electric Buses:-
Hybrid-electric buses combine two energy sources, one an electrochemical or electrostatic storage device and the other a fuel-burning prime power source. The prime power source could be any device which converts chemical fuel to mechanical energy, and is most often a diesel or gasoline engine, as this allows the buses to use the same fueling, maintenance and storage infrastructure as conventional ICE buses. Thus, today’s hybrid-electric buses combine the elements of the battery bus – the electric motor, controller, battery packs – with an ICE which is most typically a diesel engine, and less frequently a gasoline engine, coupled to an electric generator.
In addition, a number of hybrid buses in service today replace the battery packs with an ultracapacitor, an advanced energy storage option.
The hybrid system allows the engine to operate in a more efficient mode, by sharing the energy and power demands of vehicle operations between the batteries and engine. The batteries can provide the traction motor with extra power as needed for acceleration or steep grades; this allows the engine to operate in a more steady-state mode, increasing the efficiency of in-use engine operation. The electric motor and energy storage also allows for energy recovery through regenerative braking, as described in the battery-electric bus section. Regenerative braking allows the propulsion system to apply a retarding load on the drive axle during braking, thus converting the vehicle’s kinetic energy into electrical energy. The vehicle stores that energy onboard, to be used to drive the wheels at another time.
The overall efficiency of a hybrid-electric system depends on which system elements are selected, how these various systems are integrated, and the electronic control strategy.
Currently, there are two major configuration strategies for hybrid-electric vehicle systems: series or parallel.

Series: In a series hybrid, the engine is completely mechanically decoupled from the drive wheels. All of the energy produced from the engine is converted to electric power by the generator, which powers one or more electric traction motors as well as recharging the energy storage device that provides supplemental power. The electric motor system – by itself – provides torque to turn the wheels of the vehicle. Because the combustion engine is not directly connected to the wheels, it can operate at a more optimum rate, and can be switched off for temporary all-electric, zero-emission operation.
Parallel: In a parallel hybrid, both the combustion engine and the electric motor have direct, independent connections to the transmission. Either power source – or both of them together – can be used to turn the vehicle’s wheels. These vehicles are often designed so that the combustion engine provides power at high, constant speeds; the electric motor provides power during stops and at low speeds; and both power sources work together during accelerations.

Today’s commercial hybrid bus products generate all the electricity they need on-board and do not need to be recharged. The battery pack is simply recharged during the course of normal driving. This allows the bus to avoid the range problems of pure battery buses. It is possible for a hybrid bus to be designed to be “charge depleting” which would mean that the vehicle batteries (or energy storage device) would need to recharge on a regular basis. The all electric range of a plug-in hybrid-electric vehicle would be longer than that of nonplug-in hybrid, but the bus would also run with the ICE engine, giving it a better range than a battery-only bus.

Benefits of Hybrid Electric Buses:-
The appeal of a hybrid-electric system is that the electric drive can improve drive system efficiency, reduce emissions, and reduce energy consumption. This results from the optimized integration of the system, to best capitalize on the efficiency of the electric drive system. This efficiency improvement is achieved by two primary means: the ability to operate the engine in a more efficient mode, and the recovery of regenerative braking energy.
Regenerative braking can also save wear-and-tear on the brakes. In addition, hybrid buses, while not virtually noise-free like battery buses, are quieter than conventional diesel buses since the buses typically use a smaller diesel engine and operate the engine in a more steady-state mode, reducing the noise associated with acceleration of typical heavy-duty diesel engines. In addition, depending on the hybrid system configuration, some buses can operate in an electric-only mode in low speed operations. Hybrid-electric buses are also able to utilize electrically driven accessories, thereby further increasing the overall bus efficiency.
Fuel Cell Buses:-
Fuel cells for commercial transportation applications have generated an enormous amount of attention over the last several years, as they offer the promise of a clean, efficient transportation system no longer dependent on petroleum. Fuel cells combine hydrogen and oxygen in an electro-chemical process, with water and heat the only by-products of this electricity generation if pure hydrogen is being used. The production of hydrogen itself can produce emissions, but it is also possible to produce hydrogen from clean sources like wind or solar generated electricity. Fuel cell power is attractive because it provides the potential to dramatically reduce air pollution, greenhouse gas emissions, and petroleum-based energy use. However, transportation fuel cells are still many years away from competing with current transportation technologies, due to cost, robustness and durability, as well as fuel storage issues. In addition, the lack of a hydrogen infrastructure is a deterrent for widespread fuel cell vehicle deployment, as is the expense of, and potential emissions from, hydrogen production. Nevertheless, there is intense interest in pursuing fuel cells as a commercial transportation technology, with major commitments being made by governments around the world to invest in research, development and demonstration of fuel cell and hydrogen technologies. A propulsion system using a fuel cell as a prime power source directly generates electricity from hydrogen – stored on the vehicle – and oxygen taken from the air. The fuel cell can be used as a stand-alone prime energy source in an electric drive system (essentially replacing the battery) or as the prime power source in a hybrid-electric drive system (replacing the ICE/generator). A fuel cell can be considered similar to a battery in that it produces electrical energy through an electrochemical reaction, not from combustion. Unlike a battery, a fuel cell can produce electricity continuously, without needing to be recharged, as long as it is supplied with hydrogen and oxygen. Fuel cell buses will require a substantial new infrastructure, with some similarities to that used for compressed or liquefied natural gas buses. The fuel cell powered, electric drive vehicle is very different from the standard diesel bus. Infrastructure, support, and training requirements will depend on what type of fuel is used for the fuel cells. Most demonstrations and available buses use pure hydrogen stored in compressed gas form. Infrastructure will be required for the hydrogen fuel either in bulk storage or for on-site production. Maintenance of the fueling infrastructure will need to be considered as well. As with natural gas fuel systems, the maintenance and vehicle storage facilities will need to be reviewed for mitigation of hydrogen leaks inside buildings. This will mean, at minimum, proper air ventilation and leak detectors that control emergency equipment inside the buildings as well as explosion proof wiring. Hydrogen has some safety issues beyond natural gas including the potential ability to detonate, rather than just combust. Its ability to embrittle certain metals also needs to be taken into account.
Hybrid-Electric Vehicle Batteries:-
Batteries are typically moderate power, high-energy devices. Battery-only energy storage systems have good energy storage capability, but are limited with respect to power density, efficiencies (typically less than 75%), cost, cycle life (typically less than 1,000 charge/discharge cycles), and slow discharge. Batteries usually have a thermal management system to operate beyond -3 C to +35 C temperature environment. Lithium batteries may solve some of these problems but at a high cost. Battery life (and replacement costs) in a hybrid-electric vehicle is usually defined by the number of 100% charge/discharge cycles. Reducing the depth of discharge (DOD) increases the number of cycles in a battery’s life. This relationship varies from somewhat linear to exponential. For example in a squared relationship, limiting the DOD to 10% would increase the number of lifetime charge/discharge cycles by a factor of 100. Thus, the life of a 1,000 cycle battery increases to 100,000 cycles.

Ultracapacitors:-
In a series hybrid system, a smaller engine is mated to a generator and operated at a constant, efficient speed and power output level. When vehicle power requirements temporarily increase such as during acceleration or hill-climbing additional power is drawn from an onboard energy storage system comprised of batteries and/or ultracapacitors. When vehicle power requirements are low, the energy storage system is recharged. Not only is engine efficiency increased, but also the vehicle is able to recapture energy whenever it slows down through a process called regenerative braking.
Standard hybrid electric design, which uses only batteries to provide electrical power storage, has drawbacks. These deficiencies are multiple, and they create many design challenges for automotive engineers.
• Firstly, batteries have difficulty functioning in cold weather.
• Secondly, batteries require sophisticated charge equalization management.
• Thirdly, batteries have limited cycle life under extreme conditions, which results in high-cost replacement throughout the life of the vehicle.
A new battery has to be purchased and installed; the old battery has to be removed and disposed. Battery disposal can be problematic unless the manufacturer has a recycling program. All of this adds to the cost of a battery-based system, not to mention downtime of the vehicle itself.
Perhaps most importantly though, batteries are limited in their ability to capture and regenerate energy, or provide bursts of high power during short duration events, such as acceleration and braking. This high power limitation reduces the efficiency of the hybrid electric drive system design.
Capacitors are known to accept high power levels and store energy quickly. Ultracapacitors are high power, low energy devices with high power density and cycle life, but cannot store the energy needed for long term or extended range all-electric operation. Ultracapacitor packs for hybrid vehicles offer the storage and delivery of hundreds of kilowatts at efficiencies greater than 84%. Ultracapacitors have a greater than 500,000 charge/discharge cycle life and a -35 C to +65 °C operating temperature environment. In heavy-duty hybrid vehicle applications ultracapacitor packs are projected to have a 10 to 12 year life. Furthermore, known economies of scale manufacturing techniques could reduce future ultracapacitor cost by more than 60%.

Batteries and Ultracapacitors:-
Hybrid-electric ground vehicles require an advanced, compact, high energy-density electrical storage system to provide both high power and high energy. A combination of multiple types of energy storage, such as batteries combined with ultracapacitors, may provide the best overall performance and offer superior power, energy and cycle life. The concept of combining the two systems offers potential advantages of both, while minimizing the disadvantages of either type of technology to meet the demand of storing and releasing energy. There is potential to improve the performance, reliability, and extend the operating environment of all hybrid-electric power trains, especially for heavy-duty vehicles. Typical advantages are longer range for quiet operation, better acceleration, higher fuel mileage, and lower maintenance.
The dielectric performance of stronger glass will make it the material of choice for next-generation high-energy capacitors. The new technology will drive miniaturization and cost reduction in power electronic and pulse power applications. Specifically, products like hybrid electric vehicles and implantable medical devices will utilize the new glass capacitors. The increased availability of these items will be of great benefit to the environment, economy, and human health. The new glass capacitors will replace bulky electrolytic and polymer-based capacitor technologies in power electronic applications. The most important practical use of the new capacitors will be in hybrid electric vehicles (HEVs). In HEVs, a power electronic circuit is required to convert the DC input from the battery or fuel cell into an AC output to apply to the electric motor. The new glass capacitors will provide for the miniaturization and extended lifetime of the power converter in HEVs. The associated cost reduction will accelerate the already robust market for these vehicles, which has grown nearly ten-fold in the past five years. The corresponding increase in the use of fuel cell technology in transportation will help reduce the economy’s oil dependence and curtail greenhouse gas emissions. Capacitors are critical components in the power electronic circuit. DC bus capacitors (CE) are responsible for energy storage, discharge and voltage smoothing. They require a large capacitance (100-2000 μF) and energy density. Snubber capacitors (CC) are used to reduce power dissipation in a solid-state inverter, and have a capacitance of 10- 1000 nF. Filter capacitors (CR and CF) remove unwanted frequencies and harmonics from the output AC signal, so their capacitances depend on the output frequency.
Power converter miniaturization and cost reduction will be achieved by the use of the new glass in DC bus and snubber capacitors. Currently, these components are made from electrolytic and polymer technologies, which are volumetrically inefficient. Depending on the dielectric constant, the new glass will reduce the required volume by a factor of 2-3. Furthermore, under elevated operating temperatures, the glass capacitors are expected to have a longer lifetime than electrolytic capacitors, which are prone to evaporation of the electrolyte solution. Finally, the high dielectric strength of the glass will allow the capacitors to be rated for much higher voltages than allowed by current technology. This will provide more alternatives for fuel cell and battery voltage selection.

Conclusions:-
Hybrid Electric buses provide a clean option for public transport. This technology must therefore be kept on the fast track of development. Rapid advancement in the field of Ultracapacitors and batteries is therefore of the utmost importance. We must therefore strive towards achieving the goal of a clean mode of public transport through research in Power Electronics.