Hybrid Electric Vehicles

Hybrid electric vehicles (HEV) typically incorporate an internal combustion engine, an electric motor, a generator, and a battery pack. The nature, arrangement and integration of these components can be varied to maximize performance and efficiency, and reduce emissions levels.

For internal combustion engines, hybrid systems may use diesels or lean burn gasoline engines. Different types of batteries, fuel cells, ultra-capacitors, flywheels and other means of storing energy can be used as the "battery pack". Engines and battery packs can be arranged to operate in parallel, in series, or a combination of the two.

A parallel hybrid powertrain configuration, as shown in Figure 1, has a direct mechanical connection between the internal combustion engine and the wheels, as in a conventional vehicle, but also has an electric motor capable of driving the wheels directly. The internal combustion engine alone, the electric motor on its own, or, a combination of the two can then power the vehicle.

Figure 1
Hybrid Electric Vehicle Parallel Configuration

In a series hybrid (Figure 2), the internal combustion engine runs a generator, which charges the battery pack to power an electric motor that drives the wheels. The main advantage in this system is that the vehicle can be operated largely as an electric vehicle without the combustion engine running in urban areas, thereby reducing vehicle emissions.

Figure 2
Hybrid Electric Vehicle Series Configuration

Combining a series and parallel hybrid system, sometimes called a combined or a series/parallel design, allows the internal combustion engine to directly drive the wheels but also has the ability to charge the battery pack through a generator.

The ability of the control system of a hybrid vehicle to manage how much power flows to or from each component means that the vehicle designer has considerable flexibility in how components are combined and used. Components can be integrated with a control strategy to achieve the optimal design for a given set of design constraints.

Hybrid drivetrains, with varying degrees of electrification, offer very substantial fuel consumption improvements. Hybrid vehicles can offer 20% to 30% improvements.

Battery Electric Vehicles (Energy Storage and Battery Technology)

Research continues to improve the batteries, range, overall performance, efficiency, and recharging time of battery electric vehicles (BEV). These vehicles have zero tail-pipe emissions and for this reason have come to be known as "zero emission vehicles". However, the electricity needed to charge the vehicle batteries is usually supplied from the power grid. Depending on the method of power generation, various environmental impacts can be involved. Even electricity generated using water, wind, or nuclear methods is not without environmental impacts despite the fact that these sources of power have no direct air emissions.

Energy storage devices are key to the optimal performance of BEVs and HEVs. These vehicles each require different battery power to energy ratios and use battery power differently. Alternatives to traditional lead acid batteries include; lithium polymer batteries, nickel metal hydride batteries, flywheels and ultra-capacitors.

Batteries:

Chemical batteries are used to power electric vehicles. Batteries vary in the amount of driving range they allow a vehicle to travel based on their energy and power densities and their charging/discharging efficiencies. Most electric vehicles in the past century have used lead-acid batteries, but researchers are developing advanced batteries such as nickel-iron, nickel-cadmium, sodium-sulfur, zinc-air, and lithium batteries, among others.

Flywheels:

Kinetic energy is stored and released in a flywheel system by the increase and decrease of the rotational speed of the flywheel. Advanced materials with high strength-to-weight ratios are under consideration as are configurations in which the flywheel is integrated into the motor/generator. Like other secondary power sources, cost, reliability, efficiency, and safety need to be assessed fully.

Ultracapacitors:

Ultracapacitors are devices for storing electricity like a battery. Unlike a battery, however, they are designed to release their energy in a quick burst (ideal for starting or accelerating a car) and they store energy quickly (ideal for capturing the energy available when a car is braking). Current work is concentrated on improving performance and reducing costs.

Hydropneumatics:

This is a mechanical type of energy storage. Hydropneumatic systems store energy by using a high-pressure liquid to compress a gas. These systems have high power densities allowing a quick burst of energy, ideal for vehicle acceleration. However, they have low energy densities and can only store a small amount of energy.

A report from the Electric Vehicle Association of Canada (EVAC) shows that the use of BEVs in Canada can provide significant advantages to local air quality and global greenhouse gas reductions. The study indicates that every province in Canada would experience CO₂ reductions through the replacement of gasoline-fuelled vehicles with battery electric vehicles. On average, a BEV operating in Canada will reduce CO₂ emissions by 75% compared to a comparably sized conventional vehicle. Nevertheless, range, charging times, battery life and consumer acceptance remain challenges.

Fuel Cell Vehicles

This power source has excellent fuel efficiency, high power density, quiet operation and no harmful tailpipe emissions. Fuel cells convert a hydrogen rich gas and oxygen into electricity, which then powers electric motors to drive the vehicle. In an ideal system, one that is operated with pure hydrogen, the only by-product of the fuel cell is water vapour. There is no direct air pollution. However, there can be emissions in producing the hydrogen fuel.

Hydrogen could be derived from:

• Electrolysis of water to form hydrogen and oxygen. Clean sources of electricity (wind, solar, geothermal and possibly nuclear) would be preferred.
• Reforming of a hydrogen-rich feedstock such as ethanol, methanol, natural gas or even gasoline. Biomass could be used for ethanol production. CO₂ emissions are a byproduct of reforming natural gas and gasoline.
• Gasification of coal.

The hydrogen used by fuel cells can be stored as a compressed gas or a liquid in a cylinder directly on-board the vehicle, or can be manufactured on-board the vehicle through a process of reforming using a mini refinery.

The more promising fuel cell technologies should offer powerplants that are twice as efficient with half the greenhouse gas emissions of a conventional spark ignition gasoline vehicle.

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