Hybrid and Electric Vehicles

What are HEVs?

Hybrid electric vehicles (HEVs) combine the internal combustion engine of a conventional vehicle with the battery and electric motor of an electric vehicle, resulting in twice the fuel economy of conventional vehicles. This combination offers the extended range and rapid refueling that consumers expect from a conventional vehicle, with a significant portion of the energy and environmental benefits of an electric vehicle. The practical benefits of HEVs include improved fuel economy and lower emissions compared to conventional vehicles. The inherent flexibility of HEVs will allow them to be used in a wide range of applications, from personal transportation to commercial hauling.

Why HEVs?

Hybrid power systems were conceived as a way to compensate for the shortfall in battery technology. Because batteries could supply only enough energy for short trips, an onboard generator, powered by an internal combustion engine, could be installed and used for longer trips. In the old days, we thought that by biasing the system toward battery-electric power and operating on wall-plug electricity as much as possible, efficiency and emissions would then be about as optimal as we could hope for until better batteries came along. The natural conclusion of this concept was that, with better batteries, we probably would not need hybrids at all. But after 20 years of study, it seems that hybrids are taking center stage and electric vehicles are being used in niche market applications where fewer miles are traveled per trip or daily. 

More efficient cars can make a big difference to society in terms of environmental benefits, and the serious deterioration of urban air has motivated regulators to require cleaner cars. Use of production HEVs will reduce smog-forming pollutants over the current national average. Hybrids will never be true zero-emission vehicles, however, because of their internal combustion engine. But the first hybrids on the market are cutting emissions of global-warming pollutants by a third to a half, and later models may cut emissions by even more.

HEV Advantages

HEVs have several advantages over conventional vehicles:

• Regenerative braking capability helps minimize energy loss and recover the energy used to slow down or stop a vehicle.
• Engines can be sized to accommodate average load, not peak load, which reduces the engine's weight.
• Fuel efficiency is greatly increased (hybrids consume significantly less fuel than vehicles powered by gasoline alone).
• Emissions are greatly decreased.
• HEVs can reduce dependency on fossil fuels because they can run on alternative fuels.
• Special lightweight materials are used to reduce the overall vehicle weight of HEVs.

The HEVs available for sale are very cost competitive with similar conventional vehicles. Any cost premium that may be associated with HEVs of the future can be off-set by overall fuel savings and government incentives.

Auto manufacturers are making these HEVs with comparable performance, safety, and cost because they know that these three elements are most important to consumers. And by combining gasoline with electric power, hybrids will have the same or greater range than traditional combustion engines. The HEV is able to operate approximately two times more efficiently than conventional vehicles. Honda's Insight can go 700 miles on a single tank of gas. The Honda Civic hybrid can go 650 miles on a tank of gas, and the Toyota Prius can go about 500 miles. For the driver, hybrids offer similar or better performance than conventional vehicles. More important, because such performance is available now, hybrids are a practical way for consumers to choose a cleaner drive today.

Types of HEVs

Many configurations are possible for HEVs. Essentially, a hybrid combines an energy storage system, a power unit, and a vehicle propulsion system. The primary option for energy storage is batteries. Hybrid power unit options are spark ignition engines, compression ignition direct injection engines, gas turbines, and fuel cells. Propulsion can come entirely from an electric motor, such as in a series configuration, or the engine might provide direct mechanical input to the vehicle propulsion system in a parallel configuration system.

The HEV components and system design is very critical for the success of the project and the fuel economy gains and the emission reduction. Components sizing should be based on the target vehicle and the vehicle mission, where as the control system is based on the drive cycle and the use pattern of the vehicle.

Parallel Configuration

An HEV with a parallel configuration has a direct mechanical connection between the engine and the wheels, as in a conventional vehicle, but also has an electric motor that drives the wheels. For example, a parallel vehicle could use the power created from an internal combustion engine for highway driving and the power from the electric motor for accelerating. Some benefits of a parallel configuration are:

• The vehicle has more power because both the engine and the motor supply power simultaneously.
• Most parallel vehicles do not need a separate generator because the motor regenerates the batteries.

Because the power is directly coupled to the road, it can be more efficient.

A combination alternator/starter is also being considered for parallel HEVs. This is essentially an electric machine that can start the engine, and take power from the engine and turn it into electricity. It could also provide extra power to the driveline when power assist is needed for hill climbing or quick acceleration.

Series Configuration

An HEV with a series configuration uses the engine or turbine with a generator to produce electricity for the battery pack and electric motor. Series HEVs have no mechanical connection between the engine and the wheels; this means that all motive power is transferred from chemical energy to mechanical energy, to electrical energy, and back to mechanical energy to drive the wheels. Here are some benefits of a series configuration:

• The engine never idles, which reduces vehicle emissions.
• The engine drives a generator to run at optimum performance.
• The design allows for a variety of options when mounting the engine and vehicle components.
• Some series hybrids do not need a transmission.

The series hybrid system is most suitable for applications, where the vehicles has lots of stop and go drive cycle such as transit busses and local delivery trucks. The downside is that series HEVs require larger, and therefore, heavier battery packs than parallel vehicles. The system may be less efficient for constant highway driving due to the generator and drive motor deficiencies.

What's next for HEVs?

HEVs are now at the forefront of transportation technology development. Hybrids have the potential to allow continued growth in the automotive sector, while also reducing critical resource consumption, dependence on foreign oil, air pollution, and traffic congestion.

Hybrids are a hot subject today and they are beginning to show up globally. Their widespread penetration into the automotive market hinges mainly on the economics of producing a complex hybrid power system, rather than the inherent capabilities of the technology itself. The hybrid's complexity, and the fact that some of the best storage and conversion systems have yet to be fully developed, is responsible for varied opinions on hybrids' ultimate impact in the marketplace. The Honda Insight, Honda Civic hybrid and Toyota Prius is available for sale today and other manufacturers stated models in 2004 and beyond.