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Hybrid Electric Vehicles

Summary

This section explains the definition the terms “hybrid” and hybrid electric vehicles, why we need this kind of technology, a brief history, current hybrid electric vehicle applications and their architecture.

Why Hybrid Electric Vehicles

Before we begin, we should know what the term “hybrid” means. The term hybrid can be stated concisely as the use of two or more power sources to operate any kind of system. In relation to the terminology “hybrid electric vehicles”, it means to propel the vehicle using two or more power sources such as an electric motor, an internal combustion engine or solar energy.

The world’s population is increasing day by day. This will give rise to an increase in the mobility requirements of our population. In contrast, natural resources are moving to the verge of a dangerous balance point due to increased air pollution and its detrimental effects on nature. This is because current vehicles are mostly propelled by internal combustion engines that use fossil fuels. The contribution of road vehicles to air pollution and global warming due to the their carbon monoxide emission is very high. For this reason, scientists and researchers are trying to find new energy sources to alleviate the exhaust emissions emitted by road vehicles.

Figure 1. Global warming and effects

(Source: www.augustachronicle.com )

There are clean alternative energy sources such as hydrogen and electricity. But the infrastructure is not mature enough and the travel range with these sources cannot compete with the range available by fossil fuels. An intermediate solution is thus required. The remedy is hybrid vehicles.

Brief History of Hybrid Electric Vehicles

Although the history of hybrid electric vehicles dates back to the 17^th century, the first mass produced hybrid electric vehicle, the Toyota Prius, was introduced in 1997 to the Japanese market. After this step of Toyota, Honda introduced the Insight model in 2000.

How a Hybrid Electric Vehicle Works

Hybrid electric vehicles use two propulsion units. These are an electric motor and an internal combustion engine. During vehicle motion, these two hybrid electric vehicle power sources are stepped in from time to time to achieve low fuel consumption and low greenhouse gas emissions. The main components of hybrid electric vehicles are the electric motor also used as a generator, the batteries and the coupling devices (see Fig. 2).

Figure 2. Components of Hybrid Electric Vehicles

(Source: http://www.fueleconomy.gov/feg/hybridtech.shtml)

The electric motor supplies its energy from the batteries whereas the internal combustion engine uses the fuel tank for this purpose. Electric motor and internal combustion engines are connected to each other via mechanical devices. Battery energy is replenished in two ways during travel of hybrid electric vehicles. One of them is regenerative braking. The latter one is operating the electric motor as a generator to generate electricity during normal operation.

There are mainly two different architectures of a hybrid electric vehicle which are the series and the parallel architectures.

Series Hybrid Electric Vehicles

In the series hybrid configuration, the internal combustion engine is only used to charge the batteries. There is no mechanical connection between the electric motor and the internal combustion engine (Fig 3.). When the charge of the battery reaches it’s allowed minimum level, the internal combustion engine starts to charge the battery. When the battery is fully charged, the internal combustion engine shuts off. In this configuration, the internal combustion engine runs at its most efficient region. The main disadvantage of this configuration is that, there are energy losses during the conversion of energy from fuel energy to electricity and from electricity to mechanical energy.

Figure 3. Series Hybrid Electric Vehicle Configuration

(Source: http://www.toyota.co.jp/en/tech/environment/ths2/what.html)

Parallel Hybrid Electric Vehicles

In the parallel configuration, both the electric motor and the internal combustion engine drive the wheels according to preferred efficiency paths. The electric motor and the internal combustion engine are coupled in a parallel manner. If the charge of battery is low, the electric motor acts as a generator to replenish the battery.

Figure 4. Parallel Hybrid Electric Vehicle Configuration

(Source: http://www.toyota.co.jp/en/tech/environment/ths2/what.html)

Energetic benefits of vehicle hybridization

The positive impact of vehicle hybridization on fuel economy is twofold. Fist of all, the presence of battery allows operating the engine mostly at its best efficiency. For the series structure, this is achieved by designing the powertrain in such a way that the engine works at one operating condition, corresponding to its max conversion efficiency. Therefore, the peaks in power demand are met by the batteries. For the parallel lay-out, the engine is better operated with respect to conventional vehicle, mainly because of two reasons: downsizing and opportunity of turning off the engine during urban driving, where ICE fuel economy is usually very low. In this case, the depletion in battery state of charge is compensated by turning on the engine in highway routes and demanding more power to recharge batteries.
The second important benefit relates to regenerative braking. Through such process, the negative torque required to slow down the vehicle is provided by the electric motor/generator, in both series and parallel structure. Therefore, the vehicle kinetic energy, which is generally lost in heat in conventional vehicles, can be recovered and restored in the battery pack. Regenerative braking significantly contributes to decreasing fuel economy. As an example, in urban driving (i.e. ECE driving schedule) the fuel savings obtained via regenerative braking reaches up to 15/20% with respect to same power-to-weight-ratio conventional cars.

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