Evolution of Hybrid Electric Vehicles

The burning of fuel for combustion engine-based transportation has a big effect on the environment. Global warming is caused in large part by the burning of fossil fuels in factories and cars. Electric vehicles are the best way to reach the goals of saving energy and having road vehicles with no emissions. Therefore, the governments of most developed countries are encouraging people to buy electric cars to reduce the amount of CO2 and other greenhouse gases in the air.

At the moment, hybrid electric vehicles (HEVs) are a popular solution because renewable energy sources are not always available, and batteries do not last forever. People are not as interested in electric vehicles (EVs) as they could be because EVs are more expensive than fossil-fueled vehicles, they do not have as much range, it is easier to refuel a fossil-fueled vehicle than an EV, and people do not want to spend money on current EV technology when much better technology could be available in 2 to 3 years.

People are becoming more aware of how the weather changes and how important it is to save energy. This is leading to the development of new technologies for green vehicles, such as plug-in HEV and full electric vehicles (FEV), which could be an economical and environmentally friendly solution. In this article, plug-in HEVs are discussed in detail.

Vehicle Powertrain Strategy

Alteration of the power source connections enables the implementation of a wide variety of topologies on the powertrain, each of which can accommodate a certain set of capabilities. It is possible that these connections are made using mechanical or electrical means. HEV powertrains have four major configurations: series connection, parallel connection, series-parallel connection, and plug-in connection. On the other hand, FEVs are further divided into three parts according to the source of energy battery-based, solar-based, or fuel cell-based. The vehicle powertrain strategy of both HEVs and FEVs is outlined in Fig. 1, given below.

Fig. 1. a) Classification of vehicle powertrain strategies (b) Various configurations of an HEV. Source: IEEE Access

Hybrid Electric Vehicles and Their Types

In the HEV, engine power is transferred across the ring gear, which is mechanically connected through the drive shaft. Another part of the engine power is turned into electrical power to drive the motor, which is hinged with gears and connected back to mechanical power. The first way is called a "parallel path," and the second is a "series path." The main goal of making HEVs is to reduce how much fuel they use and how much pollution they put out. Depending on how the vehicle's powertrain is set up, HEVs can be divided into series, parallel, power-split/series-parallel, and plug-in HEVs. This can help save energy and get better gas mileage. Fig. 1 (a and b) shows how the vehicles are set up and how they are built. Table 1 shows a summary of HEVs and how they can be used.

Table 1: Summary of HEV architectures and their applications. Source: IEEE Access

1) Series Hybrid Electric Vehicles (S-HEV)

In a stop-and-go driving style, a series-HEV topology gives the best performance. There is no mechanical link between the internal combustion (IC) engine and the wheels. The IC engine is mostly used to make electricity by driving the generator, which is then combined with the power from the electrical storage and sent to an electric motor that drives the wheels via a DC bus. With this strategy, the engine works well at different speeds. S-HEVs are good for driving on the highway. Buses are not good for driving in urban centers because they have a high conversion loss.

2) Parallel Hybrid Electric Vehicles (P-HEV)

An electric motor is utilized alone in parallel HEVs at low speeds, while the engine and wheels are mechanically connected directly. As a result, the combined torque of the engine and wheels is transferred to the wheels through a regular moving shaft and, most likely, a different gear. When compared to the series HEV topology, this technique results in the least amount of energy loss; however, it is not as well suited for traffic patterns that involve frequent lane changes and stops.

3) Series-Parallel Hybrid Electric Vehicles (SP-HEVS)

Both series-parallel and power-split HEV feature an additional mechanical connection between the motor and generator that runs through the transmission. This connection is mechanical in nature. This configuration offers the advantages of both series and parallel HEVs, which complement one another. As a consequence of this, one of the most significant challenges presented by SP-HEV is the regulation of power flow when splitting power. This is because the system includes functional components from both series and parallel systems, which results in an increase in the complexity of the system.

4) Plug-in Hybrid Electric Vehicles (P-HEV)

The configuration of plug-in HEVs is identical to that of conventional HEVs, with the exception of an additional electric charging socket and higher capacity electrical components, as shown in Fig 2. In this way, the energy storage system (ESS) is considered to be the primary source, which provides a new dimension to the EMS method in PHEVs for superior fuel economy by operating in two modes known as charge-depleting and charge-sustaining modes. 

Because of the enormous capacity of its electrical components, a PHEV may remain in the full-electric mode for a considerable amount of time. The plug-in HEV is a promising concept for lowering global emissions because of its high performance and low fuel consumption potential, whether operating in electric or hybrid mode. Nissan's first plug-in electric vehicle, the 'Nissan Leaf,' was released to the public in 2011, while Ford's plug-in hybrid vehicle, the Fusion Energi, made its premiere the following year in 2013.

Fig. 2. Plug-in HEV Configuration. Source: IEEE Access

Summarizing with Key Points:

Some of the takeaways from the article are as follows:

• CO2 emission and other greenhouse pollutants are driving the development of plug-in HEVs and FEVs, which could be cost-effective and environmentally friendly.

• A powertrain is an assembly of all of the components that work together to propel a vehicle forward. EV powertrains are categorized as HEVs and FEVs.

• There are three main configurations for HEVs powertrains: series connection, parallel connection, and series-parallel connection. On the other hand, there are three categories of FEVs according to their energy source: battery, solar, or fuel cell-based.

• In the HEV, the power from the engine is sent across the ring gear, which is connected to the drive shaft mechanically. Another part of the engine's power is turned into electricity to drive the motor, which is hinged with gears and connected back to mechanical power.

• Series HEV topologies perform best in stop-and-go driving. The IC engine and wheels are not mechanically connected there. The IC engine drives the generator to generate energy, which is combined with power from the electrical storage and transferred to a DC bus to move the wheels. 

• Parallel HEVs use an electric motor at low speeds and mechanically connect the engine and wheels. So, a conventional shaft and, possibly, a separate gear convey the engine and wheel torque to the wheels. This design loses less energy.

• Series-parallel HEVs have an extra mechanical link between the motor and generator that runs through the transmission. Splitting power and controlling how the power flows is a challenge.

• Plug-in HEVs are promising due to their high performance and low fuel consumption in either electric or hybrid mode. They have an additional electric charging socket and higher-capacity electrical components.