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Hybrid electric vehicles: What is a hybrid vehicle drivetrain?

When people talk about future mobility and alternative drives, they often hear the term hybrid. But what exactly is a hybrid or a hybrid car? What is the difference between a serial and a parallel hybrid drive? What are the advantages of a plug-in hybrid and what is the function of a range extender?

Everything about the hybrid car

  • Differentiation between serial and parallel hybrid drive systems
  • Differentiation of micro hybrid, mild hybrid and full hybrid
  • Explanation of plug-in hybrid, range extender and recuperation

Explanation of the term hybrid

In general, one speaks of a hybrid or a hybrid drive when several different types of drive are combined in one vehicle. Basically, a hybrid car combines at least one electric motor with an internal combustion engine under the sheet metal. The official designation for such a car is hybrid electric vehicle (HEV).

BMW i-Family i3 i8
BMW i-family: Serial hybrid in the BMW i3 with range extender and parallel hybrid in the BMW i8 (Source: BMW)

Difference between serial and parallel hybrid drive

A distinction is made between a serial and a parallel hybrid drive according to the structure or the way in which the conventional drive is combined by a gasoline or a diesel engine with one or more electric motors.

In the serial hybrid drive, the wheels are driven solely by the electric motor. The combustion engine has no mechanical connection to the drive axle. Thus, in combination with a generator, the conventional engine serves to provide the electrical energy for the electric motor and, if necessary, also to charge the batteries.

Serial Hybrid Structure Function
Serial hybrid: Structure and function of a serial hybrid drive (©

A serial hybrid car is often referred to as an electric car with range extender (REX), since the actual drive is purely electric and the combustion engine can be used for indirect propulsion and charging when the battery charge level is low, thus extending the range. A good example of a serial hybrid is the BWM i3 with a range extender that can be ordered as an optional extra.

In a parallel hybrid drive, the combustion engine and the electric motor have a mechanical connection to the wheels. The drive can therefore be provided by the electric motor alone or by the gasoline or diesel engine alone, and also simultaneously by both. The power and torque of the two drives are added together.

Parallel Hybrid Structure Function
Parallel hybrid: Structure and function of a parallel hybrid drive (©

The combination of the serial and parallel hybrid drive is called a mixed hybrid, also known as a power-split hybrid. In this type of hybrid, the combustion engine is used both for propulsion and, with the help of a generator, for charging the battery. The two engines are connected to the wheels and can act individually or together on the drivetrain. Most hybrid vehicles are designed as mixed hybrids, and a good example of this hybrid system is the Toyota Prius.

Mixed Hybrid Structure Function
Mixed hybrid: Structure and function of a mixed hybrid drive (©

Distinction between microhybrid, mild hybrid and full hybrid

The parallel hybrid can also be divided into three levels according to the share of power provided by the electric drive in the total power. However, the transitions are fluid and the power levels are merely for orientation purposes.

The micro hybrid is better known as an automatic start-stop system. In a microhybrid car, there is no additional electric motor for propulsion; rather, the starter generator is designed to be stronger, with up to 6 kW, and is essentially used to start the combustion engine. The electrical energy is provided by an ordinary starter battery and can be recovered by using the braking energy, which is called recuperation.

In a mild hybrid, the additional electric motor acts in support of the internal combustion engine to ensure greater efficiency of the overall drive system, especially in partial load ranges such as during starting and acceleration. The electric motor in a mild hybrid car generates between 6 kW and 20 kW, which is added to the system power together with the power of the combustion engine. The battery is usually larger and functions such as automatic start-stop and recuperation are available. Pure electric drive is not possible in a mild hybrid, as in a microhybrid.

In a full hybrid, the electric motor not only supports the combustion engine in low-torque power ranges, but can also drive the vehicle on its own. With an output of more than 40 kW, a purely electric drive is possible. The range and top speed in electric mode are determined by the capacity of the battery and the settings of the car manufacturer. A full hybrid drive represents the highest level of complexity when combining a conventional combustion engine and an electric motor.

Toyota Prius Family Hybrid Cars
Toyota Prius: Probably the best-known hybrid car is already being built in its 4th generation (Source: Toyota)

Distinction between hybrid and plug-in hybrid

In a plug-in hybrid electric vehicle (PHEV), the battery can also be charged by an external power source, and not just by the combustion engine using a generator or by recovering braking energy (recuperation) as in a hybrid car. This means that longer distances can be covered purely electrically and the battery can be charged while the vehicle is stationary.

A plug-in hybrid usually has a larger traction battery and is a hybrid between a hybrid and an electric car. The combustion engine and the electric motor can be combined in the body as a serial hybrid (BMW i3 with range extender) or as a parallel or mixed hybrid (GTE models from Volkswagen).

Volkswagen VW GTE Logo
Hybrids from Volkswagen: VW’s plug-in hybrid models can be recognized by the GTE logo (Source: VW)

Conclusion: Advantages and disadvantages of hybrids and plug-in hybrids

The advantage of cars with a hybrid drive is that they are often more efficient. For example, hybrid cars have lower fuel consumption and lower pollutant emissions, especially in the city, where frequent starting and acceleration is required. In addition, energy that would otherwise be lost during braking can be recovered as electrical power through a process known as recuperation.

The full hybrid cars also allow purely electric driving, which means zero local emissions and less noise generation. In the case of the plug-in hybrid cars, the battery can also be charged externally, which effectively turns the plug-in hybrids into electric cars for short distances, but for longer distances they can be moved by the existing combustion engine without the range restriction.

The disadvantages of hybrid cars stem from the use of two powertrains. First, multiple engines and a larger battery in a vehicle mean higher costs in terms of acquisition and, if necessary, maintenance. Second, the more complex technology of two drives may also lead to more frequent breakdowns and more complicated repairs. In addition, the higher weight due to multiple motors and a more powerful battery can have a negative impact on fuel consumption as well as emissions in certain driving situations.

Title image: Toyota

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