Series vs Parallel vs Series/Parallel Drivetrains

What is a drivetrain?

A drivetrain is the collection of components that deliver power from a vehicle’s engine or motor to the vehicle’s wheels. In hybrid-electric cars, the drivetrain’s design determines how the electric motor works in conjunction with the conventional engine. The drivetrain affects the vehicle’s mechanical efficiency, fuel consumption, and purchasing price.

Hybrids that use a series drivetrain only receive mechanical power from the electric motor, which is run by either a battery or a gasoline-powered generator. In hybrids with parallel drivetrains, the electric motor and internal combustion engine can provide mechanical power simultaneously. Series/parallel drivetrains enable the engine and electric motor to provide power independently or in conjunction with one another.

Both conventional hybrids and plug-in hybrids have models with series, parallel, and series/parallel drivetrains. Since battery-electric and hydrogen fuel cell vehicles don’t have internal combustion engines, they utilize different drivetrain assemblies (though some components are shared).

Series drivetrains

Series drivetrains are the simplest hybrid configuration. In a series hybrid, the electric motor is the only means of providing power to the wheels. The motor receives electric power from either the battery pack or from a generator run by a gasoline engine. A computer determines how much of the power comes from the battery or the engine/generator. Both the engine/generator and the use of regenerative braking recharge the battery pack.

Series hybrids perform at their best during stop-and-go traffic, where gasoline and diesel engines are inefficient. The vehicle’s computer can opt to power the motor with the battery pack only, saving the engine for situations where it’s more efficient.

The engine is typically smaller in a series drivetrain because it only has to meet certain power demands; the battery pack is generally more powerful than the one in parallel hybrids in order to provide the remaining power needs. This larger battery and motor, along with the generator, add to the vehicle’s cost, making series hybrids more expensive than parallel hybrids.

Parallel drivetrains

In vehicles with parallel hybrid drivetrains, the engine and electric motor work in tandem to generate the power that drives the wheels. Parallel hybrids tend to use a smaller battery pack than series drivetrains, relying on regenerative braking to keep it recharged. When power demands are low, parallel hybrids also utilize the motor as a generator for supplemental recharging, much like an alternator in conventional cars.

Since the engine is connected directly to the wheels in parallel drivetrains, the inefficiency of converting mechanical power to electricity and back is eliminated, increasing the efficiency of these hybrids on the highway. This reduces, but does not eliminate, the efficiency benefits of having an electric motor and battery in stop-and-go traffic.

Series/parallel drivetrains

Series/parallel drivetrains merge the advantages and complications of the parallel and series drivetrains. By combining the two designs, the engine can both drive the wheels directly (as in the parallel drivetrain), and be effectively disconnected, with only the electric motor providing power (as in the series drivetrain). The Toyota Prius helped make series/parallel drivetrains a popular design.

With gas-only and electric-only options, the engine operates at near optimum efficiency more often. At lower speeds it operates more as a series vehicle, while at high speeds, where the series drivetrain is less efficient, the engine takes over and energy loss is minimized.

This system incurs higher costs than a pure parallel hybrid since it requires a generator, a larger battery pack, and more computing power to control the dual system. Yet its efficiencies mean that the series/parallel drivetrain can perform better—and use less fuel—than either the series or parallel systems alone.

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