Hybrid Cars

Hybrid Drivetrains: Series vs Parallel

Hybrid Drivetrains: Series vs. Parallel

The term “drivetrain” is used to describe how power is delivered to the wheels in any type of car. With a gasoline drivetrain, acceleration requires fuel. That fuel comes from one source – the gas tank. In hybrids, it’s not so simple. Drivetrain design is important in hybrids because it determines how the vehicle’s mechanical components work with the various electrical systems to ensure the vehicle operates efficiently.

The purpose of a hybrid drivetrain is to conserve fuel without sacrificing performance. To achieve this, hybrids use an onboard computer that determines how much power comes from the battery, the gas engine and when. In hybrid electric vehicles (HEVs), there are two primary drivetrains used: series and parallel. Let’s explore each in more detail.

Series Hybrids

The series hybrid configuration is not as complex. There is an electric motor and a gasoline engine. When power is needed to move the wheels, the electric motor (and ONLY the electric motor) supplies the energy. The electric motor can use power from the battery pack or from a generator run by the gasoline engine, but not both. Think about the name series: one, then the other. Makes sense, right?

So, if the battery is fully charged, the car might opt to use the battery’s energy first. When the battery is nearly depleted, the vehicle will switch on the gasoline engine to generate additional power to keep the electric motor going. The transition from one energy source to the other is designed to be seamless.

Below, we’ve outlined the advantages and disadvantages of a series hybrid:

Advantages:

  • Battery pack provides majority of the power
  • Ideal for urban and suburban driving
  • Gasoline engines are smaller and more efficient than conventional

Disadvantages:

  • Costly to purchase as it requires a larger battery
  • Not as efficient when driving longer routes or for highway driving

Vehicle Examples:

  • Chevrolet Volt, Honda Insight, Hyundai Ioniq, BMW i3 w/ Range Extender

Parallel Hybrids

Unlike series hybrid drivetrains (where only the electric motor can drive the wheels and the gasoline engine really only serves as its backup generator), a parallel drivetrain can put power to the pavement through the electric motor or the gas engine directly. An onboard computer will determine which is best for the given set of circumstances and make the choice for the driver.

Since there are two motors, they can work side by side to propel the car. This enables the onboard computer to continually monitor the need for acceleration and decide which motor (electric or gasoline) would be best.

For example, if you’re already moving but need a boost of power, like merging onto the freeway and getting up to speed, a parallel hybrid might put the gas motor to work. But when you’re sitting at a red light and it turns green, the same car might opt for the electric motor and its readily available torque, to get you from 0-30 mph ASAP. Plus, when the gas motor isn’t being used directly for putting power to the wheels, it will turn on to act as a generator for battery charging when needed.

There is also the concept of a Series/Parallel Hybrid which allows the electric motor and gasoline engine to drive the wheels simultaneously. This type of hybrid is discussed below when we cover the Prius.

Here are the advantages and disadvantages we’ve outlined for the parallel hybrid:

Advantages:

  • Engine and electric motor can directly supply torque to the wheels at the same time
  • Gasoline engine is less compact than those used in series hybrids

Disadvantages:

  • Complex structure can be complicated to repair
  • Not as efficient when driving locally – for example, stop-and-go traffic

Vehicle Examples:

  • Audi Q8, Chevrolet Malibu Hybrid, Honda Accord Hybrid

Dawn of the Prius: Combining Series & Parallel

Prius is derived from the Latin word Primus, meaning “first.” The Toyota Prius has certainly lived up to the name. It was the first mass-produced hybrid. It also is somewhat unique because it was the first vehicle to combine series and parallel drivetrains, meaning it can be powered by the gas engine or the electric motor, or by both simultaneously. This is sometimes referred to as a “series/parallel” hybrid – which if you think about it, is like a hybrid within a hybrid!  

This power split is possible because of the planetary gear that was introduced in the second-generation Prius. Without getting too technical, the electric motor is attached to a gearbox that connects the gasoline engine, electric motor and generator together into one unit. The gear moves based on the power required by the car. The engine is the last of the gears, so initially the electric motor and generator run as long as possible; it is only when the two are unable to meet power demands that the gasoline engine starts up.

With more than 6.1 million Prius vehicles sold as of 2020, the Prius has continued to dominate the hybrid market, but was surpassed in sales this year by the RAV4 SUV. In fact, Toyota has sold over 15 million hybrid model vehicles to date including its Lexus line and produces 44 hybrid models globally. However, there is trouble ahead for hybrids as all-electric vehicles continue to increase in popularity due to reduced complexity, cost and carbon footprint. Next, let’s explore how HEVs are different from electric cars.