What Are Hybrid Vehicle Power Trains and How Do They Work?


There are a lot of misconceptions about what is and what isn’t a “hybrid” car or truck. If you’re looking to get the most efficient vehicle available and think a hybrid might be the answer, you’ll want to know which type of hybrid uses the least amount of fuel and what the differences are between one power train and another.

Hybrid vehicles can be broken down into three basic power train types: series, parallel, and series-parallel. There are at least one of each type of power train available from major vehicle manufacturers as production vehicles.

Series Hybrid

This is an electric power train for which an internal combustion engine acts as a generator to charge batteries and/or provide power to the electric drive motor. These vehicles usually have a larger battery pack and larger motors with smaller internal combustion engines (ICE). They are referred to as “plug-in hybrids” (or a plug-in electric vehicle, PHEV) and “range-extended electrics” (RE-EVs).

The drive train for a series hybrid is relatively simple, mechanically, compared to other hybrids.

Disadvantages to this drive train are the somewhat lower efficiencies at greater trip distances and the higher cost of batteries and components, since the vehicle is all-electric. Regardless, of the three hybrid options, it is the most efficient in fuel use.

Parallel Hybrid

More mechanically complex than a series hybrid, the parallel power train is dual-driven, allowing both the combustion engine and the electric motor to propel the car. The ICE and motor operate in tandem, usually with the combustion engine as the primary means of propulsion and the electric motor acting as a backup or torque/power booster.

The advantages of this are smaller batteries (less weight) and generally more efficient regenerative braking to both slow the car and capture energy while doing so. Another advantage is that it can easily be incorporated into existing vehicle models. Most hybrids on the road are of the parallel type.

The major disadvantage of this power train is that it adds more weight to the vehicle without necessarily shrinking the engine and other components. While the addition of the motor does increase fuel mileage by allowing the engine to operate at lower rotations per minute (thus using less fuel), most in the industry view the parallel drive train as a stop-gap that won’t last long. These vehicles are poor highway performers, gaining most of their efficiency in-city at lower speeds.

Series-Parallel Hybrid

This drive train is a combination of the two drive train types, allowing for the vehicle to operate as all-electric (as a series hybrid), as an all combustion vehicle, or as a combination of the two (as a parallel hybrid). This is the most complex and least efficient power train for most applications. Despite that, most of the latest-generation hybrids on the road currently are Series-Parallel types. It’s popularity is mainly due to the high cost of batteries (for which it needs few) and the marketing potential as a “plug-in hybrid”.

So Now You Know

The technology will change drastically over the next decade and more range-extended models will become available, probably trumping the other two types. Especially since a range-extended model does not have to use a combustion engine as the generator – hydrogen and methane fuel cells, turbines, and other future tech is coming soon.