A Crash Course on Hybrid Powertrains

There’s a lot of misconception out there about what is a hybrid vehicle, what isn’t, and how “green” and efficient these vehicles really are (or aren’t).  It’s really annoying for me as a writer about these things for public consumption, because what is popularly believed and what is actual aren’t always the same.  I just get annoyed with having to explain something and waste a paragraph when it can be encapsulated in two or three words – and probably was when I received the original information from its source.

The 1901 Lohner-Porsche Mixte Hybrid, world's first gasoline-electric hybrid. Image courtesy of Wikimedia.

Most people don’t know the difference between a parallel, a series, and a series-parallel hybrid system.  Of course, if you’re on the Toyota lot looking at  Prius, you probably don’t really care because they only offer one kind of hybrid.  When the Chevy Volt appears later this year, and if you consider buying one and don’t look at a Toyota, then you probably won’t care about the difference either.  When the Nissan Leaf enters the market in the next few months, you won’t care about hybrids at all if that’s your car of choice.

Regardless, when you read or research hybrids, you’ll find that there is a lot of difference between one powertrain and the next in hybridization methods.  Each has strong and weak points and I’ll point those out as we go along here.  As usual, I won’t be afraid to offer my expert opinion on the matter as well.  Just remember that opinions are like assholes: some are bigger than others.

The Three Hybrid Powertrain Types

Series Hybrid – is an electrically-driven powertrain in which the internal combustion engine (ICE) acts only as a generator to give power to the batteries and electric motor.  The electric motor is the only means of propulsion in the car itself.  These vehicles usually have a larger battery pack and smaller engine size and are extremely efficient.  They are often referred to (appropriately) as range-extended electrics.  The Fisker Karma and Chevrolet Volt are examples of this technology.  This type of drive train allows for the most fuel efficiency and least amount of complex engineering of the three hybrid drives.  But don’t tell GM that, because the Volt is highly complex and technologically advanced, dontcha know?

Parallel Hybrid – is a dual-driven powertrain in which both the combustion engine and electric motor can move the vehicle.  Usually they operate in tandem with the electric motor acting as a power booster or torque-enhancer for the drive train.  Usually, this configuration allows for very efficient regenerative braking and increases mileage by taking some of the weight off the ICE.  The Honda hybrids (Civic, Insight) are examples of this, as are most heavy vehicle ICE-electrics such as the Detroit Diesel prototypes.  This is the simplest type of hybrid drive to use in conjunction with existing vehicle platforms and ICE drive trains.  Thus it is the cheapest to implement, but not necessarily the most fuel efficient.

Series-Parallel Hybrid – is a combination of both of the above drive trains into a single unit.  It allows the vehicle to operate as either an all-electric series hybrid, driven only by the electric motor and using the ICE as a power source, or as a parallel in which both the engine and motor work together to propel the vehicle.  In most light-vehicle applications, this is the most common type of propulsion because of its versatility, but it ads significantly to the cost of engineering the vehicle as well.  The Toyota Prius and Lexus models are series-parallel hybrids as is the Ford Fusion Hybrid. These are the emerging “elite” versions of hybrid power trains, but are often the least efficient and most complicated of the three types.  This drive train combines all of the good points of series and all of the bad points of the parallel.  In the short term, it is economically popular because of the price of batteries – a significant down side to pure series hybrid or battery-electric vehicles.

There are a lot of different ways to explain these hybrid systems.  Terms like “full” or “mild hybrid” and “power-assist” hybrids are used interchangeably (and often incorrectly so) with the above.  These are engineering terms rarely used in sales except to sound knowledgeable and make their product seem more technologically advanced than it is.

Plug-in Hybrids (PHEV)

This is a fun term that is used a lot.  It’s sort of the marketing term of choice for any series or series-parallel hybrid.  All it really means is that the vehicle has a hybrid drive train and a power cord.  It can be plugged into the wall (or similar) and have its batteries recharged.  Technically, it doesn’t even have to be a vehicle capable of all-electric drive, since the term really just refers to a hybrid-electric vehicle that can be plugged in.

Normally, the term is used to refer to series-parallel- or series-hybrid vehicles that are capable of all-electric drive and can be recharged from power sources other than their internal combustion engine.

Nevertheless, the term is often misused and abused by politicians, salesmen, and under-educated green hypsters.  The next time your local Gorebot touts the excellence of a PHEV (probably their own Toyota Pompous), ask that person whether the plug-in in question is a series or parallel hybrid vehicle.  Trust me, they’ll be baffled and will probably make stuff up to cover their ignorance.

This entry was posted in Hybrids