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Aug 19 2014

Auto Technology: Understanding Hybrid Technology 

By now you've most likely seen hundreds of hybrid vehicles - also referred to as hybrid electric vehicles (HEV) - on the road. They've been in the US market for over a decade and have continued to grow as a more popular and available option for consumers. Whether you're in the market for a hybrid or just want to learn more about this growing vehicle segment, there are a couple things you need to know to understand this auto technology.  

Though most people are aware of their fuel economy and environmental benefits, often that is the extent of their hybrid vehicle knowledge. Unless you've done research on various hybrid models or read Autoweek at the dentist's office, there's a good chance you may have generalized hybrid vehicles as using a universal technology. However, that is a big misconception among many drivers, including myself at one point. Before we establish how hybrid vehicles differ, I want to make sure everyone understands the basics.

What is a Hybrid Vehicle?

Within the automotive industry, a vehicle is considered a hybrid when it uses a combination of two different power sources to move the vehicle. The two most common power sources combined are an internal combustion engine (powered primarily by gas) and an electric motor (powered by one or more batteries).

Doesn’t seem too confusing, right? Where it gets hazy is that although these two components both contribute to the functionality of hybrid vehicles, the way they contribute varies between hybrid models.

Below I've organized different hybrid vehicle technologies into two main categories - drivetrain options and electricity generation. This will make it much easier to explain and understand differences between various hybrid vehicles on the road today. 

Drivetrain Options

To preface the different hybrid drivetrains, here’s a clear definition of what a vehicle drivetrain is: “A system in a motor vehicle that connects the transmission to the drive axles.”

In traditional gasoline or diesel vehicles, the internal combustion engine powers the wheels, however, that is not always true for hybrids. Some hybrids operate in parallel, where both the internal combustion engine and the electric motor can power the wheels either individually or simultaneously. Other hybrids use the electric motor as the main source of power and the combustion engine only as a generator for the battery, known as a serial operation. However, all hybrids fall into three drivetrain categories: Mild, Full and Plug-In Hybrids.

Mild Hybrids (HEV)

Mild hybrids, classified as a parallel operation, use an electric motor to assist the smaller internal combustion engine when an extra boost is needed. For example, this "boost" could be applied during rapid acceleration or steep inclines. Honda calls their version of this technology “Integrated Motor Assist,” since the electric motor is used just for assistance and not to propel the vehicle on its own.

You may be wondering how mild hybrids could be beneficial with the electric motor incapable of powering the wheels on it's own. Because additional torque it applied by the electric motor, a smaller, more efficient gas engine can be used to power the vehicle. This results in a significantly higher MPG.

In addition, mild hybrids typically use Stop-start technology.  This causes the engine to shut off at a complete stop. The more robust battery is then utilized for powering equipment such as the vehicle lighting, sound system, and heat or AC. Once the accelerator is applied, the engine instantly starts back up using the electric motor to power the starter.

Full Hybrids (HEV)

Full hybrids, also classified as a parallel operation, can be powered by both the internal combustion engine and the electric motor. With full hybrids, the electric motor is used to propel the vehicle at lower speeds, with the power supply transitioning over to the internal combustion engine at higher speeds. In addition, when extra power is needed, both sources may be used in unison. Full hybrids are not considered a serial operation since the gas also powers the wheels.

Plug-in Hybrids (PHEV)

Similar to full hybrids, plug-in hybrids or PHEVs can propel completely off of the electric motor. In fact, plug-in hybrids generally use the electric motor as their primary source of energy and are equipped with a larger battery (or batteries) for further range. There are some exceptions however, such as the Porsche 918 Spyder plug-in hybrid, where the gas is often used as the main source of energy and the electric motor can be used to provide significantly enhanced performance.  In general though, Plug-In Hybrids are known for their impressive efficiency and often boast MPG ratings that may double full hybrid vehicles ratings when run off the electric motor only.

Plug-ins can operate in either parallel or serial. For parallel PHEVs, the internal combustion engine is used to move the wheels when the battery is depleted, provide additional power when necessary, and/or compensate for the excessive use of heat or AC in extreme weather.  

For serial plug-in hybrids, such as the Chevy Volt, the internal combustion engine is never used to power the wheels, but rather acts as a generator to charge the battery when it runs low. Serial plug-in hybrids are sometimes referred to as an  Extended Range Electric Vehicles (EREV).  

Electricity Generation

Image_courtesy_of_canstockphoto.com_(2)-2As previously touched upon, another big difference between hybrid models is how the batteries are replenished. With traditional gasoline and diesel vehicles, the internal combustion engine recharges the battery. However, since the battery in these vehicles is never used to provide additional power or actually drive the wheels, they don’t require another source of electricity generation.

All hybrid vehicles require frequent charge of their batteries, since they are much more dependent on electrical power than traditional vehicles.

Mild and Full Hybrids

For mild and full hybrids, regenerative braking is the main source in which the battery is replenished. Regenerative braking functions by using the motor as an electric brake, thus transferring energy to the battery that is wasted during braking or coasting with a conventional vehicle. This energy is stored up in the battery and later used for whatever task the electric motor provides. If the battery is running low without much braking occurring, then the combustion engine is used for recharging. 

Plug-in Hybrid

As the name implies, the batteries in plug-in hybrids can be charged by plugging into an electrical outlet at home, and can be fully recharged or "topped-off" at public charging stations. On the road, parallel plug-ins are charged using regenerative braking. With serial plug-ins, the internal combustion engine works as a generator for the batteries during extended range driving in addition to the regenerative braking. Unlike mild and full hybrid electrics, as long as the battery is charged to its full capacity on a regular basis, drivers predominantly commuting short distances may never need to use the gas in a plug-in hybrid.


Hybrid vehicles do not all function the same. Mild, full, and plug-in hybrids have various benefits and challenges, mainly determined by the driver's habits. Some hybrids are more Eco-friendly and fuel efficient than others, however, practicality is also a very important contributor when purchasing a hybrid, since hybrids are not a one-size-fits-all technology.

If you're considering a hybrid as your next vehicle purchase, we hope that this article helped increase your knowledge of hybrids and will weigh in to your decision making process. As for those of you just curious about this unique breed of cars, now you can better understand all the hybrids that surround you. 

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Check out our previous article on Alternative Fuel Sources

Proceed to the next article 6 Reasons Why You're Not Driving a Fuel Cell Vehicle... Yet

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