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How Hybrids Work


A hybrid vehicle is any kind of vehicle that uses two or more propulsion systems. Current hybrids integrate an internal combustion engine and an electric motor and battery.

Depending upon the type and design—and kind of usage—hybrid designs can range from operating mostly on the internal combustion engine with some assistance from the electric motor, to almost the opposite—operating predominantly on the electric motor, using the internal combustion engine only when significant power is needed.

The Basic Types of Hybrids:

  • Mild – uses the electric motor and battery as an assist to the internal combustion engine
  • Full – the two propulsion systems (electric motor and internal combustion engine) can work independently or in conjunction with each other
  • Plug-in – the internal combustion engine acts only as a back-up to the main rechargeable motor and battery system

Read on for more detailed explanations of how each of these hybrid types operates:

How Mild & Full Hybrids Work

Mild and full hybrids never need to be plugged in.

A mild hybrid is one that cannot drive on the electric motor alone—it always needs the internal combustion engine to propel the vehicle while the motor acts as an assist.

A full hybrid, unlike its mild counterpart, has the ability to propel the vehicle solely on its electric motor—without the internal combustion engine running. However, it is only able to do this under certain conditions (usually low load conditions). Under very light cruising load and under light acceleration, a full hybrid can run on just the electric motor. As soon as additional power is needed, the internal combustion engine will kick-in to provide full acceleration power.

Full hybrids tend to get much better fuel mileage than mild hybrids, plus much better city mileage, since the electric motor is used much more in city driving.

Regenerative Braking

To recapture energy that would normally be lost when slowing down or coasting, hybrids use regenerative braking. This is a fancy term that basically means the electric motor runs “in reverse” and acts as a generator to help recharge the NiMH battery. This is how it works: Whenever the driver lets off the throttle or applies the brakes, the computer runs the electric motor backwards to recharge the battery. Under extended periods of use such as highway cruising where little braking occurs, the engine itself can run the electric motor to recharge the battery also.

Hybrids are always in a state of flux—either drawing from, or recharging the battery. Hybrid batteries are not lead acid, like the starting battery. They are predominantly Nickel Metal Hydride (NiMH), which is a more sophisticated and reliable battery system. NiMH batteries charge better and hold charges longer. Both mild and full hybrids have the ability to shut the internal combustion engine off. Mild hybrids only turn the engine off at an idle to save gasoline and emissions—as long as the brake pedal is depressed. For example, when stopped at a traffic light, the internal combustion engine shuts off, but as soon as the brake is released, the engine re-starts in an instant, ready to go.

Complex Computer Systems

Because of the complexity of merging two different drive trains to work seamlessly as one, a hybrid requires a sophisticated computer management system. Hybrid control systems need to be heated and/or cooled, depending upon the climate and weather conditions, to maintain certain operating temperatures. The computer gathers data from many different sensors throughout the car for optiumum functioning: vehicle speed, engine RPM, engine load, gear selection, temperature, etc. It determines when the battery needs recharging, when the motor can run the vehicle, when it needs to start the engine back up—the whole shebang.

Yet driving a hybrid is just like any other car, thanks to this sophisticated computer system. Yes, the driver simply pushes the accelerator and/or brake as needed, and the computer takes care of managing all the systems to create a seamless driving experience.

Learn more about how two-mode hybrids and plug-in hybrids work.

More Hybrid Information:

Hybrids 101
Why is electricity an alt fuel? How exactly do mild, full and plug-in hybrids work? Plus FAQs about hybrids.

Hybrid Buying Guide
Check out the photos and test drives of current hybrid models, plus the lowdown on up-and-coming models.

Hybrid Maintenance and Safety Issues
Get the scoop on hybrid maintenance: from routine maintenance and professional repairs to safety issues and “beware the orange.”

Hybrid Tax Credits & Rebates
They go hand-in-hand: reduce your taxes and greenhouse gas contribution when you buy a hybrid vehicle.

Learn More at Hybrid Central.

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