GreenCars 101

How Hybrid Cars Work

Laurance Yap
Jul 2023
Hybrid cars combine components from traditional gasoline drivetrains with electric components including a motor and a power management system. Here, we take an in-depth look at hybrid technology.
Skeleton of car

How Hybrid Cars Work

If you open the hood of a hybrid car, you’ll see that it looks a lot like a gasoline-powered car – because in many ways, it is! You’ll see an engine, a transmission, a timing belt, and other familiar mechanical parts.  

But you’ll notice that the engine in a hybrid car might be smaller than usual, taking up less space. A hybrid car supplements the gasoline engine’s output with a battery and electric motor. Sometimes, the electric motor is integrated into the housing of the transmission; sometimes it is contained in its own housing or connected to a different axle.  

In both cases, the electric motor, or motors, reduce the need for a larger gasoline engine. Electric motors are compact, simple, and powerful; with just one moving part, they can produce a lot of torque, which supplements what comes from the gasoline engine to deliver better fuel economy and reduced emissions.

Tech looking at engine of car

Hybrid Battery

The hybrid battery, sometimes called the traction battery, is what gives a hybrid car improved fuel economy and acceleration. It is typically composed of lithium-ion batteries stacked together and located near the rear axle so that, in the case of a collision, the battery is well-protected.

You might be wondering whether the battery is flammable or explosive. Automakers and independent testing have indicated that the batteries are not dangerous, as they are packaged in a secure, fireproof box to keep passengers safe.

Series vs. parallel battery

Series or Parallel, or Both

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 from a gas tank, which powers the engine. In hybrids, it’s not so simple: a hybrid can combine gasoline and electricity in different ways. In order to conserve fuel without sacrificing performance, hybrids use an on-board computer that determine how much power comes from the battery, from gasoline, and when.  

The two major types of drivetrains are series and parallel. Let’s explore each in more detail.

In a series hybrid, the electric motor handles most of the work of supplying energy to the wheels. It can use power from the battery pack or from a generator run by the gasoline engine – but generally not both. Think about the name series: one, then the other. 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. Examples of series hybrids include the Chevrolet Volt, Hyundai Ioniq, and BMW i3 with Range Extender.

Series hybrid vehicles are ideal for urban and suburban driving; they can make excellent use of a larger battery and more powerful electric motor, while their gasoline engines can be smaller and more efficient. However, they tend to be more costly as they have larger batteries – and they are not as efficient when driving on longer routes where the gasoline engine does more work. For those situations, a parallel hybrid may be better.

Unlike a series hybrid, a parallel hybrid can put power to the pavement through the electric motor or the gas engine directly – or a mix of both. An onboard computer will determine which is best for the given set of circumstances and will make the choice for the driver. 

Since there are two types of propulsion, either can propel the car. The onboard computer continually monitors the need for acceleration and decides which motor (electric or gasoline) would be best. If you’re already moving but need a boost of power, like merging onto the freeway, a parallel hybrid might put the gasoline engine to work. But when you’re sitting at a red light and it turns green, it might opt for the electric motor. Plus, when the gas motor isn’t being used directly for putting power to the wheels, it can 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. Many vehicles like the Toyota Prius combine the series and parallel concepts, meaning they can be powered by the gas engine or the electric motor, or both simultaneously! A planetary gear set connects the gasoline engine, electric motor, and generator together into one unit. The gears move based on the power required by the car.

How Hybrids Charge

Since a hybrid car can’t be plugged in, it uses a process called regenerative braking to recharge its battery. To understand regenerative braking, we must step back and understand the principle of kinetic energy. Kinetic energy is the energy of motion – and cars in motion have a lot of kinetic energy.

When you hit the brakes in a traditional vehicle, the energy is transferred from the body of the moving vehicle to the braking system. Kinetic energy (from the car’s motion) turns into friction and heat as the vehicle slows. After a long drive, you might notice your brakes will be hot. While effective at stopping the vehicle, friction leads to wear and tear over time, which is why brake pads and/or rotors on any car will require eventual replacement. 

The braking system on a hybrid car is different. As soon as you lift off the accelerator, even before the brake pedal is pushed, regenerative braking starts to harvest energy from the car’s moving body, slowing it down. It does this by taking the mechanical energy and converting it into electrical energy. That energy is then sent to the battery to be stored where it will be used to power the vehicle later. In a hybrid, instead of kinetic energy being released and wasted as heat, the car holds onto it and uses it to recharge its battery.

Hybrids still have the same brake pads and discs found in conventional cars, but they are secondary to the regenerative system. Therefore, brakes on hybrids can last so much longer.

Man driving 1st gen Prius

Not All Hybrids Can Be Charged

Unless it’s a plug-in hybrid, hybrids like the original Prius cannot be plugged into an electric source for charging. Bypassing the need to plug in to recharge makes hybrids great if you want to reduce their fuel consumption and reduce greenhouse gas emissions without relying on charging equipment. If you live in an apartment building without access to charging equipment, or run a business that delivers in urban areas, you can also benefit from the hassle-free efficiency of an HEV. 

(Plug-in hybrid vehicles offer even greater efficiency and electric range that conventional hybrids – and their battery and charging systems work differently. So much so that we’ve dedicated a whole section to them here on GreenCars 101.)

Front view of a Tesla Model 3 driving through canyon roads

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