Grant Maloy Smith

Monday, September 25, 2023 · 0 min read

How Do Hybrid Electric Cars Work?

Electric and hybrid electric powertrains are rapidly becoming the norm among cars sold today. The imperative of reducing tailpipe emissions is driving these developments. Most people have some understanding of how internal combustion engines and electric motors work. Still, it is not apparent how combustion engines and electric motors can work together in the exact vehicle.

From this article, you will:

  • Learn what hybrid electric powertrains are

  • Understand how internal combustion engines and electric motors work together

  • See the pros and cons of hybrid electric cars 

What is a powertrain?

The powertrain is not just a vehicle’s engine: it includes all components involved in transforming kinetic energy into propulsion. Therefore it also includes the transmission, clutch, driveshaft, differential and axles.

The internal combustion engine

Invented in the late 1700s and put into practical use in the early 1800s, the internal combustion (IC) engine found in cars and trucks today is available in two basic types:

  • Spark Ignition (SI) - this engine mixes air and gasoline (aka “petrol”) fuel in the cylinders and ignites them using a spark generated by a plug mounted at the top of the cylinder chamber. This is the classic gas engine.

  • Compression Ignition (CI) - this engine injects air into the cylinders and then compresses it. Then diesel fuel is sprayed into the hot compressed air, causing it to ignite. A spark plug is not needed. Most trucks are equipped with diesel engines because they are more durable and provide more torque, which is important when moving heavy loads.

Figure x. Spark ignition combustion engine diagram showing a single cylinder and piston connected to a camshaft. TDC and BDC refer to the top and bottom dead center positions of the piston.

SI (gasoline) and CI (diesel) engines work via combustion. When the fuel-air mixture is ignited (either by spark or compression), an exothermic process occurs in which the chemical energy of the fuel is released, producing high temperature and pressure. This expansion pushes the pistons down inside their cylinders.

Animated crankshaft. The cylinders are blue, and the crankshaft is red. As the pistons (gray) are fired by the camshaft, they are pushed down. The shape of the crankshaft converts the vertical motion of the pistons into a rotational force, turning it. NASA, Public domain, via Wikimedia Commons.

When the driver presses down on the accelerator, the cylinders fire more rapidly, and the crankshaft turns faster. 

Typical rear wheel drive combustion drive train

The figure above represents a typical rear-wheel drive vehicle. Front-wheel drive and all-wheel drive configurations are also commonly available today.

The electric motor

Automotive electric motor. Image courtesy of Renault Group

There are two basic types of AC electric motors available today:

  • Synchronous motor - the rotor turns at the same speed as the rotating magnetic field created by the stator. Advantages: high torque at low speeds, and can be made physically smaller and lighter than asynchronous motors.

  • Asynchronous (induction) motor - the rotor does not turn at the same speed as the rotating magnetic field created by the stator. It is always trying to “catch up” in that sense.

While some systems in the car work on DC power, such as the headlights, entertainment and navigation electronics, etc., the motor runs from three-phase AC power. Therefore an inverter is used to convert DC power from the batteries to AC for the motor.

Synchronous electromotors consist of a stator and a squirrel-cage rotor.

Stator from a BMW iX-M60-Generation-5. Image courtesy of BMW

A stator is a three-coil winding. “Stator” refers to the word “stationary,” i.e., the part of the motor that does not turn. Windings are precisely arranged in the slots of the stator, which consist of highly permeable laminations inside the steel or cast-iron frame. When a three-phase AC current is passed through the windings, it produces a rotating magnetic field (RMF). 

As we know from basic physics, when an electric current passes through a wire it generates a magnetic field. When three-phase power is supplied to coils arranged 120° apart, the magnetic field produced will “rotate,” as shown in the figure below:

The rotating magnetic field from a synchronous stator

A freely spinning squirrel-cage rotor is placed inside the fixed stator. The rotating magnetic field generated by the stator induces torque on the rotor, causing it to turn. When the driver presses down on the accelerator, more current is sent to the stator, and the rotor turns proportionately faster.

Squirrel cage rotor. Image courtesy of Motor Trend
Stator and rotor, axial view. Image courtesy of Motor Trend

The hybrid powertrain

At the simplest level, a hybrid powertrain is one that combines an internal combustion engine (ICE) with an electric motor (EM). Hybrid cars typically use electric driving at low speeds, while the ICE takes over at higher speeds. The best-known hybrid on the market today is the Toyota Prius. All major car manufacturers have already or soon will add hybrid and pure electric vehicles to their product offerings.

While it is possible that the ICE in a hybrid could be a diesel engine, nearly all Hybrids today incorporate a gasoline engine.

Typical hybrid propulsion system

Special features of hybrid vehicles

Smaller combustion engine

Because the electric motor provides so much of the propulsion work, the internal combustion engine can be smaller. A smaller IC engine means improved fuel economy and less pollution.

High torque and peak power

Electromotors provide high torque and high peak power. IC engines are not terribly efficient at overcoming inertia and moving a vehicle from a stop, but electromotors excel in this area. Higher vehicle performance is the result.

Energy recuperation

With traditional internal combustion engines, the liquid fuel can only be replaced by stopping the vehicle and refilling the fuel tank from an external source.

However, the electric powertrain system is “bidirectional.” The electromotor can put energy back into the battery system during system operation. This is achieved via regenerative braking, which converts kinetic energy to electricity. The process of capturing this brake-generated electricity is called recuperation

The dynamic braking used by internal combustion cars uses brake pads and friction to slow and stop them. In this case, the kinetic energy that the car has generated is dissipated in the form of heat and is lost, resulting in less fuel efficiency.

However, regenerative braking uses the electromotor. When reversed, the electromotor places a load on the driving wheels, slowing them. When reversed, an electromotor becomes a generator – converting the kinetic energy of the slowing vehicle into electricity. This energy is directed back into the battery system.

EV Energy Requirements Infographic from the US Department of Energy

According to the US Department of Energy, a regenerative braking system can realize up to 22% recovered energy during a combination of urban and highway driving. Electric vehicles have only 15% to 20% energy loss compared to 64% to 75% for ICE vehicles. They consider electric vehicles to be 60% to 73% efficient. If we additionally consider the positive effects of minimal energy loss during idling and regenerative braking, this efficiency improves from 73% to 100%.

It should be noted that conventional friction brakes are still used in parallel, because regenerative brakes may not slow the vehicle as fast as required. Also, they may not be able to stop the vehicle completely or prevent it from rolling when the car is on a hill. Because of their operating principle, regenerative brakes are referred to as Kinetic Recovery Systems (KERS).

Redundant propulsion sources

Unlike conventional internal combustion or purely electric vehicles, hybrid electric vehicles can be propelled by the combustion engine, the electric motor, or both. 

Faster refueling

Unlike completely electric vehicles, a hybrid vehicle has an ICE system on board that can be refuelled in a few minutes. The electric motor’s batteries can be recharged by the internal combustion engine during operation, or when the vehicle is stopped and connected to an electric power source.

Types of hybrid engines today

There are several configurations of HEV (hybrid electric vehicles) today:

Series HEV

In series hybrid electric vehicles, the internal combustion engine is not directly connected to the drivetrain, but rather it is used to power the electric motor. 

Parallel HEV (PHEV)

In parallel hybrid electric vehicles, the internal combustion engine and electric motor are independently linked to the vehicle’s transmission, so they can both provide propulsion at the same time… in parallel. When the battery runs low, the internal combustion engine can take over to drive the car.

Series-parallel HEV (SPHEV)

In series-parallel hybrid electric vehicles, the internal combustion engine and electric motor are both used for propulsion in parallel. A second electric motor is used to charge the battery. Toyota’s best-selling Prius HEV is a series-parallel hybrid vehicle.

Plug-in hybrid (PHEV)

With standard hybrid cars, the battery system is recharged by the gasoline engine and by regenerative braking. But a  PHEV can be connected to an external power source to charge the batteries. Plug-in hybrids have a larger battery capacity than standard hybrids and therefore have a longer driving range when powered only by the electric motor.

Pros and cons of hybrid electric vehicles


  • HEVs are more fuel efficient than ICE vehicles

  • HEVs produce less CO2 pollution than ICE vehicles

  • HEVs have better low-speed torque than most ICE vehicles

  • Unlike pure electric vehicles, HEVs can operate from gasoline for long-range driving

  • Acoustically quieter than an ICE vehicle due to the smaller gasoline engine 

  • Less expensive than most electric vehicles (EV)


  • Higher up-front cost than standard ICE vehicles

  • Repairs to the electric system are generally more expensive than with ICE vehicles

  • Less high-speed performance than most ICE and pure electric vehicles

  • Not as acoustically quiet as a pure electric vehicle (EV)

  • HEVs produce more CO2 pollution than EVs, which produce none*. 

* Note: EV batteries are typically charged from the electric grid. In 2021, 61% of the electricity in the USA was generated by fossil fuels. In Europe, this number was 76%, and in Asia/Pacific this number was 85%. See for more details.

Industry trends

Hybrid electric cars were introduced globally in 2001, and electric vehicles in 2010. By 2021, sales of hybrid electric cars in the USA exceeded 800,000, representing 5% of U.S. light vehicle sales. More than half of these were from Toyota. 

Pure electric cars also increased to almost 500,000, and plug-in hybrids approached 200,000. But while impressive, less than 1% of all vehicles on the road in the USA are HEV, PHEV or EV. According to estimates, by 2050 about 50% of vehicles on the roads in the USA will be HEV, PHEV or EV. In terms of percentages, Japan leads the world in the adoption of HEV and EV cars, the USA is second, and Europe is third.

US DOT Bureau of Transportation Statistics


HEV and EV vehicles are here to stay. They are rapidly increasing in popularity, and are an important way that we can reduce and someday eliminate our reliance on fossil fuels and their CO2 emissions.