Before we jump into how gas-electric cars work, let’s get some terminology straight so we can better communicate about them. You’ll likely have seen these terms in news media and literature before.
Full Hybrids
Full Hybrids have both a combustion and electric propulsion system (e.g. a gasoline and electric motor) and can operate on one, the other, or both, depending on the hybrid drivetrain type.
Mild Hybrids
Mild Hybrids are vehicles that rely on combustion fuels to move the vehicle, but which stop the engine when the vehicle is stopped (anti-idle) and they often store braking power as electricity (regenerative braking) to power vehicle components and save the engine power required to power accessories. These are the least efficient of the hybrid vehicle types, but they are a step in the right direction and can give fuel efficiency bonuses of 10-15% in most combustion cars.
Parallel Hybrid
Parallel Hybrid vehicles are those that can move using both electricity and combustion through the vehicle’s transmission or gearbox. So, while the two propulsion sources are separate, they operate the same gearbox and drivetrain. The illustration below, from Wikimedia, shows how this works.
Series Hybrid
Series Hybrid power trains are those which allow both propulsion systems (electric and combustion) to act separately. The electric propulsion system is the only means of moving the vehicle while the engine acts as a generator to provide electricity for the motor. A Range-Extended Electric (REEV), Plug-in hybrid electric (PHEV), or similar vehicle is a series hybrid.
Series-parallel hybrid, or Power-split,
Series-parallel hybrid, or Power-split, is a combination of the Series and Parallel powertrains. These allow power paths from the motor or engine to power the wheels. Usually, they will have one axle run by the engine and another by the motor. For instance, the Audi hybrid system usually puts the engine at the front, powering the front wheels, while the motor is in the rear, powering the rear wheels. These are becoming more common in PHEVs because of their simplicity and redundancy. This power train is illustrated below in an image from Wikimedia.
Hybrid Power In Gasoline Electric Cars
In gasoline-electric cars, any of the three power train types listed above can be used. In the Toyota Prius (non-plug-in models), a Series Hybrid system is being used. In these common hybrids, the gas engine provides most of the vehicle’s propulsion with the electric motor acting as a power booster to the transmission and regenerative braking generator to recharge the vehicle’s batteries.
In the Fisker Karma, released just this year, however, a Parallel system is being used with the engine and motor acting separately to allow the car to be a REEV with the gasoline engine acting as a generator for the all-electric powertrain. The Series-Parallel hybrid is the Chevrolet Volt, which acts as a REEV as well but requires the combustion engine to power the drive train at higher speeds. Currently, most manufacturers are opting for all three drive train options equally. Some, like General Motors, are focusing on the Series-parallel system while others, like Toyota, are focused on the series system.
In all types of gasoline-electric cars, the hybrid system allows for much greater efficiency while still letting the driver use commonly available combustion fuels that can be used to “recharge” the system immediately. This gives obvious advantages over all-electric (or battery-electric) cars, which take hours to recharge and have a limited range.
In the interim, hybrid gas-electric cars fill the gap until infrastructure and technology get to the point where combustion engines become a thing of the past.