Petrol–electric transmission

Petrol-electric vehicles follow a series hybrid architecture. A spark ignition internal combustion (IC) engine acts as the prime mover, powering a generator which converts the rotational energy into electrical energy. The generator charges a battery pack and drives a traction motor that provides tractive effort for the vehicle to move.[1]

A schematic representation of the power-train of a series hybrid vehicle

The engine is usually smaller that what would be required for powering a conventional petrol vehicle of the same size. The engine usually runs at its optimum high efficiency RPM, powering the generator. When extra power is needed for acceleration or for climbing gradients, both the engine and the battery pack powers the motor. When the engine produces more power than is required at the road wheels, the surplus is used to charge the battery.

Petrol-electric vehicles typically doesn't require any stepping up or transmission as electric traction motors can operate at a wide rpm range at peak efficiency. The Engine generator pair is a compact unit that isn't connected mechanically to the road wheels. The connection is purely electrical.

• Electric traction motors are more efficient than IC engines in stop-and-go driving, typically the case in urban and suburban routes.[2]
• The IC engine can be made smaller than what is usually required to move a similar sized pure gas-powered vehicle. IC engines are their own air pumps and have poor efficiency at low speeds. So IC engines have to be made larger than what is required for conventional gas-powered vehicles. This is not the case for petrol-hybrid vehicles where an electric motor provides traction. Electric motors can deliver peak torque at a wide range of RPMs. Small engines result in a compact design and more space.[3]
• IC engines can operate at their peak RPM range throughout and improve mileage. IC engines operate at peak efficiency at higher RPMs, In petrol-electric vehicles, the engine can keep running at these speeds as the motor is the one delivering traction. This results in lesser fuel usage and harmful emissions. This isn't the case with conventional vehicles where the engine has to slow down when the vehicle brakes.
• Petrol-electric vehicles have an idle-stop function where the engine can be shut off during idling or during long deceleration.
• Regenerative braking is possible with Petrol-electric vehicles. When the brakes are applied, the traction motor can be switched to act as a generator and charge the battery pack. A magnetic resistance force acts on the wheels to slow them, while the battery is charged. Conventional brakes use frictional braking force only, which dissipate and waste rotational energy as heat. Regenerative braking saves energy, they are used in combination with friction brakes for anti lock braking (ABS).[4]
• Petrol-electric vehicles can deliver additional power when it is demanded by the driver, like during hard acceleration or climbing a gradient. They also have automatic transmission and can deliver step-less power.[5]

• Electric motors are less efficient than conventional IC engines for long distance driving at consistent high speeds, as is seen in highways and expressways.
• Petrol-electric vehicles have many energy-conversion steps, which can result in reduced efficiency due to energy loss at each stage of the transition.
• Petrol-electric vehicles, and typically most series hybrid vehicles need two electric machines, a generator coupled to the engine and a motor/generator coupled to the wheels.
• They need larger electric traction motors.[6]

In the 21st century, petrol–electric transmission has gained a new lease of life in hybrid electric vehicles. Examples include: Ford Fusion Hybrid; Honda Civic Hybrid; Toyota Prius.

• Diesel-electric transmission
• Turbine-electric transmission
• Turbo-electric transmission