Basics

A vehicle’s transmission is the set of all mechanic and electronic components, through which the internal combustion engine (ICE) and/or the electric motors can drive the vehicle wheels. One can differentiate between traditional transmissions, hybrid transmissions and electric drives, but the basic transmission functions are the same for each of them:

1. to enable smooth vehicle drive-off and maneuvering at very low speeds
2. to enable the vehicle to drive backwards (reverse)
3. to expand the speed range of the vehicle wheels so that the vehicle can reach reasonable speeds
4. to enable operation either with high power output or with low energy consumption, depending on the driver’s intention

The realization of the transmission functions show a great variation, resulting in different transmission types with merely different charachteristics.

The following types of transmissions cover most of the road vehicles, including motorcycles, passenger cars and commercial vehicles. Click on the transmission types for detailed information.

*gearshift without torque interruption

Traditional transmissions

Traditional transmissions are meant for vehicles driven by an internal combustion engine and are composed of two main components: the clutch (for transmission basic function 1) and the gearbox (for functions 2, 3 and 4).

The placement options of the engine and those transmission components result in a high variety of driveline layouts. The main layout options are the following:

The most typical driveline layouts are shown in the bellow figures.

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Hybrid transmissions

A vehicle is said to be a hybrid, if it is capable of storing and converting energy in two independent ways. The first energy source is always a fuel tank and the chemical energy stored in the fuel is converted into mechanical power through the internal combustion engine. Hybrid vehicles are dominantly hybrid electric vehicles (HEV), which means that the second energy storage is a battery, the energy of which is converted to mechanical power through one or more electric motors.

The additional electric drive increases the overall efficiency, through supporting the internal combustion engine in avoiding such operating conditions, where the specific fuel consumption is high (typically vehicle drive-off, low speed maneuvering and low power output at high engine speeds). The additional power of the electric motors enables also the application of smaller engines while keeping the same total peak power.

During travel, the battery can be charged either through recuperative braking or by the engine. Recuperative braking means that the kinetic energy of the vehicle is not dissipated to heat by the braking system, but saved for the battery using the electric motors as generators. In some driving conditions, the engine can also be used to charge the battery. In this case, the energy is indeed only transferred from the first energy storage (fuel tank) to the other (battery) with some reasonable losses during the energy conversion. But all things considered, this can also have a positive balance, if the energy from the battery is utilized later in such a driving situation, when the overall efficiency of the internal combustion engine would be even worse.

Hybrid electric vehicles can be classified based on the layout of the hybrid powertrain. There are 5 main concepts for the hybridization of the powertrain, click on them for detailed information.

The other aspect for classification is the power of the electric motor, which has a very strict correlation with the required battery capacity and the possible functions of the electric drive (typical value ranges):

• Micro hybrid: < 5 kW
• Mild hybrid (MHEV): 10 – 20 kW
• Full hybrid (FHEV): 20 – 40 kW
• Plug-in hybrid (PHEV): 60 – 120 kW (battery can be charged from external power source as well)

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Electric drives

Coming soon.

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