What does a transmission do?
Transmissions convert speeds
In cars, transmissions convert speeds and torques along the drivetrain. The objective is to maximise the efficiency of the engine which means to achieve low fuel consumption with the greatest performance or torque – and, therefore, driving dynamics.
Today’s petrol engines run most economically at a speed of 1,250 revolutions per minute, while diesel engines are at their most economic at 1,150 revolutions. For electric engines, the figure stands at anything from 7,000 to over 20,000 revolutions – although these speeds would be much too high to move a car.
Even the torque provided by an engine, that is to say the force of rotation, is dependent on rotational speed. In road traffic, we find cars with a maximum torque from about 90 Nm for a small car and up to 750 Nm for a luxury sports car. Both need a transmission to convert the power provided by their engines into propulsion.
In city traffic and on the motorway
Due to its weight, to get underway in inner-city traffic, a vehicle initially requires a relatively high torque on the drive wheels at low rotational speeds.
But this is the problem: it is not possible to use speeds below their idling speed because at that point the drive wheels receive no torque and therefore cannot provide any power.
The solution is to use a transmission with a high gear ratio in combination with a starting element, for example a clutch.
Hybrid and electric vehicles use an electric motor as the drive source at low speeds. In this case, no starting element is required because an electric motor can deliver sufficiently strong torque at low rotational speeds.
For high speeds, a car needs gears with low ratios. This is mainly about lowering the rotational speed of the engine to a fuel-efficient level for a longer period of time. In addition to driving economy, comfort now also comes into play: an “economically” driven engine is also quieter than one rotating at high speed. In the case of an electric motor, lowering the rotational speed by selecting a higher gear increases the overall level of efficiency of the electric drive.
Hybrid drives generally combine a combustion engine with an e-machine. This takes on a number of tasks: When accelerating it acts as a motor to support the combustion engine (boosting). When braking and during thrust phases the e-machine acts as a generator to create current (recuperation). Depending on the design, the e-machine is also able to drive the vehicle - including pure electric driving over longer distances (plug-in hybrid).