Comparison of Propulsion Units
Selection and Assessment Criteria
In order to choose a suitable propulsion system, the required power output for the respective vehicle and the torque characteristics against speed are of primary importance. A number of other factors are also to be considered. They can be summarized into the following main groups:
Operating behavior,
Economy, and
Environmental influence.
Initially, these criteria will be explained in more detail and then applied to different drive concepts.
a) Operating behavior
The demand characteristics for the motion of a vehicle which is given by the sum of the individual road resistances, is set against the delivery map in a drive-end way. Initially, it is determined by the characteristics of the drive concept. A general presentation describes the possible power or torque characteristic of the propulsion aggregate against speed. This area is limited at its upper end by a characteristic line, the so-called full-load curve. Beyond this curve, no higher power output or higher torque can be generated.
The torque delivery takes place along a characteristic in the form of a hyperbola for the respective full-load curve of the delivery map.
Due to a limited traction between tires and road (Module 1-3), only a finite torque can be transferred on to the driven axle of a vehicle. As a result, the torque of the propulsion engine can also be limited to a corresponding maximum value.
The maximum speed represents a further limit which is either dependent on the engine or the maximum speed of the vehicle. A delivery map results from these limits, as shown in the figure.
It is also an advantage for a motor vehicle to use a relatively high share of the engine power for so-called engine braking. This means that the delivery map should be expandable in the negative range of power output and torque, possibly a mirror-image against the speed axis. Ideally, the kinetic energy that is set free can be completely regained and returned to the vehicle's energy sources.