Starting element

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A start-up element is a component that sits in the torque flow in mechanical drives between the force or torque source (typically a motor) and the part to be accelerated when starting up (typically wheels). The starting element allows the transmission of torque even at different speeds.

background

All common internal combustion engines (ICE) ( Otto , Diesel, etc.) have only limited elasticity; they can not deliver any torque and therefore no power at very low speeds . When vehicles (or machines) start up, however, the wheels are at zero speed.

To start from a standstill, the internal combustion engine would have to be accelerated to a corresponding minimum speed and then suddenly coupled; the result would be intermittent stress on the drive and corresponding wear. Even extremely squat gearboxes could reduce this effect, but not eliminate it.

Requirement

A starting element must enable the transmission of torque even with a variable speed difference between input and output. For this purpose, the ICE runs at a stable speed and delivers the corresponding power, the starting element only transfers the torque to such an extent that the counteracting moment of inertia (actio = reactio) neither brakes the ICE down from the stable speed range nor causes an unacceptable jolt of the vehicle.

solution

From a physical point of view, the engine power is transferred, the "excess" part of the power (which cannot be converted into acceleration) is diverted to another power path. This almost always happens through friction ( dissipation ):

  • Mechanical: The classic case here is the disc clutch in motor vehicles. Two rotating disks (connected with input and output) are pressed together with a variable force; the excess energy is converted into mechanical friction.
  • Hydraulic (see converter ): This is what happens in most automatic transmissions . A compressor drives an oil circuit, which in turn drives a turbine. The excess energy heats the oil through fluid friction.
  • Electric: Typically in diesel-electric locomotives. The diesel engine drives a generator, which in turn drives the electric drive motors of the axles. When starting up, the electrical components heat up particularly strongly.
  • Mechanical-pneumatic: aircraft engines run stably at idle speed, but do not emit any power through propulsion; they just heat the air. Mechanical power is only released at higher speeds.

During the start-up process, the dissipated part is gradually lowered until at some point almost the full engine power is transferred to the drive. This control is done manually (e.g. by the driver using the clutch pedal) or automatically (by the design of the drive element or by active control systems).

interpretation

The following rules apply to the design of all starting elements:

  • The transmitted torque must exceed the moments of inertia and friction (“breakaway torque”) at all times in order to enable acceleration (i.e. starting).
  • The starting element must be infinitely variable or at least so finely adjustable that mechanical loads and "jerks" remain acceptable.
  • The dissipated energy must be dissipated in such a way that no thermal damage occurs over the expected service life.

Energy balance

If the friction can be completely eliminated (for example with a disc clutch in full frictional connection ), the starting element has an efficiency of 100%; After starting up, there are no more energy losses. Fluid-based start-up elements (typically hydraulic), however, always cause a certain internal friction in the fluid and thus thermal losses.

Loss-free starting elements

Theoretically, it is possible not to dissipate the “excess” energy when starting up in friction, but to store it; for example in a torsion spring . However, due to the limited capacity and lack of variability, such solutions remained rare exceptions.

If the drive axle is thin and flexible, it can act as a torsion spring to cushion jolts when starting. Some cars have a rubber buffer element in the drive axle. Example: the old Fiat 500.

Others

In the course of the development of hybrid vehicles , various new concepts for starting elements are being pursued. One of these is, for example, using the electric drive to accelerate the vehicle so far that the ICE - at the appropriate speed - can be directly engaged without a starting element.

Electric drives normally do not require a starting element, as electric machines are comparatively elastic and deliver sufficient torque even at zero speed.