d) Comparison of Continuous Service Brakes
The figure shows in a comparative diagram the torque characteristic of discussed continuous service brakes.
The braking torque of a diesel engine with exhaust brake is far below that which is achieved by non-engine related continuous service brakes. It can however be converted to a higher level using the transmission. In this case however, the basic engine speeds (maximum permissible engine speed) have to be taken into account. If for example, a maximum speed of 60 km/h (37 mph) is achieved at a transmission ratio of 1:2 and at a maximum engine speed, at the transmission output twice the maximum engine braking moment is available. The retarders act as so-called secondary retarders directly on the cardan shaft. For both retarder designs, there are nearly the same maximum braking moments, however at different cardan shaft speeds. Eddy current brakes generate their maximum braking torque already at low speeds which only slightly decreases in the range of higher speeds. The maximum braking effect of the hydraulic continuous service brake, on the other hand, is achieved in the average speed range and falls steeply particularly at low speeds.
The characteristics of retarders and eddy current brakes show the variation of the braking torque at the maximum utilisable volumetric efficiency of oil or at the maximum current input. In general, new developments allow for the constant adjustment of the braking torque to that desired by the driver. This is achieved by varying the volumetric efficiency of oil and the supply current in broad ranges, independent of the speed, using electronics.