Abstract
In today´s passenger cars only two designs of car gearboxes are found: Transmission with five or six gears and a dry clutch on the one hand and automatic transmission with planet wheels and torque converters on the other hand.
In the middle of the eighties first racing began to look for new systems to increase there own competitive position and the driver´s comfort. The racing teams used a hydraulic system which took over the control of the mechanical gear change and the actuation of the clutch.
These so-called automated gearboxes (ASG®) make things much easier for the driver and reduce fuel consumption compared with automatic transmission.
A disadvantage of automated gearboxes in comparison with conventional automatic transmissions is the fact that the gearbox must be free of torque while changing gears, i.e. the force flow between engine and drivetrain is interrupted. This traction interruption appears in particular while shifting up under acceleration. This loss of comfort is usually experienced as disturbing, because traction interruption and the jerk caused by changing the gear arise unexpectedly and are uncomfortable in particular in the full automatic mode, in contrast to hand operation.
In order to help automated transmissions to achieve faster and more comfortable shift times an active reduction of the engine speed (exhaust brake) must be realised for up-shifts, because the drag torque of the engine is not sufficient for a synchronization of engine and transmission speeds within the desired shift time. Due to a small number of revolutions between engine and transmission speed after braking, the clutch can be closed with identical shift comfort with higher gradients, which is subjectively felt as a reduction of the shift time.
In order to accelerate the mechanical gear change of the gearbox and thus shorten the pure traction interruption time, a transmission brake can be added in support of or as a substitute for the synchronization. By this method, the desired gear is synchronized in a shorter time and inserted faster. The traction interruption time and also the shift time are reduced.
Both procedures, engine and transmission brake, improve the acceleration of the gear change with equal or better shifting comfort.
Different concepts of engine and transmission brakes, their design, control and arrangement in the vehicle will be presented and examined for shift time improvement and under economic aspects.
Keywords:automated gearbox, shift time, ASG, transmission brake, engine brake