Rapid Electric Shift; Reducing the Cost of Integrated Electric Drive Systems
The automotive industry is driving a great deal of exploration into powertrain configurations for electric vehicles, however, the availability of electric passenger vehicles is dominated by designs that use single speed gearboxes. The performance of the traction motor across the vehicle speed range can be improved using a multispeed gearbox; motor torque requirements can be reduced, speeds increased and efficiency performance optimised.
Dr Nick Rivara, Senior Controls Engineer, Electrified Powertrain, Drive System Design
Simon Shepherd, Head of Electrified Powertrain, Drive System Design
Issue: #December 2017
Batteries remain the dominant cost in hybrid or electric powertrains and so opportunities for cost reduction in the remainder of the powertrain need to be utilised to improve affordability of a total system. Genuine advances in optimisation can be realised if a true system design level approach is taken. This article looks at how this type of systems thinking could bring transmission benefits to electric vehicles by designing the motor and transmission to compliment the requirements of each.
The potential negatives associated with the inclusion of a multi-speed transmission with an electric motor are increased weight, complexity, packaging volume and losses through drag and actuation power.
Parallel shaft dog-shift transmissions are the simplest, cheapest and most efficient multispeed options. Electric traction motors can potentially achieve the shaft speed synchronisation function, negating the requirements for conventional synchronisers with associated drag. However, the interruption of traction torque during a shift is inherent in a non-powershift multispeed design.
The challenge is making the shift acceptable to an electric vehicle driver who is accustomed to the seamless nature of single speed system. Minimising the torque interruption duration by rapid shifting, utilising the traction motor to synchronise the shaft speeds, is a possibility: electrically actuated systems, if designed to undertake highly dynamic motion control, can provide response times in orders of magnitude faster than conventional hydraulically actuated systems.
DSD has built up significant project experience in modelling and developing systems utilising highly dynamic electric actuation and combining this experience with our expertise in transmission design have set out to investigate if a dog-shift multispeed transmission could work for an electric vehicle by applying a system design approach.
A simple two speed design employed in the study. Shifting is achieved through a dog clutch on the input shaft connected to the electric motor. Shaft speed synchronisation is undertaken by the motor, avoiding the drag associated with open synchroniser clutch packs and conventional clutches. The output shaft drives torque to the wheel through a conventional differential.
The concept aims to achieve a shift time which is as short as possible. The stipulation for an unnoticeable transmission shift lies in the period of torque interruption being reduced below the limits of human perception for audible noise or perceived jerk (rate of change of vehicle acceleration).
Figure 1 Example Two Speed Transmission with Dog Clutch Engagement
This is an impossible task in a conventional torque-interrupt shift sequence. As the shift duration is reduced, the torque gradients increase, and the theoretical jerk exceeds a level deemed acceptable by conventional thinking. The challenge for a rapid electric shift concept is to make the interrupt at a frequency well above the natural frequency of the driveline and further providing damping of any resulting oscillations with the electrical machine. Stiffness, damping and inertia of the driveline components can also be modified to support the system response.
Initial evaluation of the concept is to be undertaken by dynamic modelling of the transmission and electric motor. The shift sequence is composed of:
- Motor torque reduction
- Dog clutch moves to neutral
- Shaft Speed synchronisation using motor torque
- Engagement of next dog clutch.
- Restore torque to driver demand.