Parallel hybrid vehicles are now commonplace on the roads on which we travel but still principally use internal combustion engines for traction. In series hybrid electric vehicles the electric motors replace the internal combustion engine in the drivetrain, and the energy distribution is electrical. Electric motors offer much greater efficiency than internal combustion engines and further energy can be recaptured with regenerative braking. However, thermal limits of the motor and the battery state of charge may prevent regenerative braking from occurring so an additional form of retardation is still required.
This thesis primarily focuses on the braking system requirements of a front or rear wheel drive series electric vehicle and analyses the performance characteristics and technology required to supplement the regenerative braking capacity of the electric motors. A novel electric braking system which combines friction and eddy current braking is proposed that can meet the performance requirements of modern vehicles. The electric brake operates entirely from DC electrical power with all mechanical linkages between the driver and brake eliminated. This is commonly referred to as a brake-by-wire system and enables traction control and antilock braking capability. However more importantly, the proposed braking system can maximise regenerative energy recovery through dynamic brake distribution unique to series hybrids with electric motors on a single axle.
An adaption of a combined friction and eddy current electric train brake into a disc brake structure has been analysed in this thesis. The torque, speed and temperature response of the brake have been investigated with sophisticated Finite Element Analysis, which can model the complex non-linear behaviour when eddy currents are generated in a ferromagnetic reaction plate. A toothed and crescent geometry of the electric train brake are simulated for a lightweight series electric vehicle and its performance is verified for the first time since their invention in the 1890’s. Significantly, the inadequate performance of the toothed design is compared with the superior performance of the crescent geometry verifying the claims of the original inventor.
The crescent geometry is subsequently investigated for its thermal performance to determine whether the limits of the hard anodising conductor insulation or the ferromagnetic materials’ Curie temperature are reached. When simulated against standard brake test procedures, the brake has been shown to reach temperatures well below these limits. Only under extreme conditions of brake failure where braking would occur with only the friction brakes, are the temperatures raised to levels that approach these limits.
The important parameters that drive an optimised solution to the mass and power requirements of the brake are presented with guidelines on methods for adaptation for any size vehicle. The significant finding is that the brake is most suited to lightweight vehicles with a large wheel diameter, which can accommodate the disc brake.
To complement the electric train brake performance, a novel engagement mechanism is integrated to increase the reliability by eliminating all moving parts that require lubrication. This has the added benefit of drag free operation essential to lightweight vehicles where minimising all losses can increase the driving range. The transient response of the brake during engagement is investigated as well as the response to modulation of the braking torque for traction control. The results indicate a brake response that is equal to or better than equivalent hydraulic systems.
To complete the integration analysis of the electric train brake, various drive cycles are simulated to highlight the benefit of a regenerative braking strategy unique to series electric hybrid vehicles with electric motors on only one axle. Significantly, the investigation shows that using only regenerative braking up to the limit of traction on a single axle can substantially increase the energy recovery and increase the life of the friction brake. This mode of operation can only be achieved with a brake-by-wire system that can individually control the torque on each wheel and can quickly revert to ideal braking distribution in adverse conditions that threaten passenger safety and vehicle stability.
The drive cycle analysis confirms documented statistics that the majority of braking occurs at low deceleration demands and the electric train brake is practically never engaged when the maximum amount of regenerative braking is utilised. When the electric train brake does engage to supplement regenerative braking, between 70% and 90% of the braking component is generated by eddy currents. This further increases the life of the brake by reducing wear and provides a reliable braking torque independent of the friction surface coefficient. Importantly, this highlights the need to minimise the weight of the brake at the expense of higher power requirements due to its exceptionally low duty under a strategy that maximises regenerative braking to increase driving range.