Lightweight Electric/Hybrid Vehicle Design

Design for optimum body-structural and running-gear performance efficiency 239
Such metering is also essential for automatic braking and Adaptive Cruise Control (ACC).
Current cruise control, it is argued, can suffer a lack of sufficient engine deceleration on steep
gradients such that the vehicle gains speed; EAS can supply precisely metered braking pressure to
prevent this. Beyond this, EAS can generate pressure for ACC so that a vehicle is kept at a safe
distance from the one ahead, without having to rely on engine braking which might prove inadequate.
For stage three the problem of varying brake pressure requirement with degree of vehicle payload
is tackled. Here, not only is brake booster pressure metered but also the pressure level boosted
proportionally to the pedal force (measured by piston rod sensor), so that constant deceleration is
obtained with constant pedal force irrespective of payload.
Brake-by-wire systems have now been developed which provide for not only a wired connection
between pedal and brake actuator but also for electromagnetic actuation of the brake itself. The
Delphi Chassis Galileo system allows dynamic brake effort apportioning, tunable pedal feel and
variable boost ratio, without the need for hydraulic/vacuum assistance, as well as integration
possibilities with stability enhancement systems. Potential exists for closed loop control of braking
for front-to-rear and side-to-side braking balance; active independent wheel brake control allows
ABS and ASR to interface with collision avoidance system, (a).
‘Electrohydraulic apply’ is one variant of the system, shown at (b), in which closure of the stop
lamp switch, as braking is applied, causes the ECU to close normally open solenoids. Upstream of
these brake fluid pressure caused by the application is sensed and transducers emit a proportional
signal to actuate piston-displacement motors on the front brakes. Pressure is continuously modulated
by the size of this signal and the rear brakes applied proportionately to provide a balance dictated
by a complex slip control algorithm. The ‘feel’ on the brake pedal is achieved by displacing fluid
from the master cylinder into an emulator device which provides a predefined force/displacement
characteristic tunable by the brake algorithm. If a fault is detected during powered (motor-actuated)
braking, the solenoids switch to the open position so that unassisted direct braking is available.
ABS and ASR is achieved by overriding the driver input with a control command based on wheel
speed.
The rear electric brake system (c) is intended for small weight-critical cars where removal of the
park-brake cable assembly can save between 3 and 6 kg and works in conjunction with the brake
system just described or a conventional hydraulic brake. The rear brake is a high-gain
electromagnetic actuator designed to maximize torque capability with minimum electrical input.
Closed-loop control ensures stability of the gain so that at an operating torque of 400 Nm, say,
each wheel brake can respond at rates up to 4000 Nm/sec, to continually adjust dynamic brake
output. The wheel brake incorporates a PM DC motor, gear train and ball-screw/nut mechanism,
actuating the brake friction surfaces through a lever system. A ‘backdrive’ spring incorporates a
park-brake latch mechanism. This is a bi-stable clutch device which locks the main motor shaft on
demand. The park-brake holds until the switch deactivates, without electrical input. Closed loop
control generally reduces the effect of component and operating condition variability on braking
performance, algorithms based on differential wheel speed information compensate for load
distribution variation.
8.11.3 ELECTRONICALLY CONTROLLED CONTINUOUSLY VARIABLE TRANSMISSION
While in series hybrid-drives batteries and IC engine typically power motor/generators, to drive
the road wheels, in parallel configured, hybrid-electric/IC-engine drive vehicles there is mechanical
drive between IC engine and road wheels, usually via continuously variable transmission, Fig.
8.21. A widely accepted CVT is the variable-pitch pulley and belt-drive type originating in the
Van Doorne design; this transmission heralded the steel drive belt having separate tension and