Environmental Engineering Reference
In-Depth Information
3.3.2 Parallel RBS
A parallel RBS is easier to implement than series RBS because full EHB is not
required. As Figure 3.9 illustrates, the parallel system immediately activates the
vehicle service brakes anytime the brake pedal is depressed. An algorithm is nee-
ded to blend M/G torque with service brake force so that the total deceleration
effect is smooth and seamless to the driver. Front-rear brake coordination is similar
to that for series RBS so that vehicle stability is maintained.
0.45
0.30
Service brakes
0.15
M/G torque
0
Compression braking
25
50
75
100
Brake pedal position (%)
Figure 3.9 Parallel regenerative brake system
With parallel RBS, the fuel economy benefit is not as pronounced as for series
RBS because some fraction of vehicle kinetic energy is always dissipated as heat
rather than used to replenish the battery.
3.3.3 RBS interaction with ABS
For normal rate of application of pedal position, the total braking effort in a hybrid
vehicle will be as shown in either of Figures 3.6 or 3.9 depending on whether the
brake system implemented is series or parallel RBS, respectively. However, if the
pressure applied to the brake is brisk, there is a tendency for wheel lock-up and skid.
The ABS was introduced as a mechanism to intervene in the braking process should
the operator engage the brakes too harshly and cause loss of longitudinal stability as
a result of a skid. With ABS the brake line pressure is modulated at a frequency of
about 15 Hz so that wheel skid is avoided. Also, braking distances are shorter,
vehicle stability is better managed and the tendency to excite vehicle yaw motion is
minimized, especially on low mu surfaces (i.e. snow, ice, wet pavement etc.).
In all hybrid propulsion systems, the engagement of ABS pre-empts M/G
regeneration torque. The M/G is commanded to free-wheel as brake line pressure is
modulated by the ABS system. Another important aspect of any vehicle, including
hybrid electric, is the stopping distance from some initial velocity,
V
0
. In the case of
braking, all the friction loss components contribute to stopping and that includes