Lightweight design of regenerative braking frictio

Electric vehicle regenerative braking friction brake lightweight design

preface

different from traditional vehicle braking, the braking of electric vehicles is completed by friction braking and regenerative braking. However, in the current design of electric vehicle friction brake, the influence of motor regenerative braking is generally not considered. In fact, the motor can provide part of the braking force during braking, so the maximum braking torque provided by the designed friction brake exceeds the maximum braking torque required in the actual braking process, that is, the friction brake has surplus braking capacity

for electric vehicles, after adopting regenerative braking, the regenerative braking torque provided by the motor bears a part of the total braking torque, so the braking torque provided by the traditional friction brake is required to be reduced accordingly. Moreover, because a part of the braking energy is converted into electric energy through motor power generation, the braking energy dissipation task undertaken by the friction brake is correspondingly reduced, and the requirements for the brake to bear the thermal load will also be reduced. Therefore, it is feasible to reduce the size and weight of the friction brake. In this paper, the lightweight design of front wheel caliper disc brake is carried out

1 brake performance requirements

the brake design should not only consider providing sufficient braking force for the whole vehicle braking, but also make the vehicle meet the attachment conditions during the braking process. It is required that the thermal stability of the braking capacity is good, and the friction lining should have sufficient service life, etc

the braking torque provided by the friction brake. On the one hand, in order to ensure the braking performance and meet the braking regulations, the braking torque provided by the friction brake cannot be less than a certain value, that is, the lower limit of the braking torque of the brake; On the other hand, limited by the adhesion conditions of the road, to prevent the wheels from locking and dragging, the braking torque provided by the friction brake cannot be greater than a certain value, that is, the upper limit of the braking torque of the brake

the lower limit value of brake torque will be determined according to relevant braking specifications. Table 1 lists some requirements of China's car braking specifications on the braking performance of service brakes

according to some requirements of the braking specification, the braking simulation is carried out by using advi-sor, and the set braking conditions are shown in Figure 1. The car accelerates to 80km/h first, and then brakes at a deceleration of a=5.8m/s2

the simulation results show that (Figure 2), the braking distance s=s2-s1=427.77-388.88=38.89m, in line with the requirements of braking regulations

s1 - the distance the car has traveled when starting braking (m)

s2 - the distance the car has traveled at the end of braking (m)

it can be seen from Figure 3 that in the braking process, the braking force provided by the front wheel friction brake in the front wheel braking force demand is ffric_ f. Min=1796.71n, the braking torque required to convert into a single front wheel friction brake is:

ffric_ f. Min - the minimum friction braking force required by the front wheel; R - tire radius

through the simulation test, it is determined that the minimum braking torque at the beginning of front wheel friction braking is TB, min=253.34n · m

2 adhesion conditions

for front wheel drive electric vehicles, when the sum of motor regenerative braking force and front wheel friction brake braking force reaches the adhesion value between the front wheel and the ground, the ground braking force received by the front wheel will reach its maximum value (i.e. adhesion value), The front wheels will lock and not turn to drag and slip

take the braking force of the front wheel friction brake when the front wheel just locks up as the upper limit of the braking force generated by the front wheel friction brake. Generally, the maximum braking force of the brake is determined according to the emergency braking of the car on the asphalt and concrete pavement with a full load (the adhesion coefficient is 0.7 ~ 0.8) to the wheel locking and sliding

use advisor software to conduct braking simulation test to determine the maximum braking force of front wheel friction brake. The adhesion coefficient of the selected road surface is 0.8, and the set braking condition is shown in Figure 4. The car accelerates to 80km/h first, and then brakes at a deceleration of a=0.8g

the simulation results show (Figure 5) that during braking, the braking force fric provided by the front wheel friction brake in the front wheel braking force demand_ f. Min =3996.34n, converted into a single front wheel friction brake braking torque:

ffric_ f. Min - maximum braking force of front wheel friction brake (n); R - tire radius (m)

through the simulation test, it is determined that the maximum braking torque of the front wheel friction brake is TB, max = 563.48n · M

3 energy load

from the point of view of energy, the regenerative braking process of electric vehicles is to dissipate part of the vehicle's mechanical energy (kinetic energy and potential energy) by converting it into heat through friction brakes, and the other part into electric energy through motor power generation. In the process of emergency braking with great braking intensity, friction brake undertakes the task of dissipating a considerable part of vehicle kinetic energy. At this time, because the braking time is very short, in fact, the heat is absorbed by the friction brake before it escapes into the atmosphere, causing the brake temperature to rise, which is the so-called energy load of the friction brake. The greater the energy load, the more serious the wear of the lining

at present, the specific energy dissipation rate is commonly used as an index to evaluate the energy load in various countries. The specific energy dissipation rate is the energy dissipated per unit time per unit of pad friction area, usually in w/mm2

use advisor software to conduct braking simulation test to determine the friction heat absorbed by the front wheel disc brake during braking. The set braking conditions are shown in Figure 6. The vehicle accelerates to 100km/h first, and then brakes at the deceleration of the high-speed linear reciprocating friction and wear tester (oscillating friction and wear tester) with a=0.6g

the simulation results show (Figure 7) that the friction heat absorbed by the front wheel friction brake during braking is E1_ fric=72.378KJ。

in the design of disc brakes, constraints must be met:

4 constraints

constraints must be established according to the performance requirements of the brakes and the positional constraint relationship between dimensional parameters:

(1) the braking torque generated by friction brakes must provide sufficient braking force and meet the attachment conditions, That is,

g1 (x) =2uf0re Tmin ≥ 0 (4)

g2 (x) =tmax-2uf0re ≥ 0 (5)

tmin - minimum braking torque of front wheel brake (n · m), see formula (1); Tmax - maximum braking torque of front wheel brake (n · m), see formula (2); U - friction coefficient of brake friction lining, taken as 0.3; F0 - braking driving force (n); Re - equivalent radius (m)

(2) energy load constraints must be met, that is,

[e] -- upper limit of energy load (w/mm2), generally taken as 6.0w/mm2; E1_ Fric - heat absorbed by the brake during braking (kJ), see formula (3)= 100/0.6g - braking time (s); A1 - working area of brake lining, calculated by the following formula:

d - lining diameter (mm)

(3) the thermal load constraints must be met, and the temperature rise of the brake disc after one brake should not exceed the allowable value[ Δ t] , i.e.

[ Δ t] -- on the one hand, study the material deformation and residual stress caused by humidity induced expansion. The upper limit of residual stress brake disc temperature rise (℃) mentioned above is taken as[ Δ t]=15℃； E - heat absorbed by the brake disc during braking (kJ); J -- thermal work equivalent, j=4180n · m/kcal; C - specific heat of brake disc (kcal/(kg ℃), for steel and cast iron, c=0.113kcal/(kg ℃); σ —— Density of brake disc (kg/mm3), take 7570kg/mm3; D - diameter of brake disc (mm); A - thickness of brake disc (mm)

(4) the diameter of the brake disc should be within its specified range Du, that is,

g5 (x) =du-d=du-x6 ≥ 0 (9)

du - the upper limit of the diameter of the brake disc (mm), usually 79% of the rim diameter DH, dh=380.2mm

(5) the installation position of the lining should not exceed the range of the brake disc, that is,

(6) the lining should not interfere with the wheel hub, that is,

dg - the given wheel hub diameter (mm), dg=76.2mm

(7) the oil cylinder should not interfere with the hub, that is,

tc - the wall thickness of the oil cylinder (mm), which is generally taken as 5mm

5 genetic algorithm solution

in genetic algorithm, it is necessary to specify the value range of design variables. In empirical design, the value range of disc brake design variables is shown in Table 2

genetic algorithm operation can be easily realized by using the genetic algorithm toolbox in MATLAB software. Compile the M-file program of the objective function and constraint conditions for the lightweight design of caliper disc brake, and solve it on behalf of genetic algorithm. Select floating-point coding, the number of individuals in the population is the key technology of bio based polyester preparation 20, the mutation probability is 0.08, and the maximum number of iterations is 100. The results are shown in Table 3

Table 4 shows that after the lightweight design, the volume of the front wheel brake disc is reduced from 1258cm3 to 872.04cm3, the mass is reduced from 9.5233kg to 7.5271kg, and the mass is reduced by 20.96%

6 summary

due to the participation of regenerative braking in the braking process, the braking torque and thermal load provided by the traditional friction brake are required to be reduced accordingly, so the size and weight of the friction brake can be reduced. Based on this idea, this paper studies the design theory of caliper disc brake, and uses advisor simulation software to carry out braking simulation experiments under specific working conditions under the premise of regenerative braking control strategy, so as to determine some performance requirements for the front wheel disc brake of the research model. Under the guidance of the disc brake design theory, the front wheel brake is lightweight designed by using Matlab genetic algorithm toolbox, and the mass of the front wheel brake disc is reduced by 20.96%. (end)