Source: The following is the second part of a two part TBR Technical Corner article written by Dr. Raj Shah, a Director at Koehler Instrument Company, Kora Farokhzadeh, Ph.D., an application scientist at Bruker Nano Surfaces, Tribology, Stylus and Optical Metrology Group, and Blerim Gashi, a student in the Department of Material Science and Chemical engineering at State University of New York (SUNY), Stony Brook. The authors open by examining the environmental impact of brake wear, then follow by analyzing emerging technological trends which might mitigate these issues. The first part of the article can be viewed by clicking HERE.
The most promising braking technology, being widely commercialized in hybrid electric vehicles (HEVs) and battery-electric vehicles (BEVs), is the regenerative braking system (RBS). RBS operation is based on cutting power to the motor so that it acts as a generator and recycles the otherwise wasted kinetic energy into electric energy for the battery. Much of the research surrounding this innovation focuses on quantifying the amount of electrical energy generated from the motor due to RBS, which enables greater driving range. A majority of HEVs, however, lack an electric motor that is able to provide sufficient torque for deceleration, making cooperation between mechanical and regenerative braking systems a necessity. As such, quality technical analysis of RBS braking system designs focus on optimizing the distribution of required braking forces between the RBS and mechanical friction brakes to maximize conserved kinetic energy, as well as distribution of the required braking forces between the front and rear axles in order to minimize stopping distance and preserve vehicle stability.
The traction motor must be controlled to produce the proper amount of braking force for recovering the kinetic energy as much as possible and, at the same time, the mechanical brake must be controlled to meet the braking force command from the driver.
Considering energy efficiency and effectiveness, the RBS has been classified in three different brake control strategies – also applied in hybrid electric buses: (i) series braking with optimal feel, (ii) series braking with optimal energy recovery, and (iii) parallel braking.
Series braking with optimal feel systems strive to minimize the braking distance and ensure steady-state braking. A braking controller controls the front and rear wheels braking forces based on the braking pedal position. For decelerations less than 0.2 g, the required braking force is low enough to be supplied by the electric motor, and thus only the RBS on the front axle is triggered. Whereas decelerations greater than 0.2 g require braking force higher than the regenerative braking force, so the electric motor will generate its maximum braking torque, and the mechanical brake system picks up the slack. In order to ensure desired braking feel, the braking forces on the front and rear axles should follow the ideal braking forces “I-Curve” distribution.
Series braking system with optimal energy puts the RBS priority and the electric motor is controlled to produce its maximum regenerative braking force at all times. For braking conditions (vehicle acceleration, road friction, etc.) where regenerative braking force produced by the electric motor is sufficient to bring the vehicle to stop, only the RBS on the front axle is applied. However, if the available regenerative braking force cannot meet the total braking force demand, additional braking force is provided by mechanical braking and the braking forces on front and rear axles are varied within motor’s maximum braking force to optimize braking feel and reduce braking distance.
Contrary to series brake strategies, active control of RBS and mechanical braking on the front and rear wheels is not required in parallel braking. In parallel braking mechanical brakes have a fixed ratio of braking force distribution on the front and rear wheels and RBS is applied simultaneously as additional braking aid to the front wheels. The RBS forces are a function of mechanical braking forces and master cylinder’s hydraulic pressure. Thus, at low motor speed, the regenerative braking force at high vehicle deceleration is designed to be zero so as to maintain braking balance. When the demanded deceleration is less than this deceleration, regenerative braking is effective. When the braking deceleration commanded is less than a given value, say 0.15 g, only regenerative braking is applied. The pressure signal is regulated and sent to the electric motor controller to control the electric motor to produce the demanded braking torque. Compared with the series braking of both optimal feel and energy recovery, the parallel braking system has a much simpler construction and control system. However, the driver’s feeling, and amount of energy recovered are compromised.
Figures 5a-c display the corresponding distributions of braking force for each system in hybrid electric bus. Generally, the RBS comprises a greater proportion of braking force in series as compared to parallel braking, understandably making series braking recycle a greater amount of energy. Similarly, there is a greater proportion of braking force supplied by the RBS for series optimal energy than series optimal feel.
CRUISE software was utilized by Sangtarash et al. to simulate these strategies and analyze brake and vehicle performance along a standard drive cycle, Nuremberg R36. The cycle consisted of numerous stopping conditions and enabled comprehensive and accurate evaluation of each braking strategy. A brake controller was implemented in MATLAB SIMULINK which calculated the required front and rear brake force, as well as the distribution between regenerative and mechanical braking for each strategy. As the total mechanical energy output of the drive cycle was calculated as 15966.8 kJ, the amount of energy recovered by series optimal feel, series optimal energy, and parallel was 7932.9 kJ, 8626.2 kJ, and 3710.4 kJ, or 50 percent, 54 percent and 23 percent of the output, respectively. The amount of energy that can be reused from these recycled inputs depends on the efficiency of the electric motor, however the general trend remains the same. Series optimal energy strategy resulted in the greatest amount of recaptured energy, while series optimal feel was a close second, and the parallel strategy understandably led to the least regeneration.
Despite the considerable similarity in energy input from series optimal feel and optimal energy, the difference between the two strategies became further apparent when considering brake frequency during drive cycle and vehicle weight. A drive cycle with more frequent braking would allow the series optimal energy strategy further opportunities to regenerate energy more efficiently than series optimal feel. Additionally, Sangtarash et al. compared the input energies from each brake strategy between an empty vehicle and a full vehicle. It was primarily discovered that the trend remained consistent, whereby series optimal feel, energy, and parallel recaptured 30 percent, 36 percent and 15 percent of the total output, respectively. The disparity between series optimal feel and energy was also more apparent for the heavier vehicle, as a greater torque is required by the regenerative braking during deceleration, allowing series optimal energy to recapture more energy.
The fuel consumption improvements by each strategy is depicted in Table 1. As compared to the conventional vehicle studied, series optimal feel, series optimal energy, and parallel resulted in a 32.7 percent, 34.3 percent, and 19.6 percent improvement in consumption. Clearly, each braking strategy offers improvements in fuel economy and recapturing wasted energy. Additional technical analyses comparing parallel and series RBS have also noted a slight superiority in these important parameters associated with series RBS, such as Zhang et al.. Regardless of the employed braking strategy, however, it is apparent that the RBS offers immense benefits that can be further optimized in terms of energy efficiency and fuel economy.
The increased utilization of regenerative braking directly reduces the reliance of mechanical braking, and thus reduces wear on brake pads. In order to quantify this relationship, a case study was performed by Jamadar et al.which compared the emission rate of traditional ICE vehicles and EVs using the RBS with 70 percent energy efficiency. In order to perform these calculations, the study utilized the brake wear emission rates (PM2.5 and PM10) calculated by previous studies, the average miles/day and number of registered passenger cars, commercial vehicles, three wheelers, and two wheelers. According to Wager et al., around 50-70 percent of brake wear is airborne or incorporated into the atmosphere due to road resuspension. Although this proportion of emissions can be larger depending on the road terrain, 50 percent of vehicles are considered in the calculations of emission rate in g/day, which are depicted in Figure 6.
Figure 6 represents the PM2.5 and PM10 emission rates for the four types of standard vehicles, also while taking into consideration the 70 percent energy efficient RBS. Along the calculations performed by Jamadar et al., the introduction of the RBS resulted in a 70 percent reduction of emissions throughout all vehicle types. However, other factors including driving behavior, road conditions, and the type of mechanical braking system utilized in addition to regenerative braking, a more realistic reduction value is 50 percent. Experimentally derived results on the reduction of emissions due to the RBS is also outlined in Clarke et al.. Although greater technical research is required on the precise effects regenerative braking has on brake wear emissions, there is no doubt that the RBS offers an innovative side-step from mechanical wearing of brake pads.
Future Prospects in Braking Technology
Regenerative braking is a turning point in braking innovations, as the system becomes widely utilized within the EV industry due to apparent advancements in reduced non-exhaust emissions and increased energy recovery. In competing with conventional internal combustion engine vehicles, EVs must reach a driving range >600 km. Most HEVs and EVs depend on the RBS as a major factor in further enhancing this target range, in addition to battery engineering. Currently, vehicles equipped with regenerative braking, such as the Nissan Leaf and the Tesla Roadster, also contain frictional brake systems in place in the case of emergency braking where greater brake force is required. Depending on the vehicle owner’s style of driving and driving terrain, the mechanical brake pads may be used only sparingly or not at all. Although this seems beneficial in terms of reduced brake dust emissions, safety concerns may arise as the braking pads can easily become corroded. In order to combat this problem, further analysis is required on the inhibition of corrosion in optimal brake components, such as Continental’s aluminum drum brakes.
The impact of higher weight of EVs compared to internal combustion engine vehicles on non-exhaust emissions has led researchers believe that one solution to improving range and air pollution is weight reduction actions. Ford Hybrid Electric Vehicle (HEV) makes extensive use of aluminum, magnesium, titanium and other lightweight materials applications to significantly reduce weight. Further reductions in weight and a means for brake dust capture.
A combination of battery engineering, lubrication optimization, and sophisticated braking technology offers the fundamental ways EVs may continue to inject itself into the automotive industry. Simultaneously, research efforts and focus must also be placed on the environmental effects of developed technologies and vehicle safety, with an emphasized push towards limiting non-exhaustive emissions and minimizing stopping distance during deceleration. EVs hold the key to a greener automotive industry, further technological advancements only supplement the momentum of the EV industry.