AMZ Racing – recuperation in Formula Student

@Benjamin Hildebrandt @Benjamin Hildebrandt

Gone is the time when only eco-friendly hybrid street cars used brake energy recovery to improve fuel economy. Brake energy recovery has definitely arrived in motor sports: As much as it is a performance factor in the 2014 Formula 1 season, it has been in FSE during the last few years.

The reasons are simple: With an inverter that can feed current back into the battery, it is only a matter of software to implement a basic setup. Thereby, typically 5% of the total energy in the battery can be recovered – think of it as an additional lap on the FSG circuit.

AMZ racing implement recuperation in its very first electric car furka in 2010. The solutions evolved in time with julier in 2013 as current highlight.

Basic recuperation setup for 2WD FSE cars

When implementing such tramadol 50 mg a solution, three crucial points have to be taken into account.

Your inverter has to be able to operate the motors as generators to brake electrically. The motors buy baclofen online, what does generic baclofen look like, baclofen generic drug . usually offer more power than your accumulators can handle and are therefore not critical.

Accumulators however have limited charging tramadol 50 mg currents and maximum voltages imposed by the data sheet. Most cells are chosen for their energy density and more often than not, have very little charge densities. If you have 6kWh total energy at nominal voltage and 3C charging current, you can recuperate up to 18kW. If you now brake at 78 km/h and 1g with a 250kg car, your car exactly recover one third of the dissipated energy (P = F * v, F = m * a). It is also important to know the internal resistance of your accumulator box. If you charge a nearly full accumulator with peak current in the first corner of the endurance, your voltage will exceed the maximum allowed value ( dU = R_internal * I_charge).

Longitudinal dynamics are crucial to understand in the analysis of braking. In a typical RWD car, recuperation can only be applied on the rear axis. Therefore, mechanical brake balance has to be shifted to the front to ensure unchanged overall brake balance. To get a first idea, you can compare the electric and mechanical brake torque (M_el = M_mot * i_gearbox, M_mech = mu * r_brake * p_brakeline * A_brakecylinder). Every FSE car is fitted with a brake pressure sensor. Recuperation torque should be proportional to this value. With the formula above, you get a first idea of the factor between the brake line pressure and the recuperation torque.

This leads us to ergonomics, an aspect often neglected in such setups. Most FSE drivers are not used to changing car parameters during the tramadol hcl race. A single dashboard button to switch recuperation on and off is the simplest solution in our experience. As a first improvement, different maps for different voltage levels can be implemented. As tempting as it is to use all the motor torque and the maximum charging current of the batteries, the car will be faster when the driver can concentrate on driving.

julier drivetrain

The 4WD drivetrain of julier offers far more possibilities for recuperation than a 2WD system. Brake energy recovery was a key point to designing it in different areas.

First of all, the concept change from 2WD to 4WD has been influenced by the possibility to recuperate more energy. Depending on weight transfer under braking (and therefore tire grip, COG height, etc.), the brake balance will only be between a fifth and a third on the rear axis. This means that a 4WD car can cost of dapoxetine in india can you buy 60 mg dapoxetine everlast online india dapoxetine buy dapoxetine hcl online . buy dapoxetine online usa 30mg in uk  recover three to five times more brake energy. We also decided to build four identical motors for simplicity but also to provide enough front torque during braking.

To cope with the increased power, the accumulators were chosen with high charge amoxil order cheap amoxil buy amoxil density in mind. We were able to find cells that offer high charge density, good energy density and little requirements for balancing. To minimize losses and heat dissipation, all other drivetrain parts such as inverter, HV wiring, power electronics or gearbox were constructed with high efficiency in mind.

julier drivetrain with motors, motor controllers, accumulator box and cooling system ©AMZ Racing

julier drivetrain with motors, motor controllers, accumulator box and cooling system ©AMZ Racing

julier pedal box

To harvest most buy fluoxetine online uk first, take 2 small bowls, placed some hair shampoo on it , subsequently press the saw palmetto draw out from the soft gel, and drop it on  energy, the car has to brake completely electrically. This means that no mechanical brake force should be applied and therefore, the brake line pressure should stay as low as possible.

Advanced hybrid street cars use so-called brake-by-wire systems:  a sensor measures the applied force on the brake pedal and an electromagnetic actor produces the actual brake force. However, FSE rules mandate that the brake master cylinders must be connected with the brake piston via the brake lines (T7.1.4).

To deal with this problem, AMZ designed a brake pedal preloaded by springs. When pressing the pedal, it does not move until the foot force overcomes the preload. The brake line pressure does not increase and no mechanical braking occurs. At the same time, a sensor in the pedal measures the force and brake torque is applied to the wheels.

When the foot force exceeds the preload, the pedal moves, brake line pressure raises and the car starts to brake mechanically. This is important for safety (T7.1.10), but also for the Brake Test when the tractive system is switched off (T7.2.2). The adjustability of the preload allows to fine tune the brake feeling for the driver as he should not notice any difference between mechanical and electrical braking.

Brake pedal with integrated force sensor © AMZ Racing

Brake pedal with integrated force sensor © AMZ Racing

julier safety software

To protect the vulnerable cells, several safety features have been incorporated. The most important data about the cell is its Open Circuit Voltage (OCV), which is the hypothetic voltage of the cell without any flowing current. As time constants of the LiPo cells and charge/discharge time constants are too close to each other, a simple internal resistance model is not enough for this application.

Data analysis led to a frequency domain model of the cell purchase discount medication! buy dapoxetine usa. free delivery, buy dapoxetine online . which allows online calculation of the OCV at any time and current. Starting from this information, recuperation currents can be limited and cells used to the maximum allowed limits of the data sheet.

Maximum recuperation torque limited by different factors © AMZ Racing

Maximum recuperation torque limited by different factors © AMZ Racing

julier recuperation software and dynamic torque distribution

This fine tuning is also made on the Vehicle Control Unit where the conversion factor between pedal force and brake torque is set.

The same as FSE teams envy their FSC companions for the sound of their cars, the latter are jealous of the multiple possibilities of driving dynamic controls of (multi-motor) electric cars. Several subsystems such as Traction Control and Torque Vectoring add up to complex structure which guarantees optimal torque, positive or negative, on all wheels at all times.

Recuperation adds the possibility to apply these techniques not only during acceleration, but also during braking.

The julier Traction Control reduces the torque of a single wheel when it exceeds a certain slip angle. Applied to braking, this system becomes your ABS without any additional hardware.

Torque Vectoring is the dynamic torque distribution between different wheels. Imagine a 4WD FSE car on a straight at different accelerations and implying linear tire characteristics. Torque distribution should be the same as weight distribution. At the entry of a corner, the car turns faster when more brake torque is applied to the inside wheels. At the exit, more torque should be applied to the outside. Combining these two insights, a basic feed forward Torque Vectoring can be programmed.

Again, only recuperation allows you to use the system not only during acceleration, but also during braking.

Future prospects

On one hand, recuperation was one of the top priorities if it had not been implemented in the car yet. Members from different smaller teams said that they were searching a solution for the 2013 or at latest for the 2014 season.

On the other hand, one can expect that teams who already use recuperation will continue to progress in this area. As former FSE engineering challenges such as lightweight construction are mastered by more and more teams, I personally expect continuous improvements on driving dynamics controls combined with chassis engineering.  If those two components are combined in a reliable package, new levels of performance are possible.

2014 AMZ model grimsel ©AMZ Racing

2014 AMZ model grimsel ©AMZ Racing

Marius Huber, 21, BSc Mechanical Engineering, 2013 Head of Controls and current tutor at AMZ Racing.

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