How are electric cars that can accelerate from 0 to 100 km/h in 2 seconds built?

Some time ago, the electric racing car Grimsel, built by students from ETH Zurich and Lucerne University of Applied Sciences and Arts, achieved a zero-to-100-kilometer acceleration of 1.785 seconds, breaking the previous record of 2.134 seconds held by the Delft University of Technology Formula Racing Team in the Netherlands. For reference, the Bugatti Veyron 16.4 Super Sport accelerates from 0 to 100 kilometers in 2.46 seconds. Electric vehicles have no gear shifting delay, and their natural advantage of being suitable for acceleration is perfectly demonstrated.

In order to popularize the professional knowledge of electric racing cars, this article takes the racing car DUT14 built by Delft University of Technology in the Netherlands as an example to give a brief introduction. The information is compiled from the official website of the Dutch DUT team.

Dut14 racing car in the race

Powertrain

The Dut14 racing car has a curb weight of only 149.2Kg and can accelerate from 0 to 100 km/h in 2.3 seconds. It is driven by four wheel hub motors, model AMK DT5-14-10, which can achieve traction torque control and brake energy recovery. The maximum power of a single motor is 27 kW and the maximum torque of a single motor is 28N.m. The advantages of wheel hub motors include:

1. It can significantly save space in the car, increase design freedom, and improve transmission efficiency;

2. Fast response speed, the response time of the drive motor is only 10ms, which is conducive to good control effect;

3. Greatly improve the vehicle's driving dynamics. The driving/braking torque of each motor is independently controllable. The yaw torque acting on the vehicle can be changed by independently controlling the torque of each motor to generate longitudinal force. It can even directly control each electric wheel to achieve steering differential and drive anti-skid, thereby effectively improving the vehicle's handling stability.

Dut14 related parameters

The wheel hub motor has high requirements for lightness and compactness, so the DUT team cooperated with the German motor supplier AMK, and DUT students modified the motor and designed the motor's mechanical connection and cooling system. The maximum output power of the motor is limited by thermal management, so the DUT team achieved maximum power output by integrating a water cooling system in the shell.

Dut14's suspension and motor

The output current of the lithium battery is DC, but since the AC motor has better power and lightweight performance, the final solution is the power supply combination of AC motor + inverter. The AMK KwS-26 motor controller is selected, and the IGBT inverter is applied, which is controlled by bandwidth. The four motor controllers corresponding to the four wheel hub motors are arranged under the driver's seat, with an efficiency of more than 98%. To ensure safety, it is protected by a carbon fiber shell and communicates with the ECU via the CAN bus.

The DUT team also independently developed a BMS battery management system. The lithium polymer battery was provided by Shenzhen Hongxing Technology Co., Ltd. (Melasta) and tested by the Dutch KEMA laboratory. The battery pack consists of 288 single cells, with two cells connected in parallel to form 144 battery packs in series, with an output of 85kW, a braking energy recovery power of 50kW, and can absorb 6.3kWh of energy at a maximum voltage of 600V. The total weight of the battery system is 43kg, divided into two battery compartments, and the battery pack is arranged near the center of gravity of the vehicle, which can withstand a maximum acceleration of 2G.

Automotive Electronic Systems

The ECU (vehicle control unit) uses an ARM Cortex M4 microcontroller, programmed in C language, and can control the speed of each motor separately. The system includes a power management module, a safety module, and a control module. The power management module needs to convert the relatively low battery voltage into different voltages required by each electronic system. When all subsystems are started, the peak load of these transformers can reach 250W. The safety module of the ECU is also very important. The team considered hardware-level safety design during the design. For example, when the system detects a high-voltage cable break, the ECU automatically cuts off the voltage of all high-voltage cables.

Dut14's full circuit layout

Chassis

The most important part of a car is the chassis, which needs to support and install the car engine and its components and assemblies to form the overall shape of the car and ensure normal driving.

The DUT14 car uses a carbon fiber monocoque body, and TeXtreme provides ultra-light tow expanded carbon fiber reinforcement materials, which reduces the weight of its composite parts by 20% to 30%. The body needs to meet the accuracy of fixing all parts while ensuring the safety requirements of the driver, all of which are designed and made by the student team members themselves.

Body and chassis

The design goal of the car body is to reduce weight to the maximum extent to improve performance. To this end, finite element analysis of strength and safety is required, and assembly simulation is performed in CATIA. In addition, the body design of the Formula One car must also meet ergonomic requirements. In order to allow the driver to focus more on the track, the cockpit, steering wheel and racing seat must be lightweight while also adapting to the driver's body shape. Therefore, an adjustable pedal box is used, and the steering wheel must also adapt to the hand shape of each driver to provide sufficient grip during driving.

Racing dynamics

In recent decades, electronic control systems have been widely used in racing cars and passenger cars. ABS, ESP and torque vectoring control not only improve performance, but also improve vehicle stability. The four-wheel hub motors used in the DUT13 racing car provide a broad space for advanced torque control systems.

DUT14 is equipped with multiple control systems including torque vector control, starting control, ABS, yaw control, etc., and it is necessary to establish a vehicle dynamic model in the Matlab/Simulink environment.

The controller can detect sensor failure and take action. Even if the sensor fails, the car can still drive normally.

The braking system of the racing car includes a hydraulic braking system and an electromagnetic energy recovery braking system. The brake pad materials of the four-wheel disc brake are worth a detailed introduction:

In pursuit of more extreme lightweight and higher performance, the DUT team abandoned the traditional cast iron material on the brake disc. After several months of simulation, discussion and research, DUT finally chose a metal-based composite material called SupremEX® AMC225XE, which consists of an aluminum alloy matrix and SiC particles (silicon carbide, commonly known as corundum, with a Mohs hardness of 9.5, second only to the world's hardest diamond, and excellent thermal conductivity). Compared with the commonly used gray cast iron, the aluminum alloy matrix has the advantages of low density and high thermal conductivity, which makes the brake system 50-60% lighter. However, the wear resistance of aluminum alloy is relatively poor, so it needs to be improved by adding hard particles. Through the thermal performance simulation of the brake system, DUT found that aluminum alloy has the disadvantage of a low melting point, but due to the addition of four-wheel motors, the electromagnetic braking effect is greatly improved, and this design has become feasible.

In fact, there is also a group of young people in China who are building Formula Student cars. Each participating team only needs to design and manufacture a small racing car within one year according to the competition rules and racing car manufacturing standards, and must be able to successfully complete all or part of the competition. In fact, such competitions are mainly aimed at training college students, and at the same time, they have become a platform for selecting outstanding domestic automotive talents, and there are more opportunities to communicate with international young automotive engineers.

For example, the 5th China Formula Student Car Competition held in Hubei in October 2014 attracted 60 fuel-powered racing teams and 20 pure electric racing teams from nearly 70 universities, with a total of more than 2,000 teachers and students participating, including the pure electric racing team of the University of Stuttgart in Germany and the fuel-powered racing team of the Karlsruhe Institute of Technology in Germany. In the competition, the pure electric formula car "Silver Shark III" built by students of Beijing Institute of Technology stood out and won the runner-up in the pure electric group (also the champion of domestic universities).

The "Silver Shark III" racing car uses a self-designed battery system and vehicle controller, and a fast charging system that reduces charging time to 20%-30% of the previous time. The transmission uses a self-designed planetary gear reducer; the vehicle has a peak power of 85KW, a peak torque of 175N•m, a curb weight of 230KG, an acceleration of about 3.6s per 100 kilometers, and a maximum lateral acceleration of about 2G.