Chinese students from Tongji University use FEA in Autodesk Inventor to help lightweight the aluminum and carbon-fiber frame of their ultra-mileage electric vehicle. The result is a car that weighs 55 kg (120 lbs) and can travel 347 kilometers with one kilowatt hour of power (7,250 MPGe).
Autodesk Software Used:
Chinese students from Tongji University are researching and developing electric cars to help provide solutions to our global environmental and resource use issues. Electric ultra-mileage cars are extremely lightweight and aerodynamic, and can use energy much more efficiently than conventional vehicles. The car the Tongji students built won the China Eco Mileage challenge in 2011.
This is a case study about how they optimized their frame, shell, and steering with the help of Autodesk Inventor.
Integrated Design Process
The students first thought about the car as a whole system, and identified several key aspects of the car that were critical to success: rear-wheel steering, a lightweight frame, and an optimized shell.
To create the most efficient design, the sub-teams working on the powertrain, steering, and shell had to work very closely together. And it was only after these higher-level systems were integrated that individual parts were optimized.
Aluminum + Carbon Fiber = Strong and Lightweight Frame
Their design combines the frame and the shell into one synergistic and optimized system (RMI 10xE Principle # 15. Wring multiple benefits from single expenditures).
Aluminum Base Frame
The team chose aluminum as the basis for their frame because of its favorable strength-to-weight and strength-to-stiffness ratios (See: Material Selection – Physical Properties). Stiffness and rigidity is very important for these lightweight vehicles because the car needs to maintain its precision and stability when the load changes, and avoid unwanted resonance from the engine or the road.
Using modular product architecture with standard grades of engineering aluminum also allowed for increased durability and reparability. Almost every part of the car can be connected to the frame by bolts and rivets, creating strong and serviceable joints.
The team reduced weight in the frame by using square aluminum tubing only on frame members that directly bear the load. For other frame members, they found that the mechanical properties of angle-aluminum were sufficient.
Integrated Carbon-Fiber Shell
The carbon-fiber body of Tonji University’s Z111 car is instrumental to the vehicle’s performance. It has a decisive influence on the aerodynamic performance, and it also helps the frame bear the load of the driver.
The team moved from car body that didn’t bear any loads,
to one where the body and frame are integrated so the car’s body/shell can be load-bearing.
Their previous’ ‘non-bearing body’ frame designs met the structural requirements completely with aluminum frame members. These frame members had to be larger and heavier.
While the carbon-fiber helped to reduce the weight of the aluminum frame, the shell itself is one of the largest and heaviest parts of the car. Therefore, it too had to be optimized as part of the whole system.
Based on a force analysis, the team applied different kinds of carbon fiber to different parts of the car. For example, they used thick carbon-fiber cloth to counter the weight of the driver in places where the stress and deformation are high, such as the car shell under the driver.
In other places, the car shell is only used to guide the air flow, so thin carbon-fiber cloth met their requirements.
FEA Stress Analysis in Autodesk Inventor
Improve Welding Method
Previously, the team had used a T-joint welding method. Through FEA, they found that they had a very high safety factor. The T-joints made the frame heavy, so they looked for ways to optimize the design.
Based on their analysis in Inventor, they switched to another welding method that allowed them to save weight and material.
Carbon Fiber Body
(Note that Inventor doesn’t support analysis of composite, anisotropic materials – tread with care)
The team also used FEA to help simulate the behavior of their carbon-fiber body. They defined their own carbon fiber material in Inventor after doing mechanical tests to measure density, yield strength, tensile strength, and other mechanical properties.
Because the team had used stress analysis extensively with their previous aluminum frame, they did some preliminary simulations to compare their new carbon fiber designs with their aluminum designs. Their goal was determine the new structure's advantages and disadvantages to conduct further optimization and physical testing.
The team did some preliminary FEA studies of their carbon fiber shell.
It should be noted that the FEA tools within Inventor are not built for simulating carbon fiber. Carbon fiber can behave non-linearly and the strength and stiffness of the material depends on the direction of the fibers (anisotropic), so the results should not be trusted. For example, carbon fiber has much higher Young's modulus (is much stiffer) when force is loaded parallel to the fibers (along the grain).
The team considered the carbon-fiber composite materials to be nearly isotropic because the laminates were laid in different directions and performance testing proved the approximation to be applicable. To further ensure their design was safe, the team conducted extensive physical prototyping.
Rear Wheel Steering Reduces Drag and Rolling Resistance
To reduce the frontal area and drag of the design, the team used rear-wheel steering with two fixed wheels in the front and one wheel in the back. Because the front wheels don’t need to turn, they can be kept aligned and tightly enclosed in the shell.
The shell of the vehicle can fully enclose the front wheels because they don’t have to turn.
Also, because there is only one wheel turning, their steering linkages can be much simpler. When two wheels turn, they need need to turn at slightly different angles to carve-out an even arc (this is called the Ackermann angle). If the linkages don’t account for this, it creates increased friction and rolling resistance when the car goes around curves – and can cause wheel misalignment. Avoiding this allows for more pure rolling.
The main challenge in making the rear-wheel steering system work is making the handling responsive. Unlike front-wheel steering systems, the car doesn’t return to a straight path as easily. It can require a lot of adjustments and over-corrections to get the car moving in a straight line. This wobbling is inefficient.
Dynamic Simulation helps design steering
The dynamic simulation capabilities of Inventor helped ensure that the steering system would not interfere with the shell or frame.
It also helped to estimate the torque required on the steering handles by the driver. This force comes from the weight of the vehicle being transmitted to the road by the wheels. The force the driver feels can be calculated by Inventor and is based on the geometry of the kinematic linkages of the steering system.
Inventor geometry helped validate the clearances of the steering system
Results and Performance
The performance results that this team achieved are impressive. While these cars are designed for ultra-mileage competitions and don’t have to reach the same standard of rider comfort, speed, and safety of production vehicles – perhaps these young engineers will use what they’ve learned to help improve the energy efficiency of our everyday modes of transportation.
- Shell Eco-Marathon 2011: 2nd place
- Shell Eco-Marathon 2012: 4th place
- Honda Eco Mileage Challenge China 2011: 1st place
- Weight: 55 kg
- Average speed: 31km/h
- Air resistance: 3.5N
- Coefficient of drag: 0.143
- Rolling resistance: 0.81N
- Energy Efficiency: 347km/kwh
- Distance: 11.2km in less than 23 min
The Z112 – A sister gasoline car
Tongji University continues to develop ultra-mileage vehicles and applied their learnings from the Z111 to the Z112, a gasoline powered car developed by many of the same students. With the Z112, the team focused on improvements to the aerodynamics and steering. The result is a car with a coefficient of drag of 0.13 that gets 1,762 km/liter (4,100 MPG).
At the 2011 China Honda Eco Mileage competition, this car won championships for all the categories in University Group, as well as the Most Outstanding Performance Award.
- Gas powered: 1,762 km/liter (4,100 MPG)
- Modal analysis used to reduce unwanted resonance from engine and road
- Coefficient of drag: 0.13
- Reduced the weight of the car by 0.6kg compared to Z111
- Steering System optimization – dynamic simulation prevents interference with the shell or frame, and improves driver comfort and visibility.