Whereas most of the other fuel economy vehicles burn fuel in an ICE (Internal Combustion Engine), PAC-Car II uses a fuel cell to convert hydrogen into electricity to power its electrics motors. The main advantages are greater efficiency, much more silent operation and most importantly, zero emission but pure water as the only by-product
PAC-Car II is equipped with 3 wheels; the single rear wheel is powered and steered, and the front wheels have a camber angle of -8°.
This solution allows the reduction of the frontal surface area because the room needed to steer the wheels is not needed. Some experiments on a test bench have shown that this camber angle does not provide too much rolling resistance.
The test stand used to investigate the influence of the camber angle on the rolling resistance (August 2004)
From then, 5 iterations, combining CFD and windtunnel tests have been performed to reach a Cd close to 0.09. The shape of PAC-Car II is optimized for air flow of up to 15° from the longitudinal axis
Reducing weight is a constant preoccupation because it affects the rolling resistance and the torque required for upward gradients.Body Shell
PAC-Car II does not have a chassis as its body is self-supporting. FEM structure computations have allowed us to minimize the amount of material and to choose the best direction for the carbon fibre layers which form the shell of the car
In collaboration with our industrial partner Tribecraft AG we constructed new, ultralight endplates for the fuel cell stack.
The 20 membranes of the fuel cell stack of PAC-Car II need to be pressed together with a force of more than three tons. This force needs to be evenly distributed over the whole area of the membranes. In PAC-Car I the stack was held together by long screws and Aluminium plates with a thickness of 30mm.
The system of the new endplates is based on Tribecraft's D.bow patent. The PAC-Car II endplates use glass fiber belts to press the membranes together. Due to these belts the tension part of the D.bow is not needed anymore and the resulting "C.bow" Aluminium part is stressed only under compression.
To apply the 1.7 tons on each belt (the belt is just 0.8mm thick and 20mm wide!) a newly invented construction with sledges (pulled together with a screw) has been added on top of the Aluminium bows:
The new endplates weigh only 0.855 kg which is a weight reduction of 739% from the PAC-Car I endplates!
Powertrain and Control
The whole fuel cell system (including control and power electronics) has been designed at the ETH Zurich.
The fuel cell, a by-product of the PowerPac project, is of the PEM type (Proton Exchange Membrane) and benefits from an embedded auto humidifying area specially designed by PSI. The stack efficiency is exceptional, close to 70%.
All the fuel cell system auxiliaries have been chosen for their reliability, light weight and energy efficiency
PAC-Car II fuel cell system layout
In accordance with the race regulations, some auxiliaries are powered by battery
Control of the whole system is managed by a microcontroller main board
The transmission uses a gear pair. Its speed ratio depends on the track and on the target average speed. It can be “open” to reduce the drag torque, for instance when the vehicle is gliding downhill.
The use of the powertrain is optimized by working backwards from a model based on measured efficiency maps of the fuel cell system and the motors, and on the track topography. The approach used requires simulation tools developed on Matlab Simulink
The manufacturing of the shell has been entrusted to the best specialist in Switzerland: Esoro. Other metallic or composite material parts have mainly been manufactured at the ETH
Stefano and Nicolas are manufacturing the PAC-Car II wheels
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