The third and thus already second last review of the focus project e-Sling took place on 8 April in Dübendorf in the Innovation Park. The presentations were held in a hybrid format, so the supervisors and sponsors could participate in the review both online and on site and discuss directly with the students.
Almost traditionally, the aircraft module began. At the start of the new semester, this had defined four major milestones that have to be fulfilled at certain times. The aircraft will be ready to fly after reaching the four milestones. Review 3 of the aircraft module now focused on the goals of Milestone 1: Electromagnetic compatibility should be guaranteed, the battery management system must be fully programmed, the DC/DC converter must be ready for use and the cooling must be filled and ready for testing.
Milestone 1 with its four sub-goals ran like a red thread through the entire presentation.
Elsa Wrenger, the team leader, first introduced her subject area 'Mechanics & Wiring'. The focus was initially on the DC/DC converter, which was also discussed intensively in Review 2. Successful external tests could be carried out on the DC/DC converter. It is currently installed in the test stand - the entire cabling in the aircraft is already ready for the installation of the DC/DC converter. Then the propeller control unit and the CHAdeMO charging connector were discussed; there, too, the cabling has been completed and the installation of the components in the aircraft has been prepared.
In the next phase of the project, Elsa will deal with the following tasks: Adjustments made in the test stand will be adopted in the aircraft. The conversion to the aircraft is being prepared and a charging connection for the low-voltage battery is being implemented.
The subject area 'Mechanics & Approval' was presented next by Joël Meyer. The DC/DC converter, which Elsa had already discussed, also has to be accommodated on the plane. Joël evaluated various concepts for mounting the converter and then presented the chosen solution. The DC/DC converter is now bolted directly to the aircraft's firewall, which separates the pilot's cabin from the engine compartment. This solution was also approved by the chief engineer of the aircraft manufacturer Sling Aircraft. Furthermore, a housing for the low-voltage battery was planned. This must be gas-tight and, in the event of battery outgassing, be able to conduct the gas out of the aircraft. Joël then presented the engine cover plates, which are now mounted on the engine, as well as the next steps to be taken in the approval process in connection with the authorities.
Jan Wallimann presented the 'Battery & Cooling' section. The batteries can now be better simulated with the help of a BATEMO model. Many new insights could be gained from the model, such as the temperature dependency of the cells, dependencies between power and voltage, and new insights into heat development.
The model for the State of Charge (SOC) of the batteries has also been improved. In addition to the battery voltage, the discharge current and the cell temperature are now also taken into account when calculating the SOC.
The cell voltages were always observed when tests were carried out on the drive train; it was possible to determine how a battery module charges and discharges significantly faster than the other modules. In this way, it was possible to conclude that there was a defect in the module and the corresponding module was replaced.
Finally, Jan filled the cooling system, which is now ready for the tests.
Rafael Sutter, responsible for 'Electronics', then began his presentation and immediately went into the electromagnetic compatibility in the system. The inverter emits electromagnetic waves and thus disrupts the important bus communication. This is a big problem that needs to be solved. The motor cover plates were installed, the shielding of the motor cables was applied on both sides and the current sensors were removed from the battery. All these measures led to good electromagnetic compatibility.
Rafael then presented the current status of the tests made. 20-minute tests with 55 kW power have already been carried out and the system remained thermally stable.
Patrick Benito completed the aircraft module with the 'Software' section. He first presented the innovations that have been made to the battery management system (BMS). The main task of the BMS is to guarantee protection against overvoltage, overcurrent and overtemperature. In addition, the battery life should be optimized. Balancing has now been implemented in the BMS. In this way, voltage differences that reduce the capacity of the batteries can be eliminated. Because charging and discharging is limited by the minimum and maximum voltage. Now 171 individual parallel circuits can be balanced via CAN. Flashing the BMS was not possible due to the installation in the wing battery. A BMS bootloader was created, which now allows variables to be programmed via CAN.
At the end of the presentation of the aircraft module, the four goals of Milestone 1 were shown again. A small tick has now appeared behind all of these targets, leaving only 3 more mileages until the aircraft is ready to fly. A lively discussion was then held with all guests of the review - it took over an hour until all questions from the audience could be clarified. After the discussion, it was the turn of the hydrogen module to show their progress in the project.
Robin Feuz started straight away with the 'Software, sensors and low-voltage system' area. After a comprehensive requirement profile was created for each of the required sensors, the right sensors could finally be found. The signals from the 20 sensors can be recorded via a TTControl control unit and then bundled and transmitted to the electronic control unit (ECU). The hardware of the ECU was also fully developed and manufactured in the last project phase.
To supply all components in the system with the right voltage, a low-voltage system is being developed, which is powered by two voltage sources. With the RapidHarness software, an overview of the entire system could be created so that suitable connectors and cables can be easily found.
Sander Metting van Rijn then presented the areas of 'fuel cells, cathode cycle and anode cycle'. First, the in-house design of the four input boxes for the fuel cell was discussed. These direct the volume flows optimally into the fuel cell and enable the compact installation of sensors before and after the fuel cell. In the area of the cathode circuit, the tubing and piping made with silicone and aluminum was presented. The required valves could already be found and ordered. A simulation of the entire circuit was also created and can now be used for controller design. Furthermore, the entire compressor cooling was presented and explained, which forms the second smaller cooling circuit in the system in addition to the large fuel cell cooling circuit.
In the anode circuit, Sander went into more detail about the inlet distributor in front of the fuel cell and the piping.
Colin Jüni, responsible for the areas 'fuel cell, anode circuit, cooling circuit', then took the floor and closed the topic 'anode circuit' with explanations of the self-developed water separator after the fuel cell.
The focus was then on the cooling circuit. The connecting pieces developed in-house and the expansion tank were shown first, followed by the Hydac air cooler. The right valves were also found in the cooling circuit and a simulation for the controller design could be created.
Colin then explained the test concepts of the hydrogen module before the presentation turned into a discussion.
After the discussion, the review ended with a small aperitif in the hangar, during which the students were able to have interesting conversations with the supervisors and sponsors. The core team was happy about the numerous inputs from all sides and is now trying to implement them in the next project phase. The fourth and final review will take place on June 8th.
