Challenges of manufacturing Formula One energy recovery systems
Changes in the Formula One regulations and developments in technologies often push manufacturing to its very limits. Not only is manufacturing generally near to, or at the end of, the development process it has to deliver right first time, on time – and, in motorsport, in very little time. The introduction of kinetic energy recovery systems (KERS) in 2009 combined with the current turbo era has certainly introduced new challenges and pushed ‘green’ technology far beyond the milk float.
KERS systems were originally split into mechanical (flywheel) and electrical (battery) types, harvesting energy during braking to be released subsequently for shorts boosts in performance. Flywheel technology works by using braking forces to spin a mass (5 kg) at very high rotational velocities in excess of 60,000 rpm, and to improve efficiency the flywheel is housed within a vacuum. This creates sealing challenges, increasing the complexity of the manufacturing and assembly techniques to produce and build it.
Modern Formula One engine turbos spin at even higher rates ( about 100,000 rpm) and both these and the flywheels demand incredibly high tolerances of manufacture. Bearing alignment and cooling are critical at these angular velocities, and when combined with high operating temperatures, centripetal acceleration, incredibly tight packaging and light weight there is little to no margin for error. To this end. teams have been specifying a concentricity of 1 micron (0.001 mm) on the turbo shafts to ensure alignment for seals and oil flingers, and reduce radial loading on the bearing and shaft vibration.
It is also critical that all rotating components are dynamically balanced, with this process having to be repeated at every sub-assembly level. Accuracy of components not only has an impact on assembly but also the speeds at which these systems can operate and therefore their ultimate performance. With reliability so important in Formula One components, quality is paramount, and given the amount of energy in a spinning flywheel or turbine wheel the result of a bearing failure or clash can be catastrophic.
The teams on this season’s grid run an energy recovery system (ERS), an evolution of the KERS, in combination with the turbocharged V6 engines. The ERS stores energy generated under braking in (lithium) batteries, and works with two motor generator units (MGUs) to convert mechanical and heat energy into electrical energy.
The first MGU, MGU-Kinetic, uses the car’s kinetic energy to ‘charge’ the batteries, which can then in turn power the MGU-K to drive the car via the crankshaft. The second generator, MGU-Heat, is connected to the turbo and converts heat energy from the exhaust gases into electrical energy, which is again stored in the batteries and can be used to power the car through the MGU-K as well as spooling the turbo to reduce lag.
For both generators, and the complex system they form, efficiencies and performance are related to the precision of the components and assemblies. However, there are further challenges with the manufacture and assembly of the batteries themselves.
First there are the United Nations Transport of Dangerous Good regulations that make recommendations for the transportation of lithium batteries. These guidelines are enforced to reduce the risk when shipping these high energy-density cells, which is relevant due to the air freight requirements of Formula One’s international race calendar. This results in the energy storage batteries being housed in Zylon fibre-reinforced epoxy composite cases with integrated busbars (connectors) co-cured into the laminated components. These casings have to pass ballistic tests to ensure they are safe to ship and race.
The next manufacturing challenge is the intricacy of the battery design. The batteries are made from multiple components in multiple materials with complex features including integrated cooling circuits, which are all critical to performance and safety. Creepage distance requires careful consideration and further complicates the design and subsequent manufacturing; it is not enough to consider the clearance distance between two conductive parts, measured through air. At high voltages a partially conducting path of localised surface deterioration of the insulating material can result in an electrical discharge between conducting components along the surface contours of a non-conducting part, a process known as tracking. This can be minimised by ensuring adequate creepage distance between these parts, but with available packaging space at a premium this is achieved by additional features that increase the creepage distance rather than simply moving conducting components physically further apart.
Finally there is the battery assembly process. The cells are assembled on bespoke trollies, in environmentally controlled areas usually outside of the main factory. The operators are earthed and wear rubber gloves, and all other engineers authorised into the build area are encouraged to keep their hands in their pockets to avoid the prospect of a hefty electric shock.
The reason for all these precautions is the potential for a short circuit in an ERS battery initiating a chain reaction through the cells leading to a ‘thermal event’. This reaction is exothermic and uncontrollable, and can generate enough heat to vaporise aluminium. The standard procedure to deal with an unstable battery is to move it, and hence the trolley and external workshop, as far away from buildings and people as quickly as possible!
All aspects of Formula One push the engineering boundaries in the quest for ultimate performance. ERS and turbo technology may just be pushing manufacturing that bit harder, the prize being additional performance if components can be produced and measured with greater accuracy.
Written by Dan Fleetcroft