Power electronics – packaging considerations
Looking to the future, there is a strong trend emerging in the regulations of some of the major race series to incorporate new technology to make racing power units more efficient. The internal combustion engine is inherently inefficient, and although race engines are generally more efficient than their road-going counterparts – lower friction and higher average throttle openings being two good reasons here – they still throw a lot of energy away in the form of heat. We have heat rejection from water and oil which, owing to its relatively low energy content is difficult (although not impossible) to recover energy from. We have heat lost to atmosphere through hot exhaust gases, a small proportion of which engineers have been recovering for decades through the use of turbocharger, and we also have conversion to heat of the kinetic energy of the vehicle by braking.
We can recover significant quantities of braking and exhaust energy by using hybrid technology, of which there are several types used in racing. The most common is the electric hybrid, which is also the most common type found in passenger car applications. This type of hybrid system comprises three main components – one or more motor/generators, a battery and a power electronics module. In a conventional electric hybrid which recovers braking energy, the motor/generators are usually driven by the engine or transmission.
The question of where to site the battery and power electronics on the car or motorcycle is one which needs to be considered carefully; the choice is not simply one of weight distribution alone. As detailed in issue 67 of Race Engine Technology recently, when the kinetic energy recovery systems (KERS) were introduced to Formula One in 2009, Mercedes had distinct power electronics and battery modules, with one situated on either side of the car.
For 2010 there was no KERS in Formula One, but from 2011 onwards the systems have been used again. Mercedes chose to combine the power electronics and battery into a single module. If it is possible within the packaging constraints of the car to site this close to the centreline of the vehicle, the inertia relative to the roll axis of the car is reduced compared to the two-module approach. There may also be a marginal mass reduction owing to the more efficient ‘semi-detached’ module construction, eliminating one wall from the enclosures.
However, a more significant mass reduction and packaging advantage of the combined power electronics and battery module is the elimination of the cable linking the two units. Cables carrying high voltage and current are generally made of copper or aluminium, and require both electrical insulation and metallic shielding. The shielding is to eliminate electromagnetic interference (EMI); if the EMI shielding breaks down, changes in current in the cable affect the magnetic field in the cable’s vicinity and can have a significant effect on the operation of the other electronic systems; this may have a much wider effect than problems with the KERS itself.
So, amalgamating the power electronics and battery into a single module can have a positive effect on weight distribution, KERS system mass and reliability. Reducing the number of physical connections between the systems is also desirable – there is less to go wrong and there are two fewer connections that the KERS technician must make when installing the system.
There are some disadvantages though to the single-box approach. In effect, we need to have more hardware available, as a faulty battery or power electronics module in a combined unit cannot easily be replaced in the field. If a battery is suspected of being faulty, the power electronics are removed by default as they are housed in the same box, and vice versa. Batteries are also classed as hazardous freight, so it becomes difficult to get a faulty power electronics module back to the factory quickly for investigation if there is a problem.
Written by Wayne Ward