On the other hand an electrical system will offer far more flexibility. An electrical motor has the ability to cope with the speed difference of gear changes or be directly connected to the output of the gearbox. However as that argument has been explained we will focus now on direct connection to the motor. By directly connecting a KERS motor / generator to the crankshaft of the engine, it has 100% of the rev range to operate with at any circuit, and if a team opts to omit the use of a gearing system KERS will have a better mechanical efficiency and also be lighter.
For example if the motor rotor is permanently connected directly to the crankshaft, then the rotor will not even need bearings to support it inside the motor stator windings, resulting in no mechanical losses from the KERS unit itself with the exception of brush contact. A permanent magnet DC machine offers the most stable and predictable system for use as a KERS platform. And in this example we’ll see how to make use of it as such while directly connected to the motor crankshaft. Coupled with the high energy densities that they are capable of, it would likely also lead to the most volume efficient solution as seen by the McLaren and Ferrari KERS Systems.
This being said, despite being easily predictable, DC motors look to have been bypassed for more flexible Induction motors. The above example also does not deal with extremes of energy density and subsequent KERS cooling, where by the motors are their own sealed entities. For the remainder of this section we will consider the DC motor for the purposes of this explanation.
By mounting the KERS motor ahead of the engine, the car itself can maintain a good balance and the electrical storage can be distributed around the floor of car and even under the fuel cell and furthermore distributed anywhere around the car providing the electrical connections can be maintained, with the batteries / capacitors used as a permanent / integral form of ballast. Furthermore by electrically opening the circuit to the KERS motor, the rotor itself is only negligibly effected by the motors magnetic field and as such is free to rotate, neither generating, nor motoring. By also adding mass to the engine itself there is also the possibility of increasing the engines inertia and hence torque, but this can only be achieved providing that the motor can maintain its angular acceleration. Under normal steady state conditions at most the KERS rotor will only slightly delay the spin up and spin down of the engines crankshaft. Below is a plot of a DC motor’s Torque curve along with a table showing the power and efficiency, throughout the entire rev range.
With respect to a DC motor, its characteristic is determined by a machine constant. Once this constant is known the motor can subsequently be controlled.
For a DC motor, its rotational speed is dictated by the Forward applied Voltage (Va). The Backward voltage (Vb) is the voltage that the motor is producing while the circuit is closed, and the rotor is in motion. The challenge for the KERS motor and the control system is to ensure that regardless of where the motor is in the rev range is adheres to the regulations and does not produce any more than 60kW of mechanical power.
At any time the rotational speed of the engine can be sampled, and as a result the backward voltage (Vb) can be calculated. In order to ensure that the motor is always adding torque to the drive train the forward voltage needs to be greater than the generated voltage. By being able to calculate the respective forward voltage to ‘lead’ the motor into spinning faster, we can thus determine the respective KERS torque, that this motor will feed directly into the engine itself. As such we can calculate and see where the rotor is barely moving, massive rotor currents lead to potentially car damaging torque levels, however current limiting circuits can control this to prevent damage to the cars drive train. Furthermore we also have the flexibility to vary the power not only to less than the maximum, but we can also steadily increase the power in proportion to the engine speed, thus leading to more manageable torque delivery in the drive train, something that has had a very large focus put on it since 2011. And thus leading to a longer time period of KERS use on the track per lap, while staying inside the rules set down by the FIA, as well as steadier KERS Braking; something that Lewis Hamilton experienced at Monaco earlier in 2009 on the approach to Mirabeau, When KERS fully charged, and then disengaged throwing the car’s brake balance during a time when the cars balance was shifting resulting in his crash.
Considering that we have looked at both designed thus far, there is also the work done by Williams Hybrid systems which has a motor but stores the energy in a magnetically loaded flywheel eliminating the need for batteries and leading to what would be possibly one of the most modular and innovative designs available. By eliminating the need for batteries, we overcome the challenges of providing the large voltages and currents required to assist a 740HP V8 engine. Considering that chemical batteries are too easily damaged by high discharge and recharge currents, and capacitors at this stage are still in their infancy as a mass storage media, there is much work that needs to be done to make smaller and lighter energy storage that will be able to last multiple race events and deliver consistent repetitive performance; something that in this case Formula 1 was inherently to be driving the development of!
Needless to say with the KERS ‘amnesty’ in 2010 this allowed teams to catch up and make developments away from the race track for the 2011 and 2012 seasons and once again push ahead with vital development of this technology which has now seen adoption in the automotive market, as well as maturity in Formula 1. For 2013 and onwards with ERS, not having this technology this year and having an understanding of it will hamper teams for many race seasons to come. That being said, with KERS in its last season in the current incarnation, I would expect teams, to focus on the issues of energy storage, reliability and packaging rather than outright motor development.This article is the fourth in a series where we look at the KERS systems in use from 2009 to 2013. Click to read part 1, part 2 and part 3. Text by Richard Ronc, Ronc Industries