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The KERS of power

kersTelevision coverage of this years Formula One races has captured some textbook examples of a KERS equipped driver out accelerating a conventionally powered car on the straight. And yet, as mid-season approaches no KERS equipped car has won a Formula One race, or set pole position, and only two teams are still running with it, the rest having either rejected its use or never planned to run it in the first place

The 2009 Formula One regulations restrict the energy available from KERS to 400 kJ per lap – which equates to roughly 80 bhp being available for a total of just under seven seconds. The total weight of the current systems appears be range around 30 kg and as it is well known that Formula One cars carry at least this much weight as ballast, so in theory it is possible to ‘build in’ an 80 bhp ‘boost button’ for no weight penalty. Sounds like a good deal…

kers big


80 bhp equates to a little over 10% peak power, which is significant, and we can get a very good feel for the actual benefit by watching the TV footage. With the 400 kJ limit, in practice it is only of real benefit on a reasonably long straight, and it is important to consider the effect under braking. Invariably the increased acceleration is used in a passing manoeuvre, but if this also results in a higher terminal velocity, and if the two cars carry similar downforce and drag and weigh the same, then the KERS car will have to brake earlier.
Although in theory the KERS car should still be ahead at the corner apex, in practice this is not always the case. Whereas activating the ‘press to pass’ button doesn’t make a great call on driver skill, modulating braking certainly does, and we have seen several instances where the non KERS car slips back through again either under braking, or on corner exit as the KERS car scrabbles around the corner off the ideal line.

There may be an additional factor in this. Again, in theory there is an advantage to be gained with KERS under braking, where charging the motor results in a degree of regenerative braking effect. However, it might be better to characterise this as a boost in engine braking, rather than mechanical wheel braking, as it will not be applied in the same proportion between front and rear wheels as when pressing the brake pedal. Although a sophisticated control loop system would be capable of producing such an effect, it is not allowed by regulation, and the actual effect will vary depending on the ‘harvesting’ control algorithms. Above all, a driver needs to feel absolute confidence in the consistency of the brake system to make maximum utilisation of it, and if there is an unpredictable outside influence on this he is likely to be cautious.

Turning now to the 30 kg system weight, it is obvious that it will raise the overall centre of gravity. This may be by perhaps 10 mm or 4-5%, but it will also decrease the amount of front axle weight by perhaps 1.5%-2%. These figures sound small, particularly the front axle weight reduction, but they run against current tyre trends and also put a lower limit on the amount of front downforce that can be used and retain a balance in the car. The systems also require additional cooling, in much the same way that an engine giving an additional 80 bhp would require larger radiators. So although there may not be an overall weight penalty, there might well be issues in transient conditions and aero losses.

We can begin to see that although there are obvious advantages, their practical implementation relies on maintaining a balance and driveability in the car sufficient for the driver to fully exploit its theoretical limits.

Many years ago in Formula One, there were strong theoretical arguments for using V12 engines , or even H16 engines rather than V8s. And yet time and again the V8 cars would be in front of the V12s – not because they were always lighter, but because they were better balanced. Keep it light, keep it low, keep it simple…

This is not to decry the concept of KERS for racing cars, rather it is a case of there needing to be a higher proportionate energy discharge relative to engine power, and the implementation of more sophisticated control logic, and in the case of the braking system, control valves before there is unquestionable reason for running with it.

Written by Peter Elleray.

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