KERS anatomy

With KERS being revived and expected to race again next year, let’s just recap what hardware’s involved and how its packaging affects the car design for 2011.

KERS (Kinetic Energy Recovery System) is a hybrid drive system that the FIA allowed to be raced as part of the 2009 major rules rewrite. It allows energy to be harvested under braking and stored, then that energy can be released to provide a power boost for around 6s per lap.
In 2009 most engine manufacturers developed their own KERS system, while Williams were the sole team developing a system independently. Albeit not every team raced with KERS and some teams dropped KERS at various races. For 2010 FOTA agreed to drop KERS, albeit it was still legal with in the rules, as a cost cutting measure it was best not to run or develop KERS any further. Even within 2009 season KERS was not a huge success, the system had a FIA cap on the amount of energy that could be re-used, only 400kJ could be stored, which when used for 6.7s per lap, the car gained some 80hp. Thus although a 0.3s boost to laptimes, the system was ultimately limited in its potential to improve laptimes. Thus no team could create a competitive advantage from a more powerful system. Then the weight of the system created issues, At a time when the wider front slick tyres demanded an extreme weight distribution of up to 49% weight on the front axle, the 25+Kg of a KERS system mounted behind the CofG handicapped teams being able to push weight forwards. Most teams dropping or not racing their system cited weight as the main reason for its loss.

What is a KERS system?
In essence a KERS systems is simple, you need a component for generating the power, one for storing it and another to control it all. Thus KERS systems have three main components: The MGU, the PCU and the batteries. They are simply laid out as in the diagram below:

In detail

MGU (Motor Generator unit)

Marelli MGU as used by Ferrari and Renault

Mounted to the front of the engine, this is driven off a gear at the front of the crankshaft. Working in two modes, the MGU both creates the power for the batteries when the car is braking, then return the power from the batteries to add power directly to the engine, when the KERS button is deployed. Running high RPM and generating a significant Dc current the unit run very hot, so teams typically oil or water cool the MGU.

Batteries

McLaren Mercedes Battery Pack complete with water cooling system

During the 2009 season only electrical batteries were used, although at least two flywheel systems were in development, but unraced. We will focus on the arrays of lithium-ion batteries that were raced. Made up of around 40 individual cells, these batteries would last two races before being recycled. In McLaren’s case these were mounted to the floor in the sidepods beneath the radiators. Other teams mounted them in a false bottom to the fuel tank area for safety in the event of a crash. Being charged and discharged repeatedly during a lap, the batteries would run very hot and needed cooling, this mainly took the form of oil or water cooling, and again McLarens example had them pack water cooled with its own pump and radiator.

PCU (Power Control Unit)

McLaren Mercedes PCU

Typically mounted in the sidepod this black box of electronics served two purposes, firstly to invert & control the switching of current from the batteries to the MGU and secondly to monitor the status of the individual cells with the battery. Managing the battery is critical as the efficiency of a pack of Li-ion cells will drop if one cell starts to fail. A failing cell can overheat rapidly and cause safety issues. As with all KERS components the PCU needs cooling

Marelli prototype PCU

Ancillaries

Aside from these main components the KERS system also integrates with the FIA SECU in order to control and monitor the PCU. KERS has to be driver activated; this is achieved from a steering wheel button. Although the drive has to initiate the KERS boost, the teams set the system up such that the driver knows to engage the system out of specific corners, the system then delivers the predetermined amount of boost specific to the demands of that section of track. In practice the KERS systems is being charged and discharged to this preset map of activations. Which smoothes the balance between charging and discharging, so the system does not overcharge above the regulatory limit. Again the SECU ensures no more than the capped amount of energy is delivered each lap.

KERS in 2011
With KERS return to F1 next year, the designers are faced with the same operating requirements in terms of energy storage and discharge. But the packaging requirements of the systems have changed in the two years since its introduction. Firstly the penalty of weight distribution has eased. With narrower tyres and the move to a fixed weight distribution for the Pirelli tyres, means that 25kg KERS system no longer tip the weight balance the wrong way. Plus there will be a higher minimum weight limit for next year.
But challenging the designers will be the amount of space to package the hardware. With the ban on refuelling, teams have enlarged the fuel tank into the sidepods to create sufficient capacity, already the sidepods are full of longer narrower radiators and the gearbox oil coolers have been moved to above the gearbox to save space in the sidepods. Then the aerodynamically undercut shape of the sidepods robs yet more volume.
Given the success of McLarens sidepod mounted solution in 2009 and the safety concerns that dogged the systems introduction, means that teams will probably opt for sidepod mounting of the Batteries and PCU. Especially as to expand the fuel tank area to mount the batteries as in 2009 will create a huge bulky rear to the monocoque. There will no doubt be an aerodynamic penalty to the slightly bulkier sidepods to house the hardware and additional cooling. This needs to be less than 0.3s laptime penalty in order to offset the gain from the power boost. Even with the gains and losses in laptime with a KERS system, teams may opt to run the system simply to use it for an overtaking aid in the race. Off the line and onto long straight the boost might be enough to overtake a rival.

Hydraulic KERS

A filament wound carbon fibre Hydraulic Accumulator

A further alternative to the generation and storage of energy is to use hydraulics. This system has some limitations, but with the capped energy storage mandated within the rules the system could see a short term application. Separate to the cars other hydraulic systems, a hydraulic KERS would use a pump in place of the MGU and an accumulator in place of the batteries. Simple valving would route the fluid into the accumulator or to the pump to either generate or reapply the stored power. Hydraulic accumulators are already used in heavy industry to provide back up in the event of failure to conventional pumped systems.
Using filament wound carbon fibre casing, an accumulator of sufficient capacity could be made light enough to fit into the car (see http://www.ctgltd.co.uk/page/hydraulicaccumulators/47). They might be capped in terms of practical storage with in the confines of an F1 sized system, but McLaren had prepared just such an energy recovery system back on the late 90s, but it was banned before it could race. With the relatively low FIA cap on energy storage, just such a system could be easily packaged, the hydraulic MGU would be sited in the conventional front-of-engine position and the accumulator, given proper crash protection fitted to the sidepod\fuel tank area. Saving space would be minimal control system (equivalent to the PCU) as the valving to control the system could be controlled by the cars main electro hydraulic system. McLaren have recently been quoted as saying the 2011 KERS would be more hydraulic and less electronic. Giving rise to speculation that a hydraulic storage system could be used.

Flywheels

A Flybrid Flywheel system, similar to that intended for the 09 Honda F1 car

As Li-ion batteries are still an expensive emerging technology, plus they have associated risks, recycling and transport problems. The attraction of flywheel KERS is obvious, however no team have raced such a system in F1. Flywheels can effectively replace the Li-ion batteries with in a typical KERS system, the flywheel being mated to a second MGU to convert the power generated by the primary MGU on the engine into the kinetic to be stored in the flywheel. Williams are believed to have just such a system. However the simper flywheel solution is connect the flywheel system via a clutched and geared mechanism. HondaF1 had developed this solution for their 2010 car. This solution was dropped as Honda pulled out and the renamed Brawn team need to focus development resources into the new car and its conversion to Mercedes power. The Flybrid made system would have sat with in the fuel tank area coupled to the crankshaft. This created a system simpler system of equal weight to a typical KERS. The main components being the flybrid flywheel and Torotrak system, plus a relatively small ECU, no additional cooling would be required. Thus the system could be far easier packaged into the chassis robbing just 13l of fuel tank space. A proposal was made to the FIA for a supply of this system to every team on the grid as a cost cutting measure. It seems the FIA did not take up this offer. So it seems this technology may be resigned to lower Formulae or non Motorsport applications.

Update:

Honda F1 Electric KERS

Although they also had a flywheel system in place, it transpires Honda also had an electric system.  ITs described in some detail in this link.

http://www.greencarcongress.com/2010/11/hondas-f1-kers-motor-60-kw-21000-rpm-7-kg.html

Linked images copyright
Generic KERS diagram – Craig Scarborough ScarbsF1.com
McLaren KERS – Racecar-engineering.com
Marelli KERS Highpowermedia.com