A friend has bought an Audi A6 which he said had "regenerative" braking. Well, a bit of googling suggested that this wasn't quite accurate and Audi call it "recuperation".
I'm struggling though to work out quite how it works. With no hybrid style traction motors all it can do is possibly use the drag from the alternator to generate a little bit extra during braking but that's going to be dependant on there being excess storage capacity to "soak up" this kinetic energy. This then raises the question of how the braking system works. Is it " brake by wire" and uses the alternator drag to slow the engine/vehicle more during gentle braking? If not, how are they generating and storing energy?
What happens if you're "old school" and use the gears when slowing? Do you lose all the benefits?
Seems more like marketing hype than anything else but maybe I'm being unduly sceptical. Does it make sense to anyone out there?
It has nothing much to do with assisting the vehicle to stop.
All that happens is the ECU already knows when the brakes are being applied, and also controls the alternator output. Some software ups the output of the alternator when braking, and lowers it when off the brakes. The averaged charge amount keeps the battery fully charged.
If you slowed the car using lower gears without touching the brake pedal, then the benefit would indeed be lost, but it's not really considered the correct way to drive any more.
It's not that clever, and BMW have been doing it for a while.
I was told that some cars can reduce the alternator output hence engine load when a lot (?) of engine power is required (high acceleration)? So is this idea simply that in reverse?
It is not 'real' regenerative braking as used in the traction motor sense, as used on electric fork lift trucks, for example.
With BMWs, when the car is 'off' throttle ( during overrun), then the alternator is switched on whether braking or not, so no, the effect is
*not* lost. If the lights are on, then the alternator is on all the time. The additional drag of the alternator can be felt as the alternator turns on. The alternator output of both our BMWs is around
160 amps.
If the battery is in a low state of charge, then the alternator is on until it the 'Intelligent Battery Sensor' instructs the ECU to turn it off. The same system was used on the Pug 308 we had too. It had a (BMW) Mini engine.
I often have a digital voltmeter attached to the car to monitor the alternator. On one of the cars, it can be disconcerting to go for miles and miles along the motorway and seeing the voltage fall to around 12.1v before the alternator tops up the battery. On overrun, the voltage goes up to approx.14.6v, whereas, if driving with sidelights on, then the voltage sits at approx. 13.2v, full lights takes it up to approx 14.6v.
Of course, the battery is not the old straight lead-acid type. It has to cope with large charge currents.
Energy recuperation Energy recuperation (from Latin "recuperare" = recover, regain) generally refers to the utilisation of kinetic energy when the vehicle is slowing down. That means that when the vehicle is braking or the engine is overrunning, the "free" energy produced is recovered and stored in the vehicle battery.
Power transmission mode: battery is discharged When the engine is transmitting power to the drivetrain, the alternator output voltage is lowered to a level below the battery voltage (12.5 V) and the alternator's current output drops. That reduces the load on the engine so that fuel consumption and, consequently, CO2 emission are reduced. During this phase, the battery takes over power supply to the electrical system. On Audi A8 ?10 models with extended energy recuperation (12.2 volts) an AGM battery is usually fitted, see page 4.
Method of operation The energy recuperation function is an essential part of electrical energy management by the databus diagnosis interface J533. Two different operating modes are distinguished in energy recuperation
Overrunning mode: battery is charged As opposed to the drive transmission phases, in the engine overrun phases the alternator voltage is raised again and thus the battery recharged.
The water-cooled alternator was first used on the 6.0l W12 engine. The A8 ?10 sees the introduction of the second generation with substantially improved efficiency (for reducing fuel consumption) and a rated output of 210 A. As no fan is required on the water-cooled alternator, it is very quiet in operation. Another advantage becomes evident during energy recuperation.
Although an air-cooled alternator is relieved of the electrical load during energy recuperation, it continues to use power from the engine in order to overcome the air resistance acting on the alternator fan. This is not the case with a water-cooled alternator as it has no fan.
Alternator fans are tiny, so the amount of power to drive them is trivial. Also, there will be more load on the water pump (and maybe a bigger pump needed) in order to circulate the water through the alternator.
Thanks for the extra info BTW.
Thinks: I wonder if anyone will come up with a variable speed drive (as opposed to a gearbox as some models already use)? Something like an eddy- current coupling could reduce the load to virtually zero when charge is not needed.
But, this first part of the first sentence, "Although an air-cooled alternator is relieved of the electrical load during energy recuperation,"
I'm struggling to understand how an alternator can be relieved of the electrical load during energy recuperation whilst it is generating current to charge the battery during recuperation... Have I read it wrong ???
I think some BMW alternators have a clutch on the the drive pulley, therefore less drag to drive a fan. Although Chris is correct, the energy lost to the fan is not much in terms of an Audi A8.
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