Regenerative braking: how it works

In my imperfect understanding, heat generated in braking is converted into electrical energy. This energy is fed into the battery, which reduces the demand on the alternator. The reduced effort from the alternator leads to better fuel consumption. Does that help?

In the A3:

During coasting or braking, the power management temporarily increases the charge voltage of the alternator to boost the battery charge (assuming it needs it), then during the subsequent acceleration phase, it draws its current needs back out of the battery, temporarily de-coupling the alternator of its duties.

They reckon they achieve 3% better fuel economy with this system (It's an AGM battery). I wouldn't worry about thinking whether you should brake and clutch or brake and not clutch - its not a mechanical regenerative system as such.

veeeight said:

its not a mechanical regenerative system as such.

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Where on earth would the energy come from if it weren't mechanical and taking the energy from the drive train under braking/deceleration? Newtons first law of thermodynamics

On coasting when the engine is on idle and not driving the drive train it is still mechanical as the alternator still harnesses mechanical energy from the engine. Although the engine would need more fuel to drive the alternator at a high voltage this would be a more efficient way of converting fuel into energy.
Does explain why the future is electric driven cars with petrol generators used only to charge batteries.

Leads to the question is the car harnessing more energy under braking or coasting?
I would suspect under braking as there's a lot more energy to be harvested, so my answer would be leave the clutch in and engine brake.

Sigh.

I meant, it's not a mechanical regenerative system akin to the F1 mechanical regenerative system (KERS). There is no mechanical energy store, or MGU.

If you wish to split hairs, by all means carry on, but I don't

wideboybloke said:

In my imperfect understanding

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Imperfect it truly is!
Heat into electrical energy still requires mechanical e.g. driving an alternator.
Most energy inefficiency is through heat loss at that's difficult to harness. Sound is the other.

Martin L said:

Where on earth would the energy come from if it weren't mechanical and taking the energy from the drive train under braking/deceleration? Newtons first law of thermodynamics

On coasting when the engine is on idle and not driving the drive train it is still mechanical as the alternator still harnesses mechanical energy from the engine. Although the engine would need more fuel to drive the alternator at a high voltage this would be a more efficient way of converting fuel into energy.
Does explain why the future is electric driven cars with petrol generators used only to charge batteries.

Leads to the question is the car harnessing more energy under braking or coasting?
I would suspect under braking as there's a lot more energy to be harvested, so my answer would be leave the clutch in and engine brake.

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I feel we're heading into pedantry territory here but I'd argue that you're not harvesting the energy mechanically.

You're harvesting mechanical energy, electromagnetically through the drag in the alternator. If you had it hooked up to a flywheel it'd be mechanical recovery.

veeeight said:

If you wish to split hairs, by all means carry on, but I don't

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Ok you mean flywheel storage
Semantics, wonderful thing that always starts a debate

Martin L said:

Ok you mean flywheel storage
Semantics, wonderful thing that always starts a debate

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Yes, a truly mechanical recovery system

Agree with leaving the clutch engaged - otherwise you are just putting load on the engine during idle to charge the battery = use more fuel.

The alternator could only take a small amount of that energy during braking. Kinetic energy of a car at speed is significant. If it took it all then you wouldn't really need brakes, but a massive battery to take that load and chemical heat without exploding.

Now just as simple question:
Does the brand of brake pads installed has any influence on the Regenerative Braking ?
If I install brake pads that are not from Audi (Ferodo), do I still have Regenerative Braking working on my Audi A3 Sportback 1.6TDi (2010) ?

gpecurto said:

Now just as simple question:
Does the brand of brake pads installed has any influence on the Regenerative Braking ?
If I install brake pads that are not from Audi (Ferodo), do I still have Regenerative Braking working on my Audi A3 Sportback 1.6TDi (2010) ?

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This threads done my head right in lol

Newton didn't really do thermodynamics, although his law of cooling is very useful when working out how much ice you need to keep your beer cold over a Le Mans weekend. Newtons speciality was motion, optics and mathematics, none of which are required to explain this system, so hair splitters can relax...

The simple version of what's going on is that some bright spark at VAG ( or rather their electrical systems suppliers) built upon the thinking that's been going on around parasitic engine loads for decades. First they decided that it might be good for peak power if they linked the sender indicating wide open throttle to a system that turned off the air con compressor and alternator charging circuit to make sure that all the engines power went to the wheels when you hit the loud pedal. Next they realised that when you close the throttle and are engine braking, with the engine linked to the road wheels, then the engine required no fuel at all to keep turning since you were turning it through the gearbox. This was an opportunity to win a few points on the eco-tests, so modern cars turn off the injectors in these situations. Now they've worked out that instead of charging the battery all the time (except on wide open throttle) using petrol, they can tweak the excitation voltage on the alternator at certain times so it takes more load when you're braking, and aren't using the engine to power to drive the wheels. Under these circumstances, the motive force driving the alternator comes from the road wheels, which spin the engine (and therefore the alternator) as you are slowing down.

Hopefully then it's clear that this will only ever work when you're in gear, clutch engaged. Braking like this will recover a tiny amount of extra charge from the "waste' energy, and will use no fuel. If you dip the clutch you'll use actually use more fuel, and the alternator recovers nothing.

It should also be mentioned that it's considered bad practise to disengage the clutch and coast the vehicle. Although this ideal was born in an age of drum brakes and crossply tyres, it's still applicable today. You have better control, you can get back on the power much quicker, and these days, it saves you fuel.

Depending on which view you have the virtual cockpit set to, a wee green segment on the mpg bar tells you when regenerative braking is active (segment to the right of the max marker lights up green in the pic)...

Lenny d said:

This threads done my head right in lol

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The reason I asked that is because of the explanation that regenerative breaking states that "energy wasted as heat" is used to charge the battery made me assume that some kind of conductive properties were needed in the brakes, though I thought it worked using inertia.

And I know from other experiences that if we don't use original parts some features are simply lost when it seems that everything is ok.
I was considering changing my battery when started looking that needs to be a battery fitted for Start/Stop and regenerative breaking.
I already regret not having installed Audi original brake pads so I was trying to know if that affected something else.

Gordo77 said:

Depending on which view you have the virtual cockpit set to, a wee green segment on the mpg bar tells you when regenerative braking is active (segment to the right of the max marker lights up green in the pic)...

View attachment 101255

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My A3 is not like that.
Is from 2010, 8PA

gpecurto said:

The reason I asked that is because of the explanation that regenerative breaking states that "energy wasted as heat" is used to charge the battery made me assume that some kind of conductive properties were needed in the brakes.
And I know from other experiences that if we don't use original parts some features are simply lost when it seems that everything is ok.
I was considering changing my battery when started looking that needs to be a battery fitted for Start/Stop and regenerative breaking.
I already regret not having installed Audi original brake pads so I was trying to know if that affected something else.

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The type of pads is unrelated to such a system. What's happening is that when you are off throttle and braking, a very small amout of the work done in slowing the car is now being taken on by the alternator. It's relatively tiny of course, but it is there.

Consider this very simplistic explanation. If you turn off the regen system and brake down to a stop with clutch disengaged, your brakes provide 100% of the work needed to stop the car. Slowing with the clutch engaged, the engine might carry out 5% of the work in pumping losses, so the brakes get a slightly easier time of it, doing only 95% of the work. This is the first law of thermodynamics in action.

With the regen system, the alternator will load up when you brake to charge the battery. At this point the alternator becomes much harder to turn and so it has a braking effect on the engine, and since you're in gear and the clutch is engaged it slows the whole car. Let's say it's a big alternator and so it takes on 5% of the work needed to stop the car. You now have the same 5% engine braking, plus an extra 5% regenerative braking. Suddenly the brakes have to do only 90% of the work. The car still stops the same, from the same speed, in the same distance, but you've used 5% of the energy you'd otherwise have used to heat the air around your wheels to charge your battery instead. This is the "energy wasted as heat" in the advertising blurb.

As such, there's really no specific reason why you can't use different pads. The system is unrelated to pad material.

Lenny d said:

This threads done my head right in lol

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LOL me too.
OK, simple summary.
Hamster wheel, and a dynamo.
Both very boring and simple things no matter how much you sugar coat it.