XC60 Hybrid MPG on Brake or Drive selection

[Dav555]

Hi all,

just wondering if anyone has any real world experience of the MPG when selecting "Brake" position in the automatic box.

The manual states that it will brake and therefore regenerate more electricity when you release the accelerator.

I think the Tesla has a similar function from the options.

Thanks

[I-S]

I don't have experience of it in the volvo hybrids, so apologies if this isn't fully relevant.

In my Leaf on my 12.5 mile commute I regen about 20%, and barely use the friction brakes. This is the key - to try to use regen instead of friction.

A quick trip back to school, Kinetic Energy = 1/2 x m x v^2. In other words, the kinetic energy of the vehicle is directly proportional to the speed squared. This is important - at 10mph then v^2 is 100. At 60mph then v^2 is 3600 - ie the vehicle has 36x as much energy.

The point here is that regen can capture a lot of energy at speed, but not much when slow. I'm not sure how the volvos behave, but my leaf has barely any regen below 10mph.

Regen is not entirely efficient, of course. Brakes are very wasteful of energy, but you need to use these things in the right way. When cruising along a motorway at constant speed, the most efficient way to go (hybrid or EV) is to drive with no regen at all, and if you need to slow slightly then just lift off and let the car slow naturally.

If you need to slow more than that, or coming off up a sliproad or approaching a roundabout, then let regen do as much as it can to slow the vehicle,

Friction brakes only to stop from low speed or emergency use - in other words, if stopping from 60mph then let regen take the 3500 units of energy down to 10mph, and then brake from there. This, of course, requires reading the road ahead to slow down in advance.

Putting a few more numbers to this - Regen itself is around 70-80% efficient in returning kinetic energy to the battery, so of our 3500 units we get around 2600 back into the battery (these are arbitrary units, but they show the proportionality of energy). Going from the battery to the wheels is around 90% efficient, so we get about 2350 of our original 3600 back as kinetic energy when we accelerate.

Compare to a non-hybrid where not only is the entire 3600 lost as heat in the brakes (and yes, of course you can mitigate this somewhat by lifting off sooner and letting the car slow naturally), but then you must burn around 10000 units worth of petrol (approx 25% efficient engine/drivetrain) to get back what the hybrid regen can give you.

Ultimately it comes down to use case. If you sit for 3 hours at constant speed on a motorway a hybrid doesn't help you - you're merely carrying extra weight. If you are stuck in start/stop traffic then it does help, particularly if you can remain in full EV mode.

Use the transmission brake mode to gather up as much energy that you would otherwise lose to braking as possible. Do not use it to try to charge the battery from petrol (as this is very inefficient and very much more expensive than charging the battery from the mains).

[Dav555]

Many thanks for the School Lesson!

I think my question was more about the regen mode in the volvo.

The gear selection has two drive modes - D and B (Brake)

This I think is designed to regenerate more, but I want to understand if it just regenerates on braking rather than on normal driving.

So will my MPG be impacted negatively?

Any help advice welcome.

I can give it a go to find out!

[I-S]

D only regenerates when on pressing the brake pedal, B will regen when you back your foot off the throttle.

If you use that to slow the vehicle instead of the friction brakes (and you don't know how hard pressing on the brake pedal brings in the friction brakes in addition to the regen) then you will get better mpg overall.

If travelling at a constant speed on motorway, it will make no difference.

A good use case scenario is if you were to drive down a hill, use B mode to keep the speed of the vehicle down instead of brakes, returning energy to the battery to use later. This will give you better mpg than if you press the brakes hard enough to go beyond regen in D.