Opinion on regenerating brake

There are two ways to disable regenerative braking in the SE:

1. Shift into neutral.
2. Hold the e-power needle (left gauge) in the narrow "ready" gap between accelerating and regenerating.

 

Driving at constant slow speeds is more efficient than either driving at constant fast speeds (due to greater air resistance) or stop-and-go traffic (because regen braking can't recover 100% of the energy required to accelerate). When the EPA rates EVs for highway vs city (stop-and-go) driving, city driving achieves the greater range numbers.

Edit: In contrast, my two gen-1 Honda Insight hybrids achieved better gas mileage on the highway than in the city in EPA testing. However, when a team drove a gen-1 Honda Insight at an average of 18 mph, they were able to achieve 143 mpg and go 2,254.4 miles on a single tankful of gasoline. Can you imagine driving 2,254.4 miles at 18 mph? Not me.

 

I thought this might be annoying as well. Turns out the one-pedal driving is rather nice since your foot isn't constantly moving back and forth from the accelerator to the brake.

 

Driving at a constant speed is the most efficient way to travel. For any non-trivial journey, the energy used is then basically just your friction losses (air friction and rolling resistance) and the power needed to run the vehicle systems (hotel load). When you speed up/slow down, you lose energy as you need to supply the difference in kinetic energy (ie need energy to speed up), but don't get it all back when you slow down. If your trip takes the same time, then the constant speed will use less energy. If you speed up/slow down but have a (much) lower average speed, you can of course use less energy as your friction losses will be less. Rolling resistance energy is actually the same no matter the speed (force x distance, force = m.g.r), but air friction is proportional to v^2 (force is proportional to v^2, energy = force x distance).

So yes, a constant speed is the most efficient for a given time to complete the journey. In the real world, especially in town, this is of course not possible. When there is no option but to stop/start on your journey (eg. town/city/traffic lights), regen provides a big boost as you get some of the energy back when you slow down.

 

Then your son is correct and we have done him a disservice.

When you accelerate some energy is wasted as heat, in the battery itself, in the cables, in the inverter and in the motor to name a few. However cars like Tesla’s are currently around 95% efficient or somewhere thereabouts.
So you lose a little when accelerating but when you slow down using regen a portion is put back into the battery, but again some is lost to heat.
So accelerating up to 40mph (for example) and staying at that speed for 5 miles then coming to a stop uses less energy than doing the same but slowing down and speeding back up several times in that 5 miles.

The same is true of gasoline vehicles however they suffer more in stop go traffic as they cannot recreate gasoline when they slow down, they just waste the energy as pure heat via the brakes.

In addition the slower you drive the better as the energy needed to push you through the air increases exponentially as speed increases linearly. Ie it takes a lot more than double the amount of energy to push you through the air at 80mph vs 40mph.

Both the above are why electric cars get better efficiency/range in city driving vs highway driving, whereas a gasoline car is the exact opposite.

 

For gasoline cars, sure. But a car going a constant sub-freeway speed will invariably be more efficient than the same car going a constant freeway speed, so the son’s premise there is incorrect as well.

Those annoying hypermilers demonstrate it well, when driving routes that require stopping, they coast as much as safely possible, and accelerate as slowly as a funeral cortège. I sail past these types on my daily commute along a six-lane boulevard with a light every 250 yards, as I’ve figured out how to be more efficient than them: it’s a posted 50 km/h (31 mph) zone, but if I quickly accelerate to 60 (36) from the light at the off-ramp and keep it there, I get every green for 3.2 km (2 miles). They end up stuck at most of the lights, wasting both time and gas.

 

My buddy lives about 50 miles away and used to bug me about why I’d use the back highways to get to his place instead of the freeway, which is undoubtedly quicker overall. The freeway route is around three miles shorter, and traffic flows at around 70. On the back roads, I cruise at 55, and there are a couple of crossroads at which I have to stop. I showed him my diesel smart’s Bordcomputer for both: 4.4 l/100 km (53 mpg) on the freeway, but just 3.2 l/100 km (73 mpg US) the back way. I’d use 0.89 gallons via the shorter, faster freeway route, but just 0.68 gallons the back way. It took me maybe ten minutes longer overall.

 

The EPA says 33.7 kWh is the energy equivalent of a gallon of gasoline. So I'm astounded that the 28.9 kWh available in a fully charged MINI Cooper SE equates to a mere 0.86 gallons of gasoline! My gen-1 Insight, the most fuel-efficient car ever sold in the US (EPA 70-mpg), could travel only about 60 miles on 0.86 gallons of gasoline (without annoying hypermiling).

I wonder how much regen braking figures into the EPA's test suite for EV range calculations?

 

Another major factor that makes BEVs better in stop-and-go traffic is there is no electricity used when the car is not moving (excluding HVAC and accessories), but gasoline engines burn fuel while idling.

 

You can coast in the SE...but instead of letting off the accelerator completely you can gradually lift your foot and it gradually goes from accelerating >coasting>slight regen slowing>max regen (completely off the pedal).

Takes like half a day to get used to it...most people find that they prefer it too (no going back and forth between brake and accelerator).

 

It is impressive.
It all changes when you factor in the gas station pump or EV charger to wheel energy figure though.
On the combined cycle, with engine, drivetrain and other losses you get at best 25% of the energy getting to the wheels, can be as low as 16% though.
An EV is between 86% and 92%.
That’s around 4.5x better so take your 0.86 gallons and multiply by 4.5 which is 2.7 gallons. Still makes for good mpg equivalent numbers even using that figure though!

 

Not meaning to be negative, but if you multiply by the efficiency quotient, you will get the same distance for similar cars (eg petrol vs EV MINIs) - by definition. The only difference is the efficiency of the car friction wise. By this I mean if the petrol MINI converts 20% of available energy to energy at the wheels, and the electric 90%, then the electric should go 90/20 = 4.5 times further per unit of energy. Multiplying the petrol cars mpg by 4.5 will get the same mpge as the EV. In the real world, the petrol will be ever so slightly better off (after multiplying by 4.5) as it is lighter, with roughly the same CoD.

Now comparing say a hummer and a MINI is a different story. I would expect the insight with its low CoD and weight to trounce the MINI when multiplying by the energy to wheels efficiency quotient. The entire point is that EVs make far better use of available energy

 

Darn, the EPA's 33.7 kWh equivalency to a gallon of gasoline seemed like such a simple basis for comparing EVs to ICEVs.

 

It is, just don't try to compare by multiplying be the overall efficiency, or you end up with the same figure The entire point is that with X energy available, either electricity, or petrol, a car can go Y distance. This is the total efficiency of the car. In petrol, it is normally mpg, and EV it is kwh/mile or km (either written as per 100 miles/km or Watts per mile/km). With 33.7kWh/gallon (US), you can make a direct comparison. A car which goes 50 miles/gallon uses 67.4kWh/100 miles (2 gallons at 33.7kWh/gallon). An EV which uses say 20kWh/100 miles will be over 3 times as efficient for the available energy. The figure for EVs is MPGe (miles per gallon equivalent). This shows that if we could convert the electricity to petrol (with no losses), then this is how efficient the petrol car would have to be.

This becomes interesting with hydrogen fuel cell (HFC) cars - the process is far less efficient than battery storage. To split water into hydrogen (and oxygen), then recombine it to generate electricity takes somewhere around 3 times more electricity than just storing in a battery and inverting it (with all the usual losses there). If a power station generates 100kWh, an EV will go 3 times further than the HFC car, or looking at it another way, 3 EVs could go the same distance as one HFC car (like for like car).

 

I'm curious if this factors in the weight of fuel? ICE vehicles get lighter as they drive due to the burning off of the fuel, whereas EV batteries don't change weight as they drain charge (in any meaningful way). If you assume 6 pounds per gallon for gasoline, a MINI has about a 15-gallon tank so at most the weight would change by less than 100 pounds. So maybe it doesn't matter.

 

What if I told you the reason that you don't feel a difference is because there isn't one ;-)

I've tested and found there is no application of friction brakes during regen braking even at 100%soc.

The only time brakes are engaged is when the brake pedal is used, or when TC and or DSC kicks in. Turn off DSC all the way, and brakes are not used at all unless you apply them.

 

That's so surprising. So the motor itself is able to bring the car down to 0 mph? Where does the energy from regenerative braking dissipate to, if not the battery?
Did you aim a camera on the brake calipers?

 

No, I've been curious about this for quite some time as I could hear when I applied brakes, but not with Regen.

I literally unplugged at 100%, pulled out the garage, immediately went to about 55mph down my road and then just let off, full Regen to a stop, parked and checked the temps and touched the rotors. Regen does not use friction brakes.

DSC on the hand does, and that is why everyone is seeing brake dust, especially more so on the rear wheels.

Neat trick, wash your car, and then fully disable DSC for a week, boom no brake dust.

As far as where the energy goes I do not know, possible that do to the mini only using a certain percentage of battery vs true capacity it can actually take the extra charge. Not like your gonna over charge it with Regen...

 

That's interesting! I will have to try that. I drive my car like its a Mini Cooper and although I use high regen all of the time, I get brake dust, mainly on the rear wheels. I don't know what they use for stock brake pads, but it is more brown than black.