Porsche Says One-Pedal Driving In EVs Isn't Efficient

[Tiekhan]

Interesting thread. In a year/9K miles of ownership I’ve always had regen on because I thought it was/is the best way to maximize range. Now I’m not so sure after reading this thread. I’ll try coasting for a week at least BUT it will be a major adjustment as I’ve gotten used to how the car drives and feels with regen on.

Porsche is right… the round trip from recuperation is more inefficient than coasting.

When power flows to the motors, it has to go through a DC/AC inverter, and during recuperation, there is a reverse AC/DC conversion. The motor itself also has losses. Suppose the motors are 98% efficient in either direction, and the inverters are 99% efficient. Then a round trip from battery to motor to kinetic energy and back is going to be 0.99 x 0.98 x 0.98 x 0.99 = 94%, so you lose about 6% energy compared to just coasting to avoid the round trip.

in actual practice the losses will be higher, so round trip efficiency will be lower.

 

[GileP]

Yes. Press and hold steering wheel button rather than just pressing it.

Press and hold? Great, thanks.

 

[Vercingetorix]

• Use your foot to "cover the brake" as you proceed through the intersection. By covering the brake pedal, you significantly reduce the reaction time necessary to respond to a hazard and also allow the vehicle to slow by removing your foot from the accelerator. The "covering the brake" technique can be used effectively in the following situations: (1) When driving next to parked cars, (2) when you see the brake lights of other cars, and (3) when approaching signal lights.

https://www.safemotorist.com/articles/ipde-method/

 

[JimBob]

Porsche is right… the round trip from recuperation is more inefficient than coasting.

When power flows to the motors, it has to go through a DC/AC inverter, and during recuperation, there is a reverse AC/DC conversion. The motor itself also has losses. Suppose the motors are 98% efficient in either direction, and the inverters are 99% efficient. Then a round trip from battery to motor to kinetic energy and back is going to be 0.99 x 0.98 x 0.98 x 0.99 = 94%, so you lose about 6% energy compared to just coasting to avoid the round trip.

in actual practice the losses will be higher, so round trip efficiency will be lower.

It's case dependent.

At highway speeds where you want to maintain a speed (preserve kinetic energy) then coasting would be better then going through all the conversions.

Now answer this. How is coasting is going to be more efficient in stop in and go traffic then either one pedal driving or blended braking? The literature indicates one pedal is slightly more efficient than blended. The bigger gains seem to come in safety.

 

[Tiekhan]

Now answer this. How is coasting is going to be more efficient in stop in and go traffic then either one pedal driving or blended braking? The literature indicates one pedal is slightly more efficient than blended. The bigger gains seem to come in safety.

I don’t see these as mutually exclusive. Just use the brake as in an ICE car. If you brake, you use recuperation. If you don‘t, you coast. Porsche’s blended brake system means that it always uses recuperation to slow the car, unless it can’t. So you never are less efficient than one pedal driving, but retains the potential to be more efficient.

 

[mystermykee]

In terms of kWh/distance (insert miles or kilometers here), one would think it would be better with regen on? You're putting energy back into the battery while engine braking up to a stoplight or into a corner instead of braking late? Or is more energy put back into the battery by coasting (regen off) and braking late?

I've always driven with regen on because I like the ICE engine braking aspect, but have been driving with regen off the past couple days after reading this thread lol. Please educate me!

 

[Tiekhan]

In terms of kWh/distance (insert miles or kilometers here), one would think it would be better with regen on? You're putting energy back into the battery while engine braking up to a stoplight or into a corner instead of braking late? Or is more energy put back into the battery by coasting (regen off) and braking late?

The conversion of kinetic energy to chemical energy (i.e. battery charge is not lossless. When functioning as a generator, no motor converts 100% of mechanical energy to electrical energy. This electrical energy is AC, so it then has to be converted to DC before it can be used to charge the battery. This conversion also has losses. Then, the battery itself doesn’t store all the energy that is sent to it (hence the heating of the pack).

Once you do get energy into the battery, when you want to use it, the conversion of this energy back to mechanical energy also incurs losses at each stage. The losses mean that there is never a time that recuperation will be more efficient than coasting, assuming that you don’t have to change your velocity.

If you are hypermiling, you’ll want to coast as much as possible to avoid all the losses. But, if you have to brake, recuperation will salvage what energy it can. By separating the braking/recuperation from the accelerator, Porsche is giving us control over when recuperation occurs, and how much to use.

It is helpful to remember that the brake in a Taycan is really mostly a recuperation pedal, unless your braking demand is so high that it has to blend in a physical brake.

 

[DougFrisk]

In terms of kWh/distance (insert miles or kilometers here), one would think it would be better with regen on? You're putting energy back into the battery while engine braking up to a stoplight or into a corner instead of braking late? Or is more energy put back into the battery by coasting (regen off) and braking late?

I've always driven with regen on because I like the ICE engine braking aspect, but have been driving with regen off the past couple days after reading this thread lol. Please educate me!

Regeneration is always on. Whenever you press the brake, most of the effort comes from regeneration.

The option in the PCM is perhaps mislabeled. Something like Throttle lift off behavior might be better with the options being: Coast (off), simulate engine braking (on), coast until approaching other vehicles (auto) might be clearer.

 

[TYKHAAAN]

Fully agree on the efficiency. Had this car for 2 years and about 38k miles and I’ve experimented a lot with the regen, auto regen and coasting and the coasting is by far the more efficient way for this car to drive. I like the regen/auto regen when going downhill, especially behind slow traffic with auto-regen, to gain good energy back in the car (Depending on the hill). Coasting is probably why i can manage such nice range numbers despite not having the best driving methods for energy efficiency so often (zooming out of those red lights!).

 

[mystermykee]

Regeneration is always on. Whenever you press the brake, most of the effort comes from regeneration.

The option in the PCM is perhaps mislabeled. Something like Throttle lift off behavior might be better with the options being: Coast (off), simulate engine braking (on), coast until approaching other vehicles (auto) might be clearer.

I know, I should have been more clear. I meant, recuperation on.

 

[Jhenson29]

I know, I should have been more clear. I meant, recuperation on.

That’s not any clearer. The user adjustable setting is overrun recuperation and the brake pedal is braking recuperation. It’s all recup (and all regen).

 

[Jhenson29]

It baffles me that we keep having these regen vs coasting threads. Yes, coasting is the most efficient. In all cases.

 

[Tiekhan]

It baffles me that we keep having these regen vs coasting threads. Yes, coasting is the most efficient. In all cases.

The confusion probably stems from poor labeling, as @DougFrisk points out, as well as a general misunderstanding about the conversion losses in any EV’s system. I mean, there are still people out there who think they need to top up blinker fluid!

 

[Jhenson29]

I mean, there are still people out there who think they need to top up blinker fluid!

Well, there’s people who think charging their car is filling it with electrons too so…

Personally, I think the confusion comes from hearing that recup recharges the battery and thinking that it’s desirable in and of itself as opposed to it only being desirable as an alternative to friction braking as is the case.

 

[whitex]

Well, there’s people who think charging their car is filling it with electrons too so…

Personally, I think the confusion comes from hearing that recup recharges the battery and thinking that it’s desirable in and of itself as opposed to it only being desirable as an alternative to friction braking as is the case.

There are people out there who think electric cars could have wind turbines on roofs which would charge the car while driving, but there is some conspiracy keeping this “free driving” from consumers.

You nailed it by the way - regen is just a form of energy efficient braking. It’s not 100% efficient, but way better than 0% with friction brakes. That said, I like some braking on my accelerator as it is a convenient way to keep constant speed in highways, especially with varying elevations.