Using Drive or L

[ashuri]

I can't remember if this has been discussed so here goes...

The other day I experimented with the "L" selection on the shifter, it did seem to me that I got "more" juice from this but I thought I read somewhere that overall, it is suggested for us (FFE) owners NOT to use the L selection.

Anyone have any experience or thoughts about this?

thanks


FFE Ignot Silver Owner 12/6/12

 

[jmueller065]

Here is what the owner's manual says about L:

L (Low)
• Provides maximum motor braking.
• Is not intended for use under extended or normal driving conditions
and results in less miles per charge.
• The transmission may be shifted into L (Low) at any vehicle speed.

Kind of odd that it says "less miles per charge"?

 

[WattsUp]

I think L is mainly intended to be used when there are external forces acting upon the car (e.g., coasting down a hill). It makes it easier to capture the "free" energy. Though you would be able to achieve the same result even staying in D by manually applying the same amount of motor braking at the foot pedal.

But, if the car is cruising along under its own power (e.g., on flat land), L may cause more occasions (due to human imperfection) where the car slows down more than the driver would otherwise want, causing the driver to then re-accelerate. Under such "typically imperfect" driving conditions, I think L (due to the inability of a human driver to perfectly maintain the desired speed) will have a tendency to cause the vehicle to more heavily "oscillate" between motor braking and acceleration, which is probably less efficient than having just stayed in D, which is more "forgiving" to human imperfection.

That said, I don't believe there would be any real difference between D and L if the desired speed could be maintained perfectly. In fact, a human driver might be able to get close to this level of perfection though systematic use of the cruise control. It would be an interesting experiment to drive exclusively in L for two whole charge cycles... one using "imperfect" speed control (foot pedal only), and then another using cruise control as much as possible... and see how range compares.

All this aside, someone else also mentioned that they believe that a "low gear" is simply required, because all vehicles are supposed to have some form of motor braking, not because a low gear is "better" or "worse" than anything else, or specifically beneficial for any one scenario. In an EV, a "low gear" (which is nothing more than aggressively-tuned motor braking) also just happens to be a convenient way to achieve strong and consistent regeneration down long hills.

 

[jmueller065]

All this aside, someone else also mentioned that they believe that a "low gear" is simply required, because all vehicles are supposed to have some form of motor braking

That is a very interesting point: The low gear is a legislated feature of the car not a marketing or engineering feature..

 

[DavidJohnson]

My manual has the following info in it on page 7. The only times I ever used low in a regular vehicle was engine braking down a long hill or sometimes if I was stuck in mud or snow. Seems like L just for engine braking down a hill now.

"Low: L (Low gear) is designed to mimic the enhanced engine braking available in conventional vehicles. L (Low gear) will produce a higher level of regenerative braking when your foot is off the accelerator pedal. This is normal and will not damage your vehicle."

 

[WattsUp]

While I think Ford made the right choices in creating a very user-friendly and "familiar" car in the FFE (with things like a normal-looking PRDL shifter), the way these concepts "map onto" the realities of an EV can lead to some misconceptions. (e.g. assuming that the L "gear" in the FFE had all the same ramifications as a low gear in an ICE vehicle)

You are correct in concluding that shifting into L when "stuck in mud" or going up a hill will make no difference. For all intents and purposes, most EVs (in fact, all that I know of) are "one gear" vehicles. That is, the motor is (more or less) directly connected to the wheels.

 

[mpt]

I've noticed another aspect of L that, IMO, is genius.

In L, if you're on a steep hill and release the accelerator, the car intelligently holds the speed. Not so in D.

I drive in D all the time.

wrt the thought that it's a mandated feature, I spoke to Ford during the 'beta' phase of the program, they said that the design of the car was intended to mimic, as closely as possible an old fashioned ICE car, so as not to freak out new owners. I won't go into that right now, it just makes me cross but, it seems that EVs that offer high levels of type-A regen (on the Accelerator pedal) like Tesla, BMW, etc. don't offer an L mode but cars that try to mimic old fashioned ICE with little type A regen and loads of type B regen do.

 

[dmen]

Coasting in L might lower miles per charge partly through a resulting driver-level inefficiency as suggested by Wattsup, but I don't think Ford engineers were considering that. Coasting in L, unless on a steep downhill, is less energy efficient than coasting in D for the same reasons that coasting in D is less efficient than coasting in N. The sooner motor resistance stops you, the shorter the distance you travel while slowing, and the further you must travel while not slowing. Travel while not slowing uses energy. The increased amount of energy you "regain" while slowing hard will never exceed the amount of energy you used to get closer to your slowing point. If you were able to regain more than it took, you'd be driving a perpetual motion machine.

Say you're driving from point A at a steady 55mph, and you want to stop at point D. You could either coast in neutral from point B and roll to a gradual stop at point D, or you could continue driving a bit further and begin coasting in drive from point C, coming to a stop at the same end point. (Of course no one should be coasting to a stop from 55mph in neutral while driving on public roads, nor in D without also using brakes at least gently for that matter, but the illustration is useful.) To figure out where those points are, you need to know your deceleration. From my driving on a level road, I see it takes 6 seconds to slow down 5mph when coasting in drive, and takes 17 seconds to slow down 5mph when coasting in neutral. Starting at 55mph, coasting in drive will stop you in 66 seconds, and coasting in neutral will stop you in 187 seconds. With these numbers, I calculate traveling 1.42 miles before you stop while coasting in neutral, vs 0.5 miles while coasting in drive. Coasting in drive therefore requires you to continue driving at 55mph for 0.92 miles further than you would coasting in neutral, using 220Wh more than you would coasting in neutral. But coasting in drive regenerates electricity, right? Yes, but it's certainly less than 220Wh regenerated. I can't get a precise number for how many Wh are regained when slowing in D, but I'm pretty sure it's less than 100 based on the behavior of the car's kWh counter- I've never seen it decrease more than 0.1 while slowing from 55mph to a 100% regen stop.
The same relationship would hold comparing drive coasting to low coasting, but I haven't played with low coasting enough to do the calculation.

 

[WindPowered]

The increased amount of energy you "regain" while slowing hard will never exceed the amount of energy you used to get closer to your slowing point. If you were able to regain more than it took, you'd be driving a perpetual motion machine.

While I appreciate your analytical reasoning later in your post, this first statement is the most important point that many who are new to EV's fail to grasp.

To put it even more simply, what you're ultimately trying to do is preserve momentum. Once you've spent energy getting the car moving, you ideally want to keep it moving at that pace all the way to your destination. Any other action is wasted energy.


WP