Speak EV - Electric Car Forums banner

1 - 5 of 5 Posts

·
Registered
Joined
·
8,106 Posts
Discussion Starter #1
For me, to understand the regen vs coasting issues I just go back to basics.

There is really only one basic concept that we must grasp to understand whether it is better to use regen or to coast and that is the conservation of energy and how when energy is transformed from one form to another a bit is always lost, normally as heat.

A car that is stopped has no speed, or kinetic energy. However, it does has chemical energy stored in either the petrol or the battery. When you press the go pedal you start to move forward powered by the stored chemical energy, some of it is wasted as heat but most is converted into speed, or kinetic energy.

So, once you are moving if you shift to N then you will coast and slow down by converting your speed or kinetic energy into heat and sound through friction with the road and air. That friction is always there whether you are accelerating or slowing and I call it the background friction.

If you want to slow down more than background friction allows then you have to convert more of that speed, or kinetic energy, into another form of energy. On a car without regen that is normally done with the friction brakes adding additional friction to the background friction and creating more heat in the brakes. So, on a car without regen you will go further by just allowing the car to slow naturally just using the background friction than by using the brake pedal.

When we add regen to the mix nothing much changes. To slow down we must convert the kinetic energy into another form of energy. As before, we could just put the car into N and coast to a stop thereby just using the background friction. If we want to slow down more than the background friction provides we now have two choices, regen or friction brakes. Using regen will convert the kinetic energy of our speed into chemical energy in the battery by charging the battery. This is great because instead of just using the friction brakes and converting the kinetic energy to heat in the brakes and wasting it it is stored in the battery for later use.

The key thing to note though is that it is a conversion of one type of energy to another, through generator, inverter, charging circuit etc, and in all that conversion some of the energy is lost, again as heat mostly. So not all of the energy is recovered.

So, this then answers our question as to which is best to use when slowing... friction brakes, coasting or regen?

Coasting wastes the least energy at all times. Only the minimum friction is used to slow allowing the car to travel the furthest but because you travel further to slow you must give yourself a much greater distance to do so and that normally requires driving with much more anticipation. Often, it is not even possible to do so safely.

Next is regen. It allows you to slow down much more aggressively and will save some of the extra energy needed to slow in the battery but it will waste some as heat in the conversion.

Last then, we can use the friction brakes. They will allow you to slow down as fast as tyre traction will allow but all the lost kinetic energy is converted to heat in the brakes thereby wasting it.

So, if you want to get best range, always coast to slow if you can. That is not always possible so then use regen when you can and as a last resort use the friction brakes.

As regen is a software programmable item (it must be because the go pedal regen varies between L & D) so I would like to see the level of go pedal regen user adjustable. If on a long highway trip when I have more control over my stopping distance I might want no regen on the go pedal at all allowing me to slow down just using the background friction. Remember, this is the way I will get the most range. However, in a town environment I might want maximum regen on the go pedal to allow me to ensure that I can get maximum regen braking without the risk of engaging the friction brakes and thereby wasting energy further.

I would like to see a setting, say from 0 to 5, adjustable via the config menu or even via a button on the central stack or even on the steering wheel. I shall dream on :cool:
 

·
Premium Member
Joined
·
11,319 Posts
Paul C

Are you seriously suggesting that Ampera drivers should be shifting to N when slowing down? Perhaps they can use their left hand for this while using their right hand to switch into HOLD mode :)

Being serious for a second, in many cases this would just end up with the driver realising they need to slow down more than they first thought, so they press the brake pedal which results in friction braking - remember when in N there is no regen braking. Or perhaps they desparately try to put it in L before they hit the car in front?

GM claim that up to 80% of the kinetic energy can be converted to electricty using regen breaking so I can't see the point of doing anything else, unless you need to stop quickly of course!

What do others think?

Regards

Paul R
 

·
Registered
Joined
·
8,106 Posts
Discussion Starter #3
No, I am not suggesting people switch to N ever when in motion. I suspect that this could be considered that the driver was not in full control of the vehicle in much the same way as if people changed to neutral in a manual car.

Let me be clear... I used "switching to N" as an example of coasting... nothing more. In our car, the Ampera, there is no way to determine when we are coasting... i.e. when the car is neither using power from the battery nor regenerating power to it. Using the "N" example was a way of demonstrating the state of coasting... as if it was in N.

Having said that, this issue has been discussed many times on the Nissan Leaf forums. There are many Nissan Leaf drivers that do switch to N when they want to coast. They do not believe it could be considered dangerous nor do they believe that it could be considered contrary to any law. I do and I don't do it and I have made that public on those forums. I do not in any way consider my opinion any better than others but I am a qualified commercial pilot, a flying instructor and a trained driving instructor and in my humble opinion driving any car in N or neutral cannot give anyone the necessary control over both braking and accelerating that is necessary to properly and safely control a car. Of course you are all entitled to disagree with me and that is fine but you will find me robustly defending that view.

I hope that clears that misunderstanding up. I am sorry if I gave any impression that I was suggesting people should drive in N.

There is also a suggestion on the Leaf forums that driving the Nissan Leaf in N might actually damage the car in some way and that might also be the case with the Ampera.

Now that is cleared up... my argument stands and perhaps is made even more strongly after this clarification. Because there is no way to legally or safely coast in the Ampera so people might actually be inclined to drive in N. I have always believed that all EVs should have a proper, legal and safe way to coast to extend range. The easiest and safest way would be to have an adjustable regen level on the go pedal so that if you wanted to coast you could set regen to zero. That way you would have complete control over the vehicle but could still have maximum range available. So far none of the EV manufacturers have agreed with me and made coast an easy option as far as I know. The Nissan Leaf can coast without going into N because it has power guage that shows if you are in regen or using power and you can feather the go pedal to get that zero power setting but it is a pain and difficult to do it all the time as it requires a constant eye almost on the guage and that in itself is not a good idea. The flow display on the Ampera is almost that but is way too inaccurate.

As to us not having any need to do anything else well yes, I agree with that because as an ER-EV we have a generator when we run out of battery power and so getting the last drop of EV range is not so important to us, but if when driving a full EV, like the Leaf, then getting max range is rather more important. My discussion here was a theoretical one that applies to all EVs and ER-EVs but I do agree it is one that applies more to full EVs than to ER-EVs. However I thought it interesting to raise as it was something already being discussed on the gm-volt.com forum, something I thought our members might already have seen and something I thought we might all be interested in.
 

·
Registered
Joined
·
395 Posts
Well I drive in "L" mode and I find I can use the GO pedal to adjust the rate of slowdown to just what I want give or take a yard or two. Admittedly I do have to use the friction brake right at the last sometimes but then it is a matter of practice and I have not had enough yet. I find I am getting better at working out the speed now getting 30 or 40 mph about right most of the time again a matter of practice. Over time I suspect that we will all get better at driving the Ampera and getting the best out of it.
 

·
Spiro Tech, Lecturer.
Joined
·
24 Posts
Doing this safely
After driving my volt for about three months, I've worked out a combanation of Lo and Drive can bring about the most efficient diving style and maximise battery regen. You can of cause see this with the driving efficiency screen on the Ampare/Volt in real time, as you coast and regen along the road the efficiency gauge increases to 80 or 90% or even 100% on a good down hill....
E.g. When driving along an open road (with few traffic behind) I use Drive to coast, and to slow down switch to Lo.
There is a problem here however, when coasting in Drive or Lo the brake lights DO NOT come on, so I always check behind me in case someone is driving to close, if so I tap the foot brake as a warning when in regen, as a safety, would be nice if this happened automatically in Lo gear when using the the accelerator to regen.
It amazes me how much my driving style has changed since I started driving my volt.
 
1 - 5 of 5 Posts
Top