Joined
·
9,328 Posts
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
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