I think the only way to put this topic to bed is to do an experiment and use live data from an OBD scanner and plot the relevant parameters while doing the same routine using different KERS settings.
Driving at a certain speed and taking the foot off the accelerator isn’t going to reveal any surprises, therefore I’m open to suggestions on how best test the various theories?
The quality of that data is not good enough I'm afraid to get to the level of accuracy to be definitive. One issue is that there is a delay between the data actually occuring and the data being displayed which makes it hard to correlate it with other inputs.
The car system is also brake by wire using an iBoost which means the pressure you apply on the pedal is not necessarily the pressure applied on the brake calipers. The brake position and other data is fed into a "stability system" and this controls the amount of mechanical assist and also the amount of regen. The next thing is that you need a "British Standard Idiot" to drive the car in exactly the same way and weigh the same amount. The runs need to be repeated in both directions to negate inclines and wind speed and direction. Tyres need to be at exactly the same temp and pressure.
Given that, using the motor voltage and current screen gives a indication of the efficiency. As does the short journey mi/kw screen. They can be used for a bit of experimentation but be careful you can easily get distracted! Yes it is fun trying to get the best, but it is not definitive answer.
My first EV which I converted myself was instrumented to the hilt for some post grad work I was doing to look at exactly this question, and to enable me to make it efficient as possible. It data logged anything I could think off. Battery cell voltages charging currents, etc etc. It was measuring cell voltages 5 times a second for example. I could program the KERS charging current so that it was constant or a max or min value.
The biggest efficiency factors I experienced over about 20,000 miles were:
- Altitude difference between origin and destination. One trip I could get over 8 mi/kW in one direction and struggle to get 3 or 4 in the other. 200ft difference in height.
- Wind speeds which if high would reduce the amount of KERS needed to slow the vehicle down and increase it in the other direction.
- The amount of mechanical braking. I had a twin cylinder circuit with no servo so that was easy to measure.
- The charging rate of the KERS energy recovery with slower being better for efficiency.
- The weight of the vehicle. Heavier means more kinetic energy to recover but more energy to get it moving.
I think it is because the KERS setting doesn't affect efficiency. It only changes how much kers is used by lifting the accelerator. If you need to slow down you can do it with the accelerator in KERS3 or with the brake pedal in KERS1, as long as you aren't braking hard then the car uses the same amount of KERS in either situation.
Lets get away from treating KERS as an amount. KERS is a name to describe which multidimensional map is used to calculate the amount of current that is generated by the motor in generation mode. It is a profile/preference nothing more. MG don't supply kilograms of KERS.
Also define what is efficiency? For most people it is about getting the most miles per kWh, i.e. driving a longer distance. So that means how much energy is used/recovered to slow the car and how does that change the distance travelled.
The most efficient situation is where there is no braking of any kind and the amount of power used is just enough to overcome wind resistance etc to maintain a speed. If the definition of efficiency is distance travelled per kWh, then the more braking, the more that is reduced and the less the efficiency.
If a car is in a kers profile 3 which is aggressive, it will slow down quicker and in a shorter distance. It will recover some of the energy but equally it now has to spend that energy to continue at the slower speed to the final distance acheived by the KERS 1 car. The KERS 1 profile is calmer and will take a longer time to slow but covers a longer distance and does not need to provide additional power to get that speed and distance. That is a major difference for a start.
Both have reduced the kinetic energy by the same amount as they have slowed to the same speed, but the Kers1 car has travelled further and this gives a better efficiency. Is the KERS - sorry energy recovery - the same? Well no. With the KERS3 example. the recovered energy is used to maintain the lower car speed until it has travelled the same distance that the KERS1 car does. This energy capture and use is not 100% efficient so there is an immediate loss there. The KERS1 car retains its recovered energy because it doesn't need to use the it like the KERS3 car does and can use it to travel further unlike the KERS 3 car which uses it to travel the additional distance that the KERS1 car did.
The amount of energy recovered may be the same but the KERS3 car has to use it to cover the same distance that the KERS1 car did. Are the KERS profiles the same? no. Can the driver adjust the profile? Yes. There are a lot of factors involved.
In this case, the amount of energy recovered is greater with KERS3 because KERS is providing the major braking effort. However, the KERS3 car has to use that to travel the additional distance that the KERS1 car acheived. The KERS1 car has all of its recovered energy available to travel further.
If both cars recovered the same amount of energy, the KERS1 car would still be the most efficient in terms of distance travelled. So the energy recovery defined by the KERS profile does affect the efficiency but equally it depends on the particular scenario and other factors.
