Regen ... which is more efficient?

That is my approach especially when approaching a traffic light that is red. This extends the time I can preserve my kinetic energy by coasting at say 20-25 mph. Also, if I can be rolling at all, the fuel burn measured by mass air flow seems to stay much lower.

The acceleration range from 0-15 mph seems to be an especially difficult region to minimize fuel burn. But above 15 mph and especially 25 mph, it is easy to keep the mass air flow at or around 22 g/sec. and stay with traffic.

Bob Wilson

 

Bob,

+1 on not getting good FE at lower speeds. IIRC, max theoretical FE is
at ~20 MPH. Even moving through my neighborhood, I get up to 20
MPH (PSL 25) as quickly as 2400 RPM aceleration allows.

On the mass air flow measurement, how do you measure that, CAN?
I've not seen, or at least don't remember, mention of using MAF as a
FE enhancement. What are the value ranges, most desirable readout,
etc. I'm pretty sure that I can get MAF readout on my ScanGauge.

 

Of course I do every time... there is a policeman around or other cars

 

New to this thread, but has anyone seen Attila's study?
The new 2004 Toyota Prius : How to brake

Ken@Japan

 

Thank you. I hadn't seen that before. I suppose there could be some changes since 1994, but it appears that more regen occurs with the "light but firm" braking approach.

As this braking was testing from 45mph, it comes close to the situation suggested earlier where strong braking is used to get down to a glide speed. Following that, a glide had been suggested for as long as possible, followed by a firm braking to stop. Perhaps this information is in close agreement with that suggested approach?

 

the energy view may help

Regarding the original poster's posed problem of most efficiently coming to a stop from a given distance and a given speed, it may be useful to take an energy point of view. At the initial point the car has a given kinetic energy and one wishes to arrive at the stop with as much of that energy stored in the battery as increased battery state as possible. All of the energy which does not go into the battery goes somewhere else, and it may stimulate productive thought to enumerate some of those other places, in order to consider how different profiles modify them.

1. Energy dissipated in the friction brakes
2. Energy lost to air friction
3. Energy lost to resistive losses in the battery charging network
4. Energy lost to components of the car's rolling resistance not related to air friction.

The square law relationship of the primary aerodynamic forces to speed suggests some wisdom in getting the speed down relatively early. However braking harder than the maximum which allows avoidance of the friction brakes will usually be counterproductive. As those of us who have ScanGauges and watch the battery current know, it is surprisingly easy to exceed the maximum regenerative braking at higher speeds. While the pedal force required to go above regenerative braking at 25 mph is very substantial, at 75 mph it is actually quite gentle.

The resistive loss matter is harder to get your arms around at all, even with the ScanGauge or other tool that looks at data available on the automotive data bus. Were the battery itself well modeled by a linear resistor this wouldn't even be an issue, but that is quite false. Sadly I think the data that Ken linked us to cannot make the distinction accurately. One would wish to voltage measurement points, one of which was right at the battery, and the other of which included all resistive losses. This is not even possible, as some of the resistive losses internal to the battery, and some of it is also internal to the motor generator, so the wiring and other components between are not even the entire problem.

On first consideration, I think that the non-aerodynamic rolling resistance component is sufficiently linear that it will make a constant contribution as the problem was actually posed (fixed distance to required stopping point from a known initial speed) so can actually be ignored for this particular version of the problem.

 

The ability to run the ICE as efficiently as possible is the important characteristic of the Prius. I view EV as important only in the sense that it gives a solution to the pumping losses problem of low power ICE states.

 

Don't use B for normal braking. It increases engine braking, which reduces energy recovery.

 

Obviously, the efficient ICE in the Prius a key contributor to high mpg but my point was in comparison to shooting for perfect braking, the ability to drive in EV is what puts the car over the top (it's what separates hybrid from non-hybrid).

My point was the focus should be on getting into EV mode and out of ICE vs. braking perfectly (which has little effect on regen whether you do it poorly or do it well...the energy in the battery is going to be lost at some point and you are going to regen the battery just by driving the car normally. Higher mpg from EV mode is a positive...perfectly braking is only a break even (or slightly positive at best) situation.

 

Hi Matt,
I don't think I am following you. Maybe thinking about it this way will make sense:

Step I: Get the most momentum out of burning fuel as possible. This is ICE efficiency
Step II: Go the furthest (or highest) you can from the momentum obtained. This is avoiding friction losses like braking, or power transductions to and from the battery. It should be clear that if your choice is between wasting energy as heat, or routing it through a battery and keeping some of that energy for later use, then by all means use the battery!

An example I encounter in my hilly driving is going downhill. I let the generator slow me down (and gain battery charge doing so) when my speed hits about 65 mph. My reasoning is I prefer to save some of the energy that would otherwise have been expended on air drag above 65 mph.

Best choice: get the most you can from each. Is minimizing braking more important that braking efficiently ? yes. Is gentle braking smarter than abrupt braking ? yes. Is braking early in a mandatory slow-down better than later braking ? Yes, and more so if initial speed is > 42 mph.

And ... perhaps ... your point: Efficient ICE use tends to be much more important than efficient braking, at least for drivers who are looking ahead and minimizing brake use already. I very rarely worry about battery SOC management. The Prius is autocratic in that regard; and although I sometimes think I could micromanage better than the Prius computer, I am not given a choice so I focus on other aspects of hypermiling. One exception is the long, mild incline. I've learned how to avoid SOC depletion and I do so so that the engine does not rev into an inefficient range.

 

So does anyone know if regenerative coasting is better than regenerative breaking (light but firm)? I tend to coast as much as I can and only brake when coasting will not stop me all the way.