Hill regen

Yeah, you're overthinking it So let's overthink it some more...

The Prius Hybrid system works by using an Atkinson Cycle Combustion Engine. These are super economical when ran at a very specific speed (revs). Remember, that all of the energy (unless you have a plugin) within your Prius originally comes from the combustion engine; either it powered you to the top of a hill or up to a given speed, and then you'll slow down or go downhill and some of that energy is then recovered and stored in the batteries (whereas in a normal car this energy would be entirely lost as heat on the brakes or within the engine when engine braking). Also remember that whenever you convert between the combustion engine, the wheels (braking), or the battery, this always incurs some losses. At least 20% in fact.

So, it makes sense to use as much of the energy coming from the combustion engine directly to power the wheels, if possible, as this incurs no big losses. This is fine, so long as your road speed and the incline of the road allows you to drive at exactly the right speed, all the time, to allow the Atkinson engine to run at exactly the right revs, all the time, to get that super economical combustion.

But, of course, this isn't always possible. You can't drive at a single speed the whole time. If you want to go slower, this means there will be spare energy from the combustion engine. So it uses that to turn a generator to charge the batteries, thus allowing the combustion engine to run at a faster speed than you need it to, but at the perfect speed for economy. If there is a greater demand for energy at the wheels than the combustion engine can provide (e.g. harsh acceleration) without increasing its revs, then it supplements the energy at the wheels from the electric motor powered by the batteries. So the combustion engine can still run at it's economical speed, but the additional power from the electric motor allows you to go faster.

As such, you now have more freedom in the road speeds you can use while still keeping that combustion engine running at its most economical speed. And in a nutshell, that's it.

Of course, this is still too limiting for most road users, so if you floor the accelerator the engine will still rev up to a not-so-economical speed so that you can at least have reasonable acceleration when needed.

To answer your question, you need to understand the above, but also understand that in any situation where excess energy is not stored in the battery, it is always lost as heat - either on the brakes or in the engine (engine braking). It's lost for good. Wasted.

The reason the car doesn't reduce the amount the batteries are charged on long downhill stretches, is that it has absolutely no way to know you're going to be going downhill for a long time. So while it could, say, charge the batteries at 50% rate, and lose the other 50% to heat, what would happen if you're only going downhill for a short distance and then turn off onto a horizontal road for 20 miles? You've just wasted that 50% of your energy that could have been stored in your half full battery and powered you for some of those 20 miles. But instead you lost it to heat. While it's true that what goes up must go down, you missed off the word "eventually" at the end of that statement. It makes sense to use any spare energy, whenever it's available, to fill the battery, otherwise you're always going to lose it.

Of course YOU may know if you're going to be going downhill for a long stretch, which is exactly why Toyota provided the B (for engine Braking) mode for driving. If your battery is full, or nearly full, and you're going to be going downhill for a while, slip it into B mode. The car will now become more akin to a traditional car. Instead of dumping excess energy into the battery, it can now also dump excess energy into the engine to help slow the car down. The friction of the engine, and the compression of air in the cylinders, all produce heat. So you burn off some of that excess energy as heat in the engine, just like a normal car would if you downshift it a few gears while going downhill. This takes strain off your friction brakes, and off the battery, but the trade-off is that your engine will rev up super high and super nosily (using no fuel though), just like a traditional car, and you will be losing energy as heat instead of storing it in the battery. But, that's probably what you want in this situation, otherwise the entirety of that excess energy will need to be turned into heat via the friction brakes once the battery is full.

 

DITTO!!

Only solution would be to put a bigger battery, which means more weight, less efficiency at other times. TOYOTA have hit on a fantastic "compromise" position between very small batteries (relatively useless but expensive like in SUBARU Forester Mild Hybrid) and a PHEV - which has a battery so big that it needs to be plugged in.

 

Good answer from The Professor there.

The earliest Honda hybrids did this - constant battery assist up mountain grades, and constant regen down the grades. This large fluctuation in battery SOC was one of the reasons why the early Honda hybrids have such frequent hybrid battery failures.

Another disadvantage of such a method is that the car cannot tell how long the grade is, so by applying constant battery assist up mountain grades would risk depleting the battery while still climbing, thus necessitating both high revs to climb the hill and high revs to charge the battery until the peak of the mountain was reached.

 

You should come see what a bigger (~10 times bigger than regular Gen4) traction battery can do on the hill. Yap, you can capture all the regen energy and use it later on EV mode if you had Prius Prime (aka Prius PHV).
Battery Charging After Depletion? | Page 2 | PriusChat

 

I had this same thought about a decade ago. A couple of years later I got a Ford C-Max that does remember via GPS up to 5 locations that the car frequently travels to and goes into what Ford calls EV+. This allows the car to use more the HV battery's SOC to make it to that location (home or work usually) under EV when otherwise it would fire up the ICE. Nice feature but doesn't have to do anything with hills tho'.

Toyota now does similar with GPS. It's not exactly the same as EV+ (patents?) but it's similar and promotes greater FE. Again, not for hills tho'.

In car EV charging apps apply navigation data too for range estimations.

Maybe generation 2 or 3 of these softwares will use topography data to a much greater extent. Autonomous driving tech is mapping roads now so there's going to be a mountain of data coming from those efforts.

 

I have to challenge this "very specific speed (revs)" portion, because that is not how the Prius Atkinson Cycle engine is managed. I have an RPM display up on my ScanGauge, and see that RPM is continually varying, often over a very wide range as slopes change.

Engines generally have their best efficiency at a particular RPM and torque. But in normal driving, traditional cars operate their engines over a very wide area of RPM and torque points, spending little (if any) time at that most efficient point, which generally doesn't correspond to normal cruising conditions.

Prius engines have been tuned to have a reasonably wide RPM band or plateau of near-peak efficiency at fairly high torque. And Gen3 engines achieve a wider best-efficiency plateau than Gen2, as shown on these BSFC (brake specific fuel consumption) charts:

2010 Prius 2ZR-FXE engine efficiency map | PriusChat

The dark line is the "operating line". The engine controller tries to keep the RPM and torque on that line all the time, running fairly close to peak efficiency, not wandering off into much lower efficiency territory. E.g. the Gen2 peak efficiency is probably at 2800 RPM, but anything on that line from 2000 to 3800 is so close to peak that it doesn't make sense to do extra energy conversions for storage. For Gen3 peak efficiency is probably at 2200 RPM, but anything on that line from 1000 to somewhat over 4000 is still "close enough". The key is to stay on that line, not allowing torque to go far above or below that best-efficiency plateau.

So Prius engines are managed to have widely varying power outputs, and thus RPMs, depending on current needs. But for any given power request, the controller seeks to produce it at best efficiency by running at a very specific RPM and Torque pair, computed to be on that illustrated dark operating line. Traditional cars don't keep their engines running to any specific line through their own BSFC charts, instead wandering around over a fairly wide area, spending much of their time operating in their lower efficiency zones.

 

I had read #2 and said to myself , "curses! I shall have to find time to dig up BSFC charts and respond to that!" and I thank fuzzy1 for making that not have to happen.

 

Yep, yes you're quite right. I just didn't want to over-over-over think the over-over thought reply to the over thought post. It was already getting long.

The overall point I was trying to make is that the hybrid system attempts to gain maximum fuel economy primarily by running the ICE as economically as it can by leveraging the motor and battery to provide better control of the ICE output to maximise fuel efficiency. That's easier to grasp by simplifying it to a specific rev range, but as you pointed out it's at the cost of some completeness of accuracy. My bad.

Not many people realise this though, which is why so many people think running the car in EV Mode as much as possible will produce the best long term fuel economy, when in fact that's only a short term gain at the expense of long term economy, in no small part due to the fact energy in the battery has already lost 20% of what the ICE originally produced due to conversion losses, and it may have been more efficient to use that energy directly from the ICE to the wheels without such losses.

As those in the know say: "Just drive it"

 

This may be an oversimplification of the overthought .....
Or if you prefer, just have fun with it!