Wow. Thanks for that, Stan. I think you managed to put together a very clear explanation, with a minimum of mathemetics. This post finally crystalized the concept for me!

 

Ditto.

Stan's the man !

Now I get it.

How about these for the basic (i.e., not the "advanced") FAQ ?

Q: What is Heretical mode?
A: Heretical mode is a mode of operation in which the car is in a virtual overdrive. This virtual overdrive is computer-controlled, electrically operated and variable speed (i.e., variable gear ratio).

Q: Why is Heretical Mode beneficial?
A: Like any overdrive, Heretical mode produces very low fuel consumption (high mpg) by allowing the car to cruise at highway speeds with engine speed (RPM) low enough for the engine to be in its most efficient speed (RPM) range.

Q: What Characterizes Heretical Mode?
A: Heretical mode always involves cruising at low engine RPM and normally speeds over about 50 mph, usually with a very light foot on the gas pedal, It may involve occasional discharge, but usually it involves charging of the traction (NiMH) battery, so much so that it often produces extremely high state-of-charge levels.

Q: How can you tell if you are in Heretical Mode?
A: Basically you need to determine that, although you are at highway speeds, the engine is turning at RPM levels more like idle speeds than normal mode highway cruising levels. If you have a good enough ear, you might be able to hear this, but the only way to tell for sure is to use a tachometer to determine the engine speed (RPM).

Q: How can you make the car go into Heretical Mode?
A: Some technically savvy owners with instruments may have learned clever techniques to do this, but for most of us the answer is: "You can't." The computer will decide the mode. But you can improve the odds by cruising at moderate speeds with a very light foot.

 

Yeah! A post for the 'rest of us'.
I'd add a few more Q's...

Is there any way one can determine if they are in this mode just by reading the the fuel consumption gauge (not using any audible cue, etc.)? .... and ...Is there a specific image on the battery/wheel/engine display that would indicate that you've achieved this mode? (((probably no, since Q4 only referenced the sound, if one isn't wired up to gauges...but the Q needs to be asked, in any even)))

How much will it effect your mileage?

...expanding on answer #3... What is the optimal speed and driving condition? (you mentioned >50, is that 52-54 or is it any cruising speed >50?)

How long will this mode last? (minutes or all day?)

Is there any difference between the way a driver, attempting to get into this mode, would drive and the way a hybrid highway driver who maintains a relatively constant speed and occasionally accelerates and lets the car coast (I believe some call it pulse and glide)?

(((since the term 'heretical mode' has been named, presumably, by one person in this forum, it is likely that the name might not be universally understood by those outside this forum. One can't go to a Toyota dealership and say "hey, how do I get into heretical mode?". As such, and with all due respect to the hours that the kind folk here have spent discovering, analyzing, understanding and teaching about this mode, I feel using a 'private name' for it, might not be appropriate, sort of akin to an inside joke. Or, if you deem appropriate a brief, as in very brief, explanation of how the name fits with the process it describes should be appended to the first A)))



I feel that all these Q/As would be appropriate for the new owner's FAQ, since it is on a level not likely to 'blow someone away'.

A final Q A could be 'where can I go to learn more about it'? and then it can link to a (presumably forthcoming) advanced topics FAQ

 

Smilin' Jack — I think your Q&A is pretty good! I disagree, however, with your third point. Heretical mode does not result in any greater than normal charging of the NiMH battery. Mechanical power is being recirculated electrically from MG2 to MG1, but if the battery is in its normal state of charge (~3/4-7/8), there will be no battery charging taking place, and the battery's state of charge display in the MFD will not change. No such charging is normally needed, and the energy for any such charging would have to come from the ICE (i.e., the gasoline) anyway, and so would lower the FE — there's no free lunch, energy can't be created from nothing.

haroldo — I'll have a go at working on minor modifications to Smilin' Jack's Q&A, and post it for your comments in the next few days.

In the meantime, I have added a number of additional comments at the end of my post #30.

Stan

 

The easiest way I find to see heretical mode without instrumentation is this:

Make sure the drive battery well charged (green is great, but over half charged is good enough).

Find a flat smooth road (no inclines) where traffic is 45-50 miles per hour, and no wind, at least half a mile long.

Get up to speed (45-50) from a stop, hold speed for a second or three, and engage the cruise control. At this point, the FE gauge should be in the 35-45 MPG range.

Keep your eye on the FE gauge. Typically, for me, within a quarter mile, the FE gauge will drop to well over 50 MPG quickly, and stay there. If the radio is off and all is quiet, it will get even quieter as the engine RPM drops, but that is much harder to notice than seeing the FE gauge drop.

Once you get used to how the FE just drops, with practice you will find other ways to get into this mode.

-- Alan

 

Just PM me when you're ready, since I don't often check this thread (it hurts my brain!)

 

Regarding this, I most frequently slip into Heretical mode with a rather low state of charge (e.g., 3 bars). This, together with the fact that others seem to do it frequently with a high state of charge, leads me to think, now, that the state of charge may not necessarily be a significant factor in promoting Heretical mode - one way or the other.

 

Stan,

A few comments on this:

1. I am your devoted disciple on the "no free lunch" teaching. I have read with great interest your many comments about this, particularly in the "Unpalatable ..." thread. (Talk about "pearls to the swine.") You have made the clearest and best founded statements about the source and use of energy and implications for fuel economy. I buy into all of what you have written on this subject 101%.

2. I believe that you have probably carefully and correctly observed your experience in regard to no battery charging in Heretical mode.

3. Please consider that my experience is different in this regard; i,e., I definitely observe significant battery charging while in Heretical mode. See my upcoming separate post in this thread. I believe that others have also posted as to observations like mine.

4. I do understand that fuel consumption while in Heretical mode would be higher with battery charging activity than without, but I don't think that this would mean that it (battery charging in Heretical mode) shouldn't or wouldn't be done. On the contrary - Heretical mode operation might present an opportunity to charge the battery at the highest possible engine efficiency.

If the car were to be cruising in Heretical mode with the engine operating at a very high, but not absolutely highest possible efficiency, say near the max torque RPM but not quite at full throttle, we could open the throttle some while adding some (electrical) load in such a combination as to maintain the same RPM. This would move the operating point to one of even higher efficiency.

Now if the battery happened to be at a lower than optimum state of charge, and I were the computer, I would choose to take this opportunity to charge the battery at the highest possible efficiency. Yes, somewhat more fuel would be burned, but the efficiency for the battery energy stored vs. the marginal increase in energy consumed would be truly outstanding.

On the other hand, if the battery were at a high state of charge when Heretical mode were entered, no such opportunity would present itself.

Now I don't know whether or not the system designers have the computer programmed to take advantage of Heretical mode operation to do a high efficiency battery charge if and when needed, but if I were in charge (no pun) that's what I would do. And if this were the case, it would explain why some of us see a lot of battery charging after entering Heretical mode with a low state of charge, while others who typically enter Heretical mode with a high state of charge observe no battery charging.

 

I have had a couple of recent experiences with what I interpret as "Heretical Mode"

On the way back from Galveston to Houston (very flat interstate hwy, A/C on, early evening but outside temp. about 90 F, three adults plus baggage in the car), I got stuck in traffic moving at about 50 to 55 mpg (too fast for EV mode) for about 8 miles (too far for a lot of fuel cut mode, particularly since I had EV motored my way down Broadway leaving Galveston, and the battery was down to a very low state of charge at the time i hit the traffic).

As I settled in to the traffic speed, I noticed that the fuel consumption meter was either sitting on 60 mpg or hovering in the high 50's. Check of the multifunction display showed the arrow going from the engine to the wheel. Battery level was at only 2 bars. At this point, I reset the mpg meter on the (nav) fuel consumption screen.

About 8 miles later, I arrived at the head of the traffic. It turned out to be 3 lanes of cars squeezing down to one in order to pass an escorted wide load. before I accelerated to normal highway speed, I noted that the screen said:

64.8 mpg !

And the battery level was up to 7 bars !

For reference, on the rest of the trip back to Houston driving 60 to 70 mph but with a "heavier" foot on the gas and with much more need to adjust speed to traffic, I averaged about 40mph.

Yesterday I had an opportunity to repeat this experience on a stretch of I-10 through Houston. There was fairly light traffic; so I was able to use a 'light" foot on the gas. But I did need to travel more like 60 mpg and there were frequent overpasses. Fuel consumption gauge generally showed between 50 and 60 mpg. On some of the overpasses I was able to nearly maintain speed without giving it so much gas that the mpg went below 50. In these cases I could hear the engine "lugging" (i.e., straining to produce power at low RPM).

This time I made it a point to watch the energy flow display. There were occasional appearances of various patterns of energy flow, but the most common was flow from the ICE to the wheels. Often, however there was also a flow from m/g to battery. And, once again this stretch began with a low state of battery charge (3 bars) and ended with a high one (7 bars).

I think that on both of these occasions I experienced Heretical Mode with battery charging.

 

Let me try and respond to some of the recent comments.

haroldo — The initial work that uncovered the Prius' "power recirculation mode" of operation was by Graham Davies; see his Web site:
www.ecrostech.com/prius/original/PriusFrames.htm
and scroll down to "Cruising at Moderate Speed" for his description of this discovery. I don't know for certain whether the name "heretical mode" was also coined by Davies back then, but it has stuck, and is certainly not new. It is universally understood to mean what Toyota subsequently called "power recirculation mode" — back then Toyota wasn't revealing details of this Prius operating mode, presumably because of pending patents.

Smilin' Jack — To simplify things a bit, I described only what could be called "pure-heretical mode." In this mode, there's no NiMH battery charging occurring, and all the electrical power generated by MG2 goes to MG1. But one can have "quasi-heretical mode" operation, in which some of the electrical power from MG2 is diverted to perform NiMH battery charging. The fuel consumption of the ICE would be increased to generate the additional power needed to also charge the battery. The reverse power flow would also increase, with some of it now going to the battery. The MFD would not show this power flow — it only shows power flows between the battery and the wheels (i.e., only battery power assist and regeneration). The forward mechanical power flow from the ICE would also be correspondingly increased. The increased power requirement from the ICE would generally also allow it to be operated at an even higher efficiency point than before. If the air-conditioning were "on" as well, there would be a further power demand on the ICE, its fuel consumption (as well as its efficiency) would increase still further, and MG2 would generate even more reverse electrical power flow, some of which would now be diverted to power the A/C's electrical compressor. Again, this would not show up on the MFD's power-flow display. (I don't have the NAV, and so I'm not familiar with what additional power-flow information it might make available beyond what the MFD shows.) All of these things can occur while the car remains in heretical mode, but of course, the extra demand might result in the car deciding to transition to normal mode. The only certain way to tell is to use my table relating road speed to ICE rpm.

Given that Toyota had already figured out heretical mode operation before releasing the Prius, they clearly understood how reverse power flow could enable them to further improve its fuel efficiency. They always try to operate the ICE near its most efficient point under the current road conditions and other requirements, like battery recharging. This being the case, it follows that they would indeed know about, and make use of, the increased fuel efficiency of the ICE wherever possible when recharging the battery. Their master US patent, which I'm attaching to this post, discusses their general hybrid system control strategy.

So, I'm agreeing with most of the statements in your posts #38 and 39, but I suspect that you may be in "quasi-heretical mode" when your NiMH state of charge is low — battery charging might be occurring at the same time as reverse power flow. Only a ScanGauge would let us know for certain whether MG1 is spinning backwards (i.e., the mode was heretical), and a rising battery state of charge would confirm that it was being simultaneously recharged. At the end of your post #39 you say that the arrow was "from m/g to battery." Did you mean "wheel to battery?" There's no MG icon on the MFD. Perhaps you have a NAV, and if so, does its display show an MG as well as flows between the MG and battery? Then I would guess that this single MG might generically represent either MG1 or MG2, as the situation requires.

Stan