There was a posting earlier about the heretical mode occuring between 40 and 55 mph. When it was discussed, it was indicated that it would be indicated by the ICE and battery both showing arrows to the wheels and the FE gauge dropping to nearly 60 mpg.

For those who know about this, is it possible for the heretical mode to occur at faster speeds? Today, I saw all of the above happening when I was cruising at 63 mph.

 

Yes. This mode is actually more likely at higher speeds, because if the computer does not place the vehicle in it, the MG1 speed approaches its design limit. Remeber this, when you are in the "heretical" (I prefer Electronic-Recirculation or E-Recirc) mode, the output of MG2 is used to reverse turn MG1, the change in torque at the PSD meshed gear surfaces resuts in a lower shaft speed for the ICE -- the best parallel in a conventional transmission is overdrive.

To understand it go here: http://eahart.com/prius/psd/ and scroll down until you get to Alex's PSD animated simulator at the bottom third of the page. Set the value for the ICE at 3000 RPM (Don't worry about the distortion in the nomograph), then set the value for MG2 to the same value. Note the speed of the vehicle and the fact that the sun gear is moving at very close to the same speed as the ring gear. Now, reset the value for the ICE down to 1200 RPM or so. What happened to the speed? Nothing. The ICE is now turning a little slower, and its producing less power and burning less fuel. Is this VooDoo or did Newton have it all wrong? No. The answer is that the assumption for a constant speed means that the actual power demand on the ICE is adequate at the slower RPM to sustain the desired speed even if you consider the feed back inefficiency of the MG2 to MG1 loop. Both are steady states, but the second one uses less fuel and makes less power. In other words, the first situation was like driving 50 in third gear in a four speed, and then we "shifted" into top gear. Now, if we apply that in a real-world situation, the difference will not be as extreme becasue any power reduction or increase will impact the vehicle speed to some extent, but the principle is the same.

My earlier posts on this subject dealt with techniques to lower the speed at which the mode is entered or in which it can be sustained (sort of like shifting a manual transmission when you have barely enough revs to support the higher gear). Actually, while a lot or research has been published here and elsewhere on Pulse and Glide, the steady state E-Recirc mode has not been very well researched. Further, I suspect that is much more important in the TCH and HiHy than it was in the Prius because of their additional weight and power. I would like to see some practical research, graphs and techniques, based on experience for use of this mode.

 

I knew someone would be able to explain it better than me.
I've never been able to maintain this mode of driving at speeds greater than 52mph (on the speedometer). If it does occur at higher speeds during my commute, I've never noticed it. Perhaps it's more subtle at higher speeds. The transition to this mode at 52mph is quite dramatic. The engine rpm doesn't change that much but the car goes dead quiet.
I've seen the PSD animated simulator mentioned in the previous post and have always been curious about something. As my car enters this mode when I decelerate below 52 mph, my engine rpm is always 1280. Adjusting the sliders in the animation so the ICE is at 1272 rpm, both MG1 and MG2 are spinning at equal speeds but opposite directions. Is this a necessary component of this mode or is it a coincidence? If it is a requirement then one would have to maintain about 1600 rpm at 60mph to stay in this mode. I'm not sure this is even possible.
-Ed

 

I've seen it in the 70's on a decent down grade.

 

I've noticed this mode after a few weeks of driving (didn't know it had a name) and have become pretty good at using it and keeping it there. I can definitely feel when the car is going into this mode, a drop in noise (which is hard to detect in an already quiet car!) and a gentle drop in power. I've found that it triggers mostly when I am on a flat surface running at speeds of 45-60. I can sometimes manually 'force' it (or maybe I'm tricking myself) but most of the time it starts after 15-20sec of no pedal movement and constant speed. I've treated it just as I would e-mode...it won't stay in this mode if the accelerator is pressed to hard.

 

That's it! Your there! It will "trigger" on a grade, but the necessary speed is higher on an up and lower on a down. I suspect it is related to torque demanded in some way because almost every time I have seen it, it started when I was backing off on the accelerator or starting a downgrade without any demand for acceleration. Once it is in this mode, you can use very small adjustments on the accelerator to anticpate small rises or uphills, and to "bank" better MPG on dips or small downhills, but it absolutly wil not tolerate any big inputs in the pedal --in either direction. Doing so will cause a mode change.

Incidently, there is also a false indication of this mode in the 42-45 MPH range under low torque demand. The same speedo panel indications of power from both sources, but in this case, if you back off a little more the computer will go into EV mode. I think there is a small region in the computer's software at these speeds where it keeps the ICE at its own version of "Idle" at around 1260-1280 or so while actually also turning on the traction motor -- sort of "almost EV mode". The confirmation of this false mode is that backing off the accelerator a little more will immediately move the MPG indication to the Eco position.

 

H2O Doctor — Without instrumentation more sophisticated than the car's MFD or a ScanGauge, I don't think it's easy to determine exactly when the TCH is in "heretical" mode. In addition to the ICE's speed, one needs to be able to monitor MG1's speed, and also its direction of rotation. I believe that heretical mode is commonly employed when travelling at steady high speeds on relatively flat highways. It should not be confused with "fuel-cut" mode which can occur at speeds above ~64 km/h (~40 mph) when the car is coasting (or, at least, when the accelerator is only very lightly pressed). In fuel-cut mode, all fuel flow to the ICE is stopped, but it is being spun at ~1000 rpm by MG1 (powered by electricity from MG2), in order to prevent it from the over-revving that would occur were the ICE allowed to stop completely. Fuel-cut mode can be detected unambiguously from the fact that ScanGauge shows "open-loop" ICE operation, and the dash's FE gauge simultaneously reads precisely zero fuel usage (0 L/100 km or 60 mpgUS). In both cases, electrical power is flowing from MG2 to MG1. The open-loop indication will clearly distinguish fuel-cut mode from heretical mode.

Stan

 

I was doing 50-55 trying to trigger the heretical mode and noticed a few times where the MFD showed power from the battery to the wheels, but not from the engine to the wheels. In other words, it acted as though I was in EV mode but I certainly was not. Would this mean I was still in heretical mode?

 

hamm3r — I think you were probably in fuel-cut mode. Had you lifted your foot from the accelerator completely, or almost completely? That would trigger fuel-cut mode. If the FE gauge was reading on the lowest "white" tick mark (60 mpg or 0 L/100 km), this was definitely fuel-cut. It would need to read above this mark (i.e., show actual fuel usage) and the ICE symbol would need to show in the MFD as well for heretical mode.

Stan

 

You can tell if the ICE is on by a slight (very slight) accelerator demand. It will bring the FE indication off of the 60 MPH indication and move it to some decreasing value, typically in the 55-40 MPG range. This fuel flow indication will not occur in fuel cut mode and if the accelerator demand caused the ICE to restart it will move initially to above 40MPG. From my posts you probably know my advocation of heretical mode. As mentioned in several of them, I often cruise in the 50-60 MPH speed range using what I beleive to be heretical mode. My indication for this is a fuel flow in the 40-59 MPG range, but that occasionally backs down to 60 MPG.

You could be correct that the ICE fuel flow is cutting out for these 60 MPG intervals. I think it unlikely at that speed on undulating hills with a average very small grade (in either direction). However, it matters not even if it is, because a certain amount of enery will be required over the average grade for the distance. The basic condition is the ICE spinning (under power or not) with a very efficient "gear ratio" (irregardless of which way MG1 is spinning) and an average FE of well above 40 MPG. A coast, whether in neutral or not, simply trades off a loss of inertia against the enegy demanded for the next small hill or the inertial loss that will eventually result in a drop in cruise speed. In these conditions, the expenditure of the energy to maintain a constant power demand will be more efficient.