I hate to throw a wet towel into the ring, but so many participants in this Newsgroup are doing their utmost to keep their TCH's in pure-EV mode (battery power only) for as much of their travels as possible, that I feel compelled to inject a bit of physics into this discussion. If there's one thing for sure, it's that the TCH cannot violate the laws of physics!

ALL the electrical energy stored (chemically) in the batteries ultimately came from the chemical energy in the gasoline via the ICE (internal combustion engine). This includes all the motional kinetic energy recovered through regenerative braking, for that kinetic energy was imparted to the vehicle by the action of either the ICE directly, and/or the motor-generators MG1 and MG2 and/or the NiMH high-voltage battery. The only simplifying assumptions that I'm really making here are: (a) that the car ends the monitoring period at the same altitude above sea level that it started off at (so that there's no change in its gravitational potential energy between the two cases; (b) the mass load of the driver + passengers + freight stays constant; and (c) that no other forms of energy usage (e.g., air conditioning, audio system, headlights, etc.) occur. [Actually, assumption (c) isn't a problem for my argument, as any such additional energy uses just make the situation even WORSE.]

Consider the energy conversion processes involved: (i) the mechanical energy from the ICE is converted to electrical energy in MG1 (or MG2 in "heretical" mode); (ii) this electrical energy is then converted to and stored as chemical energy in the NiMH battery; (iii) this chemical energy is then converted back into electrical energy; and (iv) drives MG2 or MG1 which convert it back into mechanical energy to drive the wheels. In some cases the two chemical conversion steps will not occur. This happens when the electrical energy being produced is immediately re-used to power the MGs, and so allow the ICE to be run in its region of greatest efficiency — this happens principally during constant-high-speed travel on level ground. [Actually, this is one of the beauties of Toyota's Hybrid Synergy Drive (HSD) system — the two MGs allow the ICE to be run close to its maximum efficiency under a variety of conditions. This, combined with the high-efficiency Atkinson-cycle ICE design, is what REALLY results in the greater fuel efficiency of the HSD system — it can't be better than the ICE ON ITS OWN! This is what I shall now argue.] But, it is the pure-EV mode that we're talking about here, and that ALWAYS involves the battery.

Consider, for the sake of argument, that each of the above energy-conversion processes is, say, 90% efficient (i.e., an efficiency factor of 0.9). [This is probably a realistic assumption.] Then the mechanical-to-electrical-to-chemical-to-electrical-to-mechanical chain will have an efficiency of only (0.9)x(0.9)x(0.9)x(0.9) = 0.656, or ~66%. That is, ~34% of the energy is actually being lost in the multiple reconversion processes! The more you use the pure-EV mode, the WORSE off you are, all else being equal (e.g., assuming that the ICE runs at the same efficiency each time it comes on), and there's no way around this!

This basic point was made in passing by someone on this Newsgroup some time ago (I'm sorry, but I don't recall who it was), but its implications seem not to have been fully understood. Perhaps my explanation will help in this regard.

CONCLUSION: Don't try to force the TCH to shut off its ICE and run in pure-EV mode. You're only LOWERING your fuel efficiency overall.

 

Stanley,

Yours was a very cogent discussion of the physics/chemistry of the TCH. Reminds me of a saying of Einstein's: "Put your hand on the burner of a stove for a minute; it feels like an hour. Talk to a pretty girl for an hour; it feels like a minute. That's relativity!" (No, he really said that!)

Anyway, while your argument seems unassailable in microcosm, some implications of it are unclear.

For example, I have always assumed that the HSD system would yield at least as high an FE under all conditions (and a higher FE under some conditions) than would a Camry of similar weight powered only by the Atkinson-cycle ICE (i.e., without MG1, MG2 and the traction battery). Do you think this is true? If so, why?

Here's my take on this issue. Suppose we take two cars - a TCH and a same-weight Camry with the Atkinson-cycle engine only - and drive them for say 50 miles at the same constant speed on a dead-flat course. Will the FE of the TCH be higher? It would seem from your analysis that it would NOT, but it would be equal to the "normal" ICE vehicle. Am I wrong?

Now, take the same two cars and have them travel from Point A to Point A on a closed 50-mile route that involves some hills. Will the TCH FE be higher? I think it will. Why? Because the HSD is able to recover energy from the regenerative phase that can be used for motive power, while the "normal" ICE cannot recover that energy because of heat losses during coasting.

Wadyathink?

OK...wait....just thought of another possibility. In the "flat-course" test have both cars follow the same speed profile, but vary the speed. I think this will result in higher FE for the TCH again because of regen vs. heat losses.

 

I'm no Physics expert and most of that is over my head.

But it begs the question: As to your conclusion, it appears to insinuate that a Camry Hybrid which did not EVER run in electric mode would be as efficient as one which operated normally, as designed.

Same goes for a Prius - if it never went into EV mode, would it get better MPG?

That does not seem possible. In fact, if it were true, it would seem to render Hybrids as completely useless.

 

My traction battery is always fully charged. According to Toyota it can be only 80 % charged and discharged to 20 %. If I did not run it off the EV mode for at least some of the driving I am wasting all the energy of regenerative braking since the extra generated potential energy is not storable. So why not use up the battery power and let it get charged while braking or while my foot is off the accelerator. I can understand if the ICE charges the battery and then the battery via the motor runs the CVT, in which case there is loss of efficiency due to the conversion process. But a traction battery that is fully charged and can absorb no more energy and is not put in use is a dead weight in the vehicle.

 

Stanley,
Your analysis appears to be missing the main point of a hybrid. Much of the energy generated by the gas that would otherwise be LOST is now recovered via the hybrid battery even if not 100% efficient. This energy would be lost entirely in the pure gas vehicle, and is now recovered in part. This is where the increased mpg comes from. The more of that otherwise lost energy you use via running on the battery, the better because less new gas has to be used.
Yes, of course all the energy is coming from the gas, but less of it is wasted because of reclamation via the battery.
Joe

 

1) You're collecting energy from regenerative braking which otherwise would be lost (in ICE-only mode). This occurs under most real world circumstances (there's no steady speed route I'm aware of for my morning commute). In your model, the efficiency factor of this reaction is effectively >1, given that you're yielding "free" potential energy (at whatever real efficiency) from a kinetic energy->heat reaction

2) There's no apparent way to prevent the HSD from attempting to charge the NiMH battery, meaning if we don't drive in a manner to utilize the energy that's being stored there, we're experiencing a less-than-optimal FE (unless you're suggesting that a Camry equipped only with the Atkinson ICE would be more efficient than one with HSD).

3) How would the Camry's Atkinson ICE function with the eCVT transmission? In short, it couldn't - you'd have to resort to a mechanical CVT (belts/cones/pulleys, etc...) which would most likely suffer worse efficiency due to increased mechanical interaction (and the corresponding loss one notices when comparing the input hp and output hp of the transmission).


(Note: I'm not mechanically inclined and my physics knowledge is fairly old, but these are a couple questions to test your proposition).

 

The conclusion that running in pure EV mode wastes energy is wrong. It all depends on particular situation. Just to give very simple example, last mile of your trip should be driven in pure EV mode if possible, leaving battery in low state of charge, the next time you start the car, the cold engine will run constantly until fully warm regardless of the state of battery charge, if the battery is fully charged, then all the energy goes to waste , if the battery is low, then engine will charge the battery almost for free so to speak. Also you never took into account the fact that gasoline engine is much more efficient under heavier load, so you may loose efficiency due to multiple energy conversions, but gain efficiency by loading the engine more, especially during lighter power demand and recharging the battery at the same time. There is something called pulse and glide, used by hypermilers to increase FE, driving partially under EV mode and partially under ICE power would be a sort of milder version of that technique. It's all academic anyway because above 42 mph, the ICE will never shutdown anyway and under 42 MPH in normal driving conditions, at least where I live, you pretty much depend on the traffic conditions and drive as other people let you, not the way you want. I write this just after driving 30 miles, actually sitting in traffic for 2 hours in rain, so I hope you know where I'm comming from. BTW I can't find it at the moment but I believe I read somewhere the Synergy drive has efficiency higher than 90%, therefore in a lot of stop and go traffic, using EV mode I believe you'll cover conversion loss of efficiency with higher energy absorbtion during breaking, empty battery will take in more energy than fully charged one.

 

Dear All:

Thanks for all the interesting and thoughtful comments in response to my provocative posting. I'll address them in a moment, but first a few clarifying statements:
(a) Of course, like you, I also enjoy driving in "stealth" (pure-EV) mode when it occurs. And, it really does impress non-Toyota-hybrid owners. The question is whether trying to force pure-EV operation as much as possible actually harms, rather than increases, fuel economy. That's what my post was about.
(b) Yes, recovering kinetic energy that would otherwise be wasted as heat in friction braking is desirable, and helps to improve the TCH's fuel economy, especially in-town. This regenerative energy recovery comes for free because one has the MGs present, but it would not be possible without having the NiMH battery in which to store this recovered energy. One wouldn't want the NiMH battery's state-of-charge to get so high that there's no more room left for regenerative braking energy storage, and indeed the TCH is careful to always leave some headroom for this. But, don't forget that this recovered kinetic energy originally came either directly from the gasoline via the ICE, or else was imparted to the car by the NiMH battery and the MGs. However, the electricity in the battery (whether regeneratively recovered or not) ultimately also came from the gasoline engine. So, ALL the car's energy sources ultimately point back to the ICE.
(c) The principal use of the NiMH battery's stored energy (apart from starting the ICE and powering the air conditioning) is to add extra on-demand torque to the ICE's limited output when the driver (or the road) requires this. This is only possible for brief periods because of the battery's linited storage capacity. But it does enable the ICE to be designed for maximum efficiency (Atkinson-cycle) without the associated reduced power output capability being a liability under normal circumstances. [This small 4-cylinder Atkinson-cycle engine's power would be inadequate in a regular non-hybrid Camry.]
(d) Toyota's control system attempts to maximize the efficiency of the whole system by trying to always run the ICE in the range where its efficiency is greatest, and letting the battery supplement the ICE's output when necessary. During acceleration the battery helps to supply power to the wheels. This is when the ICE is forced to run at less than its highest efficiency. Once steady cruising speed is reached, and assuming a level road, the ICE will be running much more efficiently, and this is when the NiMH battery is recharged by the ICE. So, the electrical energy sent to the battery by the ICE is in this sense already "higher-efficiency" energy. Moreover, the ICE is FAR more efficient than the battery as a source of motive power at cruising speed, because of the losses (guesstimated at say 34% in my previous post) associated with the multiple energy reconversions from ICE to battery to wheels.
(e) To do better than the built-in control system, one would have to generate electrical power more efficiently than Toyota has been able to program the system to do. One would have to use battery power ONLY when it would be more efficient than the ICE (taking into account the 34% loss associated therewith), and use the ICE ONLY when it would be more efficient than the battery as a source of power (again taking into account the associated 34% loss). Since this must be what Toyota is already attempting to do, I say that you have a very difficult (maybe impossible?) task ahead of you!
(f) It follows that the greatest possible fuel efficiency achievable by the TCH is that of its ICE ALONE, running at that speed at which it is most efficient at producing the needed power. You'll never exceed this on average no matter how hard you try!

I am now ready to attempt to respond to your comments:

wcmack — AsI have argued above, you CANNOT in the long run exceed the efficiency of the Atkinson-cycle ICE on its own, since it is the source of all the power, and so I think that the HSD cannot overall do any better than the Atkinson-cycle ICE. In your first 50-mile scenario, I agree with you that both vehicles would achieve the SAME FE. In your second 50-mile closed-loop scenario, I agree with you that the TCH WOULD do better. If the NiMH battery's state-of-charge is the same at the end of the circuit as it was at the start (i.e., if the "ups" use as much battery energy for MG torque boosting, as is recovered regeneratively in the "downs," and in principle this WOULD be the case), I think your argument is sound.

lars-ss — As long as the vehicle runs in pure-EV mode (taking into account the associated 34% loss) ONLY when it is more efficient than using its ICE, then it WILL benefit from doing so. This is presumably what Toyota is trying to achieve with its system control, and why the TCH DOES go into pure-EV mode when it decides that it wants to. The battery is mainly intended to store recovered kinetic energy and then use it for starting and torque boosting the ICE when necessary. I believe that most other uses actually cause overall FE losses. Any energy placed in the battery has incurred a net 34% (say) penalty by the time it is used. I have argued above that its most efficient use is for torque boosting, not for powering the car in "stealth" mode.

greenvillehybrid — The traction battery is FREQUENTLY used for torque boosting the ICE, especially when accelerating. The energy-flow display clearly shows this. That's where your recovered kinetic energy is best used. You're NOT wasting the energy of regenerative braking by not running the car in pure-EV mode. You have a better use for it. After all, the kinetic energy was produced by acceleration in the first place (that's what got the car up to speed from rest!), and so it's fitting that it be used to assist in getting the car back up to speed again. You're right that the traction battery must keep "headroom" available for storing regenerative braking energy, and indeed it does do this. The fact that it stays approximately fully charged all the time probably reflects a more relaxed driving style and/or your local terrain.

Joe.B — You're right of course, and I sincerely hope that my original post did not leave the impression that there's no point in storing and recovering energy in the NiMH battery, even though there are losses associated with this process. This energy recovery and re-use is a MAJOR advantage of a hybrid design. This is particularly valuable in in-town start-stop driving (and here the fact that the ICE shuts off frequently is also a MAJOR benefit — and don't forget that it's the NiMH battery that is used to restart the ICE each time). But since all this recovered energy came originally from the gasoline via the ICE, the ICE's efficiency alone sets the limit to what's achievable overall. Yes, indeed, use this electrical energy; but running in pure-EV mode is usually not the best use for it.

turk — (1) Yes, I agree with you — you certainly DO want to use the energy recovered from regenerative braking, even though there are losses associated with this recovery and re-use. Without a MG and battery this couldn't be done. I was not arguing against this. (2) Please see my reply to wcmack above. (3) Agreed. All in all, we're not in disagreement, but I hope that my introductory comments above will help clarify things for you.

Pete4 — Regarding your point that it's best to deplete the NiMH battery before shutting off the TCH at the end of the day, so that the ICE will do useful work by recharging the battery during its initial warm-up the following day, this seems plausible; and if this is when you try to force pure-EV mode, I could be convinced by your argument. Many TCH owners, however, appear to be trying to force pure-EV mode whenever possible. And, yes, the ICE is more efficient under heavier load, and that's the way Toyota use it! It's what I was getting at when I said that they attempt to run it so as to produce the needed power in the most efficient manner, and the HSD allows them to do so. I have been using a ScanGaugeII (www.scangauge.com) to monitor the ICE load factor for the past month, and it's usually high whenever it's running. Battery charging done when it's at its most efficient is best. This WAS all incorporated in my original post, and my elaborations above may make this clearer. Sorry if I wasn't as clear as I would have liked to have been! As regards "pulse and glide," I'm skeptical that this really does result in a FE improvement overall. Can you document this convincingly? I think that my argument shows that this cannot be the case, because of the losses associated with the multiple energy conversions involved.

Well, that's all for now. This is becoming an interesting thread. Sorry about the length of this reply!

Stan

 

You are right in that the TCH maximum efficiency would involve running the ICE all the time at a steady speed where the ICE could run efficiently all the time and avoid power conversion losses involving the battery. This car could easily get 60mpg (4L/100km) or more in such conditions. So technically I agree with your submission. However, the reason it does not apply to the most experiences in the real world is not a mistake, but an omission of a variable that has considerably more impact on FE than power conversion losses. That is the variable organic control system (ie the driver). How does a driver get a TCH to operate in Emode as much as possible?

a) avoid quick acceleration / coast as much as possible
b) operate the vehicle so that it requires less energy than the battery is capable of providing over the longest period possible
c) limit high speed driving to reduce the need for ICE to spin or run.


How do you achieve the best FE in any car?

a) avoid quick acceleration / coast as much as possible
b) operate the vehicle so that is requires the least amount of energy to travel from point to point.
c) limit high speed driving to reduce aerodynamic drag.

And more... Sound similiar?

Having drivers strive for max Emode really does result in max FE because what is really happening is driving styles are being adapted to max FE. I've had tanks where I've focused on max Emode and usually get around 5.8L/100km (~41mpg) and I've had tanks where I tend to drive without such focus and usually get around 6.4L/100km (~37mpg). You are probably right that I incur greater power conversion losses with the 5.8 tank, but my driving style conversion increases more than make up for the difference. This forum has it's share of people that drive TCHs fast, don't bother with specific hybrid driving techniques like optimizing Emode. They typically get less mileage then average, but they probably are doing wonders on limiting power conversion losses.

So I still believe striving for max Emode is the same as striving for max FE. Striving for E-mode means striving to operate the car using the least energy possible. And in these cases, battery power can often be more efficient than ICE in these very low power situations, even if a conversion loss will be incurred charging the battery later.

 

I believe you're taking into account conversion losses but forgetting to fully take into account ICE losses and here it is why: Imagine 2 TCH driving for an hour at steady 40 MPH on flat road. One car uses ICE at all times, another switches between ICE and EV. For simplicity lets also imagine the ICE needs some minimum amount of energy to run, which is totally wasted and the remaining energy is 100% efficiently used to either charge the battery, push the car or both. Now the car with ICE running all the time will waste more energy (minimum energy needed to keep ICE spinning x60 min) than the car that had ICE running part of the time, let's say minimum energy X 40 min. It is impossible for me to say if the conversion losses are higher than ICE friction losses from running 20 minuttes longer without some elaborate tests. If I had to make wild guess, I would think less friction loss from ICE not running all the time would just about cover the conversion loss with some particular cases going one way or another. So IMO it doesn't matter if you keep forcing EV mode or not, at the end it won't make much difference, so maybe why bother? I do agree that changing in driver behavior has much bigger effect on FE than the type of mode the car is in by itself.
Tell me more about the scanner, Ive been looking for something like that. Does it have specific TCH functions or just generic OBDII?