Droid13 — If what you are saying were true, you might indeed be beating Toyota's ECU programming. But is it true? The following technical paper (and indeed it is very technical!) by Dr. John M. Miller of the University of Michigan: http://www.engin.umd.umich.edu/vi/w4...Miller_W04.pdf

models the 1st-generation Prius' hybrid drive system in great detail. [Bear in mind that the TCH's components are more powerful than the Prius', so the numbers wouldn't be identical for the TCH, but the principles are the same.] The three graphs on Page 62 are particularly instructive.

The left-hand graph represents the ICE. The heavy black curve is the ICE's torque limit — it must operate below this curve in the torque-(ICE-rpm) plane. The dashed grey curves represent curves of constant power output, and the continuous grey contours represent curves of constant efficiency (labelled in grams of fuel consumed per kilowatt-hour of energy produced by the ICE). The greatest possible ICE efficiency occurs in the center of the contour labelled 200 — let's call it 195, say. The red curve with the large dots shows where the ICE's operation falls at constant road speeds on the flat — the dots are for speeds of 30, 40, 50, 60, 70, and 80 mph. Note how Toyota operates the ICE so as to approach maximum efficiency as its road speed increases, and that the Prius' engine is being operated at its maximum possible efficiency on the flat at a speed of around 75 mph. At lower constant speeds it is significantly less efficient. For example, at 40 mph its efficiency has dropped by a factor of 195/250 = 0.78, i.e., by ~22% from its peak. At 60 mph it has dropped by ~15%.

The graph at the top right shows corresponding curves for MG1 (the "generator"), and the graph at the bottom right shows those for MG2 (the "traction motor"). The bounding curves represent their torque limits, and the interior contours are labelled in % MG efficiency. Again, the red curves show where in the torque-(MG-rpm) plane the electrical components are operating at road speeds (marked by triangles or squares) from 30 to 80 mph. Note that, in this vehicle, MG1 and MG2 are being operated at efficiencies between 85 and 95% over this range of constant speeds. [So, my earlier assumption of an efficiency factor of 0.9 for each MG is indeed realistic.] The lower right graph shows the enormous torque reserve available from MG2 for on-demand acceleration at all speeds.

So, how does the Prius' pure-EV mode compare with its pure-ICE mode? I believe that the graphs show that, unless you can prevent NiMH battery charging from occurring at speeds under maybe 60 mph (which, of course, you can't), you are clearly better off NOT to try to force pure-EV mode operation, as it will definitely be LESS efficient (taking into account the additional re-conversion losses) once recharging of the traction battery by the ICE has taken place. I'm certain that corresponding conclusions can be reached for the TCH too.

Stan

 

Cool! My turn...

Using your example, the graph you are talking about nicely shows how a Prius ICE is operating at max efficiency when cruising at 75mph. Everyone knows however that this speed is not the best way to get great FE. Why is that? According to the graph driving for 1 hour at 75mph the Prius needs about 18KWh of power so even though it is getting best efficiency at 195g/KWh (3.51KG of fuel), its the amount of power it needs to generate to maintain this speed. Dropping down to 50mph for 1.5hr (same distance) reduces efficiency to about 237g/KWh but the power requirements drop to almost half (about 9.5KWH based on the huge red dots on a tiny graph ). So for a power requirement of 14.25KW for the same distance the amount of fuel needed is 3.37KG.

Even though the graph is hard to read for exact numbers, it does illustrate that max real world efficiency does not necessarily come at max engine efficiency, its a balancing act with reducing total power requirements as well. This illustrates my point. Emode is the state at which the vehicle has the absolute least power requirements with infinitely maximum engine efficiency.

Yes, you will need to charge the battery later with some conversion losses, but the graph clearly shows that the ICE operates at higher efficiency at higher power outputs, so adding some additional battery charge power requirements later will push the ICE to operate at an even higher efficiency. The graph shows that the increase in engine efficiency begins to slow as it approaches the upper power output levels, so you don't even need to drive fast to get the benefit of this efficiency increase.

So Emode gives you a low power requirement state coupled with max engine efficiency now and higher engine efficiency later during battery charge. Sounds like a good deal to me!

 

If the batteries are near fully chraged, because you have not used e mode enough, and you are running over 42 MPH, and the engine is producing more power than is needed to sustain the current rate of motion, would that energy be lost if there is no head room for the batteries to take the excess?

I just got my car last week, and I have noticed that since I do 80% of my driving at speeds between 55-70 MPH the motor is constantly switching between charging the batteries and dis-charging them. While it is doing this the indiator shows 1 bar left and the battery is green. I know there is some head room left for charging, but if I try to run in E mode as much as possible prior to doing my highway driving, I am assuming I am leaving the batteries in a state to accept the excess energy that would normally be lost if there was not battery capacity left to accept it.

I am also assuming it would allow the engine to ramp up to the most efficient spped to promote battery charging if the engine would normally run at a lower efficiency speed if there were no need to charge the battery.

 

I'm a physicist and I understand all that. If you go back far enough, the energy really comes from the sun! Or before that, the "big bang", etc. The only USEFUL discussion is the behavior of the car for a single trip - or a section of a trip. My TCH does just perfect on flat ground, open county, and up hills. On downhills and in stop and go, I wished it would take better advantage of the regen braking opportunities and let the battery get at least a little bit lower before kicking in the ICE. Or if it seems impossible for the computer to figure that out, it would be nifty to have a switch so I could tell the computer what my anticipated driving would be for the near future. In regards "one size fits all", I think my TCH overall does well. But why not go the next step and have a flip switch to select between 2 different ECU programs? Or a rotary switch to select among several? That would be SWEET!

 

Dear tedpark,

You're one up on me as to physics. I have a degree in physics, but never worked as a physicist...went into law instead.

I agree with you that consideration of how energy is supplied to the car during a trip or a section of the trip is useful. As far as I know, at any moment, there are only three sources of kinetic energy available to the car - converting chemical potential energy in gasoline by running the ICE in traction mode (the planetary (ICE input) gear is driving the ring (output) gear in the Power Split Device), converting chemical potential energy stored in the traction battery by running one of the electric motors in traction mode (the sun (electric motor input) gear is driving the ring gear in the PSD), or converting gravitational potential energy through coasting downhill in the regen mode.

The point I tried to make in my last post (and I think one of the points that SPL was making in his original post) was that, over time, the only source of kinetic energy to the car is through the combustion of gasoline in the ICE. It recharges the traction battery (no plug-in capability with the TCH) and it (or the traction battery that it has charged) provides the energy to gain altitude that creates the gravitational potential energy that it will later convert to kinetic energy.

Yes, the initial charge of the traction battery may come from a source other than running the ICE. Yes, each time the car travels net downhill some of its kinetic energy comes from the conversion of gravitational potential energy. But over time the only thing that is added to the car is gasoline with its stored chemical potential energy. This truly is the only source of energy to the car.

As to your point of making more control map choices available in the ECU, I completely agree. I believe that similar technology is already available in cars that have automatic transmissions with "sport" or "ecomony" modes. Selecting either of these really just commands the EMU to apply a different ignition map. Theoretically, Toyota should be able to make this available in the TCH.

 

I just thought I'd jump in with an observation after reading this thread over the last couple of days.

I think there is a confusion among terms. Effeciency seems to be used for both fuel economy (e.g. miles per gallon) and power losses through the drive system.

A point was made that the energy delivered to the wheels would have gone through less losses in a non-hybrid version of the TCH versus the hybrid version. Although I do believe this to be true, that does not imply that the TCH has less fuel economy than a non-hybrid version.

Take just two little parts of the system: regenerative breaking and turning off the ICE at a stop light. If the only use for the battery was to restart the engine at a light and the only place to put breaking energy was in this battery, I think people would clearly see an increase is overally fuel economy. (Idling energy that would have been lost to greenhouse gases and heat doesn't occur and breaking energy that would have just turned in to heat is instead turned in to some heat and some electricity.)

Another point on efficiency: running the engine at optimum power conversion efficiency (lowest losses) also does not imply the least fuel usage. A hybrid system can store this extra energy produced at a more efficient RPM, but since it is extra energy then extra fuel was burned. A non-hybrid system would have the car running at a different RPM and thus a different energy conversion efficiency. Although the fuel usage is less, there is no additional energy stored. This would seem to imply that a non-hybrid might use less fuel. However, most fuel (as most of us can easily see on our meters) is used during acceleration where the ICE isn't allowed to run at the best conversion RPMs but rather at RPM increases designed to flatten us in our seats (alright, maybe not in these cars...). Instead, though, in a hybrid the electricity generated during high conversion efficiency RPMs is used during this time to help flatten us in our seats while the ICE RPMs and fuel can be better optimized. And, of course, the electricity still also comes from other sources that would have previously just generated nearly pure heat.

That's what makes the hybrid systems have overall better fuel economy even though there are many more places for power losses. The smart engineers at Toyota (and Honda, and others) have figured out ways to increase fuel economy by gathering, recovering, and storing more energy than is lost in the process of the gathering, recoring, and storing of the energy.

(Think of this: 10kilowatts recovered from breaking at 90% loss of energy through various conversions is still a killowatt more than without the recovery. And that's just extreme loss to make a clear example.)

Just my 2 cents (if it's even worth that...)

 

Yeah, I have a VW Touareg. It's got that. I don't think there ought to be too many - it would be two confusing - but 2 or 3 would be really great. They already have D and B - just add one more. Don't get me wrong, we love our TCH, but we scream at it (gnashing of teeth, etc.) when it kicks in the ICE just before a long downhill that would be more than enough to top up the battery.

 

This is exactly right. Engines are typically most efficient run at a fairly high load, in the lowest RPM possible using tall gearing. At any given RPM, a certain amount of energy is always used to pump air through the cylinders (this is what causes "engine braking" when the engine is spinning, but there is zero fuel output), so low RPMs but high load will minimize both pumping losses, and friction losses.

Since driving at 75mph requires more work to overcome increasing air drag, engine load will increase, and engine efficiency will improve, but total fuel consumption will increase as well. However, the idea of pulse-and-glide however is to run at this high output, but then use the extra momentum of the car as a "battery", and then coast using the stored momentum with the engine off.

Possibly the biggest improvement available from a hybrid system is the ability to downsize the engine, but use the batteries/motor to develop peak power. When comparing say a 1 liter engine with hybrid drivetrain to a 1.5 liter car with the same acceleration specs, the hybrid will perform much better at level cruising, as the smaller engine will be running at closer to optimum load when cruising.

I woudl agree with the original poster. The times when using all electric makes the most sense are under very slow speed cruising (where the engine will be barely above idle), or heavy acceleration, where the extra torque from the motors will keep the RPMs climbing. The control laws should be very good at operating the motors when it is wisest to do so, ie, when to run the engine would require it to operate at a lower efficiency than max efficiency.

The original poster is not saying that using electric vehicle mode can be a good thing, but that using it too often may not be the best idea. Once scenario I can think of is when accelerating -- rather than accelerate extremely slowly in EV mode, and drain the state of charge, it would make more sense to accelerate moderately, with the ICE running under efficient load, using little to no battery assistance. Accelerating up to a given speed will require the same total kinetic energy no matter what, the way to get up to that speed using least fuel is to get that energy directly from the engine at its optimum output.

There are times however where it may pay to override the control laws and "force" all electric mode, such as:

1) Starting up the car and moving it a short distance -- no reason to start warming up the engine if you will only be running it a couple minutes, it will be better off to recharge that battery when the engine has a chance to warm up, later.

2) If a long downgrade is coming up -- here it will make sense to use your stored up battery capacity, as otherwise it will fill up on the descent and you will be forced to waste energy on friction braking.

Likewise, there are a couple situations where it will pay off to use less battery power, even if it means running the engine temporarily at a less efficient load:

1) Climbing a very long grade that will quickly deplete the battery -- it will be best to space out elecrtic assist evently through the whole climb than run efficiently for the fist half of it, and at some high RPM for the rest of it once the battery is spent.

2) If a stretch of prolonged slow-speed stop and go driving is about to come up -- it will make sense to save the battery capacity for later.

This is the whole problem of the car not being able to see the road, I definitely believe control over the battery logic should be under some degree of manual control, becaues of the above reasons. However, the point of the original poster is just that given such control, using the battery as much as possible and turning the ICE off as much as possible would not be the best way to use such a control.

 

Sorry about the long hiatus — I've been rather busy these past couple of weeks (work does sometimes take precedence, I'm afraid!). I'll now try to reply to your comments. Since some of us have been exposing ourselves, I guess that I also should admit to being an applied mathematician and physicist.

Droid13 and lilbyrdie — Yes, you're quite right that ENGINE efficiency (as shown in Miller's graphs in grams (g) of fuel consumed per kilowatt-hour (kWh) of energy produced) and VEHICLE efficiency {as measured in liters per 100 kilometers (L/100 km), or reciprocally in mpg} are different things. The increasing air resistance with road speed (which Miller models as being proportional to the SQUARE of the vehicle's speed) can dominate over engine efficiency at higher speeds. This means that it's not simple to estimate optimal driving parameters. However, I disagree TOTALLY with your CLAIM that you can recharge the NiMH battery later at higher efficiency, and so benefit overall from forcing pure-EV mode. This is wishful thinking! [If you can substantiate your claim by sound argument, I'm willing to be convinced.] I shall now give the results of my own calculations along these lines, based on Miller's paper. [Some of his graphs have misplaced red "dots," and there are some obvious numerical typos, which I have corrected.]

1) First, let's calculate the fuel conumption rate for driving in normal ICE mode at 40 mph (there will also be battery charging occurring). I've chosen this speed as I want to compare this scenario with pure-EV mode, and 40 mph is close to the highest speed at which pure-EV mode is possible in the 1st-generation Prius, which is the model for which Miller gives detailed data. Miller is using an efficiency factor of 0.69466 for the conversion of the pure thermodynamic power (Pe) produced by the Atkinson-cycle engine to the actual mechanical power (Pem) appearing at the ICE's crankshaft at this road speed. He calculates that the thermodynamic power required from the engine is 6.47 kW, and since this occurs at a fuel consumption of 250 g/kWh, this requires 1618 grams of gasoline per hour.

2) Second, let's suppose for the sake of argument that we force pure-EV mode at 40 mph in this model Prius. The driveline power required is 5 kW before final drivetrain losses. This 5 kW, using his efficiency factor of 0.91 * 0.94 = 0.8554 for the electrical to mechanical conversion (assuming that this includes battery storage conversion losses too — it's not clear whether this additional loss factor is actually included), the battery power required is 5.85 kW, and this needs to be made up by recharging the battery later.

3) Third, let's assume that the battery is then allowed to be recharged to its previous state at the same rate as that at which it was discharged, while driving at 40 mph in normal ICE mode. Using Miller's ICE efficiency factor of 0.69466, and loss factor of 0.8554 for the mechanical-to-electrical conversion, the combined loss factor is 0.59421. The required additional thermodynamic power from the ICE is 9.84 kW, and at 250 g/kWh this uses an additional 2459 g/h of gasoline.

4) Comparing fuel usage in cases (1) and (3), we see that forcing pure-EV mode has consumed (2459 - 1618) = 841 g/h more gasoline than just running in normal ICE mode, where the consumption rate is 1618 g/h. The conclusion is that forced running in pure-EV mode has made our fuel consumption 52% WORSE THAN NORMAL ICE MODE! I rest my case.

By the way, assuming that steady-state conditions apply, as they must if the car is being driven for a prolonged period in ICE mode, there can be no net battery charging or discharging occurring — all electricity generated is immediately consumed in the motor. Since battery capacity is only sufficient for short-term boosting (i.e., only a few minutes worth), all longer-term driving on level ground derives ALL the needed power from the ICE alone. This does NOT mean that the MGs aren't involved. They are in use almost all the time, but this doesn't show up on the simplified displays, which show only a single "motor" and can't display the complexity of what's actually happening. At lower speeds, electrical power flows from MG1 (acting as a generator) to MG2 (acting as a motor) in order to let the ICE run more efficiently. At higher speeds, electrical power flows from MG2 (acting as a generator) to MG1 (acting as a motor) — the so-called "heretical" mode. Under these circumstances, one might out of theoretical interest try to compare a "pure-ICE" scenario (NO battery charging or discharging occurring) with a "pure-EV + battery recharging" scenario. But, I'll leave that for another time!

Finally, it's of interest to see what fuel efficiency figures can be computed from Miller's graphs. I'll compute them in metric for convenience, and give mpg equivalents. Using the formula:

L/100 km = 100 * (g/kWh) * (kW) / (g/L) / (km/h)

and the fact that gasoline has a density of ~740 g/L, I find from Miller's graphs that for the Prius I:

@ 40 mph (64.4 km/h) fuel consumption is 3.4 L/100 km (69.3 mpg)
@ 50 mph (80.5 km/h) fuel consumption is 3.7 L/100 km (63.6 mpg)
@ 60 mph (96.6 km/h) fuel consumption is 4.3 L/100 km (54.6 mpg)
@ 70 mph (112.7 km/h) fuel consumption is 5.0 L/100 km (46.7 mpg)
@ 80 mph (128.7 km/h) fuel consumption is 6.1 L/100 km (38.3 mpg)

These numbers seem to be in the right ballpark, and give me some confidence in my calculations about pure-EV mode above.

tedpark — I disagree with you. I think that it is NOT meaningful to compare only portions of a trip. For example, on a downhill portion, the ICE may be off all the time (infinite mpg), while on the return uphill portion the ICE would be on all the time (low mpg). The only meaningful comparison is, as I said right at the beginning, to assume that the car returns to its original location (same altitude) at the end of the trip.

Double-Trinity — This thread is considering only the supposed "advantage" of forced pure-EV mode. I believe, like you, that "pulse and glide" methods could likely achieve higher mpg figures, but they aren't practical for everyday driving. And they are for a different thread.

Stan

 

I suspected as much from a Waterloo guy... Best I can say is I know how to add and substract , but I am a decent observer of the real world.

I'm not disputing the math, just the assumptions. If the real world could actually play out according to your assumptions I wouldn't take issue. Unfortunately my TCH operates at many speeds besides 40mph. To make near instantaneous jumps to 40 mph requires approaching the speed of light, and that would require near infinite energy. Not very fuel efficient with or without Emode, but it might help retain my youth. Ha ha. Ok, bad physics joke.

Your original post was all about "forcing Emode" being a less efficient driving style than driving normally. Yet your math, while impressively detailed, continues to ignore the different variable inputs from the driver that differentiates "forcing Emode" vs driving normally. You are ignoring the essence of your own proposal by not accounting for that most important fact that driving styles are different. You continue to assume the vehicle is being operated under identical circumstances. If the TCH had a button that allowed the driver to force the TCH into Emode while making no other input changes to the control of the car, the yes, a case might be made. However, no such button exists. The only case you've made so far is that installing this kind of button is probably a bad idea, and it looks like Toyota agrees with you.

Normal driving (as compared to forcing Emode) means driving the car according to posted limits (or thereabouts) and with typical safety regards for traffic and road conditions, etc. For example that may mean driving 65-70 kph in a 60 zone, driving 15m behind the car in front, braking and accelerating with traffic. Maintaining a steady speed if road conditions allow.

Forcing Emode means avoiding accelerating to 65kph and beyond if practical even if you would normally do 70kph. It means driving 30 m behind the car in front so you have more room to take up speed changes from the car up front. It means letting the speed bleed off gradually (maybe to 55kph if traffic allows) going up slight inclines to avoid an ICE start (ie constant speed. For some people it means avoiding A/C where practical (like me).

You can't convince me mathematically that one driving style is better or worse than another if you don't even attempt to define and account for the difference in those driving styles.

You have convinced me and it makes perfect sense without the math that driving this car at a constant speed over a decent distance is likely to be more efficient if energy comes directly from ICE and not battery, but that ain't forcin' Emode! Curiously though, if you drive a TCH at a steady speed of 60kph it will go into Emode on its own sometimes (insert Twilight Zone sound track here)

Cheers.