2016 Prius Two : Load Leveling
I used the tag A Road to Carbon Freedom for this series because: Reducing carbon (fossil) fuel usage is but a small first step toward a rational energy policy, worldwide. The Prius leads the way with advanced coding and systems controls.
Okay, maybe Toyota is not the leader, I just made that up (I could have been in Advertising!). I bought a Prius, and I am having a lot of fun groking now it works without consulting with the techs that designed the system. I am of the educational school of thought: Tis better to reason than to listen. Lectures can be so . . .
So, let us reason together.
I opined in the first installment of this series: Regenerative Braking is but the beginning of explaining the fuel efficiency of a Prius (which is really only a gasoline powered vehicle).
That is: Converting, storing, and reusing the vehicle's kinetic energy as it slows (even to a stop) is important, however the true genius in the design is the computer controlled energy flow (power) management under all driving scenarios. Load Leveling is the key to high efficiency for all types of energy conversion systems — let me explain.
Load leveling devices are of several types [distinguishable by the type of energy stored]:
- Gravitational Energy (the oldest LLD: Lift something, then drop it!)
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- You lift a rock or hammer to drive a spike into the ground.
- A seagull drops a clam onto a rock to crack open the clam.
- You pump water from a lower to an upper reservoir for later (peak demand) hydro power generation.
- Thermal Energy
- Shine sunlight onto a rock bed and it gets hot. Pass air through the rock bed to heat your house.
- Elastic Energy
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- Rotate an axle to wind a spring. The spring rotates a second axle to drive a device.
- Compress a gas into a cavity. The pressurized gas drives a turbine.
- Rotational Energy
- Spin a flywheel (gyroscope). Extract the rotational energy to drive a car.
- Chemical Energy
- Charge a battery of electrochemical cells. Dump the charge through an LED or some other machine.
Each energy production/transfer application requires its own unique control system to optimize the LLD employed. Decoupling the LLD's output rate from the LLD's input rate accounts for the efficiency gains for systems incorporating this concept.
- Two very different driving experiences:
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- Driving my Rav4 — a manual with front wheel drive only.
- When I push the accelerator ("gas" pedal) an actuator moves to increase the amount of gasoline delivered to the engine's cylinders, increasing the engine's RPM which transformed through the transmission (a particular combination of gears) increases the rotational speed of the road wheels. Internal combustion engine energetics are hampered by an "unfortunate power curve".
- Driving my Prius Two — a hybrid with front wheel drive only.
- When I push the accelerator ("go" pedal) an actuator moves to initiate a plethora of actions designed to eliminate the dynamically unstable region of the (now virtual) power curve. Over a very large dynamical range, moving the pedal causes an unexpectedly "precise" acceleration.
Most of the time I drive in "ECO" mode. During normal driving scenarios the Energy Monitor will display a variety of energy flow patterns. Occasionally the energy required by the driving situation precisely matches the output characteristics (remember the "unfortunate power curve") of the engine. This state never lasts for more than two or three seconds.
Usually the engine is generating a greater, or a lesser amount of energy than that required by the driving conditions. That's why, when driving a "normal car", you need to continually move the gas pedal.
If the engine needs to produce an excess of energy to be efficient, the excess is routed through the motor/generator to the battery. This particular energy flow configuration is somewhat transient because demand seldom remains constant for long.
This, coupled with Control System's apparent desire to maintain the battery's charge between ≅ 20% and ≅ 80% requires the flow diagram to change as the system responds to its prime directive: The engine, when running, is to do so as efficiently as possible.
- Pop Quiz for the interested reader:
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- Explain the energy flow as depicted in this diagram.
- Reference the mechanism for each segment's energy flow.
- Comment upon the ambiguities inherent within this diagram.
If the engine needs to produce a deficit of energy to be efficient, additional energy is routed from the battery through the motor/generator to the wheels. The most dramatic example of this case occurs as I enter an expressway.
As I enter the expressway I toggle from "ECO" through "NORMAL" through "POWER" to "ECO" as the Prius surges to then stabilizes at expressway speed. The response is so flat as to be unreal (read: "like a video game" or maybe "like a Willy Coyote cartoon"). The acceleration from city to expressway speed is much more constant than for any "normal" vehicle I have ever driven and is like engaging a turbocharger without the downside of sending the mileage though the floor.
Quite frequently the engine is not needed and the battery supplies all the necessary energy. [To repeat myself] The Control System seems to prefer the battery maintain a charge state between ≅ 20% and ≅ 80%, and the engine will switch on or cut off as these levels are reached. For stop and go city driving I have been able to travel about 2 to 3 miles seeing only these two flow states as I speed up
and slow down in traffic. This final diagram depicts the Regenerative Braking referenced in the first installment of this series.
The Prius' performance is enhanced by a factor of about 2.5 because of its onboard load leveling battery of electrochemical cells.
Manual Transmission
If you are driving on the "high side" of the power curve, a slight increase of RPM causes a decrease in power transmitted to the road wheels, while a slight decrease of RPM causes an increase of power transmitted to the road wheels, and the vehicle is operating in a dynamically stable mode.
Meaning: A slight deviation away from the operating point creates a tendency for the system to oscillate about that operating point.
If you are driving on the "low side"of the power curve, a slight increase of RPM causes an increase in power transmitted to the road wheels, while a slight decrease of RPM causes a decrease of power transmitted to the road wheels, and the vehicle is operating in a dynamically unstable mode.
Meaning: A slight deviation away from the operating point creates a tendency for the system to be driven exponentially away from that operating point.
Thus, there exists an optimal RPM for each speed and load condition. Finding that optimal RPM is part of the fun of driving my Rav4. And, while I think I do that well, I am certain I do not do so optimally.
The problem with the shape of the power curve is most noticeable while driving with cruise control engaged. As the vehicle encounters an increasing slope, there comes a point for which the engine can no longer maintain constant speed. As the speed decreases below a critical value the cruise control disengages. In addition: If the cruise control does not disengage by the time the vehicle reaches the summit, it will surge to and often surpass the preset speed by a significant amount. I drive in the West, and have examined this behavior often.
I need to quote my earlier statement:
Usually the engine is generating a greater, or a lesser amount of energy than that required by the driving conditions. That's why, when driving a "normal car", you need to continually move the gas pedal.
A Computer on Wheels with a Video Game in the Hole*
As we were driving on some mountain roads . . . Okay, racing . . . yea so what‽ . . . I cut corners . . . I was trained by a professional . . . okay‽ sheesh‽ my wife pointed out that the Prius was less a car than it was a computer (game‽) on wheels. I just laughed (manically‽) and continued to play with my new toy.
During the past month I have driven my Prius for more than 5000 miles. On several occasions along routes with greatly varying slopes. With the cruise control engaged, the vehicle's speed varies by no more than 1.5 miles per hour. That is: The Prius does not drive like a "normal" automobile. So . . . more to come as this series continues . . .
*This is a play on: Showing a face card, I wonder if the dealer has an Ace in the Hole?
Comments
The Pop Quiz is Tongue In Cheek
To most drivers a Prius is just a different car and like any car you only feel comfortable after driving it for some amount of time. As I noted in the first installment of this series: After driving for a short time (actually a few seconds) I started telling the "Sales Consultant" about the car's attributes. He took the loss of control gracefully because he knew at that moment I would be "making his day".
For me driving a new car is an opportunity to do what I do: From before I can remember my dad taught me to think like a scientist. He had very little formal education but he and the one I am named after loved to challenge each other with "real life" problems to solve. Given all that, maybe it was inevitable I would get locked into this peculiar type of compulsive behavior. I suppose there are worse sorts of compulsions.
w00t! Part Two :=) Ty, PriceRip. I enjoyed the first one and
look forward to diving into this one. Thanks for the explanations; you are a very good teacher. Cheers, mate,
Resilience: practical action to improve things we can control.
3D+: developing language for postmodern spirituality.
I have a 2010 Fusion hybrid. Over 100,000 miles now.
It has the regenerative brake thing.
I love the vehicle. Still decent mileage overall even using air conditioning. About 36-37 mph.
Newer ones get better mileage. Are there significant differences between Prius and Fusion with technology?
Off the top of my head.
There are two aspects that come to mind:
Command and Control
My only real (somewhat limited) programing experience is in Machine Code (you know, the ones and zeros). Small (seeming innocuous) changes can have large effects. I suspect the major advances from year to year are in the Command side of the system.
Thanks.
Love the Prius (all Gen's)
You should try 'pulse glide' a ultra-milage technique. Prius Chat has all the tricks.
I'm cheap tho. I'm looking for a Gen 1 with a bad traction battery. They can be had for less than $1k. People will rebuild the battery for ~$500.
I've always been a 'beater' guy.
I want a Pony!
Wow, me too . . .
My first was an abandoned 4-door '54 Chevy.
My most kludged together was a ford falcon.
My most successful was a Toy Hilux.
But real life and professional exigencies preclude further adventures of that sort.
I used to think Pulse and Glide was nonsense
Largely because my understanding of what was going on in the engine control module was still back in the 70's. Older cars with carbs or fuel injection were always forced to compensate for a high demand (gas pedal) when the gearing had the engine in a low rpm (low power) range by dumping lots of additional fuel into the engine (rich mixture) until a more efficient rpm range could be obtained. This could be modulated by downshifting, assuming you wern't starting off in the lowest gear, but the net result was that the car spent very little time in the "sweet spot" where the engine output was at the optimal rpm.
Optimal is a relative word in this situation since it can refer to either efficiency or power, but I digress.
The Prius is quite different in many respects, but one fundamental mechanical difference is that it's infinitely variable transmission allows for a much more precise matching of the gear ratio, allowing the Prius to spend much more time with the engine at it's most efficient rpm. However, internal combustion engines are never at their most efficient at low rpms, and the Prius is no exception.
So, if you baby the throttle with your Prius in an effort to maximize fuel efficiency, you will inadvertently be keeping the car from reaching the higher rpm range that would actually yield better fuel economy.
Thus "pulsing" to your cruising speed briskly, and then "gliding" with your foot completely off the gas, then pulsing again when your speed drops (and the F-150 behind you is about to crawl up your exhaust pipe) will, in fact, result in better fuel economy than light footing to cursing speed and then holding it steady.
For this Prius driver the "Pulse & Glide" hyper mileage technique is an interesting factoid, but not worth practicing on any road in traffic where it will definitely earn flipped birds from drivers behind you who have no idea what the f#ck you are doing and just want you to get out of the way.
“What the herd hates most is the one who thinks differently; it is not so much the opinion itself, but the audacity of wanting to think for themselves, something that they do not know how to do.”
-Arthur Schopenhauer (1788-1860)
Very nicely written, thanks.
What I find interesting about the 2016 Prius Two, Basic Package (I have never driven any other so I cannot compare) is that sometimes as I turn it on, the engine will start and run at a fairly high RPM. This suggests that Command and Control are set up to avoid:
While driving more than 5000 miles I have detected nothing to suggest that anything I do in this vehicle directly influences the engine RPM.
I haven't given any thought to trying the "Pulse and Glide" technique because it brings to mind the Port Angeles to Victoria ferry crossing. You would not want me to be a passenger in a your car doing a "Pulse and Glide" . . . Trust Me.
Unfortunately there doesn't appear to be a tachometer option to monitor this aspect of performance. Also (while I have not yet asked) there seems to be no way for users to access and modify the operating system. It is as though they don't trust us: Hum, I wonder why‽
I enjoy your explanations
I've been driving my 2010 Prius IV for almost 7 years, and quickly learned to turn off the power train display so I wouldn't get in an accident. I love the precise acceleration of the car, which is quite brisk from a start even uphill. I drive with the car in cruise control mode as much as possible, because the car can drive itself much more efficiently than I can. Even in stop and go city driving in ABQ, which is very hilly, I still get 45-50 mpg depending on season. Now that I'm taking occasional weekend trips out of town I'm running in the low 50's in mileage per gallon.
I just put down a deposit on a Tesla Model 3, which I probably won't get for a couple of years. In the meantime, I'll continue to enjoy driving the best car I've ever had.
Please help support caucus99percent!
Me too
The planned to go into this particular point in painful detail in the next installment. Most of the miles on this car are from driving between Kearney, Nebraska and Medford, Oregon. So not only City versus Expressway but also Flat Lands versus Mountain Passes. I have a boatload of data to share.
Considered one, got a Mazda3 instead
I like that little car, which may be my last. Decent gas mileage (and I am still running snows), and drive by the mpg clue on the dash. In good conditions, that indicates 45-99 mpg. One bizarre meaning counterintuitive observation is that it has traction control. Foe winter driving, especially up my hilly between-the-trees drive, I am less stuck if I switch Traction Control OFF and drive manual in second gear. Otherwise, it's an automatic (my first, evah).
Hey! my dear friends or soon-to-be's, JtC could use the donations to keep this site functioning for those of us who can still see the life preserver or flotsam in the water.
Experience versus Technology
I learned to drive by watching (while standing in the middle in the front seat) my dad drive. I like to say I was trained by the best. He designed, built, and raced cars in the dirt track circuit when he was young and stupid. His last accident (5 back and 3 neck vertebral breaks) put him out of that game. From him I learned driving techniques used in police academy training. Chief among them, how to drive Oregon passes without using tractions devices of any type. The only time I have ever used chains was when I drove a step van (Holsum Bread Truck) over Cabbage Hill one winter (company vehicle, company rules).
This will be the only problem vis-à-vis the Prius, I do not trust being disconnected from controlling the driving while traction is an issue. I will use my Rav4 during incumbent weather in the Oregon Mountains. Being able to directly control tire torque saved my life one day in Wyoming a few years ago.