## Daniel's original: http://priuschat.com/forums/kb.php?k=14&mode=article
## Google-cache recovery and minor cleanups by Hobbit
## *Note: only applies to 2004 and later Prius, not the Classics!

            Knowledge Base -> FAQs [Printable version]

   Author: daniel
   Date: Fri Sep 24, 2004 9:40 am Type FAQs
   Category FAQs

   The Five Stages of Prius Hybrid Operation
   A brief description drawn principally from my observations.

   --- The Five Stages of Prius Hybrid Operation --
   The computer that controls the operation of the Prius hybrid system
   (HSD) appears to have 5 distinct stages of operation, which I will
   call Stage 1, Stage 2, Stage 3a, Stage 3b, and Stage 4. (Abbreviated
   S1, S2, S3a, S3b, S4.) Briefly, they govern how the HSD will combine
   power from the Internal Combustion Engine (ICE) and to and from the
   battery via two motor-generators, MG1 and MG2. The following was drawn
   principally from my observation, after reading several posts on the
   subject. EV (electric vehicle) mode is distinct. I drive a 2004
   "package AM" Prius.

   Disclaimer: I am not an engineer or a technician. My understanding of
   this is not complete.

   Definitions:

   ICE: The internal-combustion engine.
   SOC: The state of charge of the battery. (How much charge it has.)
   Shown by an icon with 8 bars on the MFD (multi-function display).
   MG1: Motor/generator 1. Most commonly used to crank the ICE for
   starting, and to generate electricity from the output of the ICE,
   which can then help drive MG2 or charge the battery.
   MG2: Motor/generator 2. Most commonly used to drive the car, alone or
   in combination with the ICE, and to provide regenerative braking.
   (Occasionally MG1 and MG2 can act in arcane ways as well.)
   Golf-Cart Mode: (so called because the car is as quiet as a golf-cart)
   is my term for when the car is moving on electric power alone, with
   the ICE not running. Some have used the term "stealth" which I think
   sounds too militaristic for such a peaceful car, and is misleading
   because the car is not completely silent when moving on electric
   power. There is the whirr of the motor, and the crunch of tires on
   pavement.
   EV Mode: EV Mode is when Golf-Cart mode is entered via an EV button or
   switch. The button is standard on European and Japanese Prius. The
   button or a switch can be installed on North American Prius. For DIY
   instructions, see Dr. Evan Fusco's article in the Knowledge Base. For
   a relatively easy-to-install kit, visit Coastal Tech, coastaletech.com.
   You cannot enter EV mode if: You are in S1; your SOC is very near or
   below 3 bars; you are accelerating; your battery is too hot; or you
   are moving at 34 mph or faster. EV mode will automatically disengage
   when: your SOC reaches 2 bars; you step too hard on the accelerator;
   you reach 34 mph; or your battery gets too hot. The upper limits of
   speed and acceleration, and possibly the lower limit of battery SOC
   are different for EV mode than when Golf-Cart Mode is chosen by the
   computer. In EV Mode the upper limit of speed is lower, but the
   permitted acceleration is greater.
   Normal Mode: Any time the car is not in EV Mode. In Normal Mode, the
   car may go in and out of Golf-Cart Mode.
   I have seen references that say the transitions mentioned below depend
   on coolant temperature. I have also seen references to the importance
   that HSD gives to keeping the catalytic converter warm, to reduce
   emissions. I will simply refer to the system temperature in the
   discussion that follows.

   The Stages:
   S1:
   In Stage 1 the ICE is cold. If not prevented by use of the EV switch,
   the ICE will start a few seconds after the car is powered On. Until
   the system reaches the S1/S2 transition temperature, the ICE will run
   continuously. This accounts for the anomalous observation of electric
   regeneration or no arrows (coasting) at the same time as a low
   instantaneous mpg reading. No power is being drawn from the ICE, but
   it is running. Also in Stage 1 you will notice that battery power is
   used for even mild acceleration. In Stage 1 the HSD prefers not to
   draw power from the ICE if it is not necessary. You cannot enter EV
   mode while in Stage 1.
   S2:
   When the system temperature reaches 40 degrees C. (104 F.) the
   computer goes to Stage 2 operation. The original Japanese poster gave
   more detail than I clearly remember, but it appears that in Stage 2
   the ICE will shut off if it is not needed to provide power. At a
   stoplight, in S2, the ICE will stop immediately and fairly smoothly.
   However, the car will not go into golf-cart mode in Stage 2. It can
   draw power from the battery or it can recharge the battery, but will
   not use the battery without the ICE. I have not ascertained if you can
   enter EV mode from S2.  [Update: you can.]
   S3:
   When the system temperature reaches 73 C. (163.4 F.) the computer goes
   into Stage 3a. In S3a the ICE will not shut off unless you come to a
   full stop for 5 or 10 seconds. Paradoxically, it is more reluctant to
   shut off than it was in S2, and in S3a it shuts off very roughly. You
   will feel the car shudder. But once it does shut off at a full stop in
   S3a, it will then be in Stage 4 operation.
   Once the car is in S3a, if you reach 34 mph without having come to a
   full stop for 5 or 10 seconds, the car will go into S3b operation. S3b
   is identical to Stage 4, except that if you slow down below 34 mph and
   apply the brakes, you will be back in S3a. In S3 you can engage EV
   mode providing that you meet all the conditions for EV mode.
   I have observed that at the full stop in S3a sometimes the ICE stops
   after 5 seconds, and other times runs for 10 seconds before stopping.
   I do not know what determines the difference. I have also observed
   more than one rough shut-down in a trip, suggesting to me that perhaps
   the ICE may sometimes stop roughly in S2, and may not stop immediately
   when you let off the accelerator in S2. I repeat that I do not clearly
   understand Stage 2 operation.
   Coming to a full stop while in EV mode does not seem to trigger S4.
   You must be in normal mode when you stop in order to enter S4 operation.
   S4:
   Stage 4 is normal, full hybrid operation. The Prius is most efficient
   in S4. The car will go into golf-cart mode at the computer's
   discretion. You can encourage it to do so by lifting your foot off the
   accelerator pedal for a moment, and then pressing on the pedal very
   lightly. This is known as "feathering" the pedal. Feathering does not
   force the car into golf-cart mode, nor is it necessary for the car to
   go into golf-cart mode. It merely encourages the car to do so a little
   bit sooner than it would otherwise do on its own. The benefit of
   feathering the pedal is questionable, since the computer is deciding
   when it thinks Golf-Cart Mode is most desirable. Increasing the time
   spent in Golf-Cart Mode may only increase the efficiency losses
   involved in charging the battery and drawing power from it. On the
   other hand, encouraging Golf-Cart Mode when the SOC is very high may
   be useful, as it creates head room in the battery to accept charge
   wheen needed.
   Above 42 mph the ICE must spin, and will generally provide power. It
   is possible for the ICE to spin without providing power to the car
   above 42 mph, and this has been observed. But since the ICE is more
   efficient at these higher speeds, Golf-Cart Mode above 42 mph (Super
   Golf-Cart Mode) is unusual. So the maximum speed in EV Mode is 34 mph,
   cutting out at a tiny fraction over 34 mph, and the maximum common
   speed in golf-cart mode is 42 mph. You can go faster in non-EV
   golf-cart mode than you can in EV mode.

   Thanks to jfschultz for providing the transition temperatures given
   above. They were cited by a Japanese poster some time ago but I had
   forgotten them.

   -- Sidebar on using EV mode. The following is my personal speculation: --
   EV Mode is inherently inefficient because not all the energy fed to
   the battery comes back out of it. Heat is generated when charging the
   battery and when drawing it down. However, there are situations when I
   believe EV mode is desirable:
   Short trips: Because the car's warm-up cycle burns extra gas to get
   the catalytic converter hot as quickly as possible to reduce
   emissions, if you have enough charge to complete a trip entirely in EV
   mode I believe you more than offset the efficiency losses mentioned
   above.
   End of day: This is very speculative, but I believe that the system is
   reluctant to charge the battery if it is at or above 6 bars. Since the
   ICE will insist on running until it reaches Stage 4, more power will
   be produced than is needed to drive the car, and it is desirable that
   this energy be used to charge the battery. I believe that leaving the
   SOC at around 4 bars (or even 3) at the end of the day will provide
   the head room necessary to assure that the most energy possible
   produced during the inefficient warm-up cycle (Stages 1 through 3a)
   gets into the battery. Thus if your first trip of the following day
   will not be short enough to make entirely in EV Mode, I think it is a
   good idea to use EV at the end of the last drive of the day, if
   necessary, to leave the SOC at around 4 bars. You burn no gas during
   that last half-mile or so, and you leave head room in the battery for
   the next morning's obligatory warm-up cycle.
   Very slow driving: I sometimes observe very low instantaneous mpg at
   slow speeds even with a high SOC. And I never observe very good
   instantaneous mpg at slow speeds except in Golf-Cart Mode, when it's
   at 99.9. I therefore believe that it's always a good idea to use EV
   Mode at very slow speeds, such as parking lots or creep-and-stop
   traffic jams.
   Showing off: Sometimes you just want to show off how quiet the car is.
   It may not be the most efficient thing for the circumstances, but it's
   fun.
   -- End Sidebar --

   For some fascinating in-depth technical explanations of how the prius
   works, visit Graham's Prius Page and click on "Understanding the
   Prius." He wrote this for the Classic (2001 - 2003) Prius, so some of
   the gear ratios may be different for the 2004/2005 Prius, but the
   operation is the same.
     _________________________________________________________________

   ken1784 Posted: Sat Mar 19, 2005 9:54 am    Post subject Re: [ KB ]
   The Five Stages of Prius Hybrid Operation

   Hello all,
   I agree this article is very good.
   I would like to report that we, Japanese mileage freaks, found new
   stage called Stage-1B.
   daniel wrote...
   You cannot enter EV mode while in Stage 1.
   However, you can enter EV mode if you have EV switch by following
   process when outside temperature is higher than 0C(32F) and your
   battery level is 6 or higher.
   1. Push "START" on your vehicle.
   2. Keep vehicle in "P" mode, the ICE will run for warming up after
   10-15 seconds from pushing the "START" button.
   3. Watch your energy screen on MFD, and you'll see a green arrow for
   charging.
   4. After 56 seconds since starting engine, you'll see charging is now
   stopped.
   5. Engage the EV switch and ICE stops, now you're in Stage-1B.
   You should use ICE for accelerating by EV switching OFF, then you can
   enter EV mode for coasting and stopping at signal by EV switching ON.
   It is very efficient warming up to run ICE only on accelerating until
   Stage-2.
   Hope daniel will add this stage on the article.
   Regards,
   Ken@Japan
     _________________________________________________________________

   ken1784 Posted: Sat Mar 26, 2005 3:16 am    Post subject Re: [ KB ]
    The Five Stages of Prius Hybrid Operation

   efusco wrote (): > I think the 1B thing is an interesting find, but of
   > little practical use. Why sit still in Park running the ICE for almost
   > a full minute...what a waste.

   Our experiment shows using this method is far better total mileage
   than without the process.

   Quote: > FWIW, the comments about accelerating with ICE and then using
   EV for slow coasting and at a stop is pretty much what I do.

   You can do it on the Stage-3, but you can't on the Stage-1A.
   Regards,
   Ken@Japan
     _________________________________________________________________

   Hybrid_Dave Posted: Mon Jun 13, 2005 1:54 am    Post subject Re: [ KB ]
    The Five Stages of Prius Hybrid Operation

   What a great write-up...as I'm a little over a week from picking up
   and driving a Hybrid, it really helps to have this info on hand. I
   didn't pretend to know much about how this car worked, but now I
   understand a little better. It's things like this that make me glad I
   joined this forum...thanks again
   Dave.
     _________________________________________________________________

   tomdeimos Posted: Wed Jun 29, 2005 11:07 am    Post subject Re: [ KB ]
    The Five Stages of Prius Hybrid Operation

   Stage 1B seems to work great for me, at least sometimes.
   Starting up normally I generally get in summer, 25 to 40 mpg for the
   first 5 minutes.
   Using the EV and 1B I get about the same sometimes, but other times it
   is higher.
   Today I got 50 for my first 5 minutes, and that has never happened
   before!
   The reason it all varies so much is I have 3 long traffic lights I may
   or may not have to stop for and idle at on the way to the highway.
   So for me the 1 minute warm up in my driveway is more than made up
   for. That is preferable to idling at the lights for well over a minute
   often, and I'm usually not at a stage where the car wants to stop the
   engine.
   Plus, I have considerable coasting possible I can do and no need to
   idle the engine there, also longer than 1 minute!
   So if it works or not depends on lots of factors, but mainly terrain.
   I think on average I am benefitting from it.
   Another factor: Couple of hills make it too much for the EV mode alone
   all the way.

#####
Hobbit add-on: There is an additional short period of what appears to be
excessive engine-running after the coolant temperature reaches 85 deg C,
which is the normal thermostat-controlled engine running temp.  What
happens is that the 3-way water valve that routes coolant to and from the
thermos storage bottle opens to let hot coolant flow through the whole
system rather than just engine and radiator, and the hybrid ECU tries
to keep the ICE running long enough to make sure that coolant gets
sufficiently pumped around.  This state lasts only a minute or so,
after which normal S4 shutdowns are re-enabled.

If the coolant temperature dips below 73 deg C while the car is powered
up, which is quite possible in long periods of low demand during colder
weather, the system stays in S4 but the ICE will run [while driving] until
the coolant temp rises above 73 again.  The full-stop S4 transition isn't
necessary.  EV mode can be forced here, too.