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   === Warp Stealth: a Prius driver's guide ===

Executive summary: the third-generation Prius has a higher-speed "glide"
mode that uses no fuel and allows much longer runs of coasting, but it's
very subtle and hard to find.  It *is* a major key to higher mileage in
that tricky "mid-speed" range from 42 - 55 or so mph on secondary highways.
Here's the how and why.

Most Prius drivers are delightedly familiar with "glide" or "stealth"
mode, where the car moves solely under electric power without the engine
running.  Electric traction and the ability to bring the engine in and
out of play based on go-pedal demand is one of the major hallmarks of
such a hybrid system, and contributes to its ability to use very little
fuel at lower speeds with a foot technique called "pulse & glide".  But
alas, this game seems to cease at 42 miles per hour, when the engine comes
on and stays on as long as the car is going faster than that.  For most
people, mileage suffers somewhat since even at idle or lower power demand,
the engine is consuming *some* fuel.  The concept of a good "glide" seems
to be inaccessible at those higher speeds.

The reason for running or at least turning the engine above 41 mph is to
avoid having MG1, the smaller motor-generator, turn at ridiculously high
speeds backwards to make up for the engine sitting still.  While Toyota
improved the top-speed RPM rating for this part from 6500 to 10,000 in the
3rd-gen Prius, they don't necessarily want to *run* it at those speeds
routinely.  The easy answer is to let the engine spin, which sort of
rebalances all the component speeds in the planetary transmission in a
more sensible way for those road speeds.

However, just because an engine is spinning doesn't necessarily mean that
it is using fuel.  The concept of "fuel-cut" during coasting has been around
for a while in all cars -- when going downhill with no throttle, the fuel
injectors can simply remain closed and thus not even dribble a little bit
of the precious commodity into the engine.  No reason to; it doesn't affect
how the car runs when the engine is being pulled along by the wheels.  Fuel
injection immediately re-commences once the throttle starts opening again,
or speed drops below a certain level, etc. to make sure the engine remains
running properly.  [Some cars do produce a slight lurch when that happens.]

The Prius does fuel-cut at higher speeds too, but several other useful things
also happen when you take your foot off the accelerator:

	Engine RPM drops to around 950, maintained by the motors

	VVTi valve timing is retarded as far as possible

	Regenerative "fake drag" comes into play, charging the battery

	and of course fuel injection ceases entirely.

This is the typical coasting/slowing mode, which feels just like it would
in a conventional car.  But once we're in this state, we can take better
advantage of these low-consumption conditions.  By feathering the accelerator
back on just a *tiny* bit, a driver can eliminate the fake drag and actually
bring the battery in, just a smidge, to help keep spinning the "dead engine"
and push the car a little.  Fuel-cut is maintained, and the retarded intake
cam timing reduces the amount of air sucked in around the throttle flap and
through the engine to almost zero.  This is a very low-resistance state for
the engine to be in, and it only takes a kilowatt or two to keep all that
merrily turning.  Since the regenerative drag is reduced or eliminated, the
car will coast along a lot farther -- still losing speed on the flat, but
fairly slowly, and one can easily maintain speed or accelerate going downhill.
It's really the higher-speed equivalent of "glide" with perhaps a little more
brought in from the battery to compensate for the spinning dead weight of
the engine.  If held long enough, the HV battery charge will begin dropping
just like it would during gentle EV running at lower speed.

How do you know when you're in this state?  Having good instrumentation like
a battery-current readout helps, but you can even tell from the "energy"
display.  It looks just like electric-only drive -- battery to motor to
wheels, no engine.  At any speed over 41 mph!  This is why it's called
"warp stealth" -- stealth really has nothing to do with it, since the engine
does spin and at those speeds the car is making a lot of tire noise, but the
only power source being used at that point is electric.  Unfortunately the
display updates sort of slowly, so if you slip out of this mode by pushing a
little too hard you may not be able to tell for another second or two.  The
best seat-of-the-pants way to reach and hold this mode is to back completely
off for 2 seconds, and then feather very slowly back on until the drag *just*
goes away and you see the electric-only arrows.  There isn't any "deadband"
scenario here -- it's much easier to hop onto the "step" that draws a little
battery current.  Push a little farther, and the engine comes back on and in
fact runs fairly *inefficiently* at a low power demand.  If you reach the
state and back off again, you'll see the green regen arrows reappear --
occasional small bursts of that can be used to help make sure you're holding
just above that spot and you can probably feel the small bit of drag.  Once a
driver gets used to holding the accelerator unmoving *right there*, very long,
free, high-speed and zero-consumption coasts can be enjoyed.  And the numbers
will speak for themselves [or at the very least, say something grumpy about
the effects of air resistance].

A possible downside is that it only works in shift position "D", so the driver
*must* maintain the accelerator position.  Moving to "N" simply causes the
engine to idle, consuming more fuel.  That's about the same RPM as in warp
stealth, in fact.  If it's going to turn slowly anyway, why burn gas and
generate more waste heat doing it?

A battery-current meter makes all this much easier to see.  Removing your foot
from the accelerator brings in about 20 charging amps of regenerative drag, and
then feathering back on quickly reduces that to zero and then crosses over to
10 or so of discharge and that's exactly when you're in warp stealth.  The
really interesting thing is what happens with a little more accelerator.  Over
a certain small range of increased demand, nothing changes!  If the battery is
at its nominal 60% SOC, current drawn from it to maintain warp stealth stays
constant for a while, and then past a certain demand point suddenly the engine
re-lights and battery current goes back to its normal slight wandering high
and low of zero.  There is actually a little "plateau" in demand response, and
to make it even more interesting that plateau's width varies with battery SOC
too.  With the caveat that your instrumentation may reverse the sign of
battery current, have a look at this little chart:
   
which illustrates the "breakover thresholds" as sudden drops to zero, but
really indicates the demand point beyond which the engine goes back to running
and battery-management returns to whatever it would have ordinarily been
doing.  There is hysteresis in the system, so to get onto the plateau again
you have to back the accelerator demand completely off and slowly try again.

This has two notable implications:

	If you've just warp-stealthed a long way on a gentle downhill,
	it will become harder and harder to hold the state because the
	threshold to start the engine running again becomes tighter as
	the battery SOC drops.  At three or four bars or so, you won't
	be able to go any higher than zero battery amps before the
	system kicks back into running and charging.

	If you've just regenned up to 7 or 8 green bars down a mountain
	and now there's a long flat part, you can likely warp-stealth the
	whole way because a high SOC allows you to draw much *more* current
	out of the battery to push the car.  For a while, at least, until
	SOC starts dropping back toward 60%.  Then the plateau begins
	narrowing as above.

The fact that the response plateau exists at all and the fact that battery
SOC affects it convinces me that it is a designed-in mode.  The cruise
control can occasionally find it by simulating the same complete backoff and
gentle re-application of accelerator, but if the cruise falls off the plateau
and the engine starts running again it has no clue what happened.  It's just
luck of the terrain.  You as the driver who doesn't mind a little coasting
speed variance have much better control over how long you choose to warp
stealth -- until you run low on battery, or the right terrain, or that
annoying traffic light up ahead changes and gives everyone the "sixty to
zero surprise"...

There is no observable dependency on vehicle speed, other than falling back
to normal EV stealth below 42 mph.  Warp stealth works anywhere above that.

By the time you're finished with the long-mountain-descent-into-flat exercise,
the engine may have lost so much heat from not burning any fuel that once it
re-lights, it may insist on running a bit more until it gets back up over
70 deg C.  It really tries to stay around 80C.  Of course, poking your EV
button can rein in that behavior until you're really ready to use it.

Oh, and you don't even have to be in full stage 4 for this to work.  So if
your route immediately takes you onto mid-speed secondary highways, you can
still play some great high-mileage games.  One great use of warp stealth is
approaching highway exits -- timing the jump onto the plateau just right can
yield a graceful half-mile or more of zero consumption, through warp stealth
and then true stealth, all the way to the stop at the end of the ramp.

The data for determining all this came from various instruments I've added
to my Prius over the past year or so:

	battery current meter, center-null

	injection monitor [pickoff and LED from #1 injection coil lead]

	vacuum gauge

	coolant-temp gauge

	tachometer [but useless when ignition ceases; the 950 figure came
	from other sources and monitoring the NEO crank-speed output..]

I do not yet know if this applies exactly as set forth to the second-gen
"classic" Prius; someone needs to help me out here.

Reference:

http://techno-fandom.org/~hobbit/cars/five-stages.txt --
   (from Priuschat) -- The Five Stages of Prius Hybrid Operation

_H* 060522