New Prius - "Super Highway Mode" Technique (SHM).

[diamondlarry]

Nice work Jerad!

[pdk]

As far as what I was doing, I think I found a sweet spot keying in at 1200-1300 RPM (1248 and 1280 were a good spots). I was getting anywhere between 80 and 150 MPG instantaneous (depending on terrian) and was only losing 1 MPH every ~3 seconds on flat terrain, with accelerating back up to speed I got 25 MPG instantaneous and gained about 1 MPH every second, which roughly figures out to be about 65-70 MPG (not bad in this weather). My trip was worse than that due to suboptimal terrain, though.

I really keyed in on this on the way back and got 55 MPG on the return trip (compared to 57.5 on the go-there trip).

Note that this RPM range tends to switch between IGN 14 and IGN 15 in this weather).

[hobbit]

I'm gonna resurrect this thread, because I've got some new input
for it. I finally figured out why ignition gets retarded in
those low-RPM regions, and it hit me while I was studying a bunch
of stuff about *airplane* engines.
.
Piston-engine pilots generally have control of throttle and
mixture. The amount of fuel metered in does rise and fall with
throttle setting, but there's an additional knob to bias it fairly
heavily around either side of 14.7:1 and most of the time they
run way richer than that. Stoichiometric burn happens to mostly
coincide with the peak exhaust gas temperature, which is one of
the things they monitor so they're always talking about "rich
of peak" or "lean of peak". Besides that, many planes have a
"prop" control which is also referred to as "RPM" -- since varying
the pitch of the propeller tries to make it take a bigger or
smaller "bite" out of the air, it's the closest approximation
they have to a variable-ratio transmission -- i.e. something
that affects the balance of engine RPM vs. airspeed.
.
The kicker? They have NO control over ignition timing. It's
hard-set to somewhere around 20 deg BTDC, which for the normal
steady-state 2000+ RPM that these engines run is appropriate.
But there's a vicious trap lurking for these poor guys -- if they
let RPM drop but still place a high shaft load on the engine,
they're right into detonation land where damage starts happening.
There's no way to sense that and back off the spark timing, as
there is in almost every car made now.
.
The problem is that the rate at which a mixture burns doesn't
change that much, so at lower RPM you ignite well before TDC
but now the peak pressure pulse occurs *way* too early, while
the crank is still around high noon and it isn't going to move
the piston down. Increasing RPM gets the crank around faster
so the PPP occurs at the right place in rotation and now you get
plenty of torque. So pilots learn to strictly avoid low-RPM
scenarios at higher loads and dance delicately around the fact
that their engine control systems are 1930s state of the art
which will simply eat itself under the wrong conditions.
.
So that's why the Prius ECM backs the timing off so far when
RPM creeps down around 1200 -- it's probably mapped to do so,
and keep the peak pressure pulse in the right place. These
values are always trimmed using input from the knock sensor,
and so most car engines ride the delicate line between detonation
and torque falloff all the time. I bet you'd see this in any
other car, too, under the same high-torque/low-RPM scenario.
.
I was just stunned by learning this, but apparently the concept
of variable ignition timing and something other than the old
dual fixed magnetos is only now just barely starting to get
past the crusty FAA certification process. When you consider
that even your typical '57 chevy had a simple centrifugal system,
it just boggles the mind.
.
_H*

[xcel]

Hi Al:

___All is good except you are in the absolutely lowest torque point the engine can output while still maintaining some semblance of speed. When you see it for yourself, you are just going to shake your head and say WTF, the darn thing is barely above idle yet putting out enough HP to keep the car moving through the air at 50 mph while DWL. Maybe 20 HP at most?

___Your example insinuates the timing retardation at this low RPM is a bad thing to keep detonation from occurring but this is exactly what timing advance does. The same can be said from idle all the way up to red line if Toyota decided to tune the 1.5 that way. The Accord’s max FE bottom end comes in at or around 1,400, the Ranger around 1,300 and the MDX around 1,100 RPM’s. All with either a locked up TC (Accord and MDX Auto’s) or the Ranger in fifth. They do not perform anything like the Prius’ 1.5 at IG14 in a somewhat steady state DWL scenario however

___Let me know your thoughts …

___Good Luck

___Wayne

[hobbit]

No, timing retardation isn't a *bad* thing, it's a *necessary*
thing to keep the pressure pulse pushing at the right time *and*
prevent bad things from happening. That implies that the low-end
grunt you seek is available, whereas it wouldn't be from how an
airplane engine is set up and controlled.
.
It just suddenly dawned on me that it's *why* we're seeing the
IGN decrease to 14 or 15 down there. If it didn't, you'd start
hearing rattle, rattle, rattle...
.
_H*

[xcel]

Hi Al:

___The bad part was the connotation and connection with an older airplane engine in the post above. We want timing retardation as that is what it is for maximizing efficiency while reducing or eliminating detonation so as to not destroy anything.

___What I am wondering more about is the HP requirements at that speed and how an ICE just above idle is not only providing enough power to propel the Prius at ~ 50 mph but holding onto such great FE in a somewhat heavy vehicle at the same time

___Good Luck

___Wayne

[BillLin]

Hi Wayne and Al,

> ___What I am wondering more about is the HP requirements at that speed and how an ICE just above idle is not only providing enough power to propel the Prius at ~ 50 mph but holding onto such great FE in a somewhat heavy vehicle at the same time

Would the engine be running MG1 at that time, providing electricity for charging the battery and current for driving MG2 at the same time? Just a thought, given a partial understanding of Al's writeups on how the Prius works... Clearly, that power has to go somewhere. What does the mfd say?

cheers,
Bill

[krousdb]

I have read that the Prius II requires 20 hp at 60 MPH. Based on published weight, Cd and frontal area, that would put it at around 13 hp at 50 mph.

[phoebeisis]

Interesting stuff. I'll have to eyeball the FE of the Suburban relative to ign advance.It is a more primitive motor that the Prius, but it gets surprisingly good FE for a big 2 valve V-8 with its heritage firmly in the 60's.It wasn't my 1st choice(Pilot was) as a evac/home depot/second car, but I could have done worse.
On another related note-HAS ANYONE NOTICED THE NEW RECORD PRICE OF OIL-ALMOST $105/BARREL??
Thanks,
Charlie

[PaleMelanesian]

On our cross-country drive, it seemed to me that combinations of the following make SHM almost impossible:
- Low temps - freezing and below. Once we hit warm So-Cal, it really woke up.
- Headwind
- Cargo weight - we had 3 in the car, plus some bags in the back.