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After that really horrendous first day's rain and overnighting at
JimboK's, the next day cleared up nicely as I headed south on I-95.
Once out of Richmond the terrain is fairly level, so I decided to
do a couple of small experiments. I think we generally agree that
the right steady-state highway technique for the Prius is to lock
your foot into known efficient engine-running regions, and don't
stray too far up or down like cruise-control would. As I headed
onward that morning the MPG really wasn't doing all that well,
which I can only attribute to that morning's tankload of crap winter
gas from the Sheetz near Jim's house -- that and I was sorta moving
right along between 60 and 65ish. I got to thinking about *power*
to overcome wind resistance, possibly with this map (http://techno-fandom.org/~hobbit/cars/hp-color.gif) in mind, and
having done a small xgauge mod (http://www.cleanmpg.com/forums/showthread.php?t=11742&page=5#post162988) to read engine kilowatts, decided
that I was unlikely to salvage this tank's average much out of
mid-fifties territory and it was time to play with numbers.
.
So I brought my running kW up to 12.0, and tried to hold it right
around there as much as possible, accounting for minor deviations
one way by letting it deviate the other way for about as long.
A serious exercise in Not Moving One's Foot -- not a game for the
leg-cramp prone, certainly. In reality the RPM and torque curves
do shift around relative to car speed, so I did have to compensate
a little on the gentle ups and downs. The update rate of KW / HP
is also not very fast because the SG is doing a bunch of math
over other parameters to figure it, so one has to take a very
damped control approach and wait for it to settle.
.
I started off by trying to hold 12 kilowatts, and here are the
average surrounding running conditions as eyeballed and mentally
averaged on the fly.
_ 12 kW
_ 1660-1680 RPM
_ 5.6-5.7 ms injector
_ 25% TPS
_ 60-62 MPH on the flats
_ 51-52 MPH climbing, 65ish on backsides
_ 56F ambient
_ 17C IAT
_ elevation 100-200 feet, no major excursions
_ light variable right/front sidewinds, I think
and I held that state for about three hours, letting speed change
mildly on the small rises and falls. The MPG average by the car's
ticker settled on 56.2 at 300 miles on the tank and stayed right
there most of the way, so I figure that's how I was doing
steady-state for this.
.
This experimental phase was over as I neared Florence SC where
I'd pick up I-20, and pulled off to deal with the
Sanders truck (http://www.cleanmpg.com/forums/showthread.php?t=17923&page=2#post173809).
Then once I got on 20 it was anothe flatlands run I'd have at
least an hour on, so I went for phase two:
_ 14 kW
_ 1800 RPM
_ 6.1 ms injector
_ 26% TPS
_ 63-65 MPH
_ 55 MPH climbing, push 70 on backsides [and being 100% passed]
_ same ambient
_ same IAT
_ same terrain
_ winds were beginning to die down toward evening
for about an hour, and then the terrain started getting a little
hillier toward Columbia and it was definintely over when I came
up against a stop-n-crawl about 5 miles before getting onto I-77.
This brought the MPG average down to 55.9 at 362 miles, and if I've
done my math right, yields 54.8 MPG for the last segment as it
worked on pulling down the overall average. And the people doing
70+ are wondering why they're pulling 44 mpg all winter...
.
Under any scenarios in this range the engine is pretty much fully
loaded and running the minimum RPM it needs to sustain speed, but
I'm still fishing for any minor sweeter spots in this range and not
having much success. Torque falls off around 5.1 ms injector so
basically I try to stay in the loaded range all the time and vary
toward its endpoints to adapt for terrain. It's pretty much how
you DWL in a Prius, I imagine. It's not Wayne's 71 to NYC but it
still bests the *old* highway EPA on this car, going faster than
their idealized 50 or 55 MPH warm-weather test. I really wonder
how they settled on 51 since it's pretty easy to just sit there
like a lump and wait for a long trip to play itself out. Anyway,
I still have this nagging little suspicion that *varying* running
conditions like pulse, warp-stealth do marginally better than
steady-state, but I'm not sure of the best way to force something
like that on the flat.
.
Astoundingly, the first day's ballpark average in the screamin'
rain was 58.8, probably due to the generally lower speeds I had
to go most of the way. Not sure how many kW I was averaging,
since I was paying much more attention to the road! But I was
shoving an awful lot of water aside in the process, so I'm
surprised it was that high.
.
Too early to tell how the no-ethanol tank I put in last night
will do yet. Anything else I should try while doing a whole
lot more flatland tomorrow, through GA and FL?
.
_H*

Any chance we can get this moved to the Prius section? I think would be very useful there.

Josh

moved .

You're probably right; frankly the fact that there's a specfic prius
section escaped my notice. That's where I meant this in terms
of what info is presented. [any mod that happens to see this,
please feel free to move the thread]
.
The next leg of this was yet more flatland, especially on I-95, and
I decided to pretty much stay at the 14 kW mark as long as possible
and managed to do almost half a tank at that level. Speed seemed
to hold at 65 MPH or a little more, and MPG settled in around 57.7.
This part of the experiment pretty much ended as I reached some
construction in Georgia, which then went on for over 20 miles.
Wandering through that at 55 or so MPH popped the average back up
to 58.6 fairly quickly, and the "legs" on the MFD looked to be
well into the high sixties for half an hour.
.
I was able to derive another little sort of hack table of ballpark
kilowatts vs speed and one or two mpg notes, eyeballed from the
iFCD and secondarily from the scangauge's guesstimate:
.
kw, spd
11, 57 @ 63 mpg
12, 60 @ 59 mpg
13, 62
14, 65
15, 68 @ 55 mpg
.
pretty much showing the usual 60 @ 60 crossover. The temps gradually
rose through the fifties as I proceeded through all of this. 15
kilowatts is getting up near 2000 RPM, 6.2 ms injector, which is
where I'm trying to limit my "DWL max" unless climbing a more
serious hill.
.
No, I'm not sure where I'm going with these figures, other than
just offering more evidence on what the car is capable of by simply
keeping operation in that sweet band on the highway.
.
_H*

Hobbit it was already moved ;)

This is some really interesting data, I wonder how the new version of HSD will stack up against this one.

It would end up there eventually. It has to start some place! I enyoy reading about the Prius as well as other cars. So relax fellows.H

I'm hoping no one took what I said the wrong way, I'm a Prius driver to ya know. :)

Kacey must have moved it while I was in the process of posting,
which actually took a couple of hours since I got distracted in
the middle and it sat there in a buffer the whole time. I'm having
to do a lot of the "poke at the parental computer" stuff this
week...
.
_H*

I think I'm going to plug in the Kw x-gauge and see what I can come up with during P&G. I wish I would have had this when I took a few decent length highway trips a few months back. Anywho I'll throw it in and see what happens.

Josh

One thing I forgot to mention back then was that the second day
holding the "14 kW" conditions was on that supposedly ethanol-free
tank that Kacey took me out for the evening I got to Columbia. It
didn't seem to make a whole lot of difference, though, in almost
identical conditions -- maybe one MPG higher, hard to say. Or
maybe conditions weren't identical enough ... without doing formal,
round-trip A/B tests with full instrumentation resets in between,
it's hard to ferret out these subtle differences.
.
But one thing I did manage to get done while in FL was change the
spark plugs in the Prius, taking advantage of the warmer weather
while I have it. At 96K it's just a little early but I've been
curious about wear characteristics so I went ahead. Now I get to
see if I can tell any MPG difference *that* might make. I've got
nice close-in pix of my old ones, one of Jesse's old ones at 120+K,
and the shiny new ones that I'll get on the net at some point. The
way the iridium electrodes become pitted is pretty interesting.
.
_H*

I noticed they've had the 10% placards removed from the pumps at my last visit, and the stuttery filling was something I was able to kind of get used to, we'll see at the next fill.

The trip home was relatively uneventful, except for one particular
United van (http://techno-fandom.org/~hobbit/pix/FL08/united/) making deliberate threats. The new plugs don't seem to
make much difference; I pretty much steady-stated my way across the
southeast flatlands up I-95 to JimboK's for the night, and then kept
going up through MD and PA, with the car showing tanks at 59.3 [still
in FL], 58.8, 57.3, and 59.3. That's all at a usual push level of
14-15 kW, doing 65ish and still being almost 100% passed, and
grabbing a little warp-stealth wherever I could which was only on
the occasional overpass backside.
.
My final fill was near the NY/CT border around Brewster, and I
decided it was time for a slightly different experiment. Starting
on a fresh tank, I drove the rest of the way home like a total ass.
.
I-84 through CT is usually pretty horrendous, and I was hitting it
right around weekday rush hour or a little after. After watching
1200 miles of left-lane conga line go by me I got to thinking ...
having been driving like I drive nowadays I'm possibly *out of
practice* with handling that headlong, high-speed rush over in the
left lane, but also that it should be possible to maintain high speeds
with that flow of traffic and still do it reasonably safely and
maintain good distance ahead. So it was time to play, gain some
perspective, and just see how bad the resultant MPG would be.
.
So I swung over after a small bunch passed by, and started mixin' it
up with the left-laners. I found that the average speed they go is
about 75 - 80, with a few bursts up to 90 as one or two particularly
fast ones came by. In stark contrast to everything we talk about
here, I responded to someone coming up behind me by *letting* them
push me faster, i.e. sticking my foot in it to get away from them --
with one major difference, that being still at 4 or 5 seconds behind
any car ahead of me. To accomodate overall flow I bounced between
the middle and left lanes as circumstances seemed to dictate.
.
I cannot remember the last time I actually worried about cops and
speeding tickets, but here I was being worried about it. Sure, so
many other people were flying along too, but there I was, watching
the median in a long linear blur and thinking "y'know, it would be
*so* ironic for *me* to come home with a speeding ticket..." but in
the interest of Science, I kept my eyes open and kept at my efforts
to "be like them".
.
There actually wasn't quite as much traffic as I anticipated on 84
that evening, so it wasn't quite the meatgrinder I expected. There
were still plenty of fast "point people" to follow, so I just picked
them here and there and tried to keep up.
.
To maintain that 75+ MPH level, it seemed to take about 25 kW at 2900
or 3000 RPM. Acceleration from there swung the needle up to around
4000, pushing 35 to 40 kilowatts -- still only 50 horsepower, but that
was plenty to scoot me down the road and reach peaks of 85 or 90 MPH
when following some of the others and passing clumps in the middle lane.
As power demand in the Prius increases substantially past 4000 engine
RPM, there seems to be sort of an inflection point at which the whole
system sort of takes a big deep breath and goes into a totally
different running state. I've referred to it as "WFO mode", with the
obvious derivation from "wide open" referring to the throttle. [It
actually only goes to 55% TPS which you can see (http://techno-fandom.org/~hobbit/cars/tb/tps/) isn't really that
near physical full bore.] But this is where the vacuum actually hits
the peg at 0, and a distinctly different roar starts coming from the
engine. If you've ever floored a Prius you know what this feels like.
I stayed out of that region, since just under 4 grand still yielded
plenty of oomph to get away from someone coming up behind, and I
still applied sensible practice with regard to terrain and momentum
and spacing. Frankly, it's really weird to be pulling a warp-stealth
glide at 85 MPH. You sit there thinking "wow, at this speed a glide
will last forever" but in reality, the air resistance saps away speed
pretty quickly and for many gentle downhills I wound up parking the
RPM back on 1500 and maybe 11 kW or so just to maintain until the
next upswing. These numbers have some significance -- 14 kW to
maintain 65, and *25* kW to maintain 75 plus. A much larger
percentage in power increase over speed increase. But this also
makes it clear to me that 50 horsepower is *plenty* for any car in
this weight class, which is a little over half of what the combined
Prius powertrain is capable of at maximum ... but even that much shot
me out of tollbooth stops briskly enough to be dangerous. Yes, I
threw in some canonically hard acceleration whenever I had to come
to a stop for something too.
.
Right, at this point you're thinking "who are you and what have
you done with Hobbit" ... but really, this was worth trying. There
is definitely a different feel when screaming past people in the
left lane, and frankly it's not entirely a good feeling. Maybe the
Prius is not one of those "well-composed" roadsters because it
sometimes felt like I was just barely hanging on to the edge of
safe maneuvering sometimes. In the left lane there's nowhere to
go, as there's no real shoulder on that side, and responses to any
slight jig of the steering are much more profound. I'm not sure if
there was a bit of crosswind going on or I've got the front wheel
spats in the wrong place, but even with the BT chassis stabilization
plate bolted in under there the car seemed like it was wandering
quite a bit more. It was definitely a more stressful, unsafe-
feeling situation, yielding decidedly more right-knee torque-up
and more butt and shoulder strain and feeling a bit more mentally
shagged, like I was just barely getting away with managing to
navigate successfully down the road. I kept thinking "okay, what
if there was any black ice on this next curve? I'd be toast!"
One mitigating factor was probably keeping that large following
distance -- as much as I wanted to reproduce the same conditions
other drivers set up for themselves, tailgating is not one of the
components of that. It is quite possible to stay well apart in a
fast chain of cars, and I still can't understand why anyone would
want to ever be as close as most of them come.
.
One thing I did *not* have to worry about that whole time was
playing the on-ramp game, where right-lane travel often requires
coming down to the 52-MPH average entry speed of oncoming traffic.
I'm fine with doing that and keeping those situations smooth is
one of the little hypermiling-mode challenges I've gotten really
used to, but once you're in middle or left it's a non-issue. There
are of course the yutzes who try to do 80 in the *right* lane and
get all pissed off when someone enters at 50 ... well, there's no
help for them other than perhaps a ball of fiery wreckage.
.
I will admit that I got home, well, a little sooner. But showing
50.3 MPG at 208 miles into the tank, quite a bit of contrast with
the rest of the trip. About 13% more speed, and about 13% less
FE. That tradeoff doesn't take the increase in risk into account,
of course. I hope all that extra air that got warmed up by being
violently pushed aside enjoyed the energy I wasted!
.
_H*

That is a fascinating account of the energy losses associated with high-speed driving. I posted a while back about a trip I took with incredible head winds that put my air-speed at close to 90mph while the road speed was 15mph under the PSL. That air speed murdered my FE. What I found most interesting in your account above was the 80% increase in kilowatts used when increasing from 65mph to 75mph. That really shows how quickly drag increases in relation to air speed.

Of course, if I saw your car (complete with the "<--- Go Around" sign in the rear window) zipping past me in the left lane at 75mph, I would call the police and report it stolen because I would never believe it was really you over there in the left lane. :D

These power figures are assuming my "kW" xgauge (http://www.cleanmpg.com/forums/showthread.php?p=162988) is returning
accurate info, of course. Since I don't know what math the SG
is using to calculate "horsepower" in the first place, simply
dividing by some different factor isn't going to remedy any
errors it might be introducing.
.
_H*