The project to build more Prius Xgauges by observing what some
enhanced-mode scantools send to the car has started up. With
the caveat that there are still some bizarre data formats, I'll
field suggestions as to what other hitherto-mysterious parameters
might be useful -- I've only really just begun looking at this.
Some of the bits-n-bytes details are in my
other running thread
about network protocol.
.
To summarize from there, I believe this pretty closely gives
fuel injector time in milliseconds for the Prius:
.
_ TXD 07E021F3
_ RXF 0461 85F3 0000
_ RXD 3808
_ MTH 000A 0008 0000
_ NAM inj
.
[I separate pieces of RXF and MTH for clarity in seeing what
they do, but you'd enter them as one long string.] Depending
on small rounding errors which the scantool[s] may or may not
have, MTH may want to be 000A 0008 0001 instead.
.
Now, here's the fun part -- why it's good to have. As many of
us know the Prius does quite a bit to optimize throttle control
and load in response to driver demand, but always stay on top
of the "mound" on the BSFC chart for best efficiency. This
translates to high torque, lowest needed RPM most of the time.
Remember the IGN 14 "valley" that one must sort of fight with
to optimize during gentle acceleration or SHM? Injector time
gives a much easier way to keep track of those boundary conditions
some of us like to run in with the engine under good load but
at the lowest RPM we can manage without letting it unload.
.
There seems to be a definite "plateau" of operation beginning
around 5.0 milliseconds, when the injector pulse widens out
to supply enough fuel to produce high shaft torque. This
happens right before RPM begins to increase appreciably to
increase overall power output; as RPM increases the injector
time slowly widens out further to 6 and slightly beyond, and
even up to around 7 as you push 3000 RPM which for many of us is
as much "emergency hill-climb warp" as we're willing to give it.
Watching injector time isn't so useful on the high side other
than to try and keep a bit of a lid on it, but is definitely a
significant factor on the low side since it indicates a> whether
you're under load or not, and b> about where you are on the
"possible RPM" scale inside of which you'll stay under load.
The magic area seems to be from 5.0 to 6.something-low milliseconds,
and trying to stay down near the low end of that range is yielding
some really good segments.
.
In other words, it appears to be about as close as an Xgauge
gets to being that "sweet spot meter" I was fooling with a
couple of years ago. That concept was in fact based on fuel
injection, but integrated over time not quite the right way.
It's easier to read than the IGN 14 thing since as you unload
the engine, IGN can jump back up toward 17-18 as conditions
head back basically toward idle and it's sometimes difficult to
figure out which side of that "valley" you're on. In addition,
IGN 14 doesn't show up at speeds above 53 or so MPH, which if
you're out there doin' the 60 @ 60 dance isn't the right way
to know your sweet spot. But injector time still shows the
loading condition up into higher speed ranges, and tooling
along above 60 MPH with the injectors locked in at 5.1 has
the iMPG bar pegged right at 75 or 80.
.
Which segues into some of the equivalent conditions that one
might observe in a stock, uninstrumented Prius, making it mostly
possible to benefit from these observations even if there's
no scangauge or tach to be had. Assuming a speed of around
55-60 MPH, here's a quick chart of rough operating parameters
within the relevant range, matched up for comparison:
.
Code:
iMPG RPM inj
70 1500 5.1 - 5.2
50 1800 5.5 - 5.7 [seen over a fairly wide range]
35-40 2000 6.0 - 6.2
.
It's the same RPM range as we've discussed numerous times as
being the most efficient band to stay in for highway travel;
there's a reasonable correlation to iMPG at 60% SOC under
steady-state conditions. This is about as close to DWL as a
Prius gets, in fact. Much under 1500 and the engine rapidly
unloads -- I've been seeing this with my vacuum gauge for
years, so I'm intimately familiar with that "knee" that happens
with insufficient foot pressure. But the injector time really
shows that dropoff very sharply, and a quick glance to determine
if we're at 5 or better allows riding that RPM down as far as
we absolutely dare to hold a speed on the interstates in best
fuel-sipping style.
.
Now, that "loaded RPM" range definitely shifts downward with
vehicle speed. Even a fairly gentle startoff from the line
yields injector times of high sixes or 7.x at ridiculously low
RPMs like 1250, as high torque is produced to help takeoff.
Wayne observed this when he was driving the Classic around. Once
you hit about 30 MPH, the "I'm just loaded at 40 iMPG" axiom seems
to hold true, so if conditions allow acceleration at that sort of
rate [it's still pretty slow!] it will do much to *not* destroy
your running average. I began playing with this by a bit of
guesswork [not watching "inj" yet] back at the "automotive
challenge" event in NJ on the Pine Barrens flatlands, trying to
hold 40 iMPG and just letting the car slowly build speed -- even
on those 50 mph roads, I came back showing 72.something average in
the MFD after 140 miles. If I'd had the more immediate indication
of "inj" at the time I might have done even better.
.
Now, unfortunately it's not all that immediate since it's an
active OBD-II query. I don't know if injector time is one of
the bits of data flying around the bus we could listen to --
if it was, the number would change almost instantly. I've
already found that placing my foot at the right place involves
a little guesswork and then waiting for the update, and
adjusting from there. A glance at the tach or the MFD can
help confirm what's going on a little faster.
.
What I'd appreciate if some Prius pilots want to try this out,
is to see if you don't find some really sweet tipping points
around 5.1 ms that's probably the direct equivalent of SHM.
And keeping track of this for applying minimum expenditure
during low speeds and accelerations can really help overall
averages -- like I've said before, there are some big
differences to be had depending on how the front end and
back end of a trip segment are handled. Hopefully this can
help.
.
_H*
Linear Mode
