Throttle control

[hobbit]

Disclaimer: this is primarily Prius-oriented, since that's what I'm
most familiar with these days. But after watching a little driving
by some of the esteemed colleagues hereabouts at Tour de Sol,
I've been having more thoughts about ideal running conditions
and how to achieve those with the Prius HSD. 14 months after
taking ownership, and I'm *still* trying to solve the highway-mileage
problem...
.
Anyways, my chatting with various Insight drivers [including a
local friend who's had one for a while but doesn't do TdS] seems
to have consensus that low-RPM lugging is the highest efficiency
mode that engine runs in. Lean-burn aside, I suspect that this
is true for many engines, especially given the techniques I read
about for other vehicles here. Unfortunately to get that in
a Prius, some compromises have to be made in how the HSD spins
its components, and I fear that the situation is never quite
like throwing a step gearbox into fifth and going for it.
.
One thing I notice, having a vacuum gauge in my car, is how the
Prius controls its throttle at lower speeds. Initial requested
torque comes mostly electrically, but when it decides to add
some ICE oomph in, it basically *snaps* the throttle open.
Well, not really snap because in the absence of throttle-opening
"accelerator pump" style enrichment that would cause a stumble,
but fast enough to get past the low-torque range trivially
fast. In fact under 35 mph or so it's very hard to get the
vacuum to hold at the inefficiency-telltale ranges between
say 10 in-Hg and 15 or so. It's high load or nothing, basically,
and that's exactly how it should be in those scenarios. The
driver can then enhance this behavior even more by adding P&G
on top of that, so it's run-under-load or shut off entirely.
.
Fine, but at higher road speeds, things begin to damp out a little.
The vacuum begins to rise slightly, and starts tracking the
driver's foot a little more linearly. I think this is the killer
in the mid-speed ranges -- drivers who get up to 50 or 55 and
then just back off enough to gently cruise are now running
totally off the efficiency curve at light loads, even if the
RPM is down around 1000. Now, the throttle opening never really
gets that high at any time during all this, unless you really
nail it -- its baseline is 14.5% at fully closed, and 15.?%
seems to be about nominal idle level, and I'd be lucky to get
it up to 35% even during "brisk" acceleration. In fact according
to the OBDII laptop next to me, the throttle opening seems to
track fairly closely to *RPM*. Which is interesting but
understandable -- engine RPM is based on *power* demand, across
a fairly flat *torque* curve. If torque is relatively constant,
then more power comes from letting it spool up more RPM.
.
So what I've been trying now is while on the interstates, trying
to let my RPM drop as far as possible but keep the vacuum down
around its higher-speed baseline of 5ish in-Hg. Theory being that
that represents higher shaft load, and the telltale for torque
starting to back off is increasing vacuum. You can't *feel* any
of this in your butt, it's got to come from instruments. The
instantaneous mileage rides somewhere north of 60 if I'm doing
this right, but that only resists about 62 mph worth of air
resistance. So it's still not a lot of kilowatts out of the
engine, and I'm wondering if I'm just fooling myself. I've tried
a bunch of other scenarios -- pulse harder around 2500 rpm and
then try to "warp stealth" for a while, but besides being way
more work and uneven speed, seems to return lower mpg segments.
This "maintain torque" [still somewhat theoretical, mind you]
strategy, with departures into higher RPM [aka power] bursts to
get up hills, seems to be doing just a shave better in general.
.
What I'd like to see in response to this [besides "geez, this
guy does just *go on*, doesn't he"] is some of the accumulated
knowledge about torque and throttle control and mpg learned from
some of the *other* cars people have been experimenting with.
And from other Prius drivers, but I realize there aren't a whole
lot of those hereabouts yet.
.
And if the whole thread dies here because all of the above was
too confusing, I'd understand. I'm still entertained by how
the prius whacks the throttle open on a low-speed pulse, though.
.
_H*