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*

[tbaleno]

Yeah. I don't think a lot of us have had any experience with the technical sides of our cars. Mostly its been about learning to drive efficiently not about how the fine details on how the car does what it does. Hopefuly this will change over time and we will become more rounded.

I'm still trying to figure out the optimum way to accelerate. All the variables like am I starting off going up a hill or down one or flat and winds and all those make it very complicated to figure out.

I loved this post and hope we can get more like it.

[psyshack]

hobbit

I think its a good thread and a possible breeding ground for interaction.

Having a 06 Civic EX with a semi atkins head. Ive been very confused with what Ive been seeing on the ScanGauge concerning the MAP. Its like throw all you thought you knew out the window cause none of its right now.

On the r18 it has a ecnomy lobe on the cam. It appears the intake valves are on this lobe between 1000 rpm and 3500 rpm. With its peak efficiency around 3000 rpm. What its doing here is holding the intake valves open long enough to push a combustion charge back into the intake manifold. The valves then close and limit the compression stroke to 1.5L for the engine insted on the normal 1.8L. Then the intake manifold has a short and a long flow path thats controled by some other outerspace device Im sure they found in Roswell. All this going on makes driving by the Manifold Air Pressure impossible.

One moment you will look down and see 5.5" Hg with a instant mpg at 90 mpg. Look again and its 10.5" Hg and your at 90 mpg. Look again and its at 7.5" Hg and your getting 21 mpg. It makes no sense to me at all. Defies all rules governing Hg and mpg.

The scan gauge is pretty quick when you have it setup on its fastist refresh rate. You put throttle imput in and you have instant reading. Or at near instant readings that seem to follow your foot.

None the less,,, Ive found this to be intresting and very frustrating. More on the frustrating of late. With enough intrest to hope this can be explaned to me.

psy

[krousdb]

Very interesting stuff Hobbitt. I never thought to hook up a vacuum gauge to see what is going on. I did ha ve one in the del sol and it changes with throttle position and tended to mimic the SuperMID readout linearly so I removed it. My experimentation with the del sol also confirms that accel at low rpms with high load, low hg is what yields the best FE. I would try to install the hg gauge on the Pruis but it is now broken.

Anyway, you are far more advanced than I am at speeds over 41MPH. Until now I just used cruise control. Now, next time I get the chance I will try warp stealth. Please let us know when you figure this over 41 thing out.

[hobbit]

Oh, right, the other thing I forgot to mention is that I believe
the slight vac rise as speed increases is because of the valve
timing changing. Unfortunately I don't have a good way of watching
the VVTi target angle right now -- that requires something that's
more aware of nonstandard OBDII parameters [it's not Prius-
specific, just possibly out of most tools' capabilities]. For a
while I was running around with an O-scope hooked to the oil control
solenoid output, but that told me nothing since it just gets
bumped one way or the other for a sec and back to center.
.
Yup, one day I really want to understand this. But having the
knowledge people have gleaned about other engine setups will help
actually *master* it someday...
.
_H*

[tbaleno]

I'm getting confused about what high and low load are. Is low load when you are not accelerating and high load when you are accelerating more briskly?

[philmcneal]

Originally Posted by tbaleno

I'm getting confused about what high and low load are. Is low load when you are not accelerating and high load when you are accelerating more briskly?

the higher the load, the better risk for lugging. but for us FE people lugging is good it seems. but your quote pretty much answered your own question. Although the brisky part is done at low rpms.

[xcel]

Hi Hobbit:

___First I want to get into a bit of theoretical guess work with the Prius II’s inner workings at those higher speeds. Please add what you can in regards to her MGSet interactions at 41 and above because I am not a Prius II tech by any means … The way the Prius II falls on its face as speeds climb above 41 mph cannot be explained because of aero and mechanical drag alone imho. The reason I say this is the 1.0 in the Insight uses somewhere in the neighborhood of 17 - 20 HP to maintain a lean burn cruise of 90 mpg at 55 mph or so. I have seen charts developed from Wayne Brown’s Palm Prius II Sim which show the Prius II holding 85 mpg from 25 to 41 mph steady state. This speed range is not only huge but almost pancake flat up to the magic 41 mph speed. The precipitous fall after 41 mph right when MG1 is screaming for help and exactly where all the trouble starts …

Speed vs. MPG

68F, sea level, 40% RH, 29.92", 225 payload, no AC, no wind

___This very unorthodox drop off after 41 mph is not normal other then an Insight or HCH-I dropping out of lean burn. What it looks like to me is the MGSet’s are starting to fight one another with that loop-current (or whatever you call it). MG1 generating electricity while dragging off the ICE to feed MG2 for torque output all the while the ICE is also generating torque for propulsion and feeding MG1 torque to generate electricity. The Prius II’s Atkinsonized ICE is certainly strong enough to maintain 60 mph with little in the way of higher RPM’s I would have to think? Again, think back to the Insight running a steady 17 - 20 HP at 55 mph with a similar Cd and maybe 70% of the frontal area. The Prius’ 1.5 should easily be able to maintain 25 HP from its peak of ~ 75 at a very low fuel consumption rates vs. the possible 15 – 20 HP needed to maintain the 23 – 41 mph range shown in the graph for maximum FE. In the real world, I think Dan told me she allows 50 mpg at 70 mph, 60 mpg at 60 mph, and 70 mpg at 50 mph. With those numbers, 20 mph is almost a 30% loss in FE due to aero drag and slightly higher mechanical friction? That just seems like a relatively large number when considering how the Accord or Corolla have consumed at those speeds although I have never truly tested a flat cruise at 70 mph in the FCD equipped Accord unless I am in some kind of draft if at all possible.

___Ok, my theory is that the act of MG1 drawing power off the ICE to feed MG2 while MG2 provides propulsion as well as the ICE providing propulsion and feeding MG1 is where the real inefficiency lye’s.

___Intakes and throttle openings. The Insight does her best work at just above lug and just below IMA assist during acceleration in my experience. Honda screwed the pooch with IMA assist as it is like a male dog in a pack of female dogs in heat. It just wants to go like a turbo with a full boost on-off switch! Enough about IMA The good thing is that just like a non-hybrid, I have found a very low RPM accel at relatively small throttle openings work for me during a standard acceleration. All the tech jockeys with the latest efficiency charts and graphs appear to have it wrong with a close to WOT for minimal pumping losses for maximum efficiency all the while the F=ma equation has shot right through the roof on every rotating component accepting maximum torque from whatever ICE one is driving! I do not know who is right but I know for maximum FE in my automobiles, the go slow with a relatively small throttle plate opening while grabbing the bottom of the first torque peak appears to do wonders! It sounds like the Prius II is running with very minimal pumping losses in the low speed ranges by the way your Vacuum is snapping off but at highway cruise, vacuum is holding up as I believe it will in all cars at a low load highway cruise.

___With what I believe to be the real Prius II inefficiency at higher speeds (42 and above) arising from MG1 drawing off the ICE and converting for MG2 somehow, the rumor mill has the Prius III running some kind of gear reduction on MG1 to alleviate the over rev and inability to restart the ICE at 41 mph and above due to limited torque concerns. This upgrade alone should allow the Prius III the ability to hold EV far beyond the 41 mph limits THS and HSD have saddled her with forever plus alleviate some of that huge drop at 42 + mph and beyond as shown in the graph?

___Ok, enough of my BS. It is time to see and hear the actual details of the Prius II’s inner workings from those that own one and know what they are talking about

___Good Luck

___Wayne

[hobbit]

Well, a bit of clarification might be in order. By "lug" I didn't
mean dangerously low RPM -- what I meant is closer to your high-torque,
low[ish]-RPM scenario. I would like to think that the fewer times
per second a charge is burnt in the cylinder, the less heat is
produced and thrown away ... but on the other hand, if *more* mixture
is introduced per stroke, maybe that's fallacious thinking. I should
try to design an experiment to track how fast an ICE warms up under
low-speed/hi-torque vs. high-RPM/light torque ... or has someone
already investigated this?
.
I've seen that diagram before too, and suspect that the sudden
drop is simply due to having to pull the engine around, whether it's
burning gas or not. One thing to remember about high speed travel
at low ICE RPM is what's called "heretical mode" -- where MG2
actually drives MG1 *backwards* as a motor to help push the ring
gear around faster per given engine RPM. If you look at the
nomographs you'll see how this must be true. Only when you bring
demand [and thus RPM] up to much more than needed for cruising does MG1 rpm
cross back over zero and go back into the usual "generator" mode.
Yup, this is hard to wrap one's brain around [and was the seed for
*much* debate back in the early days of prius_technical_stuff]
but that aside, whichever way the necessary electrical hop that power
takes is flowing at a given time, *it* seems less responsible
for the MPG drop than just having to haul all that extra metal
around along with. There will still be some air compression
or decompression -- even the ideal "valves totally closed" state
that some other engines may be able to do is not a completely
lossless air-spring.
.
Anyway, one of the original ideas of this thread was to determine
[or bring forth] whether it's better, from a mileage standpoint,
to do low-RPM/hi-torque or higher-RPM/low-torque/less-throttle.
It may be completely dependent on which engine one is talking
about, and/or other factors like how its intake is constructed,
atkinson vs. otto, etc etc. Maybe reducing all this to that
simplified question would help... then, how to best use those
results gets back into the dependencies of the driveline connected
to it.
.
_H*

[hobbit]

Tonight I took my O-scope out cruisin', and observed some injector
times at interstate speeds. Anywhere between 1400 and maybe
2200 rpm in the THS-II, injector time seems to sit right around
6 or 7 ms. Drifts up to 8 ms as I cross that critical 2400 rpm
threshold where the vacuum starts to drop from 5 in-Hg to more
like 3 in-Hg, which I suspect is more related to the ICE moving
to more of a "power" regime than "efficiency" regime via its VVTi
valve timing. The most interesting thing was the fact that
injector time remained almost constant through a wide RPM range,
which throws my prior thinking about how to meter "injector
duty cycle" into a cocked hat and now I'm thinking that I have
to monitor the actual injector time from around 2 - 12 ms to
get meaningful feedback on how much fuel is being consumed.
Clearly, what I'm up against here is largely a function of ICE
RPM, so at a wild-ass guess it seems that minimizing that is
the rudiments of a higher-mileage strategy for high speed. And
plenty of warp-stealth where appropriate, natch.
.
_H*