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 in mind, and having done a small xgauge mod 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. 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*
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
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 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 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.
These power figures are assuming my "kW" xgauge 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*
