Speed vs. Power Gauge: Request for Data

By far, the largest number of questions received regarding the iMiEV deals with RANGE. For the iMiEV, range has NOT been empirically quantified. The simplistic EPA ratings is all we presently have available, and some of us have a problem with their numbers.

Intuitively, we iMiEV drivers recognize that going fast disproportionately decreases our Range Remaining; conversely, slowing down disproportionately slows down the decrease of our Range Remaining. It sure as heck is not linear! Technically, we of course recognize that drag is proportional to the square of the vehicle's velocity.

Trouble is, we don't have these numbers, and thus I am soliciting the help of all the iMiEV drivers out there.

jjlink was kind enough to modify the photo of the iMiEV power gauge to show divisions that should be easy to assess by eyeball: breaking up each division into "thirds" ("Quarters" was felt to be too fine a division).



Now, here's what I would like us all to do: while driving our iMiEVs on a flat level windless surface and holding it steady with the red needle at a particular point, it would be great if you could record the vehicle's SPEED vs. red needle POSITION. If you could each do this for as many points as are realistically feasible for you (without breaking any laws) and I would appreciate it if you could then send the data to me.

Originally, I thought we could each publicly post our data, but in the interest of avoiding being influenced by others' data I would appreciate it if each of you could PM (Private Message) me the data directly, by simply clicking on PM at the bottom of this post.

What I will then do is compile this information into a spreadsheet with datapoints and then perform a nonlinear regression analysis to end up with a curve of SPEED vs. POWER GAUGE LEVEL which of course I'll publish. If you'd like, I could also publish the anonymized raw data y'all sent in.

This will now form the basis for quantifying our RANGE vs. SPEED (with perhaps some discussion about things like voltages and Peukert prior to that).

OK, if there are any questions then let's get them out in the open on this thread, and in the meantime I'd appreciate all your help in getting the data to me. I think you'll find that it is not as simple as it sounds to hold the car at a steady speed in zero wind on a flat level surface... but please try, and I realize this may take some time to accomplish for us to get sufficient data. I found that using a voice recorder works well for capturing this information.

Thank you for your help.

You will also need tire pressure information and road surface can also make a difference but not as much as tire pressure.

JrCRXHF, you're absolutely right about tire pressure, which I should have been including in all my mileage posts. Might as well include ambient temperature as well. Hmmm, if someone's at a significant altitude, that should also be noted. Thanks.

It seems to me that there are just too many variables for the collected data to be meaningful

Speed is easy enough measure, but temperature, a few MPH of head or tail wind (or even crosswind) a degree ot two of uphill or downhill when it appears to the driver that the road is absolutely flat, vehicle loading, condition of the pavement, etc, etc

To get something meaningful, all cars would just about have to travel the same stretch of road under the same conditions. Even then, I can sometimes do 70 on a stretch of road I travel regularly while keeping the needle right on the "1" and then other times (no matter how hard I try) I can't

Don

I would agree.

Don said:

It seems to me that there are just too many variables for the collected data to be meaningful. Speed is easy enough measure, but temperature, a few MPH of head or tail wind (or even crosswind) a degree ot two of uphill or downhill when it appears to the driver that the road is absolutely flat, vehicle loading, condition of the pavement, etc, etc To get something meaningful, all cars would just about have to travel the same stretch of road under the same conditions. Even then, I can sometimes do 70 on a stretch of road I travel regularly while keeping the needle right on the "1" and then other times (no matter how hard I try) I can't
Don

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Don - I agree that trying to get a windless perfectly flat stretch of road is difficult, which is why I'm asking everyone to do the best they can and send in the raw data. The more data points we have the better we can subsequently analyze it. Like you, I have found it very difficult to pin down. Whenever I'm driving the car (when my wife reluctantly relinquishes it) I look for flat stretches of road and try to hold steady on one of the points. It's a slow process...

Regarding the other variables: temperature is probably the least important variable, but it is available on our dash display. Altitude (less dense air) only matters if it's significant. Extremes of each of these two variables is what we primarily care about.

Don't lose sight of what we're trying to do: quantify speed vs. power consumption in order to quantitatively assess the range issue. You wouldn't believe how many people "don't think" that driving 65mph vs 55mph makes any significant difference...

If anyone has any better ideas on how to do this, I'm open to suggestions. In the meantime, sure would appreciate some more data...

I had the rare opportunity today to drive constant speeds on flat surfaces. I used D mode. Ambient temp was in the 60s. Tire pressure nominal (36). Road surface reasonably smooth asphalt. Winds less than 5mph. I tested speeds in the range of 35 to 65. In every case, maintaining the current speed, the needle was dead on 1/2 tick. It didn't matter what speed I was going, as long as I was maintaining rather than accelerating or decelerating. There was no perceptible difference in needle position for different speeds for zero dV.

Anything above 1/2 tick was under acceleration or incline surface. Anything below 1/2 tick was during deceleration or decline surface.

dumemama - I'm confused, so let me make sure I didn't misunderstand you: you are saying that you are able to hold a steady 65mph (or any other speed between 35mph and 65mph) with the red needle sitting steady on the letter "c" in the word "Eco" in the middle of the green zone, irrespective of speed?

If that's the case, then I need to re-install the ammeter and take my time re-visiting my previous readings:
http://myimiev.com/forum/viewtopic.php?p=1378#p1378
(assuming I can get the car away from my wife)

If you're right, then that gauge shows us the penalty we incur due to acceleration and it is not simply a pretty ammeter. Dang, and here I thought I was starting to get somewhere with my own data-taking, despite my wife thinking I'm my usual nutcase as I've been asking her to write down numbers as we drive along ... and then giving her different numbers :roll:

Anyone else want to chime in?

Hi Joe,

I agree with dumemama, there is a large penalty for accelerating, but holding a speed, once attained, is pretty low-cost barring changes in elevation or wind resistance. I don't think I could hold the car at a full tick for too long on a flat road with no wind, I'd be constantly accelerating.

Have you seen something different?

Jenn

Jenn, interesting, as you are corroborating what dumemama is saying.

I just recently started trying to get serious about data-taking and am finding variations which I attributed to the hilliness of all the roads around here (not to mention the windy afternoons). I do, however, have two different speed vs. needle positions that I believed to be 'solid' and repeatable, but which I will now have to revisit again.

Anyone else?

Let's not lose sight of the reason for doing all this: I am trying to quantify the iMiEV's energy consumption as a function of vehicle speed in order to empirically identify the speed vs. range penalty.

I found that I would have to travel at least 1 mile to get a steady (non accelerating) speed. I found easier to speed up to a certain speed then ease off and keep that speed constant and the needle wouldn't move and the speed wouldn't change.

JoeS, the data I sent you was taken at on dry days, at 5-5:30am, the only time that it is calm here, temp in low fifty's, tyre pressure-40psi all around.

This morning I got up at 5 intending to 'borrow' my wife's car and go for a slow-speed joyride and definitively pin down the speed vs. gauge readings. I had identified what I thought was a perfectly flat stretch of expressway (a misnomer, as our 'expressways' around here have traffic lights and 45mph max speed limits). Well, just before leaving home I decided to check the intended route on GoogleEarth using their altitude readings, and guess what: there is absolutely NO level stretch of road anywhere around here :!: :!: :shock:

I scanned all the straight-stretches that I thought were flat on all the roads around here, and there is no such animal. This entire area gently undulates and gradually slopes down to San Francisco Bay. The flattest roads are actually right by the Bay, but that's almost ten miles from my house… maybe tomorrow morning I'll give it a shot.

I would really appreciate it if more of you would send in your readings, especially at the low end. Despite my intention to quantify the iMiEV's range limitation at higher speeds primarily due to wind drag, when I have enough datapoints I think you'll all be amazed at how far the iMiEV can actually go at low speeds.

Once again, look at the pretty picture of the display on Page 1 of this thread, hold the red needle steady on one of the points and wait for the speed to settle down while driving on a windless perfectly flat stretch of road and then simply note the speed. Then PM your numbers to me. Thank you.

Joe,

The nearest real freeway is 15-20 miles away from me. So I do little to no real highway driving. Even when I do, I intentionally do not go above 65. So you are correct that my observations do not include situations where drag is a factor. This time of year, there usually isn't any wind either. I maintain that without drag, there is no observable difference in power consumption due to speed with acceleration and grade at zero. That doesn't mean the difference is zero. It just means it is small enough that it's not discernible on the analog meter, in D mode. I'm not going to test it, but I'd venture that in E or B mode there might be a discernible difference due to speed with all other factors zero (see below #1). It is indeed hard to find truly flat track for a good mile or so to get a data point. I did my tests on the PCH in spots where it is completely flat and straight along the coast.

I live in the mountains. The last 7 miles to my house is 5% grade. I live at the top and "civilization" is at the bottom, 7-15 miles away. If I'm not careful, I can get stranded in no cell-coverage, mountain lion territory... which I did 2 miles from home on my maiden voyage when I foolishly drove in B mode thinking it would be ultra power-conservative under all conditions. My regular commute is only 35mi/day. But adding errands can quickly get me in trouble esp if I have to "go over the hill" to the valley. What I've learned in my (not the car's) brief break-in so far...

1) D mode is the most efficient on ascent at any speed. E and B mode use more power on a steady ascent at the same constant speed. B is a battery killer on ascents. E isn't too bad, but noticeable. For example, on my 5% grade at 25mph constant, D mode is about 1 tick on your scale and E mode is about 1 1/4 or 1 1/3. B mode can be 1 1/3 or more. Don't ask my why, this is just my observation in comparisons done carefully enough for safety's sake. When you have variations in grade, dips and curves, E and B mode kill your momentum and you have to use more power to overcome that. But they are useful to increase regen, so I make liberal use of appropriate downshifting instead of the pedal brake only when I need to slow down for curves and dips, even on my ascent.

2) The power used (the needle) is primarily a function of pedal acceleration and overcoming acceleration due to gravity. Constant speed on the flat (needle on O in Eco) will give you average range, regardless of speed unless/until other other factors come into play such as drag, load (passengers, etc), and maybe temp and whatnot.

3) At 5% grade, every 125 feet of elevation gain uses one extra mile of range (in D mode), all else being equal or zero. So for example, at the start of my ascent home for a climb of almost 1900', I need 15mi of range just to overcome the elevation gain and 7mi for the actual distance. If I've been driving on the relative flat so my range remaining is a reasonable figure for that, I need 22miles reading before beginning that last 7mi or I'm screwed. I've made it home with RR reading 2 miles, eek! If you go one mile with 250' gain, you are likely to use 3mi of range. This is generally true going 25 mph up my last 7 mi or 45-55 mph on undulating PCH up and down 100-500' at a time. If you allow the car to slow on the uphill grade (speed not constant) the penalty is lower of course. But that only works if the grade ends in a bit.

4) The inverse of #3 is true, but not at 100% efficiency. The efficiency rate depends greatly on driving and grade and for me is about 75-80% - I don't use the brake pedal at all. The lower the grade, the lower regen. When I descend 1900' in the morning, I get about 10-12 free miles of range. Although the range remaining reading when I start is unreliable because the immediately preceding 7 miles was all uphill driving. I try not to charge fully because if I do, I waste the free charge from gravity. I wish the remote allowed charging time specified by percent charge desired.

I have no qualms about driving on the highway since I don't mind driving like a trucker, slowing on the up, letting gravity do its job on the way down, and lower constant speed on the flat (avoiding significant drag). It's the hills that kill as no amount of adjustment in driving habits can avoid the power needed to overcome gravity. Driving habits can make it much worse, but not significantly better. On average even with my hilly situation, I'm getting 80+ range.

Hi dumemama (Barb?) - thank you for your extensive comments. Sounds like you live in a lovely area above Malibu. Hopefully no forest fires.

Your red needle shows power consumed, irrespective of whether you're in D, Eco, or B - it's the same. Pedal position is different for Eco. Both D and B easily give you max power, but you really have to stomp on it to achieve the same level in Eco. Regen is progressively greater going from D to Eco to B, as "measured" by the red needle. The scale of the red needle power gauge does not change with shift lever position.

On the level, your red needle shows the initial high power draw as you are accelerating, then backs off and settles down at some level as you maintain constant speed. Since roads are rarely flat and wind influence is significant, that needle is always moving and difficult to pin down. At low speeds, our iMiEV really consumes a miniscule amount of power which is very difficult to measure consistently and accurately. I'm still trying (e.g., it was too windy this morning to even attempt it).

It's ok as a rough approximation to add your miles driven to the RR and come up with a "Range" - hey, our iMiEV is better at it than the Leaf! Your number of "80" at the end of the trip is quite good. I'm sure the Range Remaining computer gets quite a workout from those of us who live in the hills - it simply is not reasonable to expect it to anticipate the ups and downs, although I think that in the not-too-distant future linking to a GPS and incorporating where you're going into the algorithm - and maybe identifying who the driver is in order to feed their characteristics into the algorithm as well - will produce an even more accurate estimate for RR. Reality is that ballpark numbers are good enough and "I ain't got no range anxiety", sez me, 'cause I love my batteries and I simply never cut it close (two bars is my personal lower limit).

Finally, there are a lot of variables that go into determining how "efficient" our iMiEV's regeneration is, and my impression is that we recover a good percentage of what we put in (thus hills are not quite the killer if we go back down). Separate topic.

First, let's see if we can pin down more closely the speed vs. red-needle position, and I welcome everyone's inputs as a PM.

Your red needle shows power consumed, irrespective of whether you're in D, Eco, or B - it's the same. Pedal position is different for Eco. Both D and B easily give you max power, but you really have to stomp on it to achieve the same level in Eco. Regen is progressively greater going from D to Eco to B, as "measured" by the red needle. The scale of the red needle power gauge does not change with shift lever position.

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I know what the red needle indicates. I understand that you believe in theory that what I'm saying should not be the case, which is why I've tested it myself over and over again. My test case is a pretty steady 5% uphill grade at 25mph. When I do that in D mode, I can hold the speed and the power is steady at about 1 tick. In E mode when I hold that speed the power is steady at about 1 and 1/3, about 30% more power used in Eco to achieve the same exact climb, same speed. If I do the entire 1900' climb over 7 miles in E mode, it consistently takes 5 bars on my charge gauge. If I do it in D mode, it takes 3-4. If I do it in B mode, it takes 6 or more! Sorry I don't have a clamp-on ammeter for more accurate data.

Unlike on the flat, speed does indeed make a big difference in power use on ascent... the higher the speed, the higher the power use when maintaining that speed going uphill. For example, a 5% grade up at 45mph in D mode will be about 1.5 ticks, about 50% more power than 25mph. Eco and B still uses more power than D on the same stretch at the same speed (any speed) when going uphill. I've only had my car two weeks, but I drive these hills every day once or 3 times, and I'm confident enough in my findings that I often make it home with 2-5 miles RR.

I love my i and I love my hills. But you won't catch me going up in anything but D mode. Not only that, Eco makes my nimble little car drive like an SUV!
I'm also looking forward to what others are seeing.

dumemama said:

I love my i and I love my hills. But you won't catch me going up in anything but D mode. Not only that, Eco makes my nimble little car drive like an SUV!

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Perhaps, "drive like a loaded truck" would be a better choice of words to indicate the apparent sluggishness of the vehicle in Eco for the same pedal position. That's because you are indeed using less power, which is what that setting is trying to get us to do. Somehow I need to convince you that climbing a given grade at a given speed will consume the same amount of energy regardless of whether you're in D, Eco, or B.

No question about it, our iMiEV feels very lively when driving in D when compared to Eco, but where we disagree is that I think you'll find that that liveliness is at the expense of higher energy consumption. Perhaps we should devise a controlled experiment?

Barb, I agree with you - we need others to weigh in on this, especially on the issue of red needle position...

All I want is for people to send me their flat level windless speed vs. power data so I can complete my science experiment.

As y'all might recall, I started this thread in an attempt to determine the power consumption of our iMiEV at different speeds in order to quantify what I believe is the iMiEV's greatest range robber: aerodynamic inefficiency. Enough datapoints would give us the ability to plot an energy-consumption curve and thus an empirically-derived Range at different speeds.

Unfortunately, our iMiEV is sooooo efficient at low speeds, that getting that needle on the Power Gauge to stabilize has proven to be quite difficult. For example, repeated runs in the very early windless morning on the flattest road I could find resulted in a speed of 22mph with the needle sitting right on the bottom edge of the green zone. Anything above that would take me over the speed limit of that particular road.

Due to the underwhelming response to my seemingly-simple request for data (thank you WeeJohn for your datapoints) and evident difficulty in obtaining this data (i.e., windless perfectly-level non-undulating zero-traffic long stretch of road), let me try to see if we can reach a consensus on what the tick mark "1" (upper end of green zone) translates to in terms of speed.

Anyone care to publicly offer up a number for speed at that tick mark "1" :?:

Unfortunately, I've always had traffic and a non-level road surface, and the number I've come up with so far is 66mph - but which I still have no confidence in. Anyone else?

HI Joe,
In testing here, only over the eight days though, we also found that we could steady the current gauge on the line between recharge and eco, and the weird thing was, that we could achieve this at differing speeds and maintain it, even though logic would suggest that current must be higher at higher speeds.

I am sure there is a logical reason for it, but it seemed like magic to us !

I would say, if you looked for a racetrack oval that would let you do a test in the still air day, that one could get a rough idea of current compared to speed and help me figure out if the best speed to travel for long distance is also a safe speed to travel at.

I am expecting the speed for longest range to be about 20 km hour and the speed that is acceptable but with a 200 km range is about 50 km / Hr.

By acceptable I mean on non highways, but open road 100 Km speed limits, what would be acceptable to travel at without getting run over ! Cyclists do only 25 Km Hr but are easy to pass as they are on the very side of the road, the iMiEV being wider might make people get grumpy if I am too slow, though I am planning on doing the long distance travel at between 1 am to 7 am.

I wonder if I should do what pilots do and look for a tailwind for most of the flight ?.

JoeS said:

Anyone care to publicly offer up a number for speed at that tick mark "1" :?:

Unfortunately, I've always had traffic and a non-level road surface, and the number I've come up with so far is 66mph - but which I still have no confidence in. Anyone else?

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67mph-definite! :?: