Coast Down Testing - 2000 Honda Insight

[Hubert Farnsworth]

Originally Posted by drmiller100

seems to me there are 3 major uses of fuel.

One is overcoming aerodynamic drag.
One is overcoming rolling resistance.
One is accelerating the vehicle.

Of the three, I suspect the middle one is the least significant.

I agree aero drag would be MUCH easier to calculate at higher speeds.

Also, I suspect some of your curve is actually from the wheels energy being non linear with respect to speed - the wheels are rotating, so they give energy to the system as a function of the square of the mph.

accelerating the vehicle is a function of rotational inertia and weight of the vehicle.

Rolling resistance is not the least significant, at least not at lower speeds, one really needs to do the calculations, but below a certain speed the road load force due to the vehicle mass and rolling resistance plays a larger role than the aerodynamic drag. After this point then the contribution remains fairly consistant where as the aerodynamic drag increases as a squared function of the velocity of the vehicle.

[Daox]

You have to be going pretty slow to make aero insignificant though. In my Paseo, at about 35 mph its a ~50/50 division between aero and RR and it has decent aero and a very small frontal area. Even at 25 mph its still ~30% of the energy used.

[drmiller100]

Originally Posted by Hubert Farnsworth

Rolling resistance is not the least significant, at least not at lower speeds, one really needs to do the calculations, but below a certain speed the road load force due to the vehicle mass and rolling resistance plays a larger role than the aerodynamic drag. After this point then the contribution remains fairly consistant where as the aerodynamic drag increases as a squared function of the velocity of the vehicle.

I did a bunch of calcs, and rolling resistance is ALWAYS the least significant of the three items I listed. At low speeds we assume city traffic with start/stop, and in that case the energy to accelerate the car is by far the biggest contributor.

At high speed (over 40 mph) aero is the biggest issue.

If you have a heavy car at low speed, then acceleration of the car is the biggest issue. If you have a light car at low speed, then acceleration of the car is still the biggest issue.

Your thoughts?
What con

[jime57]

Originally Posted by 3-Wheeler

It is this author’s opinion that the conditions and/or measurement methods during these coast down tests are still too variable to be of much use regarding aero improvements to our cars.
Jim

Jim,

I've been studying your data and I have one suggestion as to a possible error source.

We all know how very sensitive the Insight fuel economy measurement is to tire pressure. Looking over the part of the year you were testing, I'm wondering if you may have had significant tire pressure variations. Tire temperature changes tire pressure. On sunny warmer days, the road would have been warmer, and on cold overcast days the road surface would have been colder. Cold stiff tires have higher rolling resistance.

I have a slight hill in front of my house which runs for .4 miles. When I give my Insight a little push, I gain speed to 8 MPH at the first intersection in the summer. In the winter, I can barely make 5 MPH at that point. You say that the car is thoroughly warmed up, but perhaps the tire temperatures are still varying significantly on different days.

From my racing I also know this. We use a bit more pressure on cold days to put the traction into the sweet spot.

[3-Wheeler]

Originally Posted by Old Tele man

...can't "explain" ALL the variances, but these 'variables' came to mind:

• differences in wind velocity
• differences in wind direction
• differences in driver control (we're seldom 100% repeatable)
• differences in air temperature
• differences in air density (cold/dry winter vs. hot/humid summer)
• differences in GPS satellite lock-on's (jumping between different satellites?)

...individually, these might be small, but their "interactions" might be significant?!?

...what kind of regression correlation coefficent (R^2) do you get when you tell the Trendline function to "set intercept = 0", which forces the "average" line to go through zero at Y = X = 0.

Hi Tele,

Yes, I understand what you are saying about "influences" that could affect the coast down numbers.

I absolutely agree that wind and temperature have affected the repeatability of these number presented, and I have made no attempt to correlate them with individual coast down decelerations towards the end of this study.

Why?

The GPS readings are simply too few, and littered with too much scatter.

I have probably spent well over 200 hours or so "trying" to get more stable or properly "filtered" data from the raw GPS output. All in vain in my opinion however. The GPS data is simply too scattered to be of much use for this precision application.

As for the statistical R^2 value, in this case I think that "goodness of fit" number is of not much use with this much data variation.

As for the Instuctables.com posting, you can now see that much more data needs to be collected to get really accurate results.

Jim

[3-Wheeler]

Originally Posted by Aveomiler

Wow. This is very interesting. I've been wondering myself about the reliability of my testing. Does anyone know of the reliability of this compared to measuring fuel economy while holding speed to test improvements? I suspect coast down testing would be a little more reliable, but as many of us know, even the tiniest variations in a driving commute can contribute significant variations in fuel economy. Ideally, it would be nice to average 39 runs in A-B-A style for testing, but that is very time (and gas) consuming.

Hi Aveo,

As for the reliability of straight-line A-B-A testing, I would rely on that type of testing more than coast down testing until I can develop some type of automated setup that takes the VSS speedometer readings from the Insight and logs it onto an SD card for later retrieval.

MetroMPG and others have seemed to provide reliable data by this method.

Jim.

[3-Wheeler]

Originally Posted by JRMichler

.....Your top speed was only 40 MPH, which is not fast enough for accurate results. A 5 MPH headwind increases aero drag by 27% at 40 MPH, and more at lower speeds. A 5 MPH head or tailwind will change my trip mileage by about 2 MPG.

Averaging several runs is good. Starting at a much higher speed is even better. The best starting speed is where the aero drag is at least ten times higher than the rolling resistance. Coasting down until almost stopped will also improve accuracy.

MTA: OK, I see that you already spotted some of this. You calculated that air drag is equal to RR at a speed higher than your highest test speed. That means that it is difficult to accurately estimate Cd from your data. Your Crr should be pretty good because the test data is almost all rolling resistance, with very little contribution from air drag.

Actually my top speed was closer to 60 mph (over 25 m/s), so that should be pretty useful for getting the split between Cd and Crr.

The rest of your comments are based on the assumption that my maximum speed is only 40 mph, and because of this, these are not correct because my start speed was actually higher than that.

Jim.

[3-Wheeler]

Originally Posted by drmiller100

seems to me there are 3 major uses of fuel.

One is overcoming aerodynamic drag.
One is overcoming rolling resistance.
One is accelerating the vehicle.

Of the three, I suspect the middle one is the least significant.

I agree aero drag would be MUCH easier to calculate at higher speeds.

Also, I suspect some of your curve is actually from the wheels energy being non linear with respect to speed - the wheels are rotating, so they give energy to the system as a function of the square of the mph.

accelerating the vehicle is a function of rotational inertia and weight of the vehicle.

One of the nice things about "accelerating" a vehicle, is getting all that momentum back in the form of "coasting".

This where serious MPG's can be made. Much more than straight line driving with the motor running continuously.

Jim.

[3-Wheeler]

Originally Posted by NeilBlanchard

Holy moly Batman! That is some serious data crunching.

How did you determine the Crr? In my coastdown tests using the Instructables spreadsheet, this is the hardest part to get -- it is basically an educated guess?

@drmiller100 -- I suspect you are correct. The aerodynamic drag swamps everything else at a constant speed. And accelerating (from a stop and/or uphill) is huge but if you can limit the time it lasts in proportion to the whole drive time, then it becomes less critical.

Hi Neil,

Well, I did spend a ridiculous amount of time "trying" to filter data that was very noisy. I like working with math, and this study was a serious study in noise filtering, and definitely shows the need for more and more stable data.

The Crr was calculated using the same technique as posted in the Instructables site. And please note that ambient air temperature make a "huge" difference is exactly what Crr number is generated.

Colder temperatures in the winter, compared to summer, changed the Crr value by about 20% or so. I don't know how accurate this calculation is because of the current GPS accuracy. But the the calculated number was generated by a "best fit" calculation was iterated many times until the error reached a minimum from the actual data compared to the "model" data.

The assumption that air drag swamps everything else at a constant speed, is only correct if the speed happens to be 150 mph or above. If however the car is a Honda Insight, and the speed is 45 mph, then the Cd and Crr are equal to each other. Obviously at slower speeds, the Crr predominates.

Jim.

[3-Wheeler]

Originally Posted by jimepting

....I have felt for a long time, since 2008 when I first joined, that the easiest measurement would be MPG improvement over long two way courses. With such controls as low wind, constant driving techniques, constant average speed, the small variations of conditions and driving technique average out if the course is long enough. I have achieved good repeatability with this technique. True that it doesn't serve up the number we would all like to get - the improved Cd - but it does point to which mods work and relatively how well they work.

Hi Jim,

At this point in time I would have agree with you.

A constant speed gas mileage test such as what MetroMPG or others have done, seems to be relatively simple and yet very effective to producing relative gains in aero changes and other effects.

In fact I was quite enamored by Metro's testing of his cardboard tail and later, the mirror, radiator block and other details, using the ABA technique.

I have not given up on coast down testing, but for now I will focus on other aspects of energy savings until a better approach to speed recording can be implemented.

Jim.