A bit of background on these:



I was curious as to just how efficient cars are, and how much energy
is actually takes at the wheels to move them. The energy in
hydrocarbons is easy to look up; but what about the drag forces?



It occurred to me that since drag is simply the force that must be
balanced to keep up ones speed, and speed is acceleration integrated over
time, that it should be possible to find out what I want to know by taking
a tape recorder out on the road and finding a straight, level portion, then
bring the car up to speed and put it in neutral, calling off speeds into
the recorder. Thus I got plots of speed versus time, the derivative of
which of course is acceleration. Given that plus the mass of the car, at
that time available on the registration, I got numbers for drag, and hence
required power, and energy per mile.



I did these tests first on a '77 Impala, then on a Monte
Carlo, the figures for which I remember as 25 kW and 17 kW respectively
to maintain 60 miles per hour. I no longer have the data for these vehicles;
but I do for others that I've tested. When I asked a regular poster at
grex to run such a test on his
car, it occurred to me that I should also do the same. So I did, and posted
the results there.



By the time I got my latest vehicle, I'd had for a while a small digital
video camera, which makes it relatively easy to get a continuous timed
record of the speed gauge.



And so, here are the test results:




Windstar class minivan

Test conducted 2007:05:20; temperature 63 degrees F, barometric pressure
30 inches of mercury, for an air density of 1.219 kg/m³.
Intellicast
reported wind from the north at 11 MPH, which I judged small enough compared
to the test speed range, or at least as light as it usually gets.



Beforehand I had determined that the speedometer and odometer both
read about 15% short. The other vehicles that I tested had accurate gauges.



The highway in the test location runs along course 71 degrees (ENE) and
251 degrees (WSW). For this test I managed to get data for each direction.

Mass
Curb mass  3711.4  lb
Fuel         75
Self        150
Misc         50

Total:     3936.4  lb

Polynomial approximation: V = sum( Ak t^k );  a = sum( k Ak t^k )

East-Northeast run:
  Speed      Speed    Time   Poly    Accel    Accel   Drag   Power   kW*hr
(MPH, raw) (correct) (sec)   calc  (MPH/sec) (grav)  (lbf)    (W)    /mile
-----------------------------------------------------------------------------
   77.5      89.13    16.0  88.82   -1.577  -0.0719  282.99  49985  0.56274
   75        86.25    18.0  85.72   -1.521  -0.0694  273.01  46539  0.54289
   70        80.50    21.4  80.71   -1.427  -0.0650  256.04  41094  0.50914
   65        74.75    25.4  75.23   -1.316  -0.0600  236.06  35313  0.46942
   60        69.00    29.6  69.95   -1.199  -0.0546  215.09  29917  0.42771
   55        63.25    35.5  63.36   -1.034  -0.0472  185.62  23387  0.36910
   50        57.50    42.7  56.63   -0.834  -0.0380  149.64  16852  0.29756
   45        51.75    50.3  51.10   -0.622  -0.0284  111.65  11345  0.22202
   40        46.00    59.4  46.59   -0.369  -0.0168   66.14   6128  0.13153

West-Southwest run:
  Speed      Speed    Time   Poly    Accel    Accel   Drag   Power   kW*hr
(MPH, raw) (correct) (sec)   calc  (MPH/sec) (grav)  (lbf)    (W)    /mile
-----------------------------------------------------------------------------
   76.5      87.98    14.5  87.78   -1.381  -0.0630  247.80  43255  0.49276
   75        86.25    16.0  85.74   -1.348  -0.0614  241.89  41238  0.48100
   70        80.50    19.8  80.77   -1.264  -0.0576  226.90  36443  0.45119
   65        74.75    24.6  74.96   -1.159  -0.0528  207.96  30997  0.41354
   60        69.00    29.2  69.86   -1.058  -0.0482  189.81  26367  0.37744
   55        63.25    35.8  63.35   -0.913  -0.0416  163.76  20631  0.32564
   50        57.50    43.5  56.98   -0.743  -0.0339  133.36  15110  0.26518
   45        51.75    52.8  51.02   -0.538  -0.0245   96.62   9803  0.19213
   40        46.00    63.5  46.52   -0.303  -0.0138   54.34   5026  0.10805

I don't have figures for actual fuel economy yet - getting them requires
filling the tank - twice. The behavior of the fuel gauge suggests
a rate of about 19 MPG on surface streets and maybe 25 MPG on
nonstop highways. I plan to get better instrumentation at some point.



25 MPG at 75 MPH is 3 gallons per hour, or about 100 kW. 20 MPG would be
3.75 gallons per hour, or about 130 kW. This implies a total drivetrain
efficiency ranging from 23% to 31%. While still not exactly stellar
(the ideal for a 9:1 compression ratio air standard otto cycle is 58.5%),
it's significantly better than those of the Escort and Grand Am.

Other vehicle test results

Escort Wagon

  Mass (curb): autos.msn.com/Vip/Specifications.aspx?modelid=1064&src=vip
               2513 lb
  Crew and
  cargo:        160 lb approx
  Fuel:          60 lb approx

  Total:       2733 lb

     Speed     Time     Poly    Accel    Accel     Drag    Power    kW*hr
     (MPH)    (sec)     calc (MPH/sec)  (grav)    (lbf)      (W)    /mile
  ------------------------------------------------------------------------
        80     7.87 79.76637 -1.15239 -0.05253    143.6    22954    0.287
        75     11.7 75.43812 -1.10717 -0.05047    137.9    20675    0.276
        70     16.8 69.95688 -1.04123 -0.04746    129.7    18148    0.259
        65       22 64.73187 -0.96724 -0.04409    120.5    15654    0.241
        60     27.2 59.90930 -0.88644 -0.04040    110.4    13243    0.221
        55     32.7 55.28575 -0.79357 -0.03617     98.9    10867    0.198
        50     40.1 49.91168 -0.65658 -0.02993     81.8     8174    0.163

Fuel economy: 25 MPG (at 75 MPH, 3 GPH = 100 kW.)

Efficiency: 20% to 21%





Safari Wagon:

  Mass (curb): carpoint.msn.com/Vip/Overview/Pontiac/Safari/1988.asp
               4182 lb
  Crew and
  cargo:        150 lb approx
  Fuel:          30 lb approx

  Total:       4362 lb

     Speed     Time     Poly    Accel    Accel     Drag    Power    kW*hr
     (MPH)    (sec)     calc (MPH/sec)  (grav)    (lbf)      (W)    /mile
  ------------------------------------------------------------------------
        85      3.7 85.25442 -1.42615 -0.06501    283.6    48174    0.567
        80      8.1 79.40030 -1.23836 -0.05645    246.2    39370    0.492
        74     12.5 74.32544 -1.07195 -0.04886    213.2    31523    0.426
        70     16.8 70.02861 -0.92997 -0.04239    184.9    25870    0.370
        65     22.7 65.03736 -0.76838 -0.03502    152.8    19848    0.305
        60       30 59.99955 -0.62164 -0.02833    123.6    14822    0.247
        55       39 54.92147 -0.52172 -0.02378    103.7    11403    0.207
        50     48.6 50.03282 -0.51370 -0.02341    102.1    10207    0.204

Fuel economy: 13 MPG (at 75 MPH, 5.8 GPH = 192 kW.)

Efficiency: 16% to 17%





Grand Am:

  Mass (curb): carpoint.msn.com/Vip/Specifications/Pontiac/Grand%20Am/1993
               2756 lb
  Crew and
  cargo:        150 lb approx
  Fuel:          45 lb approx

  Total:       2951 lb

     Speed     Time     Poly    Accel    Accel     Drag    Power    kW*hr
     (MPH)    (sec)     calc (MPH/sec)  (grav)    (lbf)      (W)    /mile
  ------------------------------------------------------------------------
        80      7.6 80.00630 -0.86589 -0.03947    116.5    18624    0.233
        75     13.6 75.07199 -0.78044 -0.03557    105.0    15736    0.210
        70     20.8 69.78537 -0.69032 -0.03146     92.9    12991    0.186
        65       28 65.09877 -0.61376 -0.02797     82.6    10726    0.165
        60     36.6 60.15105 -0.54008 -0.02462     72.7     8712    0.145
        55     47.1 54.83992 -0.47636 -0.02171     64.1     7044    0.128
        50     57.6 50.04656 -0.44146 -0.02012     59.4     5934    0.119

Fuel economy: 30 MPG (at 75 MPH, 2.5 GPH = 83 kW.)

Efficiency: 18% to 19%

Tags: automotive, energy