Tuesday, November 4, 2014

Day 3: Into the Great Wide Open

Today’s plan: drive west and stop at every SuperCharger on I-90 in Minnesota and South Dakota. Okay, so, there was a bit more to today than that but not much.

Since we started from a full battery in La Crosse, WI, LJ made a speed run across the Wisconsin border and the eastern part of Minnesota.  The skies were a beautiful blue with only a couple tiny white clouds off on the horizon.  Winds were steady at 15 mph from the northwest with gusts to 25 mph.  Between the strong winds and the blistering pace, our average energy usage for this first led was an astounding 431 Wh/mi.  If you remember back to previous posts, “typical” driving conditions are supposed to be 300 Wh/mi and our first two days we averaged 361 Wh/mi. 

The next leg of the trip used even more power. I was averaging more than 450 Wh/mi cruising at highway speeds when I started to get worried about the ever-closing gap between the Projected Range and the distance to the next SuperCharger.  I drafted behind an 18-wheeler for about 10 minutes as we approached the stop in Worthingon, MN, which brought it down to the 441 Wh/mi shown in the photo below.

Day 3: winds were howling on this leg


Today’s physics lesson starts with the word draft (transitive verb): to stay close behind (another racer) so as to take advantage of the reduced air pressure created by the leading racer.

Draft is a key verb for Tesla energy efficiency.  Out here on the wide open roads, drafting behind an 18-wheeler is critical for minimizing the time spent at charging stops.  The physics is pretty simple: the truck pushes most of the air out of the way, which means less energy expended by our car.  Per the Wikipedia article on drafting, following within 100 feet of an 18 wheeler can lead to an 11% increase in mpg efficiency for a gasoline-powered car. 

The first photo above shows the stats from the second leg of the trip.  (The Trip A is supposed to be a daily update but we didn’t reset it until 100 miles into the trip today.)  The wind was howling across the plains, leading to the high usage of 441 Wh/mi.  For the next leg of the trip (Mitchell to Murdo), I drafted behind a truck going 74 mph for about half the trip.  As he was leaving the highway, I fell in behind another one going just a bit faster.  I managed to drop the average energy usage down to 327 Wh/mi for that leg of the trip.  While some of the efficiency increase was due to the wind dying down, the instantaneous power change that occurred when I got out from behind the truck showed that drafting was clearly making a difference.

Day 3: why drafting matters


The one touristy thing we did today was to stop by the World’s Only Corn Palace in Mitchell, SD.  Since we already had to leave the interstate to use the Mitchell SuperCharger, it was just a short detour to see the palace.  After a dinner and charging stop in Murdo, SD, we headed to Kadoka for the night.  

The World's Only Corn Palace in Mitchell, SD

Here at the America’s Best Value Inn, LJ found a standard 120V outlet just outside our first-floor window.  I’ll be interested to see how much power we’re able to add overnight over that super-slow connection.

We crossed into the Mountain time zone somewhere in the middle of South Dakota, which means we stopped reasonably early for the night as compared to the last two nights.  Probably a good thing, given how long tomorrow will be.  A key error in our original trip planning was that we didn’t think about how few hours of daylight there are in a day when we’re this far north in November.  We had planned to do the Badlands on Tuesday night until we realized that we would be driving through them in the dark.  So tomorrow we check out the Badlands and the Black Hills as we continue on our way west.

Day 3 stats:

562.7 miles in about 11.5 h for an average speed of ~49 mi/h overall (including stops)
4 SuperChargers stops:
  • ·         Albert Lea, MN
  • ·         Worthington, MN
  • ·         Mitchell, SD
  • ·         Murdo, SD

Overall trip: 1915.4 miles miles in 3 days, which means we’re more than halfway to Palo Alto

Day 3: Kadoka, SD

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