Finally Did It ...

I finally made my entire morning commute (including 10 miles of freeway driving) on battery power alone.

 

Highway speed about 57 mph.

It's downhill mostly, but barely. There are two steep uphill slopes I have to take at about 25 mph to avoid burning all my charge. The rest is about a 0.9% downhill grade (a 10-inch drop for every 100 feet). I have to pass through 26 traffic signals on the street portion of the commute.

 

It's very dependent on all of the planets and stars lining up in addition to technique. If one extra traffic light had been red, or if traffic had been moving 5 mph faster on the street, I wouldn't have made it. When I first got the car I could barely go 10 miles on EV, but you learn how to massage the miles.

 

The return trip does not "cancel" the effect of elevation. You will get less range going up and down an 8% hill than you will a 2% hill. The bottom line is that you can't directly compare any route against any other route because they're all different.

Even if the difference is 1 inch in 20 miles? That's still downhill.

 

Indeed. At some point the slope becomes a fairly small factor. A 0.9% grade is less slope than your average rain gutter (which looks pretty level to the naked eye). In my particular case the record for round trip MPG stands at 108 MPG (2.17 L / 100 km). If I could charge at work I should be able to get about 15.8 miles on EV on the return if I directly convert the energy required for each leg of the trip. Assuming the two legs cancel out the effect of elevation, I would calculate a "flat" equivalent of about 18.4 miles in EV mode.

 

In an ideal world all of our engines and regeneration are 100% efficient. The real-world problem is that the system isn't 100% efficient. For the sake of easy illustration consider the following two examples:

1. A car takes 30 kW to travel up a 1-mile slope of 5% grade at 60 mph. That's 0.500 kWh to climb the hill. The car returns down that same hill with 40% regeneration efficiency, so generates about 0.144 kW/h. The net power required to make the 2-mile trip is 0.356 kWh for 2 miles, or 178 Wh/mile.
2. A car takes 21 kW to travel up a 1-mile slope of 2.5% grade at 60 mph. That's 0.350 kWh to climb the hill. The car returns down that same hill with 40% regeneration efficiency, so generates about 0.061 kW/h. The net power required to make the 2-mile trip is 0.289 kWh for 2 miles, or 145 Wh/mile.

Because the drive train isn't 100% efficient, the more power required to move, the more power is wasted. You'll notice that even if there was no regeneration on the second hill it would still be slightly more efficient than the first hill.

The laws of thermodynamics are a pain

 

My "normal" is somewhere around 500 mpg, but can be as high as 600. I usually have to engage the ICE to climb the last steep hill without depleting all of my EV range prior to arriving at the office. If the ICE would supply full power the minute it started (instead of some weird EV-assist mode for the first minute or so), and if it would shut off as soon as I switched back to EV mode, I would regularly get better mileage.