Prius – fuel consumption improving, but highways are still a challenge


Finally, I’m starting to get better fuel consumption numbers.  the last two fills were:

  • 13 May 2012 – 782km, 36.26l for 4.64l/100km (60.88 mpg(UK) 50.93mpg(US) 21.55km/l)
  • 26 May 2012 – 660km, 31.49l for 4.77l/100km (59.22 mpg(UK) 49.54mpg(US) 20.96km/l)

The last slightly short tank was on account of filling up before driving to Sydney early the next day.  The trip meter at the end of the journey read 4.5l/100km which if I add the 6.5% error becomes 4.8l/100km.  That is not such a bad results given the conditions:

  • Morning temperature was 0.3°c
  • Foggy, so I had to use the front and rear defoggers regularly
  • Lots of steep hills from Canberra to Sydney
  • Stopped to let the dogs have a wee
  • Stopped to let a goat run across the Hume Highway (more on that story later)
  • Used cruise control and Power mode to climb hills and ECO mode elsewhere

Power mode + cruise control + steep hill = ?

As many people who drive the third biggest-selling car in the world would know, the best economy comes from “Pulse and Glide” driving.  Instead of maintaining a steady speed, you should pulse to a high-speed and then glide by coasting until the speed drops and then pulse again.  Here’s a very instructive video to explain…

I haven’t figured out how to do that at 110km/h.  And it isn’t good form to vary your speed on a busy highway as there’s cars and trucks about.

So I just set the cruise control to 114km/h (true 110km/h) to drive to and from Sydney.  In a previous post I mentioned the elevation profiles from Canberra to Sydney and back again (the two ways aren’t the same).  The hills are a challenge since they are steep and long.  The average altitude doesn’t change for about 100km, but you do a lot of climbing nonetheless.  There’s also a fair bit of descending, so it is often possible to regenerate the HV battery while maintaining 110km/h.

As anyone who uses cruise control would agree, hills can cause a rapid drop in speed followed by a surge of power, a slight jump over the crest and a nervous wait to resume normal service.  When I’ve been in ECO mode with cruise control and I reach a steep hill, the car slows down a lot and then the car makes all efforts to get back to the set speed.  This is despite accelerating in anticipation of the hill.  This results in the engine and motor dumps full hybrid power to resume normal service.  This is not great for fuel consumption.

So I tried switch from ECO mode to Power mode before the slope affected my speed.  The [hardly rigorous] results were:

  • car only slowed 1-3km/h from the set speed on most hills before it recovered;
  • just touched the power zone on the HSI on most hills, instead of hitting the top of the power zone on almost all hills;
  • maximum instantaneous fuel consumption was 15.5l/100km on the steepest hill, compared to 17l/100km in ECO mode;
  • car felt safe and confident going up hills

So for now, I’ll use ECO mode for better A/C efficiency and gentler throttle response and Power mode to climb hills.  Not quite the same as pulse and glide, but might be effective for the steepest sections.

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