Recently there was a posting in a USENET group about driving in hilly terrain. But this is another approach, assuming regular highway traffic mix including large trucks. It is based upon keeping the engine in a fuel efficient power range and how large trucks have power-to-weight ratios that match Prius efficient power levels.
Find a large truck or semi-trailer, moving vans are excellent candidates, and use them as the pacing vehicle. As they climb a hill, follow them at the same speed at least 200 ft. behind. On steeper grades, this will be in the 50-55 mph and tends to keep our Prius engine in fuel-efficient power ranges. This is especially true for the 1.5L models that use fuel enrichment to avoid overheating and damaging the catalytic converter. For example, this chart shows the fuel burn up an 8% grade hill at different speeds:
It may be tempting to go up the hill very fast but this is a false economy. The traction battery is providing the extra hill climb power while the engine is struggling at maximum power, a fuel wasting power setting.
When the truck crests the hill, follow and match speed on the descent shifting between "N", "D" and "B" with preference to "B" and "N". The reason is descending a hill in "D" and braking will put a significant charge and heat load on the traction battery ... heat is the enemy of traction batteries. This is especially true with the older NHW11, 2001-03 Prius. Maintain the 200 ft. and take pains to make sure the truck driver can easily see that you are following at a safe distance.
If you see evidence that the truck driver is acting anxious, back off and find another truck pacing vehicle. Always be friendly and smile and 'thumbs up' to the truckers. Whatever you do, keep the distance at least 200 ft., way back so you can react to road debris and won't be seen as 'drafting' the trucker. After all the trucker is providing 'cover' for your fuel efficient driving.
Following traffic will see the truck as soon as they see you and prepare to pass. If you are out there by yourself, following traffic will often come right up behind your bumper before realizing you're driving your speed, not theirs. So let the following traffic curse the truck and snicker that you're stuck and move on down the road. You'll pass them soon enough when they are refueling. <GRINS>
What if there isn't a 'big buddy' pacing vehicle?
Climb the hills at 55 mph and descend shifting between "B" and "N" at a safe and reasonable speed for the conditions. Don't crowd the bumper of slower traffic and generally be courteous.
Now if you have instrumentation that shows engine RPM, try to keep the RPM under 2,400-2,600 (NHW11.) During a climb, try to keep it under 3,800 rpm (NHW11). Anything over 4,000 really burns the fuel for not that much extra power so if traffic permits, bleed off speed. Here is a chart of mixture and brake specific fuel consumption:
This is properly called "load driving" because you are keeping the engine always in a fuel efficient region. Speed will vary but not nearly as much as ballistics driving. Some of us have ideas for adaptive cruise control that would handle the set points automatically but that is something for another day.
Bob Wilson
Today
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