Hypermiling on a continually hilly terrain

NIce but there 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.

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

The technique that I've describe is a variation of what is known in Hypermiling terminology as NICE and DWL. For the 2010 Prius (gen 3), I've found that the Scangauge II's Gallons per Hour (GPH) Xgauge to be a better indicator of fuel efficiency than the Revolutions per Minute (RPM) Xgauge. In non-highway environments, sustained burning fuel at a rate higher than 1.50 GPH makes it much more difficult to maintain a FE of greater than 50 mpg. So the underlying premise is to increase the fuel efficiency by accelerating and maintaining the velocity while not pushing the GPH meter over 1.20 GPH for any sustained period of time. In a hilly driving terrain, the most fuel efficient method of gaining velocity to accelerate while the car is travelling downhill until the desired speed is achieve and to start driving up an uphill (from the bottom of the hill) at a speed about 10 to 20 mph greater than the targeted top of the hill velocity while lessening the *load* gradually (pressure on the accelerator) allowing the velocity to incrementally decrease as the vehicle climbs up the hill.

In non-winter and non-superhighway driving environments, the most fuel efficienct load range (+50mpg) for the 2010 Prius (gen3) internal combustion engine (ICE) appear to be when the ICE is burning from 0.80 to 1.20 gallons per hour in short burst sessions while the car is in drive mode and where the velocity is from 15 to 45 mph where the optimum speed is about 35 mph (+60 mpg) and the optimum burst load is about 1.10 GPH.

In superhighway driving (+50 mph) when there was no slow truck to pace behind, I found that DWL-NICE

-while providing better FE- was much more difficult to sustain than just setting the cruise control. I just could not keep it up for more than an hour on I270-I70 in Maryland. By the time I was in Pennsylvania I basically switched to setting the cruise control between 50 mph to 60 mph (where 50mph gets about +64 mpg and 60 mph get about +58 mpg if the car is driven for more than

2 hours per driving session in a spring/summer/fall driving environment ). For superhighway driving, GPH is not as important as acceleration, maximum speed, and the length of the route (aka length of time per trip). For the best FE on the superhighway, the maximum speed should be from 50 mph to 60 mph (with 50-55 mph giving slightly better FE than 60mph), the car must be driven non stop on the route/trip for over 30 minutes/30 miles (with the longer non-stop driving session more desirable), and car should gradually accelerate to highway speed in the beginning and gradually decelerate to nonhighway speed at the end of the trip.

. . .

Fortunately the ScanGauge can display four gauges and GPH is one of three that work well in both the 1.5L NHW11 and the 1.8L ZVW30:

o ICE coolant temperature o ICE rpm o Gallons per Hour

My GPH thresholds are similar:

0.30 GPH - the car ICE under 70C while coasting in "N" 0.60 GPH - during the first ~45 seconds of catalytic warm-up in 1.5L NHW11 and ~180 seconds in the 1.8L ZVW30. I understand the 1.5L NHW20 has the longer, initial warm-up 2.00 GPH - what I try to stay under when accelerating, traffic permitting, after reaching 70C

During the initial, catalytic warm-up, both the NHW11 and ZVW30 Prius will dip into the traction battery, heavily, if you keep the GPH at ~0.60-0.70. This window means I can often reach 35 mph on traction battery in the NHW11 and 45 mph in the ZVW30 with very high fuel efficiency.

What I do is park my commuting car near the exit at work where I can see the cross-street traffic. Hours later when I'm ready to leave, I monitor the traffic and when I can pull out without stopping, start the car and smoothly exit the parking lot and accelerate to the fastest speed while in the 0.60 GPH, EV mode. This ends at over 50 MPG and when the ICE kicks in, shift into "N", traffic permitting, and getting very nice mileage.

BTW, this early warm-up is described in the Toyota paper on the NHW11 but it takes instrumentation like the ScanGauge. If you have a cheaper scanner, the mass airflow rate can substitute for the GPH.

Bob Wilson