Fuel economy, hills, and cruise control

I agree that CC works better than might be expected in hilly terrain. Personally, I like to think of the Prius as an HGEV (hybrid gravity electric vehicle) in hilly terrain, and can usually gain back my mileage on the downhill--sometimes even improve my mileage rating by coasting. The problem with CC is that it can often really rev the engine just as you come to the top of the hill, so I take it out of CC frequently on uphills, or tap it for 1-2 mph slower, to prevent this.

 

CivicQc:

If your downhill glide using both methods was about the same at 90+ KPH, while the uphill segment was faster using cruise control, then you got the best mileage and also covered the route in the least time using cruise control.

 

The human has an advantage:
- being able to anticipate terrain ahead.

The CC has two advantages:
- precise control of the throttle: a person driving has to select the correct throttle position and can easily misjudge it. A person can also fail to keep the throttle absolutely steady.
- sensing change of speed: the CC will know quickly whether there's a change of speed and can respond quickly.

CC can also use some tricks to help:
- tolerance - by allowing some variation in speed (and I notice the Prius varying by +/- 2mph, more in one significantly undulating section) it can "smooth" the road and keep consistent throttle.
- guessing - if it assumes that a hill is "normal", it can respond pretty quickly to acceleration following deceleration and reduce the throttle without too much delay.

I would expect two things would particularly hit CC economy:
- Large variation in incline: would cause frequent throttle changes.
- Speed: the higher the speed, the more likely the CC is forced to use the power zone and the less efficient it will be.

 

Interesting, I did a recent trip and decided to use cruise control the entire trip. I beat any previous record at 59.1 MPG indicated over about 150 miles. I used DRCC but there werent very many other cars that triggered the radar. I also tried something when going down a hill, i applied slight pressure to remain at a 0 energy state (allowing it to collect speed past the cc limit temporary. I held it there until the speed burned down and cc began to kick in). I drove at 55mph the whole time, and it was VERY hilly (CT15 & I91 in CT). Part of the reason I think it was so high is because of the mental speed creep. I find myself wanting to go faster if im driving manually even up to 65, but cc keeps me under control.

 

I seem to be able to beat the CC most of the time and the steeper the hills the better I can do. But after driving a 100 miles or so I sometimes get tired of it and just turn the CC on and take a mileage hit.

I use the CC most of the time on busy freeways because it maintains a steady speed and that's what other drivers expect, which probably makes it safer.

But on back roads through the mountains I can do better because I tend to let the car coast in "warp stealth" up to whatever speed I can coming down the hills and then use that velocity to help get up the next hill. I think that's a more efficient use of energy than using the down slope to cram the battery full then pulling the Amps back out again.

 

Speed is a killer of consumption too. When you do these trial runs of CC vs. manual, could you also look at average speed?

I always wonder about pulse and glide vs. CC at the same average speed...or are the pulse and glide numbers better because you are going slower?

 

I go on 15 through CT routinely myself, and I would *never* use
CC on that road. Too many surprises, as it's one of those older
highways with really short ramps. The hills are fun, particularly
toward the southern end, as it's a sort of manic-depressive game
of 3000 RPM, warp stealth, 3000 RPM, warp stealth, ... but all
closely controlled and timed.
.
_H*

 

Hi
I use the CC also. I watch the MPG graph and when it hits 75 MPG on the down hills I click up the CC lever 1 MPH. I keep doing this until I reach the maximum speed I want to go at the bottom of the hill. Now I've built up some momentum for the up hill where I click down the speed 1 MPH each time the bar hits 25 MPG which has increased my mileage from 43 to 50 MPG in very hilly Westchester NY in my 2005 Prius.

 

I'd sure like to do that but we have state patrols hidden at the bottom of hills and mountains here. They'd love to pick off someone coasting at high speed just for the sake of the ticket. But I did learn something about my CC that wasn't apparent in the instructions: If you set it for the speed limit and you are going down a hill/mountain faster than the set speed, the CC will engage engine braking automatically to slow you down. That's cool - all my American cars would just coast out of control.

 

I've wondered about this:
In my old automatic transmission cars, going down a steep hill always caused me to speed up whether or not cruise control is on.

But in the ECVT Prius, being in cruise control keeps speeds steady even downhill, whereas the car speeds up when out of CC if the downhill slope is steep.

Why does this happen? Do ECVT or hybrid cruise controls work differently?
Or is this an illusion, and they don't work any differently and it's all in my head?

 

The previous generations of Prius behaved pretty much like your old auto trannie cars as far as the CC was concerned. The Gen3 Prius uses regen braking to slow you down when descending. I think it may use friction brakes too if needed to keep the speed at the set point. Sometimes a good feature because it can keep you within the speed limit; but it may be more energy efficient to let the speed build up and use the extra velocity. You can do that by using the throttle to keep the HSI out of regen.

 

I just made a trip diagonally across the state of Ohio. I used AC and CC in both directions. I set the speed to 61 mph on the route going SW to NE (about 300 miles). I was able to achieve 65 MPG indicated (yes I was passed by everyone including tractor trailers). On the return trip the next day, I used AC CC and set the speed to 65 mph. I only got 53.8 MPG indicated. I was surprised that is was that much different. The only other factor that I can think of that may have been in play here is wind.

 

You might consider running a test on a 10 mile flat section of a local interstate doing:

• pair of runs in opposite directions without AC, record temperatures
• pair of runs with AC, record temperatures

This would quantify the effect while taking out elevation and wind effects.

Bob Wilson

 

I read this thread with great interest.

I have tried using CC initially and find it doesn't help in fuel economy because the roads here are very undulating.

Now I am experimenting combination of P&G & CC.

I maintained at CC speed at uphill climb, then as it reaches the peak toward the downward slope, I gently apply the accelerator and maintain at lower quadrant at the HSI.

The vehicle speed will increase of course; this will enable the Prius to build momentum in anticipating the next hill climb. The speed will gradually decline to the CC set speed, then I will let go the accelerator to let CC take control when it climbs the hill.

The advantages of doing this is the following
1) The increase downward slope speed; will not inconvenient other motorists trailing behind who tend to increase speed to take advantage of downward slope

2) Take advantage of the downward slope to build up momentum for next hill climb; this will reduce engine high rev.

3) Easier to control the accelerator pedal. Making use of CC to activate the Pulse when doing hill climbing, and then Glide when going down hill by gently pressing the accelerator and maintaining at lower quadrant.