Cruise Control and Hills

I would second that question.
Also, what is the most efficient Prius speed for climbing a long, yet shallow grade hill.

We have many of those around where I live. It makes for many opportunities to coast downhill. But I am always wondering what the most efficient climbing speed is and the best strategies to use.

 

It is bad in one sense: It hurts fuel economy. CC is very aggressive in maintaining speed, pushing ICE (internal combustion engine) RPM into inefficiently high ranges with even a modest hill. I anticipate hills and gain speed (safely and legally) before hitting them, then let speed bleed off as I climb to keep ICE RPM within efficient ranges.

As for it being "bad" in other ways, there should be no problem except possibly to the extent it can push the car a little hard during break-in.

As for PeakOil's question, instead of concentrating on speed -- which is largely determined by traffic and posted speed limits -- I focus on ICE efficiency. As a rule of thumb, for those without a tachometer and other added instrumentation, I suggest keeping the instantaneous MPG at least half the vehicle speed during ICE-on conditions like accelerating and hill climbing.

 

It's a myth that high RPMs are less efficient in the Prius. Toyota has published information that shows in graphical form the ICE in the Prius is -more- efficient at high RPMs and high loads.

Yup, you will see very low instantaneous MPGs. Yup, the car will be using more fuel per sec. The ICE is still running more efficiently. It's just producing more power.

Going uphill I'm not sure what you think you could do to improve economy. The only choice for improvement is to drop your speed to reduce aerodynamic losses.

I just let the CC take care of holding the speed. Now, in hilly country you -can- improve on the CC because you can anticipate, and so release the throttle sooner and let the car coast over the top, increase speed on the way down so you can coast up the next hill further. But you will be varying speed (upsetting other traffic if there is any) and possibly speeding.

 

I have found that there isn't much you can do on long hills but reduce your speed. However as David said in hilly areas, short ups followed by short downs, you can increase your fuel economy by holding the throttle (not speed) steady. Your speed will drop going up the hill and increase again on the far side but your instantaneous mpg will stay pretty consistent. I got over 60mpg on a 90 mile trip in hilly country doing this at an average speed of 65-70. If you do this please be considerate of anyone behind you.

I have often thought they should have an additional mode on the cruise control that lets you specify a specific percentage throttle instead of a specific speed. It might not be very useful in many situations but some of the areas I drive in would be great.

By the way I am under the impression that many old cars have this exact thing. Instead a of a gas peddle they had a throttle knob that you pulled out to the specific throttle you wanted and it would stay there.

G

 

David,

Do you have a link to the Toyota data showing more efficiency at higher loads?:ear:

 

I prefer using more of a roller coaster approach, but don't have much opportunity for using cruise anyway. Conservation of momentum should give the best mileage. Therefore, I glide down hills whenever possible using regen only if it is unavoidable. I will maintain this speed with mild electric first then moderate ICE going into the next climb--holding what feels like a relatively efficient throttle position producing enough power that the vehicle doesn't bleed too much speed too quickly. If it bleeds too much speed then I will be stuck at low velocity with the engine pulling even harder with lower indicated mpg, so in my estimation it is better to give too much throttle early on a hill than too little. However, I cut out before the crest (assuming I'm not already going more slowly than I would like) and glide over the top and down the backside, gaining a lot of speed in the process without using the less efficient path through regen-battery-electric motor.

Folks who wonder what a radar detector is for in a Prius should make note that the car can easily be going 20+ mph faster at the bottom than it was at the top of short hill and more so on a long one. The longer ones tend to require some regen either to stay within a reasonable speed range, because of traffic, or the occasional presence of a patrolman--they can really screw up a good glide.

 

I've been following this advice and believe it is a good target that at least feels like an efficient mode for the engine. Of course there are some exceptions: some steeper hills with traffic where it is impractical and I have to apply additional throttle to hold a reasonable rate of climb, but I try to minimize the time spent below the half guideline.

Do you have any rule of thumb for how to know when you are "lugging" the engine at too low of a throttle setting? I can feel it at times when the ICE kicks in and is providing such a small amount of power that it doesn't feel smooth or efficient--more like a manual transmission left in too high a gear. In those cases I either try to force the ICE to shut down, or if I'm going to need more speed or momentum I give it some throttle until I can feel the engine actually pulling.

 

If I wanted to WORK at driving a car, I'd buy one with a clutch pedal. Using the gas pedal too much amounts to work. I consider the cruise control to be well worth ANY extra expense it costs going up a few long hills.

I-77 North entering Virginia from North Carolina is about 9 miles at a rather steep grade. Uphill at 65MPH, I drop to maybe 39MPG no matter how good things had been going up until then. On the return trip, that 9 miles downhill has almost always resulted in a FULL green battery and a major MPG boost, but if it's not a complete wash I don't care.

Worrying about cruise control inefficiency is like worrying about pizza calories: why spoil the fun?...

 

Hobbit (among others) has reported that efficiency dropoffs start somewhere in the neighborhood of 2200-2400 RPM, and efficiency really tanks at 3000+. CC easily pushes RPM above 2400 on hills, and RPM well above 3000 is common on steep climbs. Whether that is "high" is in the eyes of the beholder, I guess. But from all I can discern from his and others' papers, the key seems to be keeping torque high while keeping RPM as low as conditions demand.

This is admittedly only a small part of maximizing fuel economy in hilly terrain. More important (at any speed) is what David suggests and Shawn says outright: conservation of momentum. That and an in-depth discussion of ICE efficiency go a little beyond the scope of the OP's question, so I chose not to elaborate. But since others have ....

At the other end of the spectrum, it seems intuitive that a low RPM would be inefficient (though the CVT effectively prevents lugging). But those who discovered and refined Super Highway Mode clearly have demonstrated otherwise. The Prius obviously can run under load (i.e., moderately high torque) in the 1200-1400 RPM (and keep up with traffic) on level terrain with good fuel economy results. No one using CC has ever reported comparable results at similar speeds.

More recently, Hobbit has demonstrated high-load, low RPM conditions with good fuel economy results using injector timing as a surrogate measure of ICE loading.

Shawn, to answer your question about a "near-lugging" condition, in the thread where I first proposed the iMPG > MPH/2 rule, I also suggested keeping iMPG below MPH*1.2. But with the SHM disciples regularly going well beyond that without hurting their fuel economy, and given that the mental math is a bit more complex to do on the fly, I stopped suggesting it. Now I just suggest folks make sure the ICE is on when they give it some pedal. From my own observations of injector timing (assuming Hobbit's assertion is correct that it can be used to indirectly monitor load), it actually is rather difficult to get into a low-load, low-RPM condition, especially at low speeds, when battery state of charge is adequate.

So back to Mi3ke, if you're tired of us hijacking your thread with this theoretical and possibly irrelevant (to you) discussion, just tell us to shut up.

 

According to the attached graph published by Toyota in their SAE paper the optimum RPM is about 2600RPM with the range from about 2200RPM to 3400RPM being within the 230g/KWH contour. Even down at 1300RPM it is within the 240g/KWH contour and 4500RPM does not drop below 235g/KHW.

This graph is presumably for the earlier generation Prius as it stops at 4500RPM but is probably close.

kevin

 

I just leave my cc set on hills, unless I get balked by trucks or SUVs. It pulls the hills just fine.

 

The graph I was referring to is attached. It was in the original document from Toyota comparing the THS I (first gen Prius) to the THS II (current or second gen Prius).

system_02.gif

 

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