Frozen at 49 mph.... since 1/19/14

Love it!

 

Maybe it's a suggestion...

 

I do not know. From Redwood City to Orange County (averaging 80 with bursts to three digits during passing) I averaged 48mpg, with no external power. That includes the Grapevine which is a substantial hill. So it is very possible to get 48mpg sans external charging. How you can get 55mpg sans charging at an average of 80mph suggests a very strong tailwind, methinks.

 

That was my worst mileage, not my best going the other direction. A tailwind would have made it my best mileage on the trip. My suspicion is that people reporting under 50 mpg are doing a lot of unnecessary accelerating and/or braking (my wife gets mid-forties in her Prius, and that's why). I see lots of people who do not drive a constant speed even on level highway, and that kills mileage.

If you start watching how other people drive you'll see a lot of baffling driving techniques. One I see a lot is people accelerating uphill. I'm not sure if they know they need more gas to go uphill and don't have the motor control required to adjust the throttle for constant speed, or if they're just not paying attention and mash the gas on a hill. The other really common one is the guy who rolls down the freeway onramp at a constant 50 mph, missing the opportunity for a gravity assist, only to mash the gas once they've merged onto the highway. I don't know if that's you or not, but probably not given that you're here on this forum. There are lots of opportunities while driving to take advantage of the forces of nature to improve mileage, but I suspect a lot of people don't bother or don't know enough about gravity, inertia, and wind resistance to take advantage.

 

Today, I noticed a disruption from the normal operation of the EV mode. This morning, leaving home in EV mode I noticed the range decreasing more slowly than normal, and increasing gradually at times on level terrain. I started out with 14.8 kms EV range, and after I had driven all 27 kms to work, I still had 4 kms EV range remaining. There are no major changes in elevation along the route.

Perhaps it had something to do with the fact that I hadn't used the remote starter to warm it up first. It wasn't as cold this morning as it had been previously.

 

My driving technique is based on 11 years of Prius Zen driving.

Again, only commenting that you got 55mpg at 80mph.

Anyone else here driving 80mph over several hundred miles get 55mpg?

 

Not me. Coming back from Florida in Nov of last year I managed to get 44.1 due to a stiff headwind. It's easy enough to get much lower mileage given the right conditions. But 55 mpg at 80 mph would certainly be quite a feat unless the measurement was from a relatively short trip downhill or riding Santa Ana winds.

 

Need a sail.

 

That's about what I got on a recent trip. Lots of gradual acceleration and deceleration on back roads with very little traffic. I don't think the speed degrades the mileage too much because of the low drag coefficient (0.25) of the Prius.

 

Back roads with very little traffic AND going 80 MPH ???

Speed degrades the mileage on EVERYTHING with a drag coefficient of greater than zero.

 

On a recent trip to the Bay Area from LA I set the cruise at 77mph traveling North. I averaged 42 mpg. Coming home traveling South I set the cruise at 80 and averaged 47 mpg. I feel like this car is ultra sensitive to wind and weight, though weight did not change in my example. I didn't feel any real wind but I'm sure it must have been a factor.

 

The other thing I've noticed is the type of hills. There's only about 30m difference in total elevation from my home to my work, but one route home involves a few short and steep climbs evened out by long gradual descents. I seem to get very good mpg on that.

 

That's why GregP507 said it doesn't reduce it TOO MUCH. If you perform some trials you will find that on level ground the mileage difference between 55 and 80 mph is pretty small. It's the hills that kill you, even slight hills.

I honestly don't know how people can make their Prius get under 50 MPG in normal driving. I've NEVER seen mileage that low except for trips that involved 50 or more miles of steep uphill mountain driving at highway speed.

 

I honestly don't know how you get 50+MPG traveling at 80mph (and distances that negate the 11 or so EV miles).

 

Drag force goes up as the square of speed, so the drag would be a little over twice as much at 80 as at 55.

I think rolling resistance is pretty important, but have no clue about quantifying it. Anyone have any stats?

Mountain driving is very important, and I wouldn't be surprised if it trumped both of these. If we had perfect recovery, it shouldn't be a big deal -- the energy conversion to potential energy should go straight back to kinetic and thence to electric. I found a document from an Oregon State grad student course in 2009 that claimed "Prius regen efficiency ≈ 30%" Doing some back-of-the-envelope, a 1.5 metric ton running at 66 mph is about a megajoule, or more importantly that 1.5 metric ton vehicle rising a 66 meters is (1500 kg)(10 m/s2)(66 m) or again about a megajoule, or about a third of a kilowatt-hour. When I come back down again, I lose two thirds of that megajoule. My entire battery is 4.4 kw-hr, or about 16 megajoules. Going up and down 24 times is the entire capacity of my battery, as compared with level travel!

 

I've done the I-5 trip to/from SJ - LA at least 15 times in 4 different Prius' since 2001. I always get higher mpg going south. I'm guessing that there are more miles of gradual downhill going south even considering the steep short uphill on the Grapevine.


Mike

 

I think it's possible to get into a deficit situation with the hybrid battery. Normally there is a reserve of electricity in the hybrid battery, which is utilized when extra power is needed. This is mostly regenerated from coasting and braking, because a direct charge from the engine-generator is the least efficient way to charge the battery.

In a deficit situation, the system has no option but to burn fuel to charge the battery back up to an acceptable level. This is not very efficient, because almost 70% of the energy is lost in the conversion from the engine, through the generator, into the battery and back out through the electric motor to the drive-train. This may account for the much lower mileage numbers.

 

Yes there IS an "option". Just wait for normal driving situations to make opportunities to recharge more efficiently. The most obvious one is when the ICE is needed but just barely (light throttle). Then you can also stuff a little into the battery at a fairly high efficiency.

I don't think there really is any urgency to recharge the battery. Why do you ??

And where do you get that 70% figure ? I think it is WAY too high.

 

I am confused how you override the computers? Mine certainly become more aggressive when I get to the bottom two bars.

We know that the engine is only 37% efficient, at best, and that the computers tend to try to use electric power when it is more efficient than the gas engine. So the 'convert to electricity, convert to chemicals, convert back to electricity, convert to rotation' must be less than 37% efficient or the computers would always use it first. (Which they do in the PIP as it starts as electricity, so saves a conversion)

 

The system cannot allow a lithium-ion battery to fall below a certain voltage level without damaging it. If coasting and braking are not sufficiently charging the battery, this must be accomplished through the engine and generator.

It's well known throughout the industry that efficiencies with regard to electric generation/storage/utilization are in the 30-33% efficiency range.

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