Most efficient speed for best MPG?

[Sheepdog]

I could really be happy if congress would lower the speed limits back to 55 and enforce them! I was doing 55 today on the interstate with a posted speed limit of *55* in that area and someone was dogging me from behind in the slow lane flashing his lights to try to move me along. I *so* wanted to slow down more!! But instead I just ignored the nasty driver. He then roared past me on the right side (on the shoulder!) and almost lost his car to a spin out before making it back to the road at 70+. My wife would say that I caused that by doing 55 in the slow lane. But then she gets 42mph in per Prii and is jealous of my average 53 mpg!


The law says the speed limit is 55 or 65 on the interstate! (depending on where) Why wont the troopers enforece the law???!

[drees]

I really don't get why people would get upset at driving the speed limit in the slow lane. There's a perfectly good lane to the left for them to use for overtaking. If it's a two lane road and holding up traffic, I'm sure you pull over at a pull-out as soon as it's convenient.

But on a multi-lane freeway/highway, it doesn't make sense. Afterall, at least here in CA, the maximum speed for big rigs is 55mph even when the rest of the traffic is allowed to go 65-70, so there is already plenty of slow moving traffic to dodge.

Doing your best to stay out of the way of the road-ragers is usually safest for all involved. But if you're already doing that and they insist on raging, what are you going to do? Just do your best to also be patient and realize that they have a mental illness called impatience.

[Sheepdog]

yep and thats only one reason I always have a pistol in the car too!

[Danny Hamilton]

Originally Posted by bwilson4web

See if you can get a copy of "SAE 2004-01-0064", "Development of New-Generation Hybrid System THS II - Drastic Improvement of Power Performance and Fuel Economy" by Koichiro Muta, Makoto Yamazaki and Junji Tokieda.

No thanks. Too much effort involved, and I'm too lazy. I'll certainly look through it if you post a link here though.

Originally Posted by bwilson4web

You realize the Prius does this automatically, right?

Yep. I wouldn't have mentioned it if I didn't realize how it works.

Originally Posted by bwilson4web

This is a little confusing:

Are you saying that my comments were confusing or were you pointing out that what you were about to say would likely be confusing?

Originally Posted by bwilson4web

Heat loss when the engine is off depends upon ambient air and air flow. The coolant temperature drop is very modest for short periods, measured in unit minutes. Any period less than 5 minutes is insignificant at temperatures of 70F or greater and low winds.

Ok, how about at 40MPH into a 15 MPH 35F headwind with an unblocked grill? How about with the coolant running through a radiator and the radiator fan on? If heat loss through the radiator is so negligible, what is its purpose?

Originally Posted by bwilson4web

ICE start/stop energy is observable but accurate measurement is difficult because it typically happens in about 0.250 second and the current scanners are too slow to measure the exact value without gathering a large number of samples.

Agreed. Which is why I prefaced my statement with "It would be interesting to know"

Originally Posted by bwilson4web

Energy loss during engine operation comes out of the engine efficiency, directly. No energy is generated without the engine working.

This is the point of my comment in the first place. When you travel a given distance without generating any energy, you reduce the amount of energy lost. As such you reduce the amount of energy that must be used to replace the lost energy.

Originally Posted by bwilson4web

I look forward to reading your 'simple calculation'

Given:
TSE=Total energy used for the purpose of stopping and starting the ICE once.
EPS=Increase in energy lost per second with ICE on as compared to with ICE off.
STB=Seconds of gliding required to benefit over extra energy used to start/stop the ICE.

STB = TSE / EPS

Originally Posted by bwilson4web

So I took a simpler approach and benchmarked P&G versus stead-state speeds. Use the search function and you can find the postings including the data.

I've seen them in the past.

Originally Posted by bwilson4web

18 mph - steady speed with the Prius handling the systems is more efficient than P&G with the same average speed

Regardless of the P&G speed range? Regardless of the weather? Regardless of the terrain?

Originally Posted by bwilson4web

Of course these are direct measurements, no modeling required. Even you could go out and replicate these tests and report the results.

direct measurements are great for proving a concept or to verify expected results. Unfortunately there are far too many variables to test the results of every possible combination (permutation?) of them. Modeling and math help to predict which situations are likely to provide a benefit and which aren't. They can save time, effort, and resources while narrowing down the number of situations that need direct measurement.

[fthorn]

This is my favorite post to-date.
Now, is the ICE 'too big' for many of the needs in these various speed states?
What if someone could employ cylinder deactivation for the ICE? Perhaps have it firing on 2 cylinders or 4 depending on what the need is? Do you think toyota is looking into that?

I did get an 88.9 mpg reading on about a 2 mile trip from the gas station last week to my home. I'd like to see more of those!

Thanks for this info. It's great.

[drees]

Originally Posted by fthorn

Now, is the ICE 'too big' for many of the needs in these various speed states?

The primary drawback of increasing displacement slightly is that it will increase friction at similar engine speeds due to the increased surface area of the rings or increase in piston stroke, depending on how they increased displacement.

Also, a bigger engine will need less throttle to produce the same amount of power, which also increases pumping losses past the throttle body.

Some of that increase in pumping and frictional losses should be offset by being able to produce the same power at a lower RPM.

Toyota has already employed the Atkinson cycle to reduce intake pumping losses at low loads.

Toyota could also go to a lean-burn catalyst which would allow air/fuel mixtures significantly leaner than stoich (14.7:1) which would also improve fuel economy.

[KandyRedCoi]

from my own experience, on mostly flat hiway driving i find 70mph to be great, its not so slow that you start road rage with your fellow motorist, and not so fast that you will get a speeding ticket

@ 70mph i avg about 43-45mpg hwy (mostly flat)

but i did find that my prius gets better mpg on up and down hiway driving at 75mph instead of 70mph about a 1.5-2mpg difference 42.5+mpg to 41mpg

i rarely drive on city streets so i "try" to do my best but dont take too much effort into keeping track of my MPGs

hope that helps

[fthorn]

Got to try out the 30 mph on the highway fuel economy today. Storm had everyone driving slow. I did very low speeds (for the highway) and got worse fuel economy than when I cruise at 55 - 60 or so. The motor never really did much for the motivation. Seemed like the engine was constantly running. But, I did get one half/bottom of a diamond. Not sure what that really means, yet, since I'm new. And the FIRST time, I got a SECOND one, but this one was a full diamond on the next bar. So, I had not seen that before. Not sure what that is indicative of. Anyway, I was hoping for 60-70 mpg, but sadly, no go.

[bwilson4web]

Hi,

Originally Posted by fthorn

Got to try out the 30 mph on the highway fuel economy today. Storm had everyone driving slow. I did very low speeds (for the highway) and got worse fuel economy than when I cruise at 55 - 60 or so. The motor never really did much for the motivation. Seemed like the engine was constantly running. But, I did get one half/bottom of a diamond. Not sure what that really means, yet, since I'm new. And the FIRST time, I got a SECOND one, but this one was a full diamond on the next bar. So, I had not seen that before. Not sure what that is indicative of. Anyway, I was hoping for 60-70 mpg, but sadly, no go.

Ken@Japan has posted the warm-up stages of a Prius and the final phase, S4, requires coming to a complete stop and letting the ICE auto-shutdown. This puts the car in a mode where it will shutdown the ICE at any speed it can below 42 mph. I've even pulled into a parking lot to get this S4 transition, it is that important to Prius mileage.

GOOD LUCK!
Bob Wilson

[fthorn]

Sorry to have missed that fact in my early readings/research of everyone's fine, appreciated posts!

Does it require or must one completely abstain, from putting it into park during that auto-shutdown 'car stopped / no motion' mode?