Most efficient speed for best MPG?

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???!

 

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.

 

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

 

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

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

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

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?

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

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.

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

I've seen them in the past.

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

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.

 

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.

 

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.

 

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 :rockon:

 

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

 

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?

 

I suspect that the rain and winds had more to do with your poor mileage than your speed. Not reaching S4 might have contributed a bit, but probably not as much as the weather. Did the traffic conditions require you to use the brake much? If so that probably ate into your fuel economy as well.

 

Hopefully Ken@Japan will chime in but what I've noticed is:

• S3 mode - the engine will auto-shutdown if at least 32+ mph has been reached and the vehicle doesn't need the ICE or deceleration. This can give a false sense of S4 because you can come to a complete stop and the ICE will be off. But if you accelerate and don't reach at least 32+ mph, the ICE will continue to run and run and run at lower speeds regardless of what you do.
• S4 mode - just come to a complete stop and if the ICE is not running, I pulse the accelerator long enough to get it spinning. If it can go into S4, the ICE will stop within ~8 seconds. Then you actually are in S4. It is something I do as part of my daily routine.

GOOD LUCK!
Bob Wilson

 

I think I am going to have to re-read this whole thread as well as some other sources several times. I may need to summarize things to get it all straight in my head. Especially the S3, S4 thing.

I have a flat, straight stretch of city road that frequently has little or no traffic. It's about 3/4 mile long from light to light. almost two if the light is green when I get there. I have tried speeds, with CC, of 30, 35, 40, 45, and 50 (50 is over the limit). I regularly get the best MPG at 45 - 50 MPH. It always seemed a little strange but hey, the car is smarter than me.

Often it goes into EV for a variety of reasons and of course that throws everything off as I am trying to get results with the ICE running.

Time for study,
Bob

 

Bah, a significant part of why the Prius gets such great MPG at slower speeds is that it shuts the ICE when it isn't needed.

If you force the ICE to idle while you drive slower and slower, then obviously your fuel economy will suffer more at the slower speeds. The ICE continues to burn through the same amount of fuel per minute while idling, and the vehicle covers an ever shorter distance per minute. Divide shorter distance by same fuel and you get a reduced MPG.

Now if you shut the ICE off while traveling some distance you have the opposite effect. You are increasing the distance covered while burning no additional fuel at all. Therefore MPG increases.

What are you trying to calculate? The amount of fuel burned per minute while the engine idles?

 

Agreed. With the Prius let the car handle it. The point is that all over these forums and others I keep reading - Slower = greater MPG. I also read it applies to non-hybrid cars.

In the Prius, I would expect the ICE to run at an optimum RPM. If this is true then at very slow speeds I would expect the ICE to send less energy to the wheels thus lower MPG. I don't think slower always translates to higher MPG.

Thus slow-slower does not always increase MPG. I would expect there to be a "sweet spot", below which you reach the point of diminishing returns. The great thing about the Prius is that it minimizes the problem by using stored energy from the battery to keep the MPG high.

Bob

 

43-47 mph seems to be the best for me. I regularly see the MPG (when on CC) at 50-70 MPG.

 

Hi Bob,
I think your explanation is perfect.
Anyway, there is a thread regarding to the stages.
http://priuschat.com/forums/knowled...perating-mode-availability-reference-aid.html

Ken@Japan

 

Every vehicle has a certain amount of energy used when "on" simply to keep everything operating. In the Prius, since it can shut the ICE off, this base energy use is much lower than in other cars.

As such, every vehicle suffers poor MPG at VERY low speeds because this base energy use is larger than the energy use to move the vehicle. As the vehicle increases in speed, it eventually crosses a threshold where the energy to keep it operating is less than the energy to overcome rolling friction and air resistance and other additional energy drains of a moving vehicle.

Because the Prius can shut the ICE off, it reaches this threshold at lower speeds than other vehicles. Through testing, some of the PriusChat regulars have found this speed for the Prius to be somewhere around 20 MPH. In other words, 20MPH will get you the best possible MPG in a Prius. As you get further away from 20 MPH in either direction (faster or slower) your MPG drops. So, for the Prius, slower increases MPG until about 20 MPH.

On other vehicles the speed at which MPG peaks is higher than the Prius, because the fuel used to keep the ICE idling needs to be compensated for with some reasonable distance covered. This number is probably specific to each individual vehicle, but I'd expect it to be more than 20 MPH and less than 45 MPH for most vehicles. So on expressways, slower will almost always mean better MPG, but on roads where the speed limit is closer to 45 MPH faster might mean better MPG in some vehicles.

Which it does. Of course it eventually reaches a point where the optimum RPM is 0 since even idleing it generates more energy than the wheels need. Actually a more accurate description might be that the optimum RPM is less than the idle RPM. Since the vehicle can't run the ICE at half the RPM, it instead runs it at full RPM but cycles it on and off so that it is only running half the time. The excess energy generated while the ICE is on is buffered up in the battery and fed back to the wheels while the ICE is off to keep the vehicle moving smoothly.

 

The only reason to shift to "P" during a stop is because your foot gets tired of pressing the brake pedal. As long as you press the brake hard enough to stop the arrows on the "Energy" display, from a fuel economy perspective it is equivalent to being in "P".

In the US the most fuel-efficient speed is 42 MPH. In Europe, Japan, and Australia it's 42 kph.
:_>

 

I'm in agreement with all of these points. The Prius does a much better job managing its available energy and thus has great FE.

It all boils down to the fact that the Prius is a GREAT car.

I did have another observation/question but I think I'll start a new thread regarding ICE vs battery at highway speeds.

Thanks,
Bob