Question on PRIUS Highway MPG

<div class='quotetop'>QUOTE(maineprius947 @ Dec 18 2006, 02:21 PM) [snapback]363947[/snapback]</div>

Actually, it is designed to emulate Los Angeles urban traffic patterns of the early seventies. The times, they have changed.

Correction, it is based on traffic patterns from the sixties.

 

Prius 2007 Touring gets 39-42mpg in town and 44-45mpg on the Interstate at 75mph. It is a far cry from the sticker, but still a good savings of dollars.

 

<div class='quotetop'>QUOTE(maineprius947 @ Dec 18 2006, 02:21 PM) [snapback]363947[/snapback]</div>

Hi Dennis.

I have too many short trips in my city driving to see the high numbers. When I lived in Fargo my commute was about 10 minutes (from the far north end of the city to downtown.) So half my drive was warm-up. Here in Spokane I have some long drives (around half an hour) but I also have some trips to the grocery store a mile and a half away. Under ideal conditions I see the high numbers. But I seldom have ideal conditions. I'm not complaining. Just commenting that for most of us, the EPA test does not represent the driving we do.

I am strongly and emphatically opposed to P&G driving. It results in erratic patterns which other drivers cannot anticipate and is therefore extremely dangerous. Drivers assume that other drivers will behave in a predictable manner, and when you P&G your speed increases and decreases in a manner the other driver cannot predict. You will anger drivers who are stuck behind you during the glide, and make it difficult for pedestrians to calculate when it is safe for them to cross because you do not maintain a constant speed. Also, pulling away from stoplights, the pulse phase accelerates so gradually that again you will hold back other drivers, who will be angered, and rightly so, and the gas you save is at the cost of extra gas burned by the cars behind you whose efficiency curve is greatly different than yours.

I am of the opinion that P&G driving is anti-social.

The drivers who pioneered pulse and glide in their famous cross-country hyper-milage trip were stopped by the cops on several occasions for erratic driving.

I encourage all Prius drivers to drive in a more conventional manner for their own safety and the safety of others.

 

<div class='quotetop'>QUOTE(daniel @ Dec 18 2006, 10:55 PM) [snapback]364017[/snapback]</div>

 

<div class='quotetop'>QUOTE(daniel @ Dec 18 2006, 09:55 PM) [snapback]364017[/snapback]</div>

Hi Daniel,

Generally where I drive there are not a lot of pedestrians and when there are I maintain a steady speed (of course I will use the brakes if they step in front of my car ). One misconception is the slowness of the "pulse" phase, since I have started using pulse and glide I accelerate more briskly from a stoplight than I did before, the idea is to get up to speed relatively quickly and then glide. If one maintains a range of say 33-38 in a 35 mph zone or even 35-40 I don't think many people would notice. In a rural setting where there isn't much traffic, it doesn't seem unsafe at all to me. When the "Prius Marathon" drivers were stopped I believe they were gliding to very low speeds (20-25 mph) in the middle of the night when they were stopped, the Police probably thought the fast and slow driving was due to intoxication. In most cases pulse and glide is not carried to these extremes, but I find it a great technique for getting better gas mileage. The technique can certainly be done in a safe manner, and I would encourage Prius drivers to try it for themselves when travelling on roads where the speed limit is 40 or less. In ideal circumstances I have managed 103 mpg over a 47 mile round trip, but I certainly cannot maintain this over a 500 mile tank, I have only managed 83.7 mpg. We are on opposite sides of the country so my dangerous driving hopefully will not bother you too much because we are unlikely to cross paths. If you are driving in central Maine just watch out for the brown Prius, and look both ways before crossing.

Dennis


Dennis

 

<div class='quotetop'>QUOTE(azguy @ Dec 12 2006, 01:48 PM) [snapback]361006[/snapback]</div>

I seem to be about the only poster here who routinely drives near the speed limit, and on my highway trips, I average mid-50s -- call it 54. That's with no effort to (e.g.) draft behind large trucks.

Your understanding of how the Prius achieves its efficiency is incomplete. Mostly, as I understand it, it keeps the gas engine (ICE) loaded at an efficient level more of the time than a straight-gas car will. At very light load, it turns the ICE off and runs off the battery. At very heavy load, it runs the gas and electric sides simultaneously. In between, it'll sometimes lug the engine by generating electricity, if that would keep the load closer to a more efficient level. So, you spend less time running the engine with an inefficiently light or heavy load.

And it shuts the engine off at stoplights, and it has regenerative brakes, but I think those are less important factors.

I'd like to disagee with the majority opinion that the HSD is a net loss on the highway. I believe that the same gas-saving factors that apply in the city, as the Prius seeks to keep the ICE efficiently loaded, also occur during highway driving. Maybe it's because I drive around 60 MPH most of the time, or maybe its the traffic and terrain around Washington DC, but I have significant periods in every highway drive where there's no gas going to the ICE and I see electricity being drawn from the battery. Slowing on level stretches, going downhill, slowing for an offramp, slowing for traffic, and so on. Driving around here, those situations constitute a significant percentage of local freeway trips.

If traffic allows, when I need to decelerate on the highway, I do so at the slowest rate that will maintain the 99 mpg reading, under the belief that keeping the car in that state for the longest period possible will improve overall mileage. No hard data to back that up. Plus, it's another fun Prius challenge to see if I can bleed off kinetic energy at just the right rate to keep the car in that low-fuel-use state.

So I think the Prius continues to manage the ICE to benefit fuel economy, even on the highway. On a flat highway with no traffic, maybe there's no clear advantage, but in typical highway driving at moderate speeds, I'm not so sure the HSD is a deaweight loss.

 

<div class='quotetop'>QUOTE(chogan @ Dec 20 2006, 01:07 PM) [snapback]364786[/snapback]</div>

I agree with this 100%, and I think most of the experienced and knowledgeable Prius drivers would also.

Additionally, the mere presence of the electric motors, which are available for added torque when needed, allow the gas engine to be smaller than it would be otherwise, and since loads on the highway are generally light, the smaller gas engine is inherently more efficient because it is more closely matched to the load.

And finally, the Atkinson cycle engine is more efficient than an Otto cycle engine, but the lower-torque Atkinson cycle engine is practical only because the electric motors are available to provide torque when needed.

In short, as Chogan says, HSD provides a significant improvement in efficiency under all driving conditions. And this can be seen by comparing real-world mpg figures. Whether highway or city, the Prius beats any other gas-burning car but the two-seater Insight.

 

<div class='quotetop'>QUOTE(azguy @ Dec 12 2006, 02:48 PM) [snapback]361006[/snapback]</div>

As a technician, I've done a couple of actual tests that may be of interest here.

First, a MPH-vs-MPG test at http://priuschat.com/index.php?showtopic=11741&hl= where I got a solid 50 MPG at 70 MPH at 70+ degrees F. That's pretty close to the EPA estimate. According to my tests, if I dropped my speed 1 MPH I would have hit the estimate, and if I drive the truck speed limit, I would have beat it.

The second test was a 'reasonably continuous' 2-hour plus test I did downtown at 30-40 MPH that netted me 72 MPG! That's better than the EPA estimate.

I have no problem with the EPA estimates, I know the car can easily do the numbers. The biggest question is how well the EPA process represents everyday drivers.

So what are the real-world numbers? Well, I live in Indiana, and I know short trips are killers. Many people may do short trips. After 15 months, my lifetime average is currently at 44.38 MPG. As a fast-food junkie with nothing in the refrigerator, I do a TON of short trips, even in the cold of Winter!

Still, name me one other 5-passenger car that can average 44 MPG lifetime under the same conditions.

 

I am newbie in this forum. But I have been wondering for long time for this question:
Why does Prius have worse MPG in the highway than the city?
I honestly don't thing because of the stop and go regenerative braking strategy alone. I think it has something to do with recirculation of energy (heretic mode) at highway speed. I also wonder if prius engine ever stop at highway speed. Let me know your opinion and experience.
cheers

 

At highway speed, more than half the energy spend is purely to push through the air. There is no way to get that energy back.

 

Holy old thread resurrection!

Just because it does better on the EPA highway test than the city test doesn't mean that it actually behaves that way for most people, esp. w/o hypermiling.

Read up on the EPA test at http://priuschat.com/forums/other-c...uth-about-epa-city-highway-mpg-estimates.html.

 

In my 1 year of driving a Prius, I have observed a sweet spot in MPG at about 50 MPH. I get about 50 MPG on a level road at that speed. I regularly drive a 300 mile round trip on the NYS Thruway and on that trip I get 43-44 MPG driving at mostly 70 MPH. Real city driving is actually worse. In NYC where you get stuck at lights, and crawl around a lot, I regularly see 30-35 MPG for that segment, and once in a while even less. I don't usually hypermile. I just got my AC working, and yesterday, found that for a 15 minute drive, with the car hot, with a few lights, and 30 MPH speed limit, I got under 30 MPG. The car usually does only 25 MPG for the first 5 minutes of the drive while the ICE warms up. (I probably should install the temp hack.) On that particular trip with the AC running the first 5 minutes was 12 MPG. (Not only AC on full blast, but a very steep hill at the start)

 

The longer your drive (time wise) the better results you will see, since the engine will be warmed up.

My commute is ~ 35 miles each way. Of this, ~ 3-4 miles are "non freeway" sections (residential streets ranging from 25 - 40 mph). I average over 58 mpg overall. My weekend "short trips" to the local baseball field and running errands actually "kills" my mileage, otherwise my overall might be above 60 mpg.

 

Yes, the Prius will indeed turn the engine off at highway speeds...depending, of course. One example is a long downhill run.

This does not mean that the engine will stop spinning (rpm=0), but it does mean that there isn't any fuel flowing into the cylinders.

 

aahh, interesting. I think during braking, engine is turning the same speed as the whole planetary gearset. As planetary gearset is rotating as a whole because there is no torque applied. The torque to brake the vehicle is totally absorbed by the motor.
So, for prius the engine on and off is only depending on the fuel injection fuel spray. From your observation, how long does the engine usually stay off? If it's depending on the state of charge of the battery and power demand you input, then, from your observation, what is the shortest time it stays off?

 

Wrong on all both counts. During high speed regeneration (42+ MPH) the engine only spins at about idle speed over a wide range of road speed. The whole idea of the planetary drive (PSD) is that the ICE and final drive can rotate at different rates.

Also no about the braking torque being totally absorbed by engine. Assuming you are driving in "D" then the valve timing is set to minimize engine loss and the braking torque is absorbed by the motor/generator as per normal regeneration.

 

Thanks, that makes sense. at high speed, engine is idling due to regeneration of energy(heretic mode). The engine is allowed to operate at low speed high torque(high efficient engine spot), but I think the transmission efficiency suffers from heretic mode, due to the energy is circulated and not being used to propel the vehicle. However, since the engine is allowed to operate at sweet spot, the overall efficient is not too bad. correct me if i am wrong

By motor, I meant electric motor, not engine. So, as I said, braking torque is absorbed by electric motor, not engine

 

I drove 500+ miles through the mountains, and because it was sunny & dry, drove between 75 & 85 Mph.
Extensive use of the Pwr mode button on the 2010 (G3) Prius.

Because of this abusive fast driving, I only got 50 Mpg (4.7l/100km).
Previously with my 2006 Prius I was getting 5.4l/100km or 45 Mpg.

That's with a passenger and luggage too.

Real world example - can't get better than this. Should I want to drive at the speed limit, and the ground is quite level, using the ECO modee, 55+ Mpg is easy to get.

 

Ok I see what you mean.

Re the "heretical mode", that's where the engine revs slower than mg2/final drive, while mg1 operates as a motor and mg2 operates as a generator. As you say this gives a seemingly wasteful circulation of energy through the PSD (the planetary power split device). The ECU will only select this mode of operation if it can produce overall higher efficiency than running the engine at higher revs but lower load (which is really what the alternative is to using heretical mode). Yes it's a trade off between higher engine efficiency and lower transmission efficiency.

My understanding was that heretical mode would only happen at moderate speed under very light load. I suppose that situation could occur fairly often at highway speed on slight downhill sections. Regardless of what mode it uses though, the underlying reason why the engine has to run more often (for example use heretical mode on a low downhill grade whereas it could have used electric glide on a similar grade at low speed) is because of the much larger aerodynamic drag at highway speed.