62.4 MPH

Probably, the best mileage occurs at a sustained 35 to 40 mph in top gear (assuming that your non-Prius vehicle operates well in that circumstance).

The 55 mph level was picked due to how rapidly wind resistence increased above that point. From my own observations (non-Prius, or computer monitored) this seems to be quite true. As you exceed 65 mph, or so, mileage drops off the faster you go.

The 62.4 mph / 100 kph observation sounds more casual, or coincidental, than scientific.

 

Remember his post said "most fuel efficient fast speed". Each cars most efficient "highway" speed is governed by the engine gearing for that car in top gear. Most manufacturers do attempt to keep this in the range of 55-65 because that is the speed limit and that is also the range at which deminishing returns would enter into the equation as air resistance is not linear and as you go faster, say 75 or 80 , the wind resistance greatly deminishes gas mileage no matter how your car is geared.

 

If the most efficient speed is 62.4 mph, does this mean if you go slower than 62.4 mph, you would get worse gas mileage? Seems a little odd...

 

Hey! Quit talking about me... Actually, that's ok. I'm so far behind with website & email stuff that delays in responses are inevitable. But once I do finally sort through my new 877 photos, figuring out which ones to publish, add the 2 new sections to the Bluetooth document, publish the first non-draft version of the User-Guide, catch up on my log, update a number of webpages, and catch up on my email, then I'll be able to reply faster.

Anywho, much of the research data for how THS/HSD works is over 3 years old and buried somewhere deep within the Yahoo archives. Fortunately, the end results weren't lost... like the stuff in the User-Guide.

The "62.4mph (yes, the missing 1 is a typo that hadn't been caught) happens to be 100km/h" concept was verified both via calculation and by obervation. The size & weight of the components hit optimum balance at that speed, which apparently is a common speed limit overseas; so engineers took that into account when designing the system.

 

Can't wait to explain that one to the State Trooper...

 

The thing to remember is that the hp necessary to overcome the air resistance is proportional to the Coeff. of drag (.26), frontal area, and the SQUARE (V X V) of the speed.

The hp to overcome the weight of the vehicle is only proportional to the velocity (not square), vehicle total weight, and frictional resistance or rolling resistance of the tires. Add to that the friction losses in the drive train, etc to keep everything turning at a constant speed.

The big one here at velocities above about 35 mph is going to be the air resistance. The faster you go, the air resistance becomes a bigger and bigger percentage of the total hp necessary to keep things moving. This increase happens very quickly because of the squaring of the velocity.

Once you know the hp necessary for any vehicle's constant speed you just multiply that by something called the BSFC to get lbs of fuel consumed per hr. This is the Brake Specific Fuel Consumption (lbs of fuel/hp-hr). The BSFC represents the efficiency of the ICE. It varies by throttle opening and RPM. So there are a series or family of curves for any particular engine. The bigger the throttle opening, in general, the lower the pumping losses and the lower the BSFC. That's where this type of ICE (so call Atkinson Cycle) shines. And the computer takes care of all of that for you to optimize this. With the Prius there are other things to consider at a constant speed, charging the battery, the ICE being supplemented with hp from the electric motor, etc.

Whew.....sorry I got carried away with all of that, my head really hurts now! But I think you can see how knowing all these variables could allow an engineer to optimize throttle openings, valve timing, etc for one particular speed. That's not to say that things will be a lot worse at say 65 mph or 70 mph. But the vehicle velocity really works against you the faster you go and there's a price to pay. But the engineers no doubt tried to minimize that price within the contraints of the entire design goal and performance.

 

Maybe John can answer this. But I really don't think you'd see a significant difference if you went 65 instead of 62.4. But you probably would at 70....at least something you might be able to measure, although probably not easily at even 70. Even with the increased hp, they might have wanted to optimize for 100 km/hr. But again, you would only see a difference at 65 with the best instrumentation, all the variables controlled on both runs and set equal for both runs. In other words, in the lab not on the road. The mpg calculator that Wayne has programmed for the palm allows you to see the differences yourself at the various speeds.... setting humidity, elevation, temperature, etc...all those other minor variables. It's the equivalent of running in the lab so your actual results on the road will vary.

 

If you go slower than the 62.14 mph your gas mileage would improve, even though the system is optimized at 62.14 mph because the dominate factor is the velocity and the air resistance. The BSFC region chosen by the computer for the ICE might not be as good as at 62.14, but it's a smaller factor in the fuel consumption calculation.

 

I've just got to say a little more about this and then I'll shut up, lol. If this stuff gives you a headache, stop reading it immediately and lay down and rest.

I had to do this for my senior project many years ago. We were constructing a potential 100 mpg vehicle. My portion was the dyno testing, mapping the engine performance, making minor tuning mods, etc and finally recommending the final gearing.

We knew the road load....hp necessary to keep the vehicle moving along at 65 mph. We also had chosen an engine that provided a lot more hp than this road load because of availability and cost. Not the best way to optimize our mpg because the resulting throttle opening would only be 10-15% but it results in a vehicle that has lots of acceleration available.

So you go to the family of HP VS RPM curves for % throttle opening and draw a horizontal line representing the road load at 65 mph. We found that anything above 10% would provide that HP. For example at 100% throttle opening you might only be turning 600 RPM for that HP. Or at 15% it might be 2500 RPM.

Next was to take those RPM values where the HP was satisfied over to the BSFC family of curves (BSFC VS RPM) and see if we could find a nice match. Something with minimum BSFC but above say 2200-2500 RPM so that we still had decent acceleration and obviously above idle RPM and not lugging the engine down too much where it didn't really want to run well. That allowed us to choose the final gearing to provide that RPM for that Speed which gave us a specific BSFC (minimized for our overly powerful engine). The result was 85 mpg, not the 100 mpg we were after. The car was never finished though, like most Senior Projects, and so we'll never know. I think we chose 15% throttle opening and about 2250 RPM.

 

This sounds right to me. It makes sense that the engineers would have decided that 100km/h is the most common speed that people drive at, and optimized the car so that it would be as efficient as it could be at that speed, at the expense of some efficiency at other speeds.

But that doesn't imply that going slower by 5 or 10 mph is going to be more efficient than 62.14 MPH. It just means that if you go 55 MPH the car would have been even more efficient if the car was designed for that speed.

I think that is an important distinction; many people seemed to have gotten the idea that driving 55 mph would lower their MPG compared to 62.14 MPH, and that does not seem to be supported.

I suspect that the Prius MPG versus speed graph looks something like figure 6 in the document:

http://www.goodyear.com/truck/pdf/radialre...rv/Section9.pdf

Where the MPG is greatest at some low speed (perhaps 30 MPH), and drops as the speed increases, dropping faster as the speed increases.

I played with Wayne's THS2 Palm application that calculates mileage from lots of variables and for the assumptions I entered (which I think were the defaults) it gave max MPG of 80.4MPG at 29-30 MPH. Here are some numbers it generated:


55 MPH: 65.8 MPG
60 MPH: 61.7 MPG
62 MPH: 59.9 MPG
65 MPH: 57.3 MPG
70 MPH: 53.2 MPG

I think this is sufficient evidence that driving 55 MPH wil give worse MPG than 62.14MPH.

John, I encourage you to clarify this in your document when you are satisfied that this is true (and when you have time!)

 

I thinkyou mean 55 MPHwill give better MPG than 62.14 MPH right? That was a very good pdf you had and it explains things very well. The thing that is unique about Pruis is that the computer knows where the engine should run for any given load and trys to maximize efficiency. When you're tied to convential transmission you're only guessing and you're very limited in your choices of throttle opening and RPM. In spite of this you can see that the Prius does have a tough time at the lower speeds when the ICE is on. The load demands are such that you just can't get a big enough throttle opening to operate the ICE in it's most efficient operating range.

That's why I believe the the EV button would be helpful because the computer has no idea that the I'll be able to recharge the battery ahead because of a downhill on my commute and so I'm sometimes forced to start and stop in the snake like traffice that I encounter. That combined with the slight uphill on my commute home uses the ICE in a very inefficient manner. There's just no getting around it. So I watch my 50+ mpg for my tank slowly drop to below 50. Last night was typical and I caught a lot of traffic so that my 51 mpg for about 185 miles slowly dropped to 49 mpg by the time I got home. I've come to the conclusion that with the traffic and all the hills around here I probably won't be the "mpg winner" of the prius chat group. If I get high 40's I'll be doing very very good.[/quote]

 

ARRRRGH

Is this what happens to people who become Prius owners? Am I joining the largest group of anal retentive, mileage obsessed geeks in the known universe? Please tell me I'll enjoy the car for other reasons. After reading Car and Driver on the airplane last night, I started to sweat a little. I've dealt with the reality of giving up my 255HP Volvo and its smooth, sweet acceleration, along with all of the enjoyment it's provided for 3 1/2 years, hoping that the Prius would merit its own pleasures. Do I have to drive with concerns about wind shear, gas consumtion and braking presure in my head in order to have a good time? AARRRRRRRRRRRGH :cussing:

 

Re: ARRRRGH

 

Yep, that's what I done meant!

In other words the slowest highway speed is the most efficient.

 

> John, I encourage you to clarify this in your document when you are satisfied that this is true (and when you have time!)

Actually, I've found it very informative observing where people's thoughts wander now verses where they did 3 years ago. So I delayed the reply...

The answer is really quite simple. If you have the choice between 60, 65, or faster, you should actually choose none of those. 62.14 is the ideal.

55 MPH didn't fall into Prius enthusiast's definition of "high speed" 3 years ago. They considered it slower, suburb type driving rather than having the trottle open like you'd do in open country.

But now that Prius has become more mainstream, it is perfectly acceptable for you to request clarificiation. And I'm more than happy to provide it... specifically what "high speed" means.