Most efficient speed for best MPG?

<div class='quotetop'>QUOTE(bwilson4web @ Mar 22 2007, 09:48 PM) [snapback]410449[/snapback]</div>

Bob, having seen your last post I would like to nominate you for the Oscar for "Best Supporting Graphical Representations Submitted as Attachments"

Only a Prius can generate this sort of data; I just love the car to death

regards

Jerry

 

Can somebody explain what the consensus is to us laymen?

 

<div class='quotetop'>QUOTE(mwbueno @ Mar 22 2007, 03:35 PM) [snapback]410344[/snapback]</div>

Do you still have the extra battery pack? I assume the tests were done with just the NiMH HV pack.

 

LOL!..I can't decide if that looks like a Pancreas or a string of eggs from a Perch!

 

<div class='quotetop'>QUOTE(Fibb222 @ Mar 22 2007, 07:30 PM) [snapback]410543[/snapback]</div>

To get 100 MPG or better:

15-25 mph - current model Prius, NHW20
15-20 mph - previous model Prius, NHW11

All it takes is a warmed up Prius, tires inflated to the maximum rated pressure, a flat driving area, no wind, temperatures at least in the mid 70s and the patience to hold the car within the steady-state speed range. Anyone can do it.

It also validates an energy model of the Prius based upon drag measurements.

Knowing the maximum MPG performance allows us to look at lower performance driving and analyze what is going on. This may lead to insights, the 'eureka' moment, when a new technique or technology improves ordinary performance. For example, it may be possible to map out more efficient highway driving profiles and develop a custom cruise control that follows such profiles.

This also lays the foundation for investigating what is going on in pulse and glide driving. The theory of pulse and glide is the pulse puts the ICE in a more efficient energy conversion range. So instead of getting the ~31% conversion of gasoline to mechanical energy, the engine is getting 35% or higher. How to achieve such efficiencies has not been explained in terms of ICE rpm and or ICE torque. However, some specific fuel consumption data indicates an rpm range from 1,600-2,600 is a good start and avoid going over 3,100 rpm.

One of the things Wayne's data shows is the importance of tire pressure for low rolling resistance. During the Prius marathon, I understand the team used nearly bald tires inflated to 65 psi. This would have given them unusually low rolling resistance. On dry pavement without other traffic, this would be OK but it is not a good idea for normal driving and in slippery conditions. However, even more radical, low-rolling resistance tires are possible.

A suitably modified Prius should achieve 125+ MPG in a marathon. All it would take would be: (1) computer controlled accelerator, (2) custom tires, (3) seriously baffled ICE compartment, (4) European or Japanese tank, (5) aerodynamic clean-up, (6) weight reduction, (7) high energy content gasoline, (8) ICE and transaxle tuning and (9) high efficiency lights.

Since it would be drop-dead boring, I would want an automatic pilot to handle steering around a limited access flat track, say an empty shopping center, large parking lot or unused airport. Ideally the track would be above 5,000 ft., say Wyoming or the upper plains. Also, a pre-heating cycle should be used before topping off for the endurance run. It would be important to have a telemetry link, WiFi, to a data analysis system to track progress and tune the accelerator profile. With this approach, a small team, say three light weight drivers and one engineer, could set a new Prius marathon record using either model Prius, NHW20 or NHW11.

It might would make sense to have two or three teams and cars to minimize the risk of mechanical failure and show this is not vehicle unique. The only problem with multiple cars, like bicycle racers, they might be tempted to draft each other and set a record that would be exceptionally difficult to beat.

Bob Wilson

 

<div class='quotetop'>QUOTE(usbseawolf2000 @ Mar 22 2007, 09:50 PM) [snapback]410615[/snapback]</div>

Hi usbseawolf2000,

Yes, the tests were performed with the OEM HV only. The 450 lbs you reference is the cumulative weight of me, the extra HV array, test gear & etc. Part of the reason it takes me 4 hours per test is that I have to 'condition' the OEM HV for a while after I unhook my extra 18Ah NiMH array. If I don't 'recondition' the OEM HV Battery before testing, it goes through a similar 'reconditioning' during testing which throws the data way out of whack (usually on the low side). The Toyota BMS (Battery Management System), like the rest of the car, is quite a marvelous piece of engineering.

Best Regards,
Wayne

 

Thanks to a recent posting, I found the log from the Prius marathon. I've add each leg to the chart:

MPH MPG
30.72 104.68
31.69 100.59
32.57 102.95
30.16 106.66
29.30 111.88
29.69 114.84
28.60 115.04
30.52 107.03
28.82 108.41
24.32 120.56
28.91 116.35
30.69 110.59

Notice that the leg with the lowest speed, 24 mph, had the best MPG, 120.56. Also the data is within spitting distance of the MPG vs MPH charts.

Bob Wilson

 

<div class='quotetop'>QUOTE(bwilson4web @ Mar 24 2007, 10:29 AM) [snapback]411242[/snapback]</div>

I believe the marathon was done using Pulse&Glide, a lot of speed changes, and we can't compare the data with Wayne's constant speed graph.

Ken@Japan

 

ZERO... that's the best speed. Just park it - and take a walk.

You NEVER have to fill your tank again. It won't matter if it's nitrogen or plain air that's in your tires... and you definitely don't need the touring model or the NAV system. Just make sure you get the DICE IPod Integration Unit - because you still need to sit in your car and listen to your IPod.

 

Ha!

I'm doing better than that right now where my Prius is at its best (in my garage).

0 MPH = 0 MPG
0 MPH = 0 MPG
0 MPH = 0 MPG

 

<div class='quotetop'>QUOTE(ken1784 @ Mar 24 2007, 07:41 AM) [snapback]411445[/snapback]</div>

Actually, I think we can but my performance models are not ready for inspection. Still, let me share the logic:

• power = drag * speed
• drag = F(rolling_force,aerodynamic_force)
• ICE_power = F(rpm, torque, fuel)

We often simplify vehicle drag into a fixed rolling drag and an aerodynamic constant times the velocity squared. This sets the power required and the amount of energy needed to move the vehicle EVEN IF the velocity changes.

Changing the velocity over any given route will always require more energy than maintaining a steady state speed. The non-linear aerodynamic effects at speeds above the steady state speed increase the drag, power and energy requirement much faster than the reduction in drag, power and energy at a speeds below the steady state. Using calculus, we can calculate the power and the energy required to move any given distance and this is where I have to dust-off my college texts to 'do the math.' (NOTE: I remember having to do this calculation in one of my engineering classes, thermodynamics I think.)

The "pulse" part works for ordinary cars because traditional gas engines are terribly inefficient at partial throttle settings (See Figure 15, "Development of New-Generation Hybrid System THS II - Drastic Improvement of Power Performance and Fuel Economy", K Muta, M. Yamazaki, J. Tokieda, SAE 2004-01-0064.) By putting an ordinary ICE into a higher power mode, it will generate power at a more efficient specific fuel consumption than partial throttle but the excess of power causes the vehicle to accelerate. The "pulse" is a by-product of trying to operate a traditional ICE at high power settings but our Prius is different.

"Pulse and glide" is critical to Honda and gas-only vehicle mileage but the Prius has not only an unusually flat, specific fuel consumption but also the ability to auto-start/stop the ICE. If the Prius ICE is kept within the 1,600-2,600 rpm range (based on current data,) it will be in the most efficient power mode. But there is an energy loss associated with starting and stopping the ICE and this is an area under investigation. To properly model Prius efficiency, the ICE start and stop energy has to be part of the model.

There may be a Prius "Pulse and Glide" profile that delivers better performance than a steady state speed. If so, it is likely to come from using the battery to extend the glide and optimum acceleration in the high-efficiency speed range as a way to minimize ICE start/stop energy loss. But the battery recharge has to happen during the ICE operation time and if done badly, it won't be recharged enough to sustain the next glide. Also, the minimum speed needs to be balanced against the 'overhead' power requirement. But this performance model with real data doesn't exist in an open source, yet.

I'm sorry if this seems long and somewhat confusing. I didn't take enough time to write it shorter. But as I refine my data and models, eventually, I'll share AND THEN TEST the results.

Bob Wilson

 

Hi Bob,

It is hard to understand your post what's the point is.

Agin, the 1400 mile marathon was based on Pule&Glide, pulse up to about 40 mph the glide down to 30 mph.
I believe the bast shift (#10) by Wayne Gerdes was slower than that, pulse up to 35 mph and glide down to 20 mph, average 24.3 mph.
The FE on pulse part was 9.84 km/L (23.1 mpg) and engine on % distance was 19%, so the rest 81% is just gliding from 35 mph to 20 mph.

I still believe we can't compare the 1400 mile marathon data to Wayne's constant speed graph.

Ken@Japan

 

<div class='quotetop'>QUOTE(ken1784 @ Mar 25 2007, 02:56 AM) [snapback]411820[/snapback]</div>

Hi Ken,

I'll see if I can't get the math model worked out this week but I'll have to review some of my old engineering texts (if I still have them.) If I don't have much luck, excel can model a successive series of approximations. But for now, I would suggest looking at the MPG vs MPH plot with the marathon data (you may need to refresh the browser to get the latest copy



The NHW20 Prius marathon used high pressure, well-worn tires, a more efficient transaxle, warmer than standard temperatures, a lower coefficient of drag and more efficient transaxle system. Still the Prius endurance test is within engineering measurement error of the NHW11 theoretical line. Furthermore, the slope of the Prius endurance data points suggests a curve that has a higher peak MPG value that Wayne has nailed that at 20 mph, backed up by testing.

The speed log for the Insight test shows an average of 18 mph and over the last days, closer to 15 mph. Now an Insight's IMA system would respond especially well to any pulse and glide protocol that uses ICE off, neutral" for the glide. But the Prius systems are different with a built-in, automatic cycling between ICE-on and ICE-off operation at speeds below 42 mph.

It is possible that the energy lost in starting and stopping the Prius ICE is significant at the peak MPG, constant speed. It is possible that acceleration at an ICE speed between 1,600-1,900 rpm followed by a glide that ends at a speed that permits recharge on the subsequent 'pulse' may be the most efficient protocol.



Understand that I prefer to start with a good model, followed by data and testing to answer this question and develop a reproducible protocol for Prius marathon driving. The ultimate answer is an automated Prius that drives the marathon under computer control with a mannequin or crash-dummy for the driver.

Bob Wilson

 

No matter what, if you're going to fast you're going to get horrible gas mileage. There's calculator at MpgForSpeed.com - Fuel Efficiency Vs. Speed that shows you the effects of your speed on your mileage.

 

Can we bring this back in to the real world where people have to drive at least the speed limit....and don't have an excel spreadsheet and DNA analyzer always with them?

If I'm taking a 250 mile road trip that will be 80% high way...driving the speed limit will produce better mpg than driving 15mph higher.

THERE! Solved.

 

True, however, keep in mind that most U.S. Interstates have a posted MINIMUM speed limit of 45 MPH. Therefore driving 45 MPH will produce better mpg than driving the posted MAXIMUM speed limit.

Then, if you get off the Interstate onto the country roads, and get yourself behind an International Harvester chugging along, you might be lucky enough to exceed 100 mpg. You can certainly improve your mpg by driving these non-Interstate roads and keeping your speed below 45 MPH. It all depends on how much of a hurry you are in to get to your destination.

 

Wow, a more efficient transaxle and a more efficient transaxle system?

All kidding aside, I just wanted to point out that while shutting down and restarting the ICE may require some amount of energy loss, the entire time the ICE is off, you eliminate the energy loss of internal friction of the various moving parts (and fluids?). Furthermore, as the ICE cools a bit while off, you reduce the rate of heat transfer out of the ICE (heat transfers faster with a higher delta, right?). It would be interesting to know just how much energy is used in starting and stopping the ICE, as well as how much energy (per unit of time) is lost during steady state to internal friction and how much extra energy is lost in heat transfer to the atmosphere. Then it'd be a pretty simple calculation to know how long a "glide" needs to be to gain advantage over steady state.

 

Now that would scare me to death. Doing 45 (legal or not) out on a highway where 90% of the traffic is doing 70-80. I know you're not really suggesting that we do it, (At least I hope you're not) but just thinking about it... shudder.

And think about how much road rage that would instill in other drivers...

I'll stick close to the posted limit, just for safety's sake.

Bob

 

I appreciate your real-world position. I also appreciate the amazing (to me, a math-deficient writer) amount of laboratory effort of Pri-fans to get the best out of their cars.

That said, can there be real-world test reports based on actual use, speed, P&G, on normally traveled roads -- nothing too high-falutin', just "we drove this fast at this temperature with a head wind on this terrain for this many miles and got this MPG", all without inciting road rage or keeping us from getting where we want to go expeditiously. Multiple tests should give a good idea of what we can expect day-to-day.

Perhaps moderators can set up a forum for that? I'm willing to contribute results of our first trip, some 460 miles after we pick up Black No. 2 in a couple of weeks.

 

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.

You realize the Prius does this automatically, right?

This is a little confusing:

• 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.
• 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.
• Energy loss during engine operation comes out of the engine efficiency, directly. No energy is generated without the engine working.
• I look forward to reading your 'simple calculation'

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. The conclusions:

• 18 mph - steady speed with the Prius handling the systems is more efficient than P&G with the same average speed
• 34 mph - P&G is more efficient than steady speed, 34 mph provided the P&G speed range is 25-43 mph.

Of course these are direct measurements, no modeling required. Even you could go out and replicate these tests and report the results. Remember, the key is to compare the equivalent steady-state speed for your P&G profile and read out the MPG for both. I look forward to a little reality training.

Bob Wilson