P&G or steady state driving?

[bwilson4web]

Originally Posted by FireEngineer

. . .
Don't forget that all of Bob Wilsons steady state in the Gen III are done with only the in car displays and no additional instruments. His Classic runs had the Mini-Scanner to instrument the tests. Take his Gen III results with a grain of salt. My own runs show P&G will be better than steady in most cases in a Gen III. Exception being where you live, nice and flat.

Actually I've got some preliminary calibration data in the mph vs MPG chart:

I'm still calibrating my ZVW30 display so it is subject to change. But the preliminary data looks good.

Bob Wilson

[bwilson4web]

Table 4. Summary of Results of 2004 Toyota Prius (added % improvement rjw)

Protocol	PnG MPG	Cruising MPG	%
. . . Lee, Nelson, Loshse-Busch . . .
30-40 mph, 10 seconds of acceleration (NHW20)	151.5	81.0 @35	87%
30-40 mph, 20 seconds of acceleration (NHW20)	99.9	80.9 @35	24%
. . . krousdb with a NHW20 . . .
30-40 mph, 'n' seconds of acceleration (NHW20)	104	93.6 @35	11%
. . . bwilson4web with a NHW11 . . .
25-43 mph, 15 seconds of acceleration** (NHW11)	87.8	79.0 @33	11%
25-43 mph, 15 seconds of acceleration** (ZVW30)*	91.3	73.0 @35	25%

pp 6, SAE 2009-01-1322, "Vehicle Inertia Impact on Fuel Consumption of Conventional and Hybrid Electric Vehicles Using Acceleration and Coast Driving Strategy," Lee, Nelson and Loshse-Busch.

* Calibration corrected values.
** Using cruise control resume function.

NOTE: I found that bringing the traction battery SOC down before the runs leads to more consistent results. I just flip it into EV mode and circle a parking lot or nearby back road. Then I can pull on the test segment and get consistent results.

[VABeachPrius]

After about 16 miles of driving home for work, I swung into a test loop that I have used in the past to learn how to pulse and glide with minimal interruption of traffic. I pulsed and glided for about 20 minutes to make sure all was in order. On battery alone I drove at 21 mph until the car dropped from 4 bars to 3. I came to a complete stop, reset the current trip of the scangage, handheld gps, and went on my way for P&G. I P&Ged for almost 11 miles and stopped without using EV after I reached my last glide. I could have gone another .6 miles or so, but I stopped.

Distance = speed * time. Average speed 24.3 mph. Distance during pulses ~0.1 mi. Average time accelerating = ((0.1 mi / 24.3 mph) *60 * 60) = 14.81 sec (for bob wilson)

Most of my pulses were started at about 22 mph with an impg of 25 mpg or better. The end of the pulse (i.e. I reached 0.1 miles with a speed of about 28-32 mph depending upon wind and battery SOC.) I glided down to 22 mph and repeated for almost 11 miles. I was interuped once (slow traffic ahead, imagine that) and had to EV for .3 miles which hurt my battery SOC and hence my overal mileage a little. I also had to come to a complete stop once to wait for a car backing out of a driveway. The test was conducted on a 2.4 mile loop through a neighborhood.

The end result = 120.xxx mpg, 14.8 sec pulses, 22-30 mph.

Krousdb,

This type of mileage falls in line with the prius marathon mileage segment for a warmed up car with no hit for the warmup.

General thoughts:

I am surprised that since I started with 3 bars that the car did as well as it did. The one time that I had to EV for extended periods. I suffered during the next few pulses as it was hard to accelerate with an impg of 22 or greater with such a battery SOC; one of the practical reasons that using EV is hard on high FE. I was surprised that with time, the car charged to 4 bars. I think that I could have slightly slower pulse rate and glided the same distance each time resulting in higher mpg. Without the interruptions the mileage might have been 123.xxx. One test, one data point.

I will try the same type of test when I have more time for steady state. I know that the final comparison needs to be done on the same day, as close to each other as practical.

It was 88F out, tires 60 psi cold, wind 8 mph from the west. Course is oval with a downwind and upwind leg. No AC, driver 204 lbs. Two car seats in the back , no kids, no wife. No radio, all four windows 1/2 way down. Sunglasses on.

Sorry to those of you who think the 11 miles were wasted fuel (.091 gal); I will make up for it with my 65 mpg lifetime average with almost 42000 miles. For those of you that only get 55 mpg, take the small wasted fuel out of the 117 gallons that I have saved by getting 65 mpg. No sarcasm, just food for thought.

[Muhahahahaz]

Wtf? I give up... My gauge is currently below 40MPG, and I've NEVER seen above 50MPG...

Most of my driving is city. I should be getting 48MPG... very disappointed.

[Dan.]

I think the PnG results are fairly understood.

Brick and I have some interesting findings on steady state. I have just about enough information to post something on it this week. Unfortunately it requires a few gauges that have only been decoded on the Gen II, but once the decodes come in I'm certain it will work on the Gen III. I do have some approximations for Gen III drivers though.

I'll link when my writeup is ready (gotta do resume first ).

11011011

[SageBrush]

I know that bw sets up his tests with reproducibility in mind, but frankly I consider his methods invalidate his results. Cruise control is simply not a reasonable driver substitute in my opinion to mimic P&G.

[bwilson4web]

Hi,

Last night I woke up thinking about the Lee, Nelson, Lohse-Busch paper, SAE 2009-01-1322, and realized there was information hiding in plain sight in three of the figures:

Fig.	text
9	Fuel Rate (g/s) of 2004 Toyota Prius for PnG Drive Cycle: 20-30 mph, 11 Seconds of Acceleration
10	Fuel Rate (g/s) of 2004 Toyota Prius for PnG Drive Cycle: 30-40 mph, 10 Seconds of Acceleration
11	Fuel Rate (g/s) of 2004 Toyota Prius for PnG Drive Cycle: 30-40 mph, 20 Seconds of Acceleration

.
Embedded in those three figures is the amount of fuel consumed to change the vehicle kinetic energy 10 mph. So I decided to go back and look more closely at the charts.

When looking at charts in a paper, I expand the view and take screen snapshots. I then put them into PowerPoint and use the embedded tools to stretch them until they have the same scales and horizontal and vertical lines to read the values (or close enough.)

Comparison of 10 and 20 Second Acceleration


The ballistics glide should be identical from 40 mph down to 30 mph. So I was surprised that the 'glide' part varied by 7 seconds out of ~51 seconds duration. This would be a 7/51 difference, nearly a 14% difference. This is why my testing involves multiple cycles so individual errors like these would average out. Two conflicting data points are not enough to tell which might be in error but shows an inconsistency that is significant.

As expected, the fuel rate times the power duration tells us the quantity of fuel burned to go from 30 mph to 40 mph. Notice that the engine didn't shutdown instantly but led to a lower rate for about 2.5 seconds (at 3:00 AM). This morning it looks to be closer to 5 seconds. Regardless, here is another fuel burn anomaly in the data. Potentially 1-2 grams out of 16, a 6.25-12.5% error in the fuel burn during the 20 second acceleration. It is easy to factor this out and for my purposes, I'm using the primary fuel burn of 16 grams for the 20 second acceleration. I'll come back to the chart in a minute.

BTW, the anomalies found in the figures from the Lee, Nelson and Lohse-Busch paper may or may not account for the exceptionally high mileage reported on the 10 second pulse and 40-30 mph glide. If the glide were actually of the 20 second pulse glide duration, the reported "151.5 MPG" (Table 4.) might be a little less.

Comparison of 20-30 PnG to 30-40 PnG


One good thing is you can see the curve in the coast down velocity from 40-30 that is nearly absent or difficult to see in the 30-20 coast down. This makes sense as aerodynamic drag is proportional to the square of the velocity. But what surprised me at 3:00 AM was the fuel burn to kinetic energy change.

As before, I calculated the fuel burn for 10 and 11 seconds and the 11.2 to 11.6 g were well within acceptable limits. But something bothered me about the change in kinetic energy. We know the familiar formula for kinetic energy (KE):

0.5 * m * (v*v)

If we calculate the change in kinetic energy as a change in velocity, the 10 mph change from 20-30 and 30-40 would appear to be the same. However, if we use earth as the reference, the power function of the velocity change is given by:

0.5 * m * (20*20)
0.5 * m * (30*30)
0.5 * m * (40*40)

You can see the mass and 0.5 constant can be taken out and the relative kinetic energy change is the difference of the squares:

_400
_900
1600

The reason these are the accurate indications of kinetic energy has to do with the speed of the vehicle when the acceleration is being applied. They would not appear if the "0 mph" velocity were taken from the position of the moving car as each pulse would be a 10 mph difference. They do appear when the absolute energy is calculated relative to the earth. The change in KE is the same force but over a longer distance because the velocity of the vehicle adds distance. So when we look at the change in kinetic energy per unit of fuel burned using an earth reference, we find acceleration is more efficient in the 30-40 mph range than in the 20-30 mph range. (There will be a brief paused while some heads explode.)

Engine Fuel Consumption

Returning to the first chart, I used the fuel burned during the 10 second pulse divided into the glide duration to come up with the ballistics glide, equivalent fuel burn rate. What this means is the amount of fuel that has to be burned to be able to glide from 40 mph to 30 mph.

I then used this rate of fuel burn to calculate the extra fuel needed to run the engine 10 seconds and found there was a 3.93 g unaccounted for. This is the fuel burned just to turn over the engine the extra 10 seconds minus what about what would have been needed to travel for that extra 10 seconds.

When the engine runs there is internal friction from the pistons, crankshaft, valves, and water pump that have to be turned. This internal overhead requires energy that has to come from somewhere and that would be the fuel burned. This is the first time I've had data that allows us to map this internal engine overhead.

Summary

In engineering there are different ways to look at a problem and energy flows happen to be one I'm fond of. Serendipity, the data derived from the Lee, Nelson and Lohse-Busch paper has shown that an energy based model of pulse and glide may account for more of the energy flows and losses and lead to a more accurate model.

Bob Wilson

ps. Facts and data is my approach to understanding how the natural world works.

[SageBrush]

GIGO. Explain the discrepancy between your CC P&G routine and the 110 mpg obtained by the Prius marathoners, or wake up and admit that your method is flawed.

How many more times are you going to post the same graphs to prove what is obvious: P&G by CC is a lousy way to P&G ?

[blippo]

I have found the cruise control (where possible) to be my best friend for great gas mileage

[SageBrush]

Originally Posted by blippo

I have found the cruise control (where possible) to be my best friend for great gas mileage

I think it does pretty well too -- for steady state speed driving over 41 mph on level routes.