Consumption Calculation Question

The computer doesn't ignore the time when the ICE is not running, but no fuel is being consumed because the ICE is not running. The calculation is just fuel burned vs miles driven over the last 5 minutes, whether the Prius is running the ICE, coasting, gliding, or getting towed by a pack of wild dogs.

Tom

 

like the previous poster said, it's simply the amount of gas consumed divided by the distance traveled within that 5 minute window (opposite for us MPG users).

Time really doesn't mean anything, except as a convenient way to separate the readings and make a cool looking graph.

 

<div class='quotetop'>QUOTE(Tideland Prius @ Oct 31 2006, 09:34 AM) [snapback]341039[/snapback]</div>

Sorry, I 've to disagree with you. Your 2.5L/100km will be true if the OP did not stop for 2.5 mins but continued in ICE-off mode covering about the same distance as the 1st 2.5 mins.

Let's assume the OP travelled at 60kmh.
Distance covered in 2.5 mins = 60/60*2.5 = 2.5km
Fuel consumed at 5.0L/100km = 5/100*2.5 = 0.125 litres

If the OP stopped for the next 2.5 mins,
Distance covered in 5 mins = 2.5+0 = 2.5km
Fuel consumed for 5 mins = 0.125+0 = 0.125 litres
Final FC = 0.125/2.5*100 = 5L/100km

Howerver, if the OP continued at the same speed,
Distance covered in 5 mins = 2.5+2.5 = 5km
Fuel consumed for 5 mins = 0.125+0 = 0.125 litres
Final FC = 0.125/5*100 = 2.5L/100km

 

<div class='quotetop'>QUOTE(efusco @ Oct 31 2006, 08:49 AM) [snapback]341176[/snapback]</div>

Here's a simple algorithm for what happens, to make things clear:

reset all totals
after 5 minutes:
- total fuel consumption since reset
- total miles driven since reset
- output to MFD total distance/total consumption --> this is a 5 minute bar


The confusion in the graph comes from the fact that not all bars are equivalent. You could spend 5 minutes in line at McDonalds, inching up on electric, and get a 5 min bar that says no fuel was consumed (100mpg), then get on the highway and travel 5 miles in the next 5 minutes, averaging over those 5 minutes 40mpg - Looking at the display, you would think you averaged 70mpg over the past 10 minutes, but we know that the distance traveled for each bar isn't the same, so the average is actually 40mpg (since there was essentially no distance in the first bar)

If you doubt this, there's one real easy way to test it: go out to the car, drive around a bit to get it warmed up (say in circles in a parking lot) get to the start of a 5 minute bar and do your best to get great mileage, then stop halfway through the bar - you'll see that the time aspect just gives you the "window" over which that average was calculated - it doesn't effect the average itself.

The formula you presented evan does not give you MPG as the final result - it gives you miles/gallon/minute, which doesn't make any sense. Also, if this was true, then in order for the Prius to get 50mpg over a tank, every 5 minute bar (on average) should show 10, not 50 (since 50MPG/5 min = 10), which it clearly doesn't...

 

On the drive home today i'll test this out, but i'm 99.9% sure that it's simply the average mpg taken over that time period - if you stop without the engine running, the average doesn't change - the consumption and distance totals don't change.

Just to be sure the testing methods are proper, i'll stop when i get to my street. when a 5 min bar rolls over, i'll travel on electric (100mpg) to my house - should take about 1 min. I'll then sit in the car for the next 4 minutes. If you're right, then my mileage should read 25mpg for that 5 minute bar, right? if i'm right, it'll read 100mpg.


You are right, though, that a mpg/mile graph is much more intuitive - The individual bars can then be averaged together, which isn't possible with the Prius display, since the distance traveled per bar is always different.

 

ummm, well, in MY experience, my 5 mins works as follows. it divides the 5 mins into sections about 10 seconds long. in each section it determines mpg in that 10 seconds. then adds them all together, discounting any segment that have no detected movement (iow its not counted so the actual 5 min segment does not have 5 mins of data) . segmnents in which gas is burned and no movement is detected will lower the average. then the total is divided by the number of valid segments and the average is displayed in the 5 min bar.

so in the scenario when you move for a few minutes and stop, your average will be what it was in the time you were moving provided you burned no gas when you were stopped.

in the event you go some where and move very little like a drive-in, your average will be zero IF any fuel was burned OR if no movement (or very little movement ) was detected.

this is something i noticed a long time ago on my 04 and the 06 works the same.

i can easily manipulate the bars by cruising the parking lot at a constant speed for 10 seconds since this is the minimum parameter of time needed to make a recording to the graph. try it once in stealth. stop wait until the bar shows up. then turn on defrost, a/c or whatever to make the engine run. do another 10 sec interval. stop. wait for that bar, you will see less than 99.9

have done a multitude of experiments on this and have found the true mpg over time is actually the tank mpg. it is accurate but gauges the overall average since you filled the tank. i tracked a few tanks when i totaled drive time and then divided miles into it and came relatively close to my tank average. if you have spare time, are anal (like me) and love to crunch numbers, try it. (its not easy, let me tell you!!) be aware that this only works for gas consumption over time (iow the canadian figures, not the mpg that americans get. this requires recording mileage driven, tank average before and after the trip and time elapsed)

in fact, the next time you fill up, reset your tank mileage and see what each says after 5 mins while driving the whole time. (realize that all variations in speed will be reflected in the two readings) then try it when you only drive for half of the time, ¼ of the time, etc)

 

Ok, will do... i've got the perfect road in mind.

Also, following the constant recommendations of Charles Suitt, i just pulled out my owners manual. Section 2-7, page 168 in the 2006 manual says:

Fuel consumption screen
Touch the "Consumption" switch. Average fuel consumption, momentary fuel consumption and recovered energy are shown.

1 Indicates past momentary fuel consumption every 5 minutes.

2 ...

3 ...

4 ...

So this doesn't exactly answer the question, but it does support what Dave said (even though that differs slightly from what i want to believe, I'd be willing to accept it since the final result is roughly the same).

 

more than willing to hear about the test results you get.

watch the update intervals, that is the key. 10 seconds...that is what is being graphed... 10 second bars, not 5 minute bars...

 

Here's the results of the test:

Started the 5 minute bar with 40 seconds of not moving (at a traffic light).
Drove for approx. 3 min, during which i observed the tank average increase from 45.8mpg to 46.0mpg, with 100 miles on the tank to begin with.
Sat for approx. 1.5 min until the consumption screen updated.

The consumption screen showed a 5 minute average somewhere between 65 and 75mpg.

Doing some easy math, we can compute the short term average over the 5 minute bar:

46*101 - 45.8*100 = 66 MPG.

Now, before you go saying the reading isn't exact, lets run the number for a 0.11mpg increase (as a minimum amount to see the average increase like that on the screen):

46*101 - 45.89*100 = 57 MPG

and for a 0.29mpg increase:

46.09*101 - 45.80*100 = 75.09 MPG

Any way you slice it, the mileage seen on the consumption graph wasn't cut almost in half like it would have been if it was calculated the way you proposed, Evan.

The results don't tell us if it's Dave's theory (with the 10 second MPG's being averaged, ignoring the ones with no movement or gas consumed) or mine (straight distance/consumption) which is correct.

In order to test that one, i think we would need someone to perform another test: spend 2.5 minutes traveling at 35 MPH with a steady mileage, then the next at 65 MPH with a different steady mileage - using dave's method (assuming i understand it correctly), you would end up with an exact average of the numbers - you have equal number of 10 second windows with the high and low mileage. With my method, you would see a number closer to that of when you were traveling fast, since you covered more distance during that period of time.

But even doing these, and using EPA numbers (51 for 35MPH, 60 for 65MPH), we don't see that big of a difference - 55.5mpg using dave's method, and 53.8mpg using mine. I'm not sure you could tell that small difference on the MFD.

 

ok but at least we have definitely concluded that no gas consumed AND no distance traveled do not affect the consumption screen and for any one who does fast foot thru the drive thru on a regular basis, they probably already knew that.

i will say that the tank average updates every 10 seconds and that is very apparent when you first fill up the tank as the numbers change for the first several segments as long as you are moving so i find it hard to believe that the Prius discards this data and creates a new data set to display the 5 min consumption bars.