Pulse and Glide

[Soylent]

After doing a bit of P&G, I felt that the amount of energy used to "pulse", or accelerate, was not equal to the amount saved while "gliding." I feel like it's artificially increasing the average MPG because it looks like the computer measures the average MPG based on the amount of time you're going that certain MPG.
So you burn gas to accelerate for 20 seconds, then you go 99.9 mpg for a few minutes. That fools the computer into thinking that you've been going 99.9 MPG most of the time, when in fact you burned a lot of fuel for that 20 seconds. This raises your MPG average, even though it may not be accurate in the end. Or am I not seeing this correctly?

[DaveinOlyWA]

sounds like your pulse is someones elses "tromping".

[c4]

You're not seeing this correctly.. It all averages out, and it's distance traveled, not the time that is used for the average (if it were time, all you would have to do is sit at a stop light with the engine off and watch the average increase to infinity)..

Because it's actually measuring fuel consumption per distance traveled, it's exactly the same thing if you get a constant 50 MPG and travel one mile or pulse for an appropriate amount and glide the rest of the way using no more fuel; you've still travelled the same distance and used the same amount of fuel.. Obviously your velocity profile will be different, with the constant consumption generally producing a constant velocity vs. the pulse-n-glide creating a higher peak velocity that slowly falls off, but that's immaterial to the fuel consumption calculation..

Some people who have very good foot control and are more tuned-in to the behaviour of the vehicle can get very good mileage at a more-or-less constant consumption rate, others use pulse-n-glide to achieve the same thing.. Personally, I can't stand the pulse driving technique and the technique I use is a targeted consumption- ie, I choose an arbitrary consumption number, eg 50 MPG, that I want to try to maintain. I ignore the acceleration phase, but once I reach cruising speed, I feather the gas to try to achieve 50 MPG while maintaining my speed. I'll accept about a +/- 2 MPH and +/- 2 MPG variation, but I try to keep everything as steady as possible. Obviously, choosing a realistic target is crucial to the success of this technique, but 50 MPG is very much achievable and it is much less stressful than the constant pulsing..

[tmgrl3]

If you calculate your mpg when you fill up, isn't it close to the MFD number...I'm only on my 3rd tank, but miles traveled and gas used bring my calculated mpg about 1 point apart.

terri

[berylrb]

<div class='quotetop'>QUOTE(c4 @ Sep 12 2006, 09:27 AM) [snapback]318273[/snapback]</div>

You're not seeing this correctly.. It all averages out, and it's distance traveled, not the time that is used for the average (if it were time, all you would have to do is sit at a stop light with the engine off and watch the average increase to infinity)..
[/b]

or ... distance = 'rate' times 'time', whereas MPG = mile divided by gallons

[theorist]

<div class='quotetop'>QUOTE(Soylent @ Sep 12 2006, 09:21 AM) [snapback]318248[/snapback]</div>

I feel like it's artificially increasing the average MPG because it looks like the computer measures the average MPG based on the amount of time you're going that certain MPG.
So you burn gas to accelerate for 20 seconds, then you go 99.9 mpg for a few minutes. That fools the computer into thinking that you've been going 99.9 MPG most of the time, when in fact you burned a lot of fuel for that 20 seconds. This raises your MPG average, even though it may not be accurate in the end. Or am I not seeing this correctly?
[/b]

I trust that any engineer employed at Toyota will know to average the MPG over gallons or fuel used, not over time or distance traveled. The pulse and glide benefit is real, evidenced by full tanks consumed using the technique.

On the other hand, we can fool ourselves by using the 5 minute time interval averages incorrectly. Suppose that I took your Prius out for a 25 minute spin and brought it back to you with the consumption screen showing 24 mpg for the first 5 minutes and 100 mpg (or more) for the next four 5-minute intervals. (The first 5 minutes' MPG is often low from warming up the engine.) Did I achieve a good average mpg on the 25 minute drive? Can we take the average of 24, 100, 100, 100, 100 and say that I achieved at least 424/5 = 84.8 mpg?

Scenario 1:
I drove an average of 72 mph for the first 5 minutes, traveling 6 miles and consuming a quarter gallon of gasoline. Over the next twenty minutes I travel at an average speed of 12 miles per hour, traveling 4 more miles, consuming 0.04 gallons of gasoline. Overall, I traveled 10 miles on 0.29 gallons of gasoline, averaging just 34 mpg.

Scenario 2:
I averaged 30 mph over each of the 5 minute intervals. In the first 5 minutes, I traveled 2.5 miles and consumed 0.104 gallons. In the next twenty minutes, I traveled 10 miles and consumed another 0.1 gallons. Overall I traveled 12.5 miles on 0.204 gallons averaging 61 mpg.

Scenario 3:
In the first 5 minutes I traveled 1 mile (averaging 12 mph) and consumed 0.04 gallons. Over the next 20 minutes I traveled 12 miles (averaging 36 mph) and consumed 0.12 gallon. Overall, I averaged 81 mpg.

We can't say what mpg was achieved by looking at the mpg over time graph on the consumption screen. Fortunately the car's computer calculates the mpg correctly.

(These three scenarios are purely hypothetical and only intended to show how the mpg over time chart can be misinterpreted, not to recommend any driving strategy for low fuel consumption.)

[Soylent]

Cool, thanks everyone. I'm still not too good at P&G, so I find that cruise control gets me the best mileage.

[tmgrl3]

<div class='quotetop'>QUOTE(Soylent @ Sep 12 2006, 11:50 AM) [snapback]318348[/snapback]</div>

Cool, thanks everyone. I'm still not too good at P&G, so I find that cruise control gets me the best mileage.
[/b]

I feel that P&G is well worth the practice...it is now becoming automatic for me and I am only on my third tank...also, I use electric only as well...my car "wants" to go into this mode at very low speeds with some curves and hills...and stays in it for some time...I let it since I am going 15 mph...I can also glide in the 20 mph range, not just in the 30-40 mph range....

For me it's fun...but I have a big improvement over my first tank and I do all very short trips with lots of hills, bunching errands.

terri

[eagle33199]

Using the OP's original scenario, accelerating for 20 seconds, coasting for 2 minutes, and assuming a steady rate of acceleration and a different steady rate of deceleration:


The pulse takes you from 35MPH to 45MPH in 20 seconds, with a shown average the entire time of 10MPG. This would imply that your acceleration is 0.5 MPHPS. we could use calculus to determine the exact distance traveled, but since it's been 3.5 years since i've done any of that, we're going to go with the simple (and decently accurate) estimate of 40MPH*20s = 0.22 miles, at 10MPG, resulting in 0.022 gallons of gas consumed

For the glide portion, we assume a linear decrease in speed back to 35MPH over the next 2 minutes. Using the same estimation, we get a distance travelled of 40MPH*2m = 1.33 miles.

This gives us a total of 1.55 miles travelled and 0.022 gallons consumed, resulting in an average of 70.45 MPG for the entire distance, which, at these speeds, is about right for those out there with the really great mileage. Personally, i don't get this high too often, as i have too much traffic and too many stops to deal with in my commute.

note to self: update mileage number on green hybrid when you get home :-p been a number of tanks...

[Soylent]

<div class='quotetop'>QUOTE(eagle33199 @ Sep 12 2006, 12:10 PM) [snapback]318374[/snapback]</div>

Using the OP's original scenario, accelerating for 20 seconds, coasting for 2 minutes, and assuming a steady rate of acceleration and a different steady rate of deceleration:
The pulse takes you from 35MPH to 45MPH in 20 seconds, with a shown average the entire time of 10MPG. This would imply that your acceleration is 0.5 MPHPS. we could use calculus to determine the exact distance traveled, but since it's been 3.5 years since i've done any of that, we're going to go with the simple (and decently accurate) estimate of 40MPH*20s = 0.22 miles, at 10MPG, resulting in 0.022 gallons of gas consumed

For the glide portion, we assume a linear decrease in speed back to 35MPH over the next 2 minutes. Using the same estimation, we get a distance travelled of 40MPH*2m = 1.33 miles.

This gives us a total of 1.55 miles travelled and 0.022 gallons consumed, resulting in an average of 70.45 MPG for the entire distance, which, at these speeds, is about right for those out there with the really great mileage. Personally, i don't get this high too often, as i have too much traffic and too many stops to deal with in my commute.

note to self: update mileage number on green hybrid when you get home :-p been a number of tanks...
[/b]

So THAT'S why they teach us math in school...oops! Anyway, thanks for that calculation, it makes more sense now.