The myth of pulse and glide

[Chuck]

So, in our example of a 20 MPG pulse and 100 MPG glide, how far must you travel during the glide to get 60 MPG total? Five times the pulse distance!

Bill Wood - SQUARESPACE - July 8, 2009

Commentable article....why is it working for so many of us then? --Ed.

The math behind "pulse and glide" seems intuitive enough. At least that's what I thought. Until I started calculating my total MPG for trips to and from work. On the way to work I have gotten 60 MPG; on the way back I have gotten close to 40 MPG - that's 50 MPG total, right? I did the math, and my MPG to and from work is only 48 in that case.

How does this apply to pulse and glide? Pulse and glide is a technique where you accelerate moderately to an upper target speed such as 45 MPH, then lay off the accelerator and glide down to a target speed such as 35 MPH, possibly putting the car in neutral during the glide to minimze fuel use. Then you repeat. The idea is that the pulse MPG and glide MPG should average out to some pretty good gas mileage.... [Read More]

[AlmightyEngineer]

Without getting lost in the semantics, this is the goal of efficient driving but two pieces of information are needed:

* Brake Specific Fuel Consumption(BSFC) - this is the amount of fuel burned to produce a given amount of mechanical or shaft energy
* drag reduction - this is the sum of the rolling, aerodynamic and what Ken@Japan calls, 'heart beat' power ... the energy that is consumed by the electronics with the car regardless of what it is doing

If you plot the drag force product with the velocity, you get an energy curve, the least amount of energy needed to sustain any given speed:



Now you'll notice in some cases, the plotted performance exceeds the theory. This isn't "cold fusion" but upon closer inspection we find:

1. tires fully inflated - in the first Prius marathon, one report claimed 50% over maximum side wall pressure and nearly bald tires.
2. warm temperatures - generally above 80F. So far, no one has reported attempting a maximum, density altitude test, say Wyoming or Colorado during the hottest days.
3. aerodynamic modifications - starting with bumper air inlet blocks and proceeding to wheel well covers and stripping off all external objects or if on public roads, putting them in a least drag position.
4. minimum weight - take out everything that can be removed.
5. lubricants - using a lower viscosity engine and transmission oil to minimize mixing losses and other properties that reduce thin film friction without leading to wear that shortens the vehicle life.

[diamondlarry]

I left the following response to Mr. Wood's article:

I still don't think you completely understand P&G. For me, the pulse lasts for 5-10 seconds but, the glide lasts for 20-30 seconds and even more. Just yesterday on the way home from my mother's, I achieved 160 mpg in a 4.6 mile trip in my '07 Prius. My pulse would last 1/8th mile or less however, the glide would last 1/2 mile or more; with one glide lasting over 1 mile. Also, the day before, I achieved 114 mpg over a trip of 18.0 miles You also are not taking into account that someone who has a good grip on the concept of P&G will use the terrain to their advantage. There are certain cases where my short 1/8th mile pulse will result in a glide of 1 mile or more. At the risk of this sounding like bragging, the above 160 mpg segment wasn't a record as I have achieved just over 170 mpg for that same trip.

[jimepting]

Fascinating chart - to an engineer. But, I could stand a little more explanation. Where does it come from? Is the "fixed" a fixed load. What is NHW11 and NHW20? I get the impression that the curves are theoretical and the data points are measurements - correct? What is What is the "hack?" Anyway, would you explain the history and the relevance of the graph a bit more.

I assume this data is all from one car and one driver. Correct?

[Jough96Accord]

Same for me. I would not try to pull a long glide if it was not at least somewhat going downhill. There are spots on my commute where I can glide for nearly 3 miles. Glide for 3 miles, how can that not save gas?

[vtec-e]

My recent attempts at p&g seemed to give me great results but my tanks weren't much better. My difficulty in dialling in my scangauge was giving me optimistic results. The 1:5 ratio, for me at least, is hard to do on the flat unless i plant the accelerator. This results in a net waste of fuel. Not to mention being harder on the clutch. It wouldn't last long if i pulsed like that every 20 seconds over an hour, twice a day, four days a week....
I'll keep trying anyway! It works for some so why not me eh?!

ollie

[fusion210]

Easy guys. I think the title is getting people slightly riled up.

He's not saying it doesn't work. The only message is that you won't instantly get the average of how much gas you're using in a pulse and how much gas you're saving in a glide as if it was 50/50.

Have you ever tried to explain pulse and glide to someone and had this happen? In my case it's getting around 30mpg in a pulse, 180mpg in a glide. The person I'm talking to usually combines 30mpg with 180mpg, getting 210mpg and then divide that by two. I must be getting 105mpg! But no, it's somewhere between 60-70mpg at freeway speeds.

Perhaps it should have been just titled "A misconception of pulse and glide."

[cpeter38]

This is the meat of my response:
The focus of the article should be different.
The point should be that a properly applied P&G will deliver VERY SIGNIFICANT fuel economy benefits. As an example, I am averaging approximately 60 MPG in my car by using P&G (a Ford Contour SVT that nearly qualifies for the cash for clunkers bill). I've actually had serious debates about whether or not I should try to go below the 1/8th mark on this tank because I am very close to 1000 miles on this tank (~850 miles).

[JusBringIt]

Fusion210 has it exactly right.

The author just wanted to explain the misconception people have of using pulse and glide.

For reference, my pulse is at about 17mpg, my glides are at 9999mpg. I avg out to 55mpg doing this. A simple calculation will show you that my pulses are less than 1/3 of the distance of my glides on average.

All he is saying is Time is NOT a factor in the equation of why pulse and glide works, it's distance.

[bryankwalton]

Originally Posted by fusion210

Perhaps it should have been just titled "A misconception of pulse and glide."

Or perhaps it should have been just titled, "A misconception about how MPG is calculated." I'm afraid that too many people think that if you get 20mpg for a 100 mile stretch of road, and then get 100mpg for a second 100 mile stretch of road, that you have averaged 60 MPG for the trip. In reality, you have used 5 gallons for the first stretch, and 1 gallon for the second stretch. This means for the 200 mile trip, you used 6 gallons of gas and averaged 33.3 MPG.

Cheers,
Bryan