(i'm writing this more because it's interesting than because it's practical...)
pulse & glide in a prius: 109 mpg
this summer, a group of five prius fanatics drove an unmodified 2nd gen prius to a record 109.3 mpg (US) over a 1397 mile marathon on a "loop" of public roads in Pittsburgh. you may have heard about this already. if not,
you can read about it here.
they used a driving technique called "pulse and glide" which initially i mistakenly thought was only applicable to the prius hybrid system. i was wrong - it works in theory on any car...
pulse & glide explained
first off, "pulse and glide" is a technique that would probably rarely be used in real world driving in a non-hybrid car. it works like this: let's say you're on a road where you wanted to go 36.5 mph. instead of driving along at a steady 36.5, you accelerate gently to 40 (that's the pulse), and then coast in neutral down to 33 (that's the glide). that's it. rinse and repeat. "pulse" up to 40, "glide" back to 33. repeat. and repeat. and repeat.
you're still averaging 36.5 mph, but it turns out that pulse and glide is significantly more efficient than driving at a steady 36.5. (i'm using 33 to 40 mph because it's the speed described in the hypermiling article linked above.)
wtf???
if you're like me, it's counter intuitive. you're asking, "how could that possibly be more efficient than maintaining a steady speed in the highest possible gear???" it violates one of the main commandments of efficient driving: conserving momentum.
how it works
the secret is in the glide. in the prius, when you release the accelerator below 40 mph, the gas engine shuts off and the CVT effectively freewheels in neutral (it's a little more complicated than that, see the above link if you want details). so, while coasting from 40 to 33, you're effectively getting infinite mpg - you're using no gas.
let's say the prius gets something like 75 mpg (US) at a steady 36.5 mph. then to achieve 109 mpg using P&G, you just have to achieve 54.5 mpg while accelerating from 33 to 40. and it turns out you can, in a prius.
so in the prius P&G trip, you "pulse" half the total distance while getting 54.5 mpg, and then you "glide" half the total distance using no fuel. it averages out to 109 mpg.
pulse & glide in a geo metro: 68 mpg (in theory)
skeptical, i went out in my metro on boxing day with the scangauge. i didn't believe it was going to work, but here are the numbers i got. i went to my "test course" - a nearly perfectly level stretch of 2-lane highway about 3 miles long. note: i used higher speeds, and there was a 20 mph tailwind (thus the high steady speed mpg); all runs were in one direction, and i didn't repeat it - so you should be skeptical of my data too.
- at a steady 80 km/h (about 50 mph) i was getting 59 mpg (US)
- "gliding" from 90 to 70 km/h took 16 seconds.
- "pulsing" from 70 to 90 km/h at a rate of acceleration that also took 16 seconds i was getting about 34 mpg (US).
- so my P&G average would be 68 mpg, vs. 59 at the same average speed. that's a 15% increase over the steady state mpg - theoretically.
i say theoretically, because the engine would have to be off in the glide to get that mpg. you could do it, but it makes it even less practical (whereas the ICE shuts off automatically in the prius).
so i took a couple more readings. with the engine idling, and the car in neutral, the average mpg shown on the scangauge in the "glide" from 90-70 km/h was 550 mpg. when you average that against the 34 mpg of the pulse, it works out to an average of 64 mpg. that's an 8% increase over the steady-state mpg.
that's it
so there you go. next time you're cruising along on a lonely road at a steady speed, you're not getting the best mileage you could. you could be pulsing & gliding to maintain the same average speed, and saving lots of gas in the process. to exceed your steady-state mpg, you just have to be able to "pulse" at a rate of fuel consumption that is greater than half of your current steady-state mpg (assuming equal length pulses & glides; you may be able to increase the proportion of glide to pulse - meaning faster acceleration in the pulse - and still beat your steady-state mpg for that average speed).
anyway... i thought it was a cool thing to learn. completely impractical if you don't have a prius (which handles the engine shutdown/startup and transmission neutral/re-engage all through the throttle pedal), but theoretically do-able in any car.
more, if you're interested:
the physics of pulse & glide
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www.MetroMPG.com
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