Posted Sunday, January 15/06 in Driving efficiently

NOTE - see also: 100+ Hypermiling / ecodriving tips for better gas mileage - added May 18/08



Q: How do you get 116 mpg (US) in a Metro XFi? A: Pulse and glide. - added May 30/07

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"Pulse and glide"

It sounds like some kind of tactical maneuver from the starship Enterprise. But in fact, it's a driving technique for more Earthly transportation. (Never mind that the car most closely associated with the term is kind of spacy-looking.)

Of course the car in question is the Prius, and this summer, a group of five efficiency aficionados drove an unmodified 2nd generation version of Toyota's hybrid to a fuel economy record of 109.3 mpg (US) over 1397 miles on a "loop" of public roads in Pittsburgh, PA.

When I first heard about their mpg marathon, I mistakenly assumed that their technique was possible only because of the hybrid system. I was wrong. Pulse and glide works - in theory - on any car (it's just easier to do in a Prius, as you'll see).

Before going into the details, I should point out that it's not a technique that can be used all the time in real world driving. Even one member of the group who made the run stated that their feat was in no way "normal", but rather a demonstration of the potential efficiency of a hybrid when driven with ultimate fuel economy in mind.

Pulse & glide overview

Pulse and glide works like this: let's say you're on a road where you want to go 60 km/h. Instead of driving along at a steady 60, you instead accelerate to 70 (that's the pulse), and then coast in neutral with the engine off down to 50 (that's the glide). That's it. Rinse and repeat. And repeat. And repeat...

By doing this, you're still averaging 60 km/h, but it turns out that pulse and glide is significantly more efficient than driving along maintaining a steady 60 km/h.

If you're like me, it seems completely counter-intuitive. You're asking, "how can that possibly be more efficient than maintaining a steady speed in the highest possible gear?" After all, it violates one of the main commandments of efficient driving: conserving momentum.

Why it works

Prius MPG marathoners at the completion of their nearly 48-hour endeavour. (Source: Toyota.com hybrid newsletter)

The secret is in the glide, and it's is best illustrated by the extreme Prius demonstration.

The Prius is particularly excellent at gliding because under most conditions when the the accelerator is released below 40 mph, the gasoline engine shuts off completely and the transmission effectively freewheels in neutral (it's actually slightly more complicated than that, but stick with me). So, while gliding, it's effectively getting infinite mpg - it's using no gas at all.

For that reason, the marathon drivers picked 40 mph as the upper limit of their pulse & glide cycle (33 mph was the bottom). So in order to achieve 109.3 mpg, they just had to average 54.65 mpg while pulsing gently from 33 - 40 mph. And it turns out, in a Prius, you can. So, assuming equal length pulses & glides, they traveled half the total distance of their trip getting 54.65 mpg, and half the total distance using no fuel at all. It averages out to 109.3 mpg.

(Their driving was actually more complex than the simple illustration above, so their pulse vs. glide proportions were not necessarily equal. But for the purposes of understanding the basics of pulse & glide, it helps to think in terms of equal halves. The drivers also took pains to ensure they were pulsing and gliding in a very specific way that minimized the flow of energy to and from the battery pack - they were in effect going to great lengths to avoid using the car's hybrid propulsion while driving. Whenever braking, however, they sought to take advantage of the hybrid system's energy regeneration. Follow the links at the end of this article for detailed Prius-specific pulse & glide info.)

Pulse & glide in a Geo Metro

Still skeptical, I went out in my car with the ScanGauge. I really didn't believe it was going to work, but here are the numbers I saw. I went to my "test course" - a nearly perfectly level stretch of 2-lane highway about 6.5 km long - but these numbers aren't meant to be taken as experimentally valid. I only did one run, so consider yourself warned. It's just a snapshot:

At a steady 80 km/h (about 50 mph) I was getting 59 mpg (US) (there was a tail wind)

"gliding" down from 90 to 70 km/h took 16 seconds

"pulsing" back up 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 pulse and glide 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 mileage. You could do it, but it adds another step in an already arguably impractical process (remember the engine shuts off automatically in the Prius when you lift off the accelerator).

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 down 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. Now we're at an 8% increase over the steady-state mpg.

I would name the difference between the two techniques "full" pulse & glide (neutral, with engine off in the glide) vs. "mild" (neutral, with engine idling in the glide).

So, now you know the next time you find yourself cruising down 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 fuel in the process.

To exceed your steady-state mpg (X), you just have to be able to "pulse" at a rate of fuel consumption that is greater than half of X (assuming equal length pulses & glides; you may be able to increase the proportion of glide to pulse - meaning faster acceleration in the pulse, or glides that are timed to take advantage of descending a grade - and still beat your steady-state mpg for that average speed). You can see how fuel economy instrumentation plays a critical role in determining the best rate of pulse to make this work.

The Pulse & Glide Workout!

But it's not really very practical, is it? When I said off the top that there's nothing special about hybrid technology that makes this work, that's only true in theory. In practical terms, the Prius enables this technique because all of its individual steps are performed through a single control - the throttle pedal.

Contrast that to my manual shift Firefly: in order to duplicate the behaviour of the Prius requires 6 discrete steps using 3 of my limbs... every 16 seconds. Glide: release accelerator, clutch in, key off, pause...key on. Pulse: pop clutch to restart engine, depress accelerator. That's quite a workout, compared to 2 steps in the Toyota accomplished entirely with the right foot: glide - release accelerator; pulse - depress accelerator.

Practical implications

What useful information comes from this for the non-hybrid driver?

(Okay, the first one is hybrid related...) Toyota ultra-efficient cruise control! The Prius could potentially have a "pulse and glide" cruise control setting to completely automate the cycle. Set your target speed, and the car pulses & glides around it.

For me, the technique simply underlines the importance of maximizing the amount of "glide" (whether engine on or off) in my driving. In other words, time spent in neutral, or with the clutch depressed. For example, when approaching a stop or a forced speed reduction, rather than maintaining a constant speed as I approach, I now try to execute the optimal pulse that permits me to coast half the distance (or more) with as little braking as possible.

It causes you to carefully consider the comparitive efficiencies of different ways of accelerating or decelerating. E.G. deceleration efficiency, from best to worst in a non-hybrid (standard liability statement applies - don't do this if it puts you or others at risk): (1) engine off, in neutral (or clutch in); (2) engine idling, in neutral; (3) engine running, in gear (it's true that some cars' fuel injectors shut off when decelerating in gear above a certain engine RPM threshold, but the savings in fuel is cancelled by the faster rate of deceleration from engine braking. Relative to coasting in neutral, it's less efficient.); (4) braking (assumes engine also running, in gear).

The pulse & glide downside: it stands to reason that the technique is somewhat harder on the machinery (in terms of wear) than simply maintaining a steady speed. This is even more true in "full" pulse & glide mode, if you are repeatedly using the starter to re-start the engine. For that reason alone I'd be reluctant to use it to boost "steady state" (e.g. hwy) mileage. Its benefits are more practically realized in situations where you have to accelerate and decelerate on a regular basis anyway (e.g. sub/urban driving).

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