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Q: How do you get 116 mpg (US) in a Metro XFi? A: Pulse and glide.
Posted Wednesday, May 30/07 in Driving efficiently
Judging by the visitor log, there's a lot of interest in the pulse and glide driving technique. It's one of the most frequent search engine queries that brings visitors here. In recent years, pulse and glide may have been popularized most by drivers of Toyota's Prius hybrid, partly because it's just so darned easy to do in that car. But contrary to a misconception held by some, this extreme fuel saving method wasn't developed by hybrid drivers, nor is it limited to hybrid cars. In fact, it has been used for decades by participants in extreme fuel economy competitions like the Shell Marathon for experimental vehicles. A group of just such people - grad students and researchers at the University of California (Davis) - demonstrated the technique (calling it "burn-and-coast") to a writer from Road and Track in 1992. With some coaching, the writer pulled off 116 mpg (US) on a closed course in a bone stock Geo Metro XFi. P&G disclaimer ... Upfront, any discussion of pulse & glide (P&G) almost always includes a disclaimer that goes like this: this is not a "normal" driving technique; it's not particularly easy to do in a non-hybrid; it may cause more wear and tear on the vehicle; and it has the potential to drive fellow motorists crazy (depending on the spread between the min/max speeds of the pulse & glide); it may even be illegal where you live! BUT... done correctly, in the right conditions, it works. From theory to "HOLY COW!" ... In January 2006, I estimated that using P&G between 70 & 90 km/h (roughly 45-55 mph) in the Blackfly had the potential to improve fuel economy by 15% over steady state cruising at 80 km/h. At that time, I could only estimate the P&G results, because my ScanGauge won't record distance travelled when the engine is switched off (skewing its MPG calculations). Since then, I have solved that problem by installing an ignition kill switch that permits uninterrupted power to flow to the ScanGauge when the engine is stopped, so the MPG calculations continue uninterrupted. I have also come to appreciate that the biggest gains from the P&G technique are to be found at a lower average speed than my initial study. Why? Because lowering the average speed minimizes the aerodynamic penalty of the glide. The longer the glide relative to the pulse, the more dramatic the results. My P&G record: 133 MPG round trip in the Blackfly ...
One warm evening last summer, the stars aligned in the form of nearly ideal conditions for a personal record-setting 28.4 km (18.3 mi.) round trip. That evening, I loaded my mountain bike into the hatchback and drove to a trail network out of town using pulse and glide almost exclusively along the way. Back at the house after my bike ride, the ScanGauge reported:
This was possible only because I was virtually alone on the road, unmolested by other drivers - and it was nearly all rural driving. This record can be broken. While conditions were great, they weren't ideal. I may have another go at it this summer. Unbelievable? Don't take my word for it ... 133 mpg (US) in a Metro? Impossible! That would have been my response too, before I understood the potential of P&G first hand. Fortunately, there are others who have achieved similar things in other cars, and documented their results:
By Kim Reynolds Smack in the middle of the world's most car-crazy populace, Davis, California, thumbs its nose and rides bicycles. Waves of 10-speeds constantly circulate its University of California campus' bike paths. Hordes back up at intersections teetering under the weight of chemistry and humanities texts. Here the automobile is transportation's bottom feeder; motorcycles are acceptable depending on displacement, mopeds are admired, bicycles command poetry. In other words, a no-man's land for gasoline-bleeders like us except for the receipt of a letter signed by Dr. Andrew Frank of UC Davis' engineering department:
Dear Kim: My response was, "That's ridiculous. Three thousand miles per gallon? Either we're going to learn a lot about mileage or we'll expose these guys. Let's go." Of course, it wouldn't do to arrive in Davis snorting and belching exhaust in a Lamborghini Diablo or a Dodge Viper, so to get into the mood we rounded up the two highest-mileage cars available: Geo's Suzuki-built Metro XFi and Honda's Ohio-built Civic VX. Of the pair, the Geo folows the vein of traditional frugal flyers, combining a light curb weight (1600 lbs.), small displacement (1.0 liter divided amont three cylinders), and beveled edges for reasonable aerodynamics. No magic, but it works at the pump -- the EPA claims 53 city/58 highway mpg. The flip side is that there's mothing magical about its performance either, the XFi requiring a humble 13.0 seconds to greet 60 mph. However, the Civic VX, which nearly matches the Geo at 45/55 mpg, emerges from an entirely different gene pool. At a substantially higher base price (roughly $10,550 versus $6999), the VX tackles frugality with performance (0-60 in 11.0 sec.) and a clutch of Honda's highest technologies. Where the Geo is light through mechanical simplicity, the Civic VX adds alloy wheels and wraps them with special low-rolling-resistance tires. Aerodynamically, there's a small, but effective underbody fairing positioned to unruffle flow beneath the Civic's stern. And instead of simply reducing displacement, the Civic adopts an inspired variant of Honda's elegant VTEC valvetrain. During part throttle (which is most of the time), one of the intake valves essentially drops shut, forcing highly turbulent combustion. Further, during normal cruising, the air/fuel ratio can lapse into extraordinary lean-ness, on the order of 24 to 1 (ordinarily, you'd expect about 14:1). Those of you mouthing, "Hey, what about NOx" get 10 full points. Our Honda test car, a 49-state version, doesn't meet California's tough nitrogen-oxides standard. In fact, California-sold Civic VXs are remapped to avoid this lean-burn extreme and sacrifice some mileage (4 city/3 highway mpg).
It's nearly 500 miles from Newport Beach to Davis, and a crow making the trip would wisely follow our ruler-straight path up Interstate 5. However, even a birdbrain wouldn't pick a Friday afternoon to line up in Los Angeles freeway traffic in the middle of a drought-busting rainstorm, but there we were. However, the idle time (literally) allowed us to appreciate how surprisingly comfortable the Metro's seats are, how the VX hesitates momentarily on tip-in (lean-burn stumble?), and that butting against a strong, rain-laden head wind can pummel mileage. Lesson One: Air drag accumulates with the square of speed. At 60 mph, a 25-mph head wind (such as ours) doubles aerodynamic drag. Thus the Geo struggled to 44 mpg; the Honda was reduced to 43 mpg. Cold, wet and beat, at midnight we fell into our rooms at the Davis Holiday Inn, perhaps landing on our beds. Eight a.m. and there's a knock at the door. Dr. Frank appears carrying chocolate doughnuts. Dollar for dollar, the world's best buy in instant credibility. However, sugar-induced popularity can have a short half-life, so I probe his background. "Growing up in Pasadena, I was into hot rods and built a '36 Ford Phaeton with a Cadillac V-12. Lousy mileage, but a real tire-burner. "Later I spent 18 years at the University of Wisconsin doing fuel-economy research and constructing two flywheel vehicles. We showed that fuel efficiency in the city could easily be doubled; in fact, computer studies indicated that the worse the traffic, the better those vehicles did. This led to further fuel-economy research, and now here we are at these super mileage contests." We follow him to the campus to meet his grad students. "Hi, I'm Mark Duvall." Like the others, there's a sparkle in his eyes I've seen before; it's unique to young engineers containing a few more psi of enthusiasm than the rest of us. And like an auto-focus lens, their interest quickly tunes in the nearest new machine. "Pop the hoods?" My kind of guys.
With their attention fixed on our cars, mine was caught by theirs -- two delicate, wispy projectiles you'd suspect are registered to George Jetson. Shamu and Side FX are student-built to contest high-mileage events staged by the SAE (Society of Automotive Engineers). Of the two, Shamu is the zanier, a 3-wheeled carbon-fiber and fiberglass shell weighing 110 lb. that constantly looks as if it's about to hatch something. Side FX, on the other hand, is pinched and sinister-looking. Perhaps it doubles as a hazing prank keeping narrow-shouldered undergrads from taking full breaths. This would reduce the noisy complaining possible in the wider Shamu, but more important, it premits FX to part and then rejoin the airstream with utmost gentleness. Shamu is credited with a drag coefficient of 0.15; Side FX, 0.14. Multiply that by their equal 3.8-sq.-ft frontal areas, and you end up with wind resistance roughly 1/20th that of the Civic and Metro. Rolling drag is similarly nil owing to 200-psi tires, which constantly threaten to pop like party ballons. Motivation, by decree of the competition rules, allows a stock or modified 106-cc, single-cylinder Briggs & Stratton (both UC Davis cars have proprietary pent-roof heads, however, and FX employs an overhead cam). At the California Highway Patrol's Sacramento training facility (actually, a pretty nice road course, and thanks for the hospitality), Shamu and FX re-create their record-setting feats: "Putt putt putt putt putt," and then nothing. Nothing except for the muffled rattle of tiny pebbles being thrown up against the thin bodywork. The technique is called "burn-and-coast." "Nice burn, nice burn," experienced watchers will sometimes say. It derives from Lesson Two: Spark-ignition engines are most efficient when run at either full throttle or turned off. What with aerodynamic drag so sensitive to speed (remember Lesson One), burn-and-coast is primarily useful at low speeds, and what do you know but the contest rules require a minimum average speed of 15 mph.
That's the bogey: Floor it up to, say, 25 mph, switch off and then coast -- seemingly forever -- down to a slow walking pace and then repeat the process. Obviously, to cover 3300 miles at 15 mph would require more than nine days of putt, putt, putt, coast -- so as as a practical matter, mileage is calculated based on a weight change in a sealable fuel bottle following three laps (5.2 miles). Burn-and-coast might seem a little impractical, but Dr. Frank and his students are adamant supporters. "In Calcutta, all the taxis drive this way. It's economical, but total chaos," Dr. Frank laughs. "Now lets try it with your cars." The hoods fly up, and a half-dozen screwdrivers start spinning counterclockwise. Off come the fuel lines. In goes the team's own pumps connected to calibrated fuel bottles. The rear seats are pulled out, giving access to the fuel pumps' leads. Chicken wire secures the temporary gear, and with the hose clamps tightened, the subversion is completed. We cruise around in 3rd gear at 20 mph to get a steady-speed baseline, and the bottles are removed and weighed. Twenty mph for the Geo - 84.1 mpg," a voice shouts from the team's van. Repeating the routine for the Honda yields 93.0 mpg. Now it's burn-and-coast time. Our first tries are a bust - I pick the wrong gears and stall the engines. Next, we follow two dead-end strategies. Switching the engine off to coast, then refiring it with the starter gulps too much gas (the injectors add an extra squirt). Just letting the engine idle during coast isn't much better. The payoff comes when we switch off the engine and then bump-start to refire it; not always reliable coasting at 8 mph, but it works.
"Honda's average at 20 mph: 104.8 mpg." A cheer goes up. We broke 100 mpg. The the Geo returns 116.6 mpg. Granted, 20 mph isn't a useful speed unless you're struggling from Narita airport into congested Tokyo. "But remember," Dr. Frank adds, "at 3300 mpg, our gasoline vehicles produce vertually zero emissions." (And, I'd add, offer rather better range than the electric cars California is currently committed to.) During dinner, I ask some of Dr. Frank's students where they are headed. "Dearborn, Michigan to work for Ford," replies Bruce Dughi, Side FX fairing designer. The others have similar aspirations. I sit for a moment and ponder Bruce, in 20 years, designing a new Ford. And I realize that Shamu, Side FX and whatever else Dr. Frank cooks up next for their UC Davis successors, aren't really vehicles. They're seeds being sown.
- A bit of trivia first: Dr Andrew Frank (who invited Road and Track to this event) has gone on to become a leading researcher in (some say the father of) the field of plug-in hybrid cars. - The statement that an engine is "most efficient when run at [...] full throttle" may not necessarily be true. Wide open throttle also induces the computer to enrichen the mixture dramatically. Depending on the vehicle, the most efficient throttle position may be somewhere below that point. --- Resources ...
darin AT metrompg D-O-T com, or here
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