driving technique: pulse & glide (long)

[JanGeo]

Yep I have no power steering on the Geo also but the brakes you should try first before you THINK you can stop without vacuum - the pedal gets really hard to push. I also have a new Scion xB - Geo rotted out at the right lower A Arm on Thanksgiving morning and on the xB you really don't want to turn the engine off and try to steer - the power assist in the rack and pinion steering is really tought and the owners manual also does NOT recommend it.

But the cooling issue is valid because stopping the engine when stationary is a lot different than while moving - air cooling is much greater at speeds and the sudden change in temperatures can cause some problems to the metal over time. Also the fuel injection may use different mixtures until the oxygen sensor and the exhost manifold warms up again so that you may be affecting the fuel economy. These things are taken into account on the Prius already as is the starter motor. I am not saying it is REALLY BAD for the motor but long term it could be.

I can't run the ScanGauge on the Geo of course but the xB has been really interesting to watch - still waiting for a tank of gas to run down more before I can reset the gas used calibration and get more accurate readings on the MPG. Getting a bit of a pump shock with the 20mpg DECREASE in the new car but I needed something a little bigger to carry the electic vehicle I am building. I am also thinking of adding a motor or two to the rear wheels of the xB to make it a parallel hybrid - with the extra weight on the downhills I could recapture more of the energy to assist on the level and slight downhills with the ICE in neutral.

[Flatland2D]

I finished my pulse and glide tank and here are the results. I had to do some calculations to compensate for a highway trip I went on. By estimating my highway mpg at 33mpg based off my previous tests and temperature I was able to calculate fuel used on that trip (130 miles) and subtract it from the total. Remaining miles and fuel would represent my pulse and glide efforts. Using this method I calculated that I got 29.38mpg, up from about 27 of my normal driving habits (a good punch every once in a while, but normally steady and coasting downhill), and 25 if I'm really feeling aggressive. Overall I wasn't too impressed. Maybe my pulses were taking up too much gas or my idle wasn't lean enough. No way to know until I make a mpg gauge. I don't think I'll continue this method because it can sometimes be annoying to always having to concentrate on the speedo when it's easier to just maintain constant speed. It's also incredibly annoying to other drivers, so you can either piss people off, or only do it when no one else is around. I really enjoy driving my car the way I normally do so I don't consider a 2 to 3mpg gain to be sufficient enough to alter my driving habits. I figure I probably have to buy an extra gallon of gas to get the same mileage as if I used pulse and glide all the time. But for me, when money's not tight right now, and gas isn't outrageously high, I'd gladly pay the extra $2.50 to be able to enjoy all 170hp every so often without having to think about how it's going to mess up my pulse and glide habits. I'll revisit this idea when I have a way to measure mpg instantaneously.

One benefit to coasting this much is that I found some really great roads to coast on that I normally didn't think would sustain a coast very well. I got one to last out to a mile. Some roads are ever so slightly downhill and it's hard to tell without rolling in neutral for a little while.

[MetroMPG]

FYI...

i just came across another description of the theory of pulse and glide and why it works the way it does. the writer has an enviable ability with words and i think he explains it more clearly than i have done.

he makes the added point that the advantages to be gained from pulse and glide are most evident at speeds below where aerodynamic losses are more prevalent.

it also indirectly addresses the issue of what type of acceleration is most efficient: slow or quick.

ICEs have a strong efficiency peak at some particular horsepower, which is usually some substantial fraction of full horsepower. For example, a 100hp engine might be most efficient at 50hp. But if you run it at 10hp or 100hp, it is less than half as efficient (uses twice as much fuel per horsepower).

The trouble is, the vehicle itself is most efficient at low speeds,
where wind resistance is negligible. So the slower you go, the more
efficient the vehicle gets but the less efficient the ICE is.

If you drive at a constant speed, then the best fuel economy occurs
where the product of the vehicle and engine efficiencies reach a peak.
This typically occurs at 30-40 mph. But the vehicle only needs 5-10hp to drive at this speed, so its efficiency is less than half its peak
efficiency.

Fuel economy champs found a strategy to beat this problem long ago. The trick is to always run the engine at its peak efficiency (say 50hp), or shut it off entirely. But keep the vehicle speed as low as possible, so aerodynamic losses are negligible. Start at some very low speed like 10 mph. Put it in gear so the car's momentum starts the engine. Run the engine at 50hp, which makes the car accellerate strongly. When you reach the speed where wind resistance losses begin (like 40 mph), shut off the engine, take it out of gear, and coast back down to 10 mph. This is "pulse & glide". On any car, you can at least *double* your fuel economy by driving this way.

The same principle applies to an Electric Vehicle, except that the electric motor has a much flatter efficiency vs. horsepower curve. A motor might peak at 85% efficiency at 20hp, but is still 75% efficient from 1hp to 50hp. This means there is less to be gained by pulse & glide. So pulse & glide would give you an extra 10% range, not double the range.

Bob Bath wrote:

... if you pull a steady current out of your batteries, the Peukert
... effect dictates that with prolonged current, you'll get less time
... at that current than if you let the batteries recover for a second
... or two as your speed drops.

Correct, but I think this is confusingly worded. The Peukert effect just says that the higher the discharge current, the lower the amphour
capacity. Peukert works against pulse & glide. The higher peak currents when the motor is on reduce the pack capacity. Since (in my above example), you only have a 10% potential gain, you could throw this all away in the batteries if your peak current is high enough for Peukert to cost you more than 10% in amphours.

I have a Prius. The Prius is specifically built for very *low*
emissions; it is 10x better than the average new car. They sacrificied
fuel economy and horsepower to get low emissions. That's why the Prius
fuel economy is good, but not as outstanding as it could have been.

The Prius automatically does pulse & glide all by itself if you drive at a constant speed under 40 mph. The engine will start, run at moderately high horsepower to improve efficiency, and use the excess horsepower to charge the batteries. Then it shuts off the engine and maintains speed using energy from the batteries.

However, there is extra energy loss in the charging and discharging of
the batteries, and motor/generator conversion efficiency. So, you can
get even better fuel economy by manually doing the pulse & glide, where you step down on the accellerator hard enough to force the engine to start; accellerate up to 40 mph or so, then position the accellerator to *just the right position* so the car is coasting; not using either the engine or electric motors. Coast down to 10 mph or so; and repeat. On my Prius, I can get 55 mpg driving at a constant 40 mph, or 80 mpg with 10-40 mph pulse and glide. I can top 100 mpg if I'm anal-retentive enough and there's nobody behind me. Obviously, pulse & glide is impractical if there is anyone behind you!

- source

[philmcneal]

Originally Posted by MetroMPG

FYI...

i just came across another description of the theory of pulse and glide and why it works the way it does. the writer has an enviable ability with words and i think he explains it more clearly than i have done.

he makes the added point that the advantages to be gained from pulse and glide are most evident at speeds below where aerodynamic losses are more prevalent.

it also indirectly addresses the issue of what type of acceleration is most efficient: slow or quick.

'



how can you tell on the rev band oncce you reach "peak torque power". With stepless gears how do we know which gear to extract most of the power from?

[MetroMPG]

your car's power specifications usually tell the RPM where peak torque and peak horsepower are achieved. if it's not in the owner's manual, it's out there on the web somewhere, or in car reviews.

[SVOboy]

Yeah, should be in the specs from the dealer somewhere or just randomly online.



Blue and red are my car now, black and green when I get it running with this swap finished.

[MetroMPG]

but... be careful. efficiency peak doesn't necessarily = torque or hp peak. but i also don't know any more about it to say more than that.

quoted above (my emphasis):

ICEs have a strong efficiency peak at some particular horsepower, which is usually some substantial fraction of full horsepower. For example, a 100hp engine might be most efficient at 50hp.

too bad magazines/manufacturers don't also publish their engine's efficiency peaks along with the other performance info.

[SVOboy]

You said torque and horsepower peak, so that's what I posted,

[philmcneal]

115 horse @ 6100
110 lbs/ft@ 4500


at 50 km/h (28 mph)

1st gear (max out)
2nd gear (4500)
3rd gear (2500)
4th gear (1800)
5th gear (1200)

I'll get my scangauge soon enough, then i'll know if its better to pulse agressively in 3rd (3000+ to 70km/h) OR switch to 4th and pulse slowly towards 70 km/h (38mph) until rpms reach (2400). I won't bother with 5th -_-;

[1993CivicVX]

Originally Posted by MetroMPG

(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.)

[snip]

You're average will actually be less than 36.5. Think about it, your glide time is longer at lower speeds than it is at higher speeds (ie. the amount of time you're gliding at speeds between 36.5mph and 40mph is shorter than the time you are gliding between speeds of 33 and 36.5) Because the slower you go, the less you are being slowed down by wind resistance and rolling resistance. You'd prolly average closer to 35mph, but yeah, that is the general gist of it. But the math is a little more complicated. I also mistakenly thought P&G was exclusively only for complete engine shut off hybrids (ie, the prius) I only recently found out otherwise.