Pulse & Glide question
 

Thinking about it, I have a question about pulse and glide...

It takes me 10 seconds on average to go from 60-50mph coasting on a flat, calm day.

It takes me 4-5 seconds in 5th gear at 80% throttle (throttle cable is adjusted so that floored = 80%) to go from 50 back to 60, same conditions.

Would it be more efficient to slow my acceleration? (Pulse)

P.S. Glide isn't 100% accurate, but Pulse is.

That sounds about right. 80% throttle on the pulse, and a glide twice as long as the pulse. You might to a little better expanding your range, to maybe 45-60 mph or so.

How to Calculate Your Optimal "Pulse-to-Glide Ratio"
 

Calculating your Optimal "Pulse-to-Glide Ratio" depends on your GPH during acceleration and glide.

For instance, Pulse in the Prerunner uses between 3.5-4.5gph. Idle during Glide uses about .35-.45gph. I want to compare that to DWL at 52mph, which uses in the vicinity of 1.3gph.

For those who want to see how to compute this:

• P&G Gas Used = PulseRate x PulseTime + GlideRate x GlideTime
• DWL Gas Used = DWLrate x DWLtime

Plug in your averages:
You'll need to choose a cycle time to compare, ex. 5 second pulse and 10 second glide = 15 total seconds.

• P&G gas used = 4.0 gph x 5 sec + .4 gph x 10 sec = 20 + 4 = 24
• DWL Gas Used = 1.3 gph x 15 sec = 19.5

So you can see that using a 5 to 10 pulse-to-glide ratio (or 1 to 2) is not fuel-efficient for me. It turns out that I can break even around a 4 to 9 ratio, and I try to shoot for 4 to 10 or 5 to 12. If I don't think I can get a 10 second glide out of it, I won't bother. Of course your numbers will be much different. Mine reflect the difficulty of accelerating and coasting in a 3600 pound brick.

You can of course use the same formulas to find ratios for any variations of P&G. For example, sometimes I use DWL at a throttle position too low to hold speed, then gas just enough to continue the cycle. This only works for me if I can limit gassing to about 1-2 seconds to get a 20-30 second slow deceleration out of it... which generally only succeeds for me downhill. If you're in a small car, your ratios will be much more favorable.

NOTE for those bothered by units of measurement: You don't need to convert to "hours" to make the above calculation useful. The final units you'll be comparing are technically "gal/3600" -- it's just scaled. You can think of the above as telling you how much gas you'll use if you do 3600 P&G cycles.

Also note: These calculations also don't take into account any other losses during a P&G cycle. For example, rev-matching. As it is, my Tacoma automatically rev-matches coming out of Neutral Glide. It's awesome. But if I'm aborting the glide at too low a speed, it'll sometimes miss overdrive, and I have to give it a "panic pulse" to get overdrive back. If I wasn't "panicking" I could probably read the GPH of the extra pulse and add that in as well.

P&G is more difficult with an automatic. I think that's where you're having trouble. My other vehicle is a V6 Odyssey minivan, automatic. I usually don't try to P&G it, choosing to DWL instead. It's worth 30+ mpg on the highway using DWL at around 60 mph.

Wow.. there's a little more to it than I expected.

PS Whoever moved this, thanks. I didn't think about it last night.

In my 4runner for highway P&G I've found (using my scanguage) that about 70% throttle up to 70 mph (in a 65 zone) before putting it in neutral and coasting down to 50-55mph (depending on traffic) gives me my best FE for using P&G. Of course, highway hills alter areas where I can P&G so I keep it around 57 mph on the highway and then Pulse up to 70mph when I see a long coast is possible on the downside of the hill or a long, flat stretch.

I used heavy P&G on a long road trip back in September (without a scanguage) and was able to pull 2 tanks of 32% and 39% over EPA. Have a Christmas trip coming up next week and now armed with my scanguage I'm actually looking forward to the 6 hour one-way trip to see what numbers I can pull (although with the colder temps I'd be surprised to match my September trip numbers).

Well, I'm not using my Civic to go up north, so P&G for me is limited to whatever I can manage in an Auto that I'd rather not risk breaking. This means that P&G will be more something like 70-55, but without ever hitting neutral. I'll do what my G-pa used to do, and gas it up without downshifting, then let it idle down. I'll see what kind of mileage I get from my wife's car, which averages 24-ish MPG mostly 10-15 minute trips in the country.

If I crack 27, I'll be happy with it.

Unless the transmission free-wheels when you let off the gas, and drops to idle rpm, you're wasting your time trying P&G in gear. DWL / steady speed is better. The momentum lost due to engine braking in gear is more than the gains. If it does free-wheel, then your job is easy. GM transmissions are good about this. Honda's are bad - they hold the gear and engine-brake your momentum into oblivion.

You will NOT damage the transmission by shifting it into neutral with the engine running. EOC might, but check your manual for flat-towing instructions.

Also, the higher your speeds, the smaller the benefit you'll see from P&G. 70 is too fast. Consider that the aerodynamic drag increases with the square of your speed. Your glides will be more like "plummets", instead.

It takes about 13 seconds to take the car down to 55 from 70 on a flat stretch, calm day, in gear. If I go over 70, the engine takes hold due to TC lockup.

70-55/13s is close to the 1mph/s decline I'd hope for, but the car is far from a hyper-miler's dream.

I could do 65-55, but I can't really see going under 55, simply b/c of the traffic in the areas that I'll be driving, and the cops that tend to patrol them. They don't mind a little speeding, but going under the speed limit frequently isn't permissable.

(Speed limit is actually 55, 70 is OK by them.)

I wasn't so much worried about whether using neutral will damage the tranny, as the thought that frequent shifting will cause fluid to overheat, and the transmission has it's share of quirks already when unlocking the TC.

In fact, frequent shifting is one of the more common reasons for Chrysler's plastic transmissions to fail. People would leave them in high gear on hilly roads, and allow them to shift into and out of OD as often as they would, which overheated the transmission, causing damage. The company caught a bad reputation b/c of their customer's ill-conceived driving styles.

How I would love to have a 55-limit highway. All my out-of-town trips are on 70mph highways, so you can guess how fast traffic goes... :rolleyes: Still, I can DWL at 60 mph with no trouble.

Ah, it's a chrysler. Yeah, I would just DWL that one, for its own safety.

pale...

i disagree that the higher the speed the less mpg gain u see from pulse and glide. my in gear sweet spot is 57 mpg according to my scanguage and at higher speeds it does go down from there.

however when i get it gradually up to 70 and neutral coast down to 55 the engine is working less in the 15 mph drop in coast than it would be starting the coast at 65. plus i coast farther going down from 70 which means im driving farther in neutral coast than starting glide at 65.

the golden rule is the longer the neutral coast the better the mpg gain. anytime the car is in neutral it is working less and using less fuel...as long as its above 20 mph for my vehicle...my scanguage confirms it.

ill also add this disclaimer...at least for my vehicle...

the mpg loss that occurs when i go above 57 to get to 70 is far exceeded by the gains on the coast back to 50. ive tried both ways on the same stretch of unobstructed highway and over a 2 mile stretch i saw more than 1 mpg gain by going to 70 instead of 65.

my rpms are only gradually affected to get to the extra 5 mph but i can coast nearly 1 mile on the flat stretch by getting up to 70

Matt, I'll not question your numbers, but I can't agree with your theory.
As Pale stated before, drag increases with the square of speed. At 70mph you are going only 7.6% faster than at 65mph, but your drag is 16% higher. By increasing your speed from 65mph to 70mph, you have to overcome additional growing drag for the whole time interval that you are accelerating. Once you switch to neutral your deceleration rate is greater between 70 and 65mph than between 65 and 55mph. The extra coasting time you get isn't enough to compensate for the extra fuel that went into accelerating over 65mph to 70.
What I remember from school physics says that P&G between 55-65mph should be better than between 55-70mph, BUT every car is different and your's just might have the right acceleration at the right speed and rpm to give you the figure you got. You definately have a slightly higher average speed when doing 55-70.

I totally agree about, "the longer the coast the better the mpg gain." It's all about maximizing the glide. I often pulse uphill so I can glide down the other side.

Higher speeds: Here's where we get into YMMV territory. My car, at 70 mph, is running 3200 rpm. There is NO WAY to make good mileage in that zone. Your automatic, I assume, runs a much lower rpm. That's half the battle right there. The other half is aerodynamics, which will still hurt you. The extra speed at the top end costs you more on the pulse, and gains you less on the glide. Whether the total is good enough, or bad, depends on the specific case - ymmv.

I find the best results in the 55-60 range in my Odyssey, dwl or nearly steady speed. I haven't been able to do better with any other techniques.

id like to say im smart in regards to aero but ill defer to you on that as i trust your knowledge more than mine...as far as my vehicle goes ive driven the same 2 mile stretch on my way to the mall probably 10 times with my scanguage.

i reset the sg current mpg as i passed an exit sign 2 miles from my mall exit going 57 and then pulsed to 65 and on another trip up to 70...ive done it at least 4 times. a 2 mile stretch is a small sample size but i got more than 1 mpg on all 4 passes in neutral coast and i coasted further from 70 than from 65...it seemed to make sense for me to start from 70 than 65.

the whole reason i tried it was to prepare for my 400 mile trip home next week.

Christ - you really need to add your vehicle(s) to the garage so people have context for your posts.

The technique I think you're describing in post #7 is what I once called "pulse and bleed", where the bleed is a very light load / high MPG gradual loss of speed (need instant mpg readout to maximize MPG while bleeding off speed). I've tried it on multiple occasions at different bleed rates in my car and was never able to make it work better than DWL at the same average speed.

Metro - Sorry about that, but I'd hate to add it, then get rid of it, and at this point, we're still unsure of what we're doing with either vehicle, hence the reason that not even basic mods have been made.

I suppose, at some point, it will become necessary to step back, take a look, and realize that I'm not benefitting anything by being undecided, and I either need to jump into something, or stick with what I have. There are large life-factors involved though.

The thing I'm asking about here, is essentially P&G - without using neutral. My wife's GP idles @ about 1200 - 1500 RPM while in drive - slightly higher than normal - when your foot is off the gas pedal, and you're coasting. The engine isn't dragging the speed down at this point, it's a GM thing, the engine idles higher while moving to compensate for potential load.. in case you hit the gas suddenly.

What I was wondering - regardless of speed - was if this is sufficient for a P&G situation where fuel mileage gains might be noted.

The problem with this, immediately noticeable, is that the TC locks to ~1500 RPM at 45-50 MPH, and doesn't go up much until around 70MPH, but under heavy accel, the TC won't lock, so I end up hitting engine speeds up to 3krpm during the acceleration, at only 30% throttle (to prevent down shifts).

Even if the TC does lock during acceleration (happens occaisionally), it unlocks as soon as there is no load - I let off the gas pedal to coast down - which means the next pulse speeds the engine to ~3krpm again, if it doesn't re-lock.

I'd love to control the locking phase of the TC manually, so I could accelerate at ~1500-2000 RPM up to 65-70MPH, then coast down at 1200-1500 RPM.

Make sense?

I'm going to try it out on the trip up to visit family, and if it doesn't work out, I'll just drive back with another method.

So Christ, it sounds like you don't have a ScanGauge, eh?

Not so much... one of my cars is non-OBD FI, the other is OBD-1 gen2 (not exactly OBD-1, not exactly OBD-2a either. It's a GM thing.)

Consequently, the SG won't do anything for me... MPGuino would, but monetarily, I can't swing it right now, especially w/ my wife having to drive Granny 1.5 hrs to school and back 4 days a week now... (carpool isn't available anymore.) More expenditures... I really hope this skool-ing thing works out for her.

Anyone here can feel free to send me one though! I wouldn't consider it out of line, or taking too much liberty AT ALL! LOL.

Was worth a shot. :turtle:

Ah, a Grand Prix - nice car. That's a great highway cruiser. Just go nice and easy on the gas pedal and it'll reward you. MUCH easier than many other cars. The transmission is eager to unlock when you let up on the gas, so you'll have little need to manually shift into neutral. Just keep it rolling. Stops are a killer on those. (actually, it's the start that follows...)

A friend here has a Buick with the same drivetrain. He gets ~25 mpg in town, but has come this close to 40 on highway trips.

Given that it's an automatic, I think your best hope is DWL in top gear with the torque converter locked. Without instrumentation, DWL is harder to do. You'd be surprised at what the slightest - almost imperceptible - changes in throttle position do to your current fuel consumption.

For DWL, a vacuum gauge would work fine.

Or wire a multimeter to your MAP sensor and aim for voltage readings that correspond to lightest load.

Pale - Tell me about the starting off again thing.. I try not to stop if I don't have to, b/c I can actually hear the car groaning under it's own fat backside when I hit the gas again. If we end up keeping this car, I intend to do something about THAT as well.

MetroMPG - Thanks for the tip, I'll have to see how this trip up north works out.. if it's pretty good, I'll probably invest in a Guino kit and go from there, since I can put it in a diff car later on. (PS I think I have a vacuum/boost gauge somewhere, actually.. :))