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Christ 12-17-2008 12:20 AM

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.

PaleMelanesian 12-17-2008 11:38 AM

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.

mobilerik 12-17-2008 12:19 PM

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.

PaleMelanesian 12-17-2008 12:26 PM

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.

Christ 12-17-2008 01:10 PM

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

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

Matt Herring 12-17-2008 01:46 PM

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).

Christ 12-17-2008 01:52 PM

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.

PaleMelanesian 12-17-2008 02:00 PM

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.

Christ 12-17-2008 02:06 PM

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.

PaleMelanesian 12-17-2008 02:11 PM

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.

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