Neutral to drive?

[palemelanesian]

600 mile highway trip last week in the Odyssey minivan. Rated 18/25 (old epa). 6 people on board, camping gear, and AC the whole time. Neutral coasting played a big part in the 28.5 mpg average for the trip.

[theholycow]

Were you doing those glides in 4wd? Unless that's the case, it's not possible for your neutral glides to affect it. It's separated from the transmission by the transfer case. With the transfer case in 2wd the front diff sees no difference between EOC, neutral coasting, cruising, and accelerating.

That front diff might be prone to such problems. What are the symptoms? My 2002 GMC Sierra front diff howls when I use 4wd. That's after 170,000 hard miles.

[sonyhome]

Neutral coasting with my CR-V saves fuel as per the ScanGage (no matter how off it is, I do arelative comparison of SG data in D vs SG data in N, so it is valid). Also SG seems about correct for MPG when calibrated right.

Coasting in "D" has 2 disadvantages on flats or light grades:
- Higher RPM (no cutoff for me since I don't get a 9999reading)
- Higher rolling resistance meaning shorter coasting

When I drive downhill on light grade, I coast in "D" to maintain constant speed.

When I coast on steeper hills I downshift to "2" or "1". However those incur higher RPMs and greater gas consumption (up to 1.2GPH if I recall right) so braking in D would be more efficient there, but would burn the brakes.

The Del Sol which has much less roll resistance coasts farther so I can milk coasting much more in city driving.
There I'm thinking cutting off the engine would save a lot of gas because the engine would be off like 50% of my commute distance. However I'm not willing to risk that even though it is a stickshift car.

What I have not figured out yet is what is the most efficient way to accelerate (soft, semi-hard, hard acceleration). I believe slow accelerations are not F/E. I want to go in coast mode ASAP, and also I believe slow accel at low engine RPMs is not efficient because you want to accelerate in the car's highest torque RPM range. Should start a thread on that!


BTW, N coasting in A/T does bug a lot passengers not used to this weird driving behavior.

[dosco]

Originally Posted by sonyhome

What I have not figured out yet is what is the most efficient way to accelerate (soft, semi-hard, hard acceleration). I believe slow accelerations are not F/E. I want to go in coast mode ASAP, and also I believe slow accel at low engine RPMs is not efficient because you want to accelerate in the car's highest torque RPM range. Should start a thread on that!

I am wondering the same thing with my automatic transmission Camry. My initial P&G tanks - where I tried accelerating "hard" to about 2000 to 2500 rpm followed by glide - were disappointing.

Since then I've been accelerating gently. On the highway I accelerate by trying to keep rpms at or below 2200 rpm. When I get to my desired cruising speed, I start to P&G, with pulses fairly gently by trying to keep max rpm at or below 2200 rpm.

So far the results are promising, although I've been interested in running some math/physics to gain more understanding of the problem.

[sonyhome]

Originally Posted by dosco

I am wondering the same thing with my automatic transmission Camry. My initial P&G tanks - where I tried accelerating "hard" to about 2000 to 2500 rpm followed by glide - were disappointing.

Since then I've been accelerating gently. On the highway I accelerate by trying to keep rpms at or below 2200 rpm. When I get to my desired cruising speed, I start to P&G, with pulses fairly gently by trying to keep max rpm at or below 2200 rpm.

So far the results are promising, although I've been interested in running some math/physics to gain more understanding of the problem.

You are driving an A/T camry, so it probably tries to be FE by accelerating in the higest gear (aka straining the engine).

Have you tried to force a downshift with a kick acceleration, then release the pedal right away to accelerate only at the right amount?
Neither flooring or slow accel IMHO are the answer. It's in between.
BTW I started a thread on that topic here:

How to accelerate in P&G mode?

[dosco]

Originally Posted by sonyhome

You are driving an A/T camry, so it probably tries to be FE by accelerating in the higest gear (aka straining the engine).

I control the engine, light on the throttle causes upshifts, harder on the throttle causes downshifts.

The point I was trying to make is that I'm not sure if it is more FE to accelerate with light throttle or heavy throttle. On my work commute, I tried heavy throttle at short duration (i.e. attempting to maximize BSFC) and got about 28 mpg (which isn't much better than my normal driving) and with light throttle and paying attention to keeping rpm at or below 2200, FE is now about 32 mpg.

Saw your thread.

Interesting to note Pale's comments that indicate light throttle to highway speed, followed by P&G with heavy throttle.

[sonyhome]

That's a 15% FE gain... For your camry I guess light is better! That's why camrys are always so slow on the road :-)

One reason may be that A/T could loose a lot of the energy in the slushbox with hard acceleration.

[theholycow]

Originally Posted by dosco

Saw your thread.

Interesting to note Pale's comments that indicate light throttle to highway speed, followed by P&G with heavy throttle.

I saw it too. I disagree with Pale with one caveat. If and only if you can control your shift points, heavy throttle is still better, regardless of whether you're accelerating to cruising speed or already there and pulsing before a glide.

With an automatic that doesn't give you a good interface/good control (or whose torque converter is too lossy) light or medium acceleration wins easily.