Accelerating and shift points

[PaleMelanesian]

From a stop, I use light-light-light acceleration. Maybe 1 second per 1 mph. Also, I'm shifting BEFORE 2000 rpm, often at 1800. That drops into the next gear at 1300-1400 rpm.

Now when I'm at speed and doing Pulse & Glide, it's a different story. Then I'm using up to 3/4 throttle and up to 2500 rpm before shifting. (although, I usually do the pulse in a single gear, no shift needed)

[tasdrouille]

I there any reason the acceleration coming off a stop should be any different than a P&G acceleration?

[PaleMelanesian]

Well, if your MPG during that acceleration stays high, there's no hit to the average. Yes, the engine is in a different, less-efficient mode than a higher rate. But overall, you're minimizing the fuel used. Just use no more than the absolute minimum. Running the engine at it's peak BSFC point is secondary.

It was a suggestion from Wayne on our cross-country hybrid vs diesel shootout. He uses this style all the time, and I've been playing with it since. Right after that trip, I posted my best tank ever, and this one is looking to be even better.

[trebuchet03]

Originally Posted by PaleMelanesian

Well, if your MPG during that acceleration stays high, there's no hit to the average. Yes, the engine is in a different, less-efficient mode than a higher rate. But overall, you're minimizing the fuel used. Just use no more than the absolute minimum. Running the engine at it's peak BSFC point is secondary.

While cruising, keeping around BSFC is secondary (unless your engine is small enough to do so)... While accelerating, it's primary.

Don't look at your average mpg while trying to figure out acceleration (effectively instantaneous) optimal. Miles/Gallon does not include a time variable - so trying to incorporate time (by watching average) will only bring false confidence. My average will stay really high if I've been driving for a long time compared to when I accelerate just after start up - but in both of those cases, it doesn't change my instantaneous fuel efficiency in the acceleration event.

Shoot for peak thermal efficiency while accelerating - then peak specific consumption while cruising (which will likely not be peak thermal). Then later, when you consider the time variable - your figure will be higher.

I there any reason the acceleration coming off a stop should be any different than a P&G acceleration?

Nope Pulse and glide just sounds better than accelerate and coast (I guess :P).

[PaleMelanesian]

Originally Posted by trebuchet03

While cruising, keeping around BSFC is secondary (unless your engine is small enough to do so)...

And none of our engines are that small, so... P&G is all about keeping the engine at its peak BSFC, or not running at all. The average is higher than running at steady-state and lower BSFC efficiency.

Originally Posted by trebuchet03

Don't look at your average mpg while trying to figure out acceleration (effectively instantaneous) optimal. Miles/Gallon does not include a time variable - so trying to incorporate time (by watching average) will only bring false confidence. My average will stay really high if I've been driving for a long time compared to when I accelerate just after start up - but in both of those cases, it doesn't change my instantaneous fuel efficiency in the acceleration event.

Of course, the same acceleration will affect your average less if you're averaging over a longer distance. That doesn't change the actual fuel consumption during that accel. I'm comparing the same points on my daily drive from one day to the next using different methods - apples to apples. At this one point, about 5 miles into my drive:

Light acceleration will bring the average down by maybe 2mpg vs 4mpg at heavier accel. The faster accel will get you up to speed 100-200 yards sooner. After 5 miles, you just can't make up that 2mpg difference in 200 yards, even if you EOC.

Here's my theory: If you could have the ideal situation and coast all the way to zero at the end of the drive, or the next light, or whatever you stop, then the higher accel rate would work. But if you use your brakes at the other end, at all, then you can't make up the deficit.

Originally Posted by trebuchet03

Shoot for peak thermal efficiency while accelerating - then peak specific consumption while cruising (which will likely not be peak thermal). Then later, when you consider the time variable - your figure will be higher.

I think we're trying to say the same thing here.

[tasdrouille]

I did a quick test on a .6 mile flat stretch between a red light and a stop sign. Every run was made in the same direction of the stretch.

"A" runs (Granny runs): Slow pace, egg under foot acceleration, shifting at 2000 rpm and finally resuming CC at 40 mph in 5th gear.

"B" runs (Jackrabbit runs): Quick acceleration (75% LOD), shifting just over 3000 rpm and finally resuming CC at 40 mph in 5th gear.

A1 39.1 mpg
B1 45 mpg
A2 39.8 mpg
B2 44.7 mpg
A3 39.5 mpg
B3 45.1 mpg

"A" runs average: 39.47 mpg
"B" runs average: 44.93 mpg

Test for yourself, you'll see.

[PaleMelanesian]

Thanks for that. I like seeing hard evidence. I'll try it out myself and see what I find. Was that in the Elantra or the TDI?

One thing about that test is that you combined a different load and different rpm shift point. I'd be interested in separating those out.

[tasdrouille]

I did the test with the elantra.

I tested low rpm / low load and high rpm / high load. I agree should have tested low rpm / high load and high rpm / low load also, but I ran out of time.

[trebuchet03]

Originally Posted by PaleMelanesian

Light acceleration will bring the average down by maybe 2mpg vs 4mpg at heavier accel. The faster accel will get you up to speed 100-200 yards sooner. After 5 miles, you just can't make up that 2mpg difference in 200 yards, even if you EOC.

In 5 miles you sure can But, I don't know what "after 5 miles...200 yards" means....

Here's my theory: If you could have the ideal situation and coast all the way to zero at the end of the drive, or the next light, or whatever you stop, then the higher accel rate would work. But if you use your brakes at the other end, at all, then you can't make up the deficit.

That is ideal - but the recovery is much faster than 1:1 as suggested.. It's situations like that that makes me hate the mpg metric - it's not really appropriate here and just leads to confusion....

[PaleMelanesian]

I wrote and rewrote that, and it still came out confusing. Let me try again:

After 5 miles of driving and building up an average, then I have this stop and an acceleration. The faster accel drops the average more, but arrives at speed sooner. Once arriving at the point where the slower accel would have been up to speed (200 yards farther along), the average is still lower than the slow acceleration would yield at that same point.

But I want to test some more to be sure...