Originally Posted by 1993CivicVX
avoid full enrichment mode for an entire tank
I didn't see this thread until a few minutes ago. I didn't even realize there was an Experiments section! But I guess great minds think alike, because yesterday I did the experiment you thought of (not for an entire tank, but at least for an entire fill). I was prompted by discussion here: http://www.gassavers.org/showthread.php?t=7400
. In particular I was thinking about the relative merits of maximizing lean burn vs. minimizing pumping losses. (My car is a '95 VX; eventually I'll get it into the garage.)
Yesterday I needed to visit a town about 100 miles away. Mostly highway. I haven't been doing much highway lately, so I thought this would be a good chance for an experiment. I had two passengers along with me (both ways).
I filled before leaving very early in the morning (call that the start of Leg A). In the afternoon I filled again, after I started back home (call this fill the start of Leg B). Then I filled again at home.
For Leg A, I used my normal P&G technique. This means alternating WOT with coasting in neutral. (Normally I use some EOC, but in this experiment I excluded that. I also excluded drafting.) When I'm doing this P&G routine, I try very hard to avoid any throttle setting other than WOT (and zero, of course). I used a speed range of 50-65 mph.
I'm using a DMM to monitor AFR. During WOT, there's obviously full enrichment. On my car, that means a reading of about -0.8v. So that's the reading I saw for pretty much the full trip (except when I was coasting, of course).
The duration of lean burn during Leg A was probably close to 0%. Aside from idling while coasting, of course. That produces a reading of about +0.2v.
For Leg B, I drove very, very differently. I tried to always use the smallest possible throttle opening. I never got anywhere near WOT. I had my eyes glued to the DMM, trying to maximize lean burn. I did this by using the smallest possible throttle opening that would maintain 57 mph. I would let my speed drop to 50 mph (which would happen going uphill, obviously), but not below (to create a fair comparison to Leg A).
Going downhill, I did some coasting in neutral, when the grade was steep enough to make it worthwhile. But mostly I tried to maintain a steady speed, with the smallest possible throttle opening.
Maximum lean on my car produces a reading of about +0.8v. For about 80% of Leg B I was looking at a reading of roughly +0.6v. Obviously there were a few moments of richness, but they were very brief, and I think never beyond -0.3v.
Leg A yielded 55.8 mpg. Leg B yielded 51.2 mpg.
I obviously would have done better (on both legs) without the two passengers, and if I had been willing to go slower. Maybe a maximum of 60 mph instead of 65.
I find the results interesting, even though it's only one trial, and even though 200 miles (round-trip) isn't a huge distance, and even though I used two different pumps. It was the same car, same driver, same passengers, same route, same day. And basically the same speed.
By the way, the ambient temp was about 20 degrees higher for Leg B.