Pumping losses

[monroe74]

"Scenario 2 has no suction losses since you have WOT."

Earlier you said this: "3. Decelerate using same shift points as 2 with accelerator at WOT." I think you're getting your numbers mixed up. That's one reason I'm finding your example hard to follow, but it's not the only reason.

Anyway, I think your example reduces to the following much simpler comparison.

A) Take an engine that's not running, and spin the crankshaft, with the throttle closed.
B) Take an engine that's not running, and spin the crankshaft, with the throttle open.

I think you're claiming that A takes less work than B. Really?

"your conclusion about WOT operation assumes the BSFC map for a VX is identical to a map for a non lean burn engine"

No, I'm not claiming it's identical. I'm just claiming that there's enough similarity to make for a worthwhile comparison.

"the map you are using as a reference is not a VX engine map"

True. But I think it's helpful, as compared with using no map at all. If you have a map or reference that you think is superior, I hope you will give us a chance to see it.

I think it's clear that it's possible to save gas by utilizing lean burn, and it's possible to save gas by minimizing pumping losses. The interesting thing is that it's just not possible to do both of those things at the same time. Lean burn is generally associated with relatively small throttle settings. Trouble is, small throttle settings are generally associated with large pumping losses. Therefore it's necessary to make a choice. And the right choice probably depends on the immediate situation.

P&G is a technique that relies on relatively large throttle settings. And there seems to be lots of evidence that P&G works (and I think it works primarily by minimizing pumping losses). Trouble is, P&G is basically incompatible with lean burn, because in the pulse phase, you're accelerating. It's generally possible to maintain lean burn while at a steady speed, but not while accelerating. So we're back to that choice again, to either minimize pumping losses, or maximize lean burn. I have a hunch that the former strategy is more effective, when driving conditions allow for P&G (ideally in the range of 25-45 mph, I think). When driving conditions require a steady speed (especially at higher speeds, like 50+), then my hunch is that it makes sense to maximize lean burn. I'm not sure about this, and I'm open to evidence that I'm wrong. But I just haven't seen that evidence yet.

"Check your BSFC map at 50 % compared to higher throttle opening positions AT LOW ENGINE SPEEDS"

I have. It shows that WOT is more efficient than 50%. Not always by a wide margin, but nevertheless the margin is positive. Therefore it's probably worth taking. It also shows that at 1000 rpm, the margin is actually quite large. And 1000 rpm/WOT is a state I use quite frequently.

"You also assume that WOT is preferrable to 70%"

True, I do. I don't believe the margin is always large, but I believe the margin is always positive. That's what I see on the graph we're discussing. On the other hand, I think you assume that 70% is always preferable to WOT. I'm open to believing this, but so far I've seen no proof.

"You also choose to ignore the fact that if you maintain lean burn you are using about 60% of the fuel you would use at WOT"

I think the number might be even less than 60%. But what's more important than the air/fuel ratio is the work/fuel ratio. In other words, it's important to approach the analysis in terms of BSFC, not absolute fuel consumption.

"Thats why I use less than 50% throttle position, because I can pulse quite a bit longer for the same amount of fuel which increases the total distance of one P&G cycle."

If I understand correctly, you're using P&G, and you're claiming that you're in lean burn during the pulse phase. May I ask how you know? My experience using a DMM to monitor my O2 sensor leads me to believe that lean burn generally does not occur during acceleration, even mild acceleration. And this is consistent with the comments of others who have monitored lean burn in this manner. How are you monitoring lean burn?

"I use the sil indicator to determine throttle position when I accelerate. It takes me 13 seconds in 5th gear to accelerate from 50 to 65 MPH, so you can duplicate that time and know exactly the amount of throttle I am using."

I'm very confused by that paragraph, because the SIL is off 100% of the time that you're in 5th gear, regardless of vehicle speed, engine speed, or throttle position.

"I only pulse in 4th gear when my initial speed is below 30 MPH and the light never comes on during the pulse."

If you are in any gear other than 5th, and using a throttle setting large enough to keep the SIL turned off, then you are almost certainly not in lean burn.