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Old 07-19-2010, 07:59 PM   #1 (permalink)
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Default Using ScanGauge to find ICE sweet spot

When you're getting up to cruising speed, obviously low revs and high throttle are good for efficiency. A wider throttle opening reduces pumping losses and increases power production to get you up to speed faster. But how far can you open the throttle without the ECU deciding to enrich the air-fuel mixture?

Here's a flawed BSFC map that at least captures the trends correctly. Note the very small number of data points in the area of interest (especially near 3500rpm, 60N-m), and the lack of lean burn.


The ScanGauge's LBN xgauge monitors the voltage of the wideband O2 sensor. It appears that LBN around 80 indicates stoich, below 60 indicates lean, and above 85 or so indicates rich.

I watched LBN while accelerating in 5th gear from 45-74mph. Up to TPS = 65, I didn't see much change in LBN, but above TPS=70, LBN rose measurably.

I propose that, at least for my car with IAT=120F and in the RPM ranges I tested, TPS around 60-65 is the right amount of throttle. Can anyone confirm this? Are there any other indicators of the ECU throwing FE out the window at high TPS, such as IGN?



I'm assuming you've decoupled IMA and ICE with CARD or MIMA. Otherwise the correct strategy is to stay out of assist however you can.
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Old 07-19-2010, 08:20 PM   #2 (permalink)
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Quote:
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But how far can you open the throttle without the ECU deciding to enrich the air-fuel mixture?
Very interesting Diagram Robert. How did you capture the info?

If the obejctive is to get upto crusing speed as fast as possible and stay in lean burn then i think the answer is simply "Not very far"

However as lean burn develops very little power it takes an age to get up to speed anyway.

Are you better off in these circumstances in a high gear to use WOT and forget about lean/rich and just get upto speed asap? The sooner you reach cruise speed the sooner you can back off into lean burn mode.
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Old 07-20-2010, 12:41 PM   #3 (permalink)
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I don't think WOT is the answer in any circumstance for high MPG.

Great job on the chart Robert. I'm going to have to come back later and make sense of it .
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Old 07-20-2010, 01:18 PM   #4 (permalink)
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I don't think WOT is the answer in any circumstance for high MPG.
WOT doesn't mean high revs, although if you need the power and are trying to avoid the regen when you hit the highway or when you turn on the lights, and are trying to avoid assist, it's the only option if everything is stock.

When figuring out the nuances of high MPG driving with my Prizm, I managed to get low 40's MPG fairly consistently on a car EPA rated for 34mpg Highway. For that car maintaining a speed above where the engine produced smooth power(which was 1600rpm for the Toyota 1.8 liter 7AFE) and shifting whenever the shift would put me at that particular engine speed and using as much throttle as would provide maximum power would produce the best results with all of the experimentation of different ways of driving that I could come up with which wasn't WOT but rather the maximum power point on the throttle or basically whatever the engine would suck on its own subtract a tad off the throttle. I pulse and glide that vehicle for anything under 50mph or anywhere where there was a downhill was something that I pretty much always do with that car because it has gobs of low end torque since it is more undersquare(long stroke compared to bore size) than the Insight is.

I'll do the same with the Insight, as soon as I search for which wire to splice to where for the clutch switch. It should produce better results than the ~85% load that I can pull while avoiding assist.

RobertSmalls, Don't forget that TPS is independant to the pressures in the engine, if you are at 2000rpm, the difference in 70TPS and 89TPS are minimal because the engine can't actually pump more than what the cylinders allow. The difference between half throttle and full throttle at 1500rpm might not even be noticeable due to this. The load is dependant on engine speed and throttle position together due to the amount of air that the engine is capable of pumping. Holding a solid number of TPS such as 50 or 70 isn't going to produce good FE unless you ramp up your TPS with engine speed to maintain the appropriate amount of load or engine pressure levels.

Try paying attention to load and the LBN value together, they will probably be more linear in comparison to TPS at varying engine speeds.
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Old 07-20-2010, 05:02 PM   #5 (permalink)
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Very interesting Diagram Robert. How did you capture the info?
I posted that BSFC plot to the BSFC Wiki at Ecomodder a while back. Iirc, I pulled it out of a research paper, maybe from the national laboratories? I've seen it at IC before. Maybe someone can name the source.

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Are you better off in these circumstances in a high gear to use WOT and forget about lean/rich and just get upto speed asap? The sooner you reach cruise speed the sooner you can back off into lean burn mode.
Check out the top two data points at 2600RPM. You'd be hard pressed to make up the indicated 16% increase in BSFC by travelling a greater distance in LB. Like Arctic says, WOT is pretty extreme. While LB is the real sweet spot, I'm looking for the semi-sweet spot that all engines have at moderate RPM, high throttle, before you press the throttle hard enough for the ECU to deprioritize FE. Here's a Ford Zetec 2.0L that has the same shape BSFC plot:



@MNDriver: I've got LOD as my fourth gauge now, so we'll see what I can figure out. Too bad LOD is so non-linear and sensitive.

Am I the only one who can't get much use out of the IGN gauge?
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Old 07-21-2010, 02:07 AM   #6 (permalink)
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I haven't really figured out what to do with IGN, it seems whenever you give it heavy throttle the number drops down and even goes negative, but the data points mean nothing to me. ...even more in the sense that I do what I can to operate at lean burn whenever I can, according to some stuff I ran into when searching about it. Things are supposed to be more efficient with more advance but if you are trying to maintain speed and use whatever throttle you need to do that but not enough to drop out of lean-burn, I don't see how to use it, clearly there is more power output vs. fuel used(better efficiency) at 100mpg than 70mpg under lean burn but it is hard to determine. It seems I get a sweet spot of power at load values under 75 and even more going to 60 but going below 70 on the highway and I lose speed most of the time unless the air is very calm and all other conditions are ideal.

I'm interested to see what others think about this too. At this point it seems that for constant speed cruising, holding lean burn and using less load seems to have better efficiency even though I would figure more load would be better but the MPGs drop without as much return in perceived power at roughly 90mpg. It's hard to calculate the difference to make an efficiency statement considering there is an unknown amount(since I don't know how to measure it) of extra power per the extra "28.5% of additional miles to that gallon" when getting 90 versus 70.

If it is indeed more efficient, I'll be happy to know this with MIMA as I provide a constant current proportaional to my highway distance(and during acceleration and to help pull in lean burn without losing too much speed) to hold the MPG numbers higher and grid charge back at home. Not sure if it would help much without a parallel pack or how well it will handle discharging half the pack over 20 minutes repeatedly for each highway leg for both directions, another thing alleviated a little by a parallel pack splitting current in half.
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