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Old 05-17-2008, 04:50 AM   #31
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Originally Posted by monroe74 View Post
I'd like to understand what you did, but I'm having trouble following this.
You have the links to the SFC and BMEP isopleth charts? The links are on this board, Pale originally posted them, I reposted in other threads.

You will note on the SFC charts that there are a number of iso curves, each iso curve represents a particular engine load - 25%, 50%, etc. The curves show that each load has a different efficiency, and that 100% load is most efficient.

The next step is to realize that the units of SFC are lb/hp-hr. So if you know what SFC your engine is operating at and how much hp is being produced, you can calculate the fuel flow rate.

I used the SFC chart for a "typical 2-valve 4 cylinder engine" and was able to find a dyno run graph for my car. Multiplying the hp number from the dyno chart with the SFC number that corresponds with a particular engine RPM yielded fuel flow rate. I calculated points on each iso curve to compare flow rates.
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Old 05-17-2008, 05:38 AM   #32
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Originally Posted by dosco View Post
Fair enough, but the next logical question: what device out there actually measures fuel flow in passenger cars?
That's under heavy discussion in the "realtime monitoring tools for pre-OBD cars" thread.

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Originally Posted by monroe74 View Post
Sorry, I know I'm a major offender in this regard. I should realize it's probably annoying. I'm just so used to doing it the old-fashioned way, but I'll give it a try.
Thanks, it's much easier for me (and I hope everyone else) to follow.

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I think one of the interesting things about open-loop is that it's something my car generally doesn't do, as far as I can tell. I think a characteristic of wideband-sensor systems is that open loop occurs only when the engine is cold, or when there's some kind of a sensor failure.
Why wouldn't it happen when you go WOT at high RPM?

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With my DMM, I can monitor my O2 sensor directly, and the response is immediate. That is, I can see AFR changing right away, as I move the throttle.
So, if I tap a wideband O2 into my exhaust, I can hook it up to a DMM and have a realtime AFR gauge? I don't think I'd risk it in my leased VW but I'll do it in the vehicles I own.

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But you're reading info indirectly, with Vag-Com getting the data from your car's computer in OBD format (I think that's how it's working; I could be wrong). That's a more complicated process, so it's harder to tell where the delay is being introduced.
It might be using OBD, or it might be using VW's proprietary protocol, I'm not sure which one it's using. You're right, with these systems being effectively black boxes to me, I can't tell where the delay is.

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I think coasting in neutral is a very powerful technique. I think it's easy to underestimate the drag created by coasting in gear, and I think this drag probably overwhelms the benefit of DFCO.
Well said! It never seemed like much drag, but in neutral it becomes obvious that it was. Now I use DFCO when I'm planning on slowing anyway, and neutral when I coast. The fuel cost to idle is less than the momentum lost using DFCO.

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EOC is great, but it requires very specific driving conditions.
My commute has a few places perfect for it. I tried it once and got three angry beeps from my car, now I'm afraid of how it could bite me in the *** in the event of a warranty claim, so I won't do it again until I can find out what the three beeps meant. Unfortunately, when I asked on a VW forum, all I got was moral judgement and hate; they were more interested in why I did that than why it beeped. There's another bigger forum where I'm not a member, but I might join to ask.
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Old 05-17-2008, 05:53 AM   #33
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Originally Posted by monroe74 View Post
If you had an instrument reading your Throttle Position Sensor, that would be a different story.
Hmmm now that's an idea... that or the MAP reading...
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Old 05-17-2008, 06:02 AM   #34
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Originally Posted by theholycow View Post
That's under heavy discussion in the "realtime monitoring tools for pre-OBD cars" thread.
Thanks for the heads-up. I'll check it out.
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Old 05-17-2008, 07:35 AM   #35
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Originally Posted by monroe74 View Post
You're claiming, literally, that there's "more resistance at WOT." I have a lot of trouble grasping how we create "more resistance" when we remove a restriction that is preventing air from entering the engine.

If you were correct that there is "more resistance at WOT," then it would be possible to maximize engine braking on a gas engine by using WOT (with the injectors off, of course). With current drive-by-wire systems, this could be easily accomplished. I wonder why no one has ever built an engine that works this way. Then again, maybe you know of one?
I can see what you guys mean about more resistance at WOT than with a closed throttle. We know that a vacuum pump will only give so much resistance, a compressor can continue to increase in the amount of resistance offered depending on the amount of pressure it needs to put out.

It makes sense that there would be more 'loss' in an engine at WOT if and only if we are talking about an engine that isn't running at all but being turned by an outside source. If the engine is running it's completely different. We all know that raising compression makes for better fuel economy and more power overall. It's the main reason diesels are so efficient.

As for making a throttle-by-wire system that engine brakes at WOT, they won't do that because all that cool air in the exhaust would cool the catalytic converter(s).
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Old 05-17-2008, 09:04 AM   #36
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Originally Posted by theholycow View Post
So, if I tap a wideband O2 into my exhaust, I can hook it up to a DMM and have a realtime AFR gauge? I don't think I'd risk it in my leased VW but I'll do it in the vehicles I own.
...and does anyone have any suggestions where I could get wideband O2 sensors cheap? Searching Google Products and eBay it looks like it will cost me ~$70 shipped.
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Old 05-17-2008, 02:00 PM   #37
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Originally Posted by dosco View Post
I calculated points on each iso curve to compare flow rates.
OK, thanks for explaining, and I understand all that. But I still don't understand something you said earlier: "In other words, at 2,000 RPM on the 50% load line I can use less fuel per hour than at 2,500 RPM at 100% load."

Trouble is, your fuel consumption (per hour) has decreased, but your engine's output has decreased even more, which means you achieved less speed and/or distance.

It's always possible to save fuel by reducing speed and/or distance. But when we optimize BSFC, we can save fuel without reducing speed and/or distance. So I'm trying to understand why you decided that 50% throttle is superior to 100% throttle.
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Old 05-17-2008, 02:00 PM   #38
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Why wouldn't it [open-loop] happen when you go WOT at high RPM?
Because in a wideband-sensor system, the sensor is able to continue to provide the ECU with useful info even in those kinds of extreme operating conditions. Therefore there is no need for the ECU to ignore the sensor (and that's what open-loop means).

Don't get me wrong; WOT/high RPM in a wideband-sensor system is still going to mean a rich mixture. It's just that it's a rich mixture arrived at by an ECU that's reading the O2 sensor, rather than an ECU that's reading values from a stored internal map (that's what happens in open-loop mode).

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So, if I tap a wideband O2 into my exhaust, I can hook it up to a DMM and have a realtime AFR gauge?
Yes. But keep in mind there are a bunch of commercial products directed at people who are asking the question you just asked. So you should look around and see what's out there before you reinvent the wheel. Try this google: wideband gauge afr.

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I don't think I'd risk it in my leased VW but I'll do it in the vehicles I own.
You're not likely to hurt the vehicle. Aside from making a hole in the exhaust system to mount the sensor.

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with these systems being effectively black boxes to me, I can't tell where the delay is.
Exactly. If you really want to monitor AFR in realtime, there are more direct ways of doing it. On the other hand, you might discover that your current monitoring system has no major lag, and your ECU is itself inducing the lag, for whatever good or bad reasons.

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Now I use DFCO when I'm planning on slowing anyway, and neutral when I coast. The fuel cost to idle is less than the momentum lost using DFCO.
Exactly. I think people tend to misunderstand that, and treat DFCO as some sort of secret magic trick, and they overuse it. DFCO is to be used only when your environment is forcing you to slow down. If you're in a situation where you would not use brakes, then you shouldn't be using DFCO, either. Rather, you should be maximizing your glide, via either coasting in neutral, or EOC. (Obviously I'm using the term DFCO as shorthand for "coasting in gear;" it actually stands for Deceleration Fuel Cutoff.)

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I tried it [EOC] once and got three angry beeps from my car
Interesting. At what point did you hear the beeps? What was the position of the ignition key at that moment? Was the engine running? Car in motion? Just curious.

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does anyone have any suggestions where I could get wideband O2 sensors cheap? Searching Google Products and eBay it looks like it will cost me ~$70 shipped.
'Cheap' is a relative concept. The wideband sensor required by the VX is $334 at Autozone, some much higher amount at the dealer, and usually about $150 on eBay.

There's a cheaper wideband sensor made by Bosch. I think that's what you're seeing for $70. I would be surprised if you ever see a wideband sensor for much less than that (aside from used ones with a questionable life expectancy).
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Old 05-17-2008, 02:00 PM   #39
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Hmmm now that's an idea [monitor the TPS]... that or the MAP reading...
I realize I brought up the idea of monitoring TPS. But it would be more direct to monitor AFR. And something more direct than monitoring AFR (and also more direct than monitoring MAP, for that matter) is monitoring actual instantaneous fuel use.

A certain throttle setting will lead to a certain AFR, which, in turn, will lead to a certain level of flow through the injectors, which, in turn, will lead to a certain level of money flowing out of your wallet. There are lots of different things we can monitor, but some are more relevant than others.

I was being a bit ironic when I hinted that Gary could use a TPS monitor. What would probably be more useful (in his particular situation, given his interest in maximizing lean burn) is an AFR monitor, which he could achieve for about $4.
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Old 05-17-2008, 02:01 PM   #40
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It makes sense that there would be more 'loss' in an engine at WOT ... [if the] engine ... isn't running at all but being turned by an outside source.
Assuming that the engine is being turned by an outside force, why is there more loss at WOT? I think you and Gary have both tried to explain this, but I understand neither explanation.

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As for making a throttle-by-wire system that engine brakes at WOT, they won't do that because all that cool air in the exhaust would cool the catalytic converter(s).
But if WOT somehow creates more loss (which I don't comprehend), doesn't that mean that less cool air, not more, would be leaving the engine?
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