Pumping losses

[dosco]

Originally Posted by R.I.D.E.

I believe the sil light uses vacuum as its principle imput (not the only imput), to show the driver when upshifting will create a greater load and lower vacuum to produce the same work, which I think we can agree is more efficient.

I think this brings up a good point. Note that the SFC chart indicates when the engine is operating at maximum efficiency. This is not the same as operating at minimum rate of fuel flow. This explains why at certain load+RPM combinations (e.g. optimum efficiency on the BSFC graph), the engine is burning more fuel than at idle (note comments by Pale in other threads about fuel use at idle).

I understand the concept of SFC, but after a few tanks of gas and use of P&G but no ridiculous improvements in fuel mileage, I was wondering what the hell was going on. Then it occured to me that the isopleths on the BSFC graph (that I was trying to emulate in my P&G technique) are of fuel flow rate per hp produced. Which I had read but didn't "compute" in my mind.

I can't directly determine what the BMEP of my engine is at any given time, so I goto the SFC vs RPM chart with the load curves. I was able to find a dyno run for a 4 cylinder Camry (believe it or not), so to get an idea of fuel flow rate I plugged in the hp number for given RPM numbers for different load lines. Lo and behold that at less-than-ideal RPM and throttle settings, I can easily reduce the rate of fuel flow. 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.

Therein lies the point - to minimize fuel use. I can maximize efficiency by operating in certain load+RPM combinations, but in doing so can actually use more fuel in total. And the point is to use the least amount of fuel for the most amount of miles - this is why idling (which is less efficient) can improve you rmileage.

I'm not sure I can hit crazy MPG numbers by worrying about throttle angle and RPM alone - the other variable is the HP created. This defines the fuel used.

<sigh>

I think I need a scangauge.

[theholycow]

Originally Posted by dosco

I'm not sure I can hit crazy MPG numbers by worrying about throttle angle and RPM alone - the other variable is the HP created. This defines the fuel used.

<sigh>

I think I need a scangauge.

Unfortunately, as I found out in another thread, the SG doesn't actually measure fuel flow and some of the other important variables it displays. Instead, it calculates them. Be aware of that as you consider buying it for precision measurement of such things.

[Dalez0r]

As for the leanburn vs P&G argument...


To me, the answer seems simple. Leanburn is obviously there to compensate for poor engine efficiency at light throttle by leaning the engine, which reduces power, which requires you to give it a bit more throttle to maintain power... which reduces pumping losses in addition to being a slightly more efficient burn. But, it also seems obvious that the engine produces more output energy per lb of fuel at high load (low rpm obviously)

So, the answer is to use them according to driving conditions. If you can P&G, DO IT! If you're not in a position to do so, you may need to DWL for a while, and leanburn is better for that than non-leanburn.

[dkjones96]

Honestly, thermal efficiency is by far the biggest player in all of this.

At part throttle there isn't much air in the chamber. That small amount of air doesn't have a lot of heat in it when you ignite it so more of the heat goes to the cylinder walls instead of pushing the piston back down, it burns slower because it isn't as dense making for timing advances that rob the engine of efficiency and at part throttle it's working against a vacuum, a partial parasitic loss only if there is enough vacuum in the chamber left to pull the piston up on the bottom portion of the compression stroke.

At WOT you have way more air in the chamber. The larger amount of air has much more heat that would have to be absorbed into the engine, it does get absorbed but the ratio of heat created to amount absorbed plummets as engine speed and throttle increases. Since there is more heat in the chamber the pressure stays higher longer and can actually perform work. The air is very dense when you get close to the top of the compression stroke and results in a faster, better burn requiring less timing advance and increasing efficiency. At WOT you have MUCH less loss from pumping on the intake stroke but you end up with more losses on the compression stroke as has been mentioned, however, when you are compressing the gases you are building potential for the event happening in the power stroke. Not just throwing it away.

[dosco]

Originally Posted by theholycow

Unfortunately, as I found out in another thread, the SG doesn't actually measure fuel flow and some of the other important variables it displays. Instead, it calculates them. Be aware of that as you consider buying it for precision measurement of such things.

Fair enough, but the next logical question: what device out there actually measures fuel flow in passenger cars?

[monroe74]

Originally Posted by theholycow

Just a guess.

I think you're good guesser.

please use the built-in quote feature

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.

I'm avoiding open loop.

OK, that's what I figured. Thanks for explaining; I think you explained it very well.

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.

But I understand why avoiding WOT makes sense in your situation.

I can't tell if it's lagging a few seconds in its readings or if it really doesn't go into open loop easily

Interesting question. I don't know what that lag is about. 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. 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.

Coming out of open loop, I could see how your car's brain (ECU or ECM or whatever VW calls it) could choose to lag and hold open loop for a few extra seconds, for good reasons. But I would think that going into open loop would be immediate, when you floor it under certain conditions. After all, the idea is to give you good throttle response, and provide the power you're demanding. Also to prevent an overly lean mixture from hurting your motor. So it should happen without a lag, I think.

I have been tried P&G (neutral, not EOC) and it looks promising.

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.

EOC is great, but it requires very specific driving conditions.

[monroe74]

Originally Posted by R.I.D.E.

more resistance at WOT

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.

Think about the Jake brake on big rigs, they are using trapped compression, not vacuum, obviously they can't use vacuum

Yes, a diesel can't create engine braking with vacuum, because it has no throttle. Engine braking in a gas engine is the result of the engine fighting the vacuum created by a closed throttle.

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?

Aside from all that, I have no idea what point you're trying to make by mentioning diesels and Jake brakes.

in my previous example with 185 pounds of compression you have over ten times the resistance that 14.7 negative pounds of vacuum would ever produce

For some reason you're comparing the compression cycle to the intake cycle, but I have no idea why. As dk pointed out, the energy put into compression isn't wasted. It's like compressing a spring. We get it back a moment later (aside from a bit of frictional loss) on the power stroke.

On the other hand, the energy that goes into fighting the throttle restriction is waste. We don't get it back.

I wish I could prove when lean burn is engaged. Since that is not possible

But it is possible. Thanks to the helpful instructions posted by TomO and others, in various places, it's not hard to do. You just need to put a DMM across D14 and D16. At Harbor Freight, DMMs start at $4.

If you monitored your lean burn, I imagine that you would notice things that would be helpful to the rest of us. Since you're not monitoring your lean burn, I have a hunch that there are quite a few moments when you think you're in lean burn, but you're not. It's very, very sensitive to throttle position; you can rapidly move from lean to rich with a very small increase in throttle angle.

lean burn was possible in lower gears and climbing grades

Yes, you can climb a hill and maintain lean burn. Provided it's not too steep and you're not going too fast. And yes, you can cruise at fairly high speed (60+) and maintain lean burn. But it's very hard to maintain lean burn in any gear, if you are conducting any form of acceleration. Unless the acceleration is exceedingly moderate.

We disagree on when lean burn is actually occuring.

Yes. But a difference worth noting is that I've monitored it directly with an instrument, whereas you're essentially making a bunch of guesses based on imprecise statements you've read in various places.

Of course the light does not come on in 5th gear, but you can still use the same throttle position percentage.

Because lean burn is so sensitive to small throttle movements, I don't think you can count on your ability to maintain lean burn based on the idea that your foot is able to remember the exact angle it was holding a few seconds ago. If you had an instrument reading your Throttle Position Sensor, that would be a different story.

Somewhere I saw a photo of someone who rigged a hand throttle, for this purpose, essentially. You would need that kind of precise control to really be able keep the throttle in exactly the best position to maintain lean burn.

To me it makes no sense that the light would be on when lean burn was working but off when it wasn't.

It might make no sense to you, but that's still what the light does (under certain conditions). If the light goes on, and then you increase the throttle setting by a certain amount, the light will go off. Even though you haven't obeyed the light and upshifted. And at this moment, lean burn is off and so is the SIL. Go figure.

I think you're oversimplifying the relationship between lean-burn behavior and SIL behavior.

Mr Honda was a pioneer in lean burn technology, going back to the stratified charge prechamber CVCC engines in the 70's.

Yes. In 1980 I bought a '78 Civic CVCC. A great car. It looked a lot like this: http://www.hondaclassiccars.co.uk/images/Dcp_1052.jpg

[monroe74]

Originally Posted by dosco

to get an idea of fuel flow rate I plugged in the hp number for given RPM numbers for different load lines. Lo and behold that at less-than-ideal RPM and throttle settings, I can easily reduce the rate of fuel flow. 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

I'd like to understand what you did, but I'm having trouble following this.

[monroe74]

Originally Posted by Dalez0r

use them according to driving conditions

Originally Posted by dkjones96

thermal efficiency is by far the biggest player in all of this.

I think these statements, and the rest of the comments from which they are taken, do a really nice job of summarizing what we're talking about.

[monroe74]

Originally Posted by dosco

what device out there actually measures fuel flow

SuperMID. And this: http://forum.ecomodder.com/forumdisp...mputer-26.html

Not much else, that I'm aware of.