best lean mixture for economy

[landybehr]

Hi,

I don´t quite understand the following graph completely.
There´s a point called "best economy". Past that there are leaner mixtures that the Megamanual recommends for economy - for a reason I presume.
Why is the fuel consumption curve raising again after the point of "best economy" ?

[66fb]

landybehr,

I'll take a shot at your question and if anyone has a better understanding please feel free to educate us both.

As the mixture gets leaner it becomes harder to ignite and also burns slower. An engine develops best power when peak cylinder pressure occurs about 14* after top dead center (IIRC). If timing remains the same and the mix become more lean (slower burn rate) than the peak pressure will occur beyond 14* ATDC and efficency drops and fuel consumption goes up.

[Mike_Robert]

"Best fuel economy" is going to vary considerably for each particular set of variables (engine type, tune, head design, multispark ign, etc.). I think that graph was showing a typical set of curves for a particular engine. I know for a fact mine won't run at all at 15.4/1 at cruise (or any other operating regime). Some of the GA aircraft folks are having reported economy related success by tuning their AFRs for a set number of EGT degrees below peak EGT. There is lots of info on this and many other subjects in the NACA archives. Old info but the basic physics haven't changed.

-Mike

[landybehr]

Right,
seems that the diagram was taken out of context. Don´t know anything about the conditions of measurement (could have been steady load testing).

While in discussion with the guy who posted it elsewhere I have been sent another link:

http://autospeed.com/cms/A_110216/artic ... larArticle

That´s quite nice information. To me at least.

[Peter Florance]

landybehr,

I'll take a shot at your question and if anyone has a better understanding please feel free to educate us both.

As the mixture gets leaner it becomes harder to ignite and also burns slower. An engine develops best power when peak cylinder pressure occurs about 14* after top dead center (IIRC). If timing remains the same and the mix become more lean (slower burn rate) than the peak pressure will occur beyond 14* ATDC and efficency drops and fuel consumption goes up.

Agreed. What I've wrestled with is making it leaner and advancing the timing. Usually at some lean mixture and advanced timing, I'll get a misfire. I'm trying to get an indication (other that experimenting) on which to push for economy. Timing or mixture (leaner).

I need to re-read Heywood on this point and see if anything registers.

[msoultan]

You guys might be interested in this thread:

http://www.thesamba.com/vw/forum/viewtopic.php?t=293837

There's another chart in there that shows something similar...

[landybehr]

Many thanks , Ken !

That link is very useful indeed. Tells me, very much compatible to the MegaManual, that it indeed does make sense to go lean on cruise. Surge is what limits it towards the lean side. No fear to induce hot exhaust gasses (unless ignition is relatively retarded by too much. Even so danger seems to be not extremely high as the folk works with carburetors and conventional dizzys).

(emissions is sth. that is not discussed. We´re tuning for economy solely here!)

NOW: something that still hurts my brain:
The autosport-link it posted tells about "Pumping Losses" because the cylinder is pulling out of a vacuum at part-throttle which is harder to do than being filled by atmospheric pressure at full throttle. This causes a good part of the lower degree of efficiency of the petrol cars compared to Diesels. BUT if the cylinder wastes some energy by sucking "against" a vacuum - doen´t it "regain" that energy when it comes to the compression stroke ? When there is less amount of air in the cylinder to compress - that´s ought to be much easier!

And second, but even more painful ;) >>
The fuel consumption curve of the graph above is raising again (in lean mixtures) after the point of best fuel consumption. WHY ?? I can understand that too lean a mixture does no longer provide enough power to turn the wheels, and, it may just take too long for it too burn (even with a lot of advance and/or the cyl.pressure could build up too slow for the piston´s liking). But if that is the case - you need to depress the accelerator further to keep the speed. By doing so you will leave the part of the map you´ve been in/measureing before.
(therefore it would make sense to me to tune like starting a little rich and leaning the cruise step by step. Adding 1-2° of advance if the power seems to be diminished too far). (I am talking about WideBand Lambda sond bases maps!).
But I cannot understand HOW someone could measure a graph where the fuel consumption raises after the point of best economy again like just mentioned.

[66fb]

NOW: something that still hurts my brain:
...BUT if the cylinder wastes some energy by sucking "against" a vacuum - doen´t it "regain" that energy when it comes to the compression stroke ? When there is less amount of air in the cylinder to compress - that´s ought to be much easier!

If you take this to an extreme (less air therefor easier to compress) you would get max. efficiency with zero air (and fuel).
VE (volumetric efficiency) is the amount of air in the cyl. divided by the amount it can hold. If you partially fill the cyl. you reduce efficiency.

But I cannot understand HOW someone could measure a graph where the fuel consumption raises after the point of best economy again like just mentioned.

If you are still referring to the autosport link -I believe they are talking about SFC (specific fuel consumption) and mention consumption increases (compared to power output) as rpm increases because of higher friction losses

This is the "purple" line in their graph.

[cygnus x-1]

Many thanks , Ken !
NOW: something that still hurts my brain:
The autosport-link it posted tells about "Pumping Losses" because the cylinder is pulling out of a vacuum at part-throttle which is harder to do than being filled by atmospheric pressure at full throttle. This causes a good part of the lower degree of efficiency of the petrol cars compared to Diesels. BUT if the cylinder wastes some energy by sucking "against" a vacuum - doen´t it "regain" that energy when it comes to the compression stroke ? When there is less amount of air in the cylinder to compress - that´s ought to be much easier!

The simplistic answer to this is yes it would; but it's not that simple. What pumping loss really is is the amount of work done by the engine just moving air into and out of the cylinder, as well as moving air around in the crank case. To account for all pumping work done you have to sum up (or integrate) over the engine cycle the pressure differentials on each side of all the pistons. It isn't just the intake stroke, but all the strokes added together. On the power stroke the pressure in the cylinder is greater than the pressure in the crankcase, so you get a positive work output. The exhaust stroke would likely be positive as well since the pressure in the crankcase is generally higher than the pressure in the exhaust manifold (in turbo applications this might not be the case), which would help pull the crank along. The intake and compression strokes will have a negative work output since in both instances the pressure differentials are operating against the piston movement.
But even beyond that there is also work involved in simply pushing around the air inside the crankcase. Even in balanced engines like 4 cylinders where the pistons are in opposing pairs, there is still some effort expended in just pushing air from one cylinder bore to another, even though the net crankcase pressure compared to atmospheric does not change. Think of sloshing water back and forth in an aquarium. It takes effort to push the water around even when there are no barriers to it's movement within the aquarium.

And second, but even more painful ;) >>
The fuel consumption curve of the graph above is raising again (in lean mixtures) after the point of best fuel consumption. WHY ??

Best efficiency is obtained when the cylinder pressure peaks at a specific point in the crank cycle (roughly 14 degrees). With mixtures relatively close to the stoichiometric ratio the fuel is easily ignited and the fuel burn times for each stroke are consistent. But as the mixture gets significantly leaner than stoichiometric, the mixture is not only harder to ignite, the burn time is also not as consistent and predictable. As the burn time varies, so varies the point in the crank cycle at which maximum cylinder pressure is achieved. So with very lean mixtures, for many of the cycles peak cylinder pressure will occur either too early or too late for optimum torque transfer. As the mixture gets even leaner you start getting misfires where no work is done at all. This cycle to cycle variation is what produces that jittery feeling known as "lean surge".
So anything that can be done to make the combustion time more consistent with lean mixtures can potentially help fuel economy. Stratified charge combustion chambers are one example of this. The mixture close to the ignition point (spark plug) is rich enough to be easy to ignite so it can get the flame started. Then the flame can spread out to other leaner mixture areas. Increased mixture velocity (swirl) in the combustion chamber can also help flame ignition and propagation.

C|

[landybehr]

Hi,

I read the whole thread again and still it´s been a very good tip.

On the bottom of the 6th page there is a draft of an AFR-table:
http://www.thesamba.com/vw/forum/viewto ... &start=100

looks like this (copy´n´pasted):
93 12.75--------------------------------------------------13.0--------------->
89 12.75--------------------------------------13.0--------------------------->
74 12.75-------------------------13.0--------16.0 -------16.5-------17.0-->
63 16.0---------------------------------------------16.5--------17.0------->
53 16.0-------------------------------------->16.5 17.0------------------->
45 16.0-----------------------------16.5-----17.0--------------17.5------->
39 16.0--------------------16.5---------17.0--------------17.5------------->
35 16.0-------------16.5-------------------> 17.0 17.5-------------------->
26 16.0-------->16.5---------> 17.0------- 17.5-------------------------->
22 16.0-->16.5--------- 17.0-------- 17.5-------------->
18 16.5 17.0 18.0 18.5 19------------------------------------------>

800 1100 1300 1700 2000 2300 2600 2900 3200 3600 4k 4.4k
the left column is the MAP [kPa]

isn´t that very lean up to the bottom of the acceleration rows (=WOT) ?
Why is there this tendency to go leaner with raising RPM´s that the guy mentions ?
That´s somewhat different to the highly rated (by me) MegaManual.

[msoultan]

isn´t that very lean up to the bottom of the acceleration rows (=WOT) ?
Why is there this tendency to go leaner with raising RPM´s that the guy mentions ?
That´s somewhat different to the highly rated (by me) MegaManual.

I think there are a few reasons for why he suggests these AFRs. The first thing that he talks about is that you can take an engine really lean during light cruise. That would be somewhere up until around 70% throttle or so. As for it leaning out for a given load at higher RPM, the engine isn't producing any more power at the higher RPMs so why try and inject so much fuel when the torque curve has already dropped off.

The other goal for all of this is geared towards air-cooled engines and keeping the temps cool. If you look at the chart that is in that thread, it shows that when you keep the AFR on either side of (but not at) 14.7, you're keeping the CHTs down. This is really important for the aircooled folks as we have a tendency to burn up our heads all the time... like me

If you don't understand why he's saying the stuff he's saying, feel free to ask. John's a pretty well experienced tuner and has a wealth of information to share. He also runs aircooled.net.

MIke

[landybehr]

Hi Mike,

that´s why I asked here. A MS-guy will "think" different engines. With a forum for spec. types of engines you´ll emphasize on certain aspects - and most important thing I´ve learnt is that there´s not ONE simple thing going on in the engine but rather a whole lot of processes which are influenced by each other so that one answer my be right for one type of engine under a certain condition, but wrong if one variable changed. Even quite interesting how .msq´s differ if made resp. tuned by different people for the very same engine in the same model of car.
Anyhow, what you and John wrote makes very much sense to me. And I did realize that John from Aircooled has knowledge way beyond VW engines.

Still though I try not to stir up too many forums (i can be quite cumbersome trying to understand)

[cygnus x-1]

Man that's lean. My Honda would run like crap with that table. Any leaner than 15.5:1 and it starts surging. 16:1 at idle would be really wheezy.
But if you have an engine that can run those AFRs, go for it. Mileage would be very good.

C|

Hi,
On the bottom of the 6th page there is a draft of an AFR-table:
http://www.thesamba.com/vw/forum/viewto ... &start=100

looks like this (copy´n´pasted):
93 12.75--------------------------------------------------13.0--------------->
89 12.75--------------------------------------13.0--------------------------->
74 12.75-------------------------13.0--------16.0 -------16.5-------17.0-->
63 16.0---------------------------------------------16.5--------17.0------->
53 16.0-------------------------------------->16.5 17.0------------------->
45 16.0-----------------------------16.5-----17.0--------------17.5------->
39 16.0--------------------16.5---------17.0--------------17.5------------->
35 16.0-------------16.5-------------------> 17.0 17.5-------------------->
26 16.0-------->16.5---------> 17.0------- 17.5-------------------------->
22 16.0-->16.5--------- 17.0-------- 17.5-------------->
18 16.5 17.0 18.0 18.5 19------------------------------------------>

800 1100 1300 1700 2000 2300 2600 2900 3200 3600 4k 4.4k
the left column is the MAP [kPa]

isn´t that very lean up to the bottom of the acceleration rows (=WOT) ?
Why is there this tendency to go leaner with raising RPM´s that the guy mentions ?
That´s somewhat different to the highly rated (by me) MegaManual.

[TPI 85 Blazer]

You really have to figure it out by trial and error. Look at your MAP and RPM under the conditions you are at the most for cruising. This is where to focus for the most fuel economy benefit. Then lean those sections out until you start to feel lean surge, then richen back up a bit and you're done. Very little power is needed to maintain your cruise through town at 35mph, so why throw a bunch of fuel through the engine? At freeway cruise you might not be able to go quite as lean. You can still idle at whatever AFR the engine likes best. Of course at low vacuum (high MAP) you need to keep the mixture rich for both power and safety. For parts of the VE map that you rarely see, there is no need to worry about this. I idle at about 13.8 but cruise at 16.5.

[mops]

Great thread.
I'll post my experiences and thoughts on the matter.
Megamanual instructs to run as lean as possible for good fuel economy (at cruise).

My engine seems to be a bit particular (i think ~260 deg cam is quite long for oem).
anyways my observations (bmw m20 engine, sohc, 2 valve, straight 6, liquid cooled)
-It will idle nicely at low 13's (AFR's), anymore it will mis, pop, etc. It will idle at 16 and 17 aft, but it's very weak, haunting, surging, might die when maneuvering on the parking lot (PS pump load can kill it). generally rich idle is strong, nice, even idle. Hence my dilema. have it nice and strong and drivable (car feels nice, smooth and strong, just like oem), or try to save fuel by idling it at 16 afr, where's it's missing, haunting, weak, etc. good idle produces about 550 deg C EGT's while once it start's popping and missing then egt's go down to 400's or even lower (depending on how much missing there is....)

-what i cant understand, on cold start it needs to be richer to idle nicely - stock ecu idles ~ 11 afr (!!!) at cold starts. with ms it needs to be into mid 12's to idle ok when cold. I understand at cold engine more fuel needs to be injected to compensate for poor vaporizing of fuel, but i dont understand why it has to be overall richer ? or perhaps wideband is reporting incorrectly when cold ?

-generally speaking, the higher the rpm the leaner it can run. i generally tune to 17's+ at cruise (35-45 deg advance depending on rpm's, 700+ deg C egt's), but at high speed cruise it can run without missing even 18+ afr. problem with running those afr's at cruise is that during normal acceleration through those bins it feels like a 'flat spot'. aswell ae must be huge as even slight incerrase of throttle at those high afr's make them go even higher and into misfiring zone.

-many tuning resources advise to 'give engine what it wants', and that's certainly (from my experience) applicable in terms of drivability/response, power, but whether fuel economy ?

I heard that optimum point for Peak Cylinder Pressure is 17 degrees, but anyways, that's not important. What is important is that ion sensing ignition can provide feedback on PCP. Yes, as you go leaner, it burns slower, etc, that's the point of electronic ignition advance - to compensate for that and make sure PCP is when it is occurring. It is visible on the dyno, and basically cruise would be tuned to max power at cruise (lean mixtures).

So basically i'm also wondering.... what's most economical:
-absolutely leanest you can run without misfiring ?
-allow some lean misfire (i dont think so, as the fuel unburned is effectively wasted) ?
-maybe there is a certain afr/advance combination that produces maximum fuel economy (at certain speed and ultimately what speed is most economical ?), but how to go about finding it ? I know that piston engine planes (at least WW2 ones and those older ones) had manual mixture adjustment. so the procedure was (cant remember where i read it) that once plane is settled in a cruise (certain rpm) mixture would be leaned out until engine rpm's begin to drop, and that point was 'best lean mixture' (economy wise). I suppose same will be application in car engines also.... the goal is to make just enough power to keep the car moving and not a bit more using as least fuel as possible....

[msoultan]

If I were you, I'd post those messages on the samba forum as then you'll probably get a reply from John and possibly from Jake Raby, who does a ton of dyno tuning and has a great reputation as a good engine tuner.

MIke

[Mike_Robert]

The aviation procedure I think you're referring to is LOP - lean of peak operation. It hinges on EGT. See http://www.avweb.com/news/airman/are_yo ... 816-1.html .
Most modern GA aircraft still have manual mixture adjustment except for the FADEC (Full Authority Digital Engine Control) birds where an ECU handles all engine management tasks. You request a power level and it gives it to you according to various algorithms.
An interesting concept may be to utilize PID control of mixture utilizing EGT as an input to a processor (for cruise only).

-Mike

[TPI 85 Blazer]

So basically i'm also wondering.... what's most economical:
-absolutely leanest you can run without misfiring ?
-allow some lean misfire (i dont think so, as the fuel unburned is effectively wasted) ?
-maybe there is a certain afr/advance combination that produces maximum fuel economy (at certain speed and ultimately what speed is most economical ?), but how to go about finding it ? I know that piston engine planes (at least WW2 ones and those older ones) had manual mixture adjustment. so the procedure was (cant remember where i read it) that once plane is settled in a cruise (certain rpm) mixture would be leaned out until engine rpm's begin to drop, and that point was 'best lean mixture' (economy wise). I suppose same will be application in car engines also.... the goal is to make just enough power to keep the car moving and not a bit more using as least fuel as possible....

I wonder if the difference between these methods yields much difference in real world fuel consumption.

I remember reading about an algorithm people were running in MegaTune to calculate fuel economy. This means that they know fuel consumption, probably based on MAP and PW. You could tune for fuel economy on a dyno by recording the engine loading at a chosen cruise speed. Then duplicate that load on the dyno and try combinations of AFR and advance to minimize the fuel flow. I guess you could do it on a road, with another driver, too. I just think that running tanks of gas through the vehicle and calculating fuel economy the old fashioned way wouldn't yield enough difference to direct your tuning.

[gfolsom]

Link to a question about the crankcase vacuum pump.
I didn't want to divert from fuel economy mixture to pumping loss questions:
viewtopic.php?f=78&t=34845&p=212176&hil ... um#p212176

Thanks