What's the ideal fuel/air ratio in a TDI?

IndigoBlueWagon

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After my last dyno Jeff looked at the fuel/air graph and said I needed more fuel. I went from PP520s to PP502s and gained significant power. Here's the boost and fuel air table:

The blue line is with the bigger nozzles. As you can see the ratio has dropped throughout the rev range, but it makes me wonder: what's ideal? The car smokes, not a lot, but it's visible at low revs under high load. Can't see it at high revs. But there's got to be a formula for this.
 

johnnloki

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Is it not 14.5:1, or somewhere thereabouts?

edit
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I mean, I suppose it'll be hard to track the info down, what with most diesel drivers preferring "SOOT!SOOT!SOOT!" to the ideal A:F ratio, but looking at your graphs there, 14.5 seems pretty likely too- the less smokier tune rides right around that area.

Further Edit
=============================
[URL="http://www.diesel-central.com/News/cackle.htm" said:
http://www.diesel-central.com/News/cackle.htm[/URL] ]

In a diesel engine, the engine speed and power is controlled by the amount of fuel injected into the combustion chamber. A diesel engine is not "throttled" as a gasoline engine is, but rather it is "governed" by a device that controls how much fuel the fuel injector injects. Injecting a greater amount of fuel into the combustion chamber will allow the engine to develop more power.

In a diesel engine, the combustion chamber is always full of pure air before the fuel is injected. When the engine is idling under load, a very tiny amount of fuel is injected. When working hard, say while pulling a big load, a lot of fuel (relatively) is injected. Because the amount (weight) of air in the combustion chamber is relatively constant (I'm not considering boost here...) and the amount of fuel is variable, diesel engines run at varying air/fuel ratios. At idle, with no load, it is not uncommon to have a diesel engine running at an air/fuel ratio of 60 or 100:1. Under full power, most diesel engines need to run lean of stoichiometric. (BTW: the stoichiometric ratio for diesel fuel is NOT 14.7:1 and it varies slightly depending upon the composition of the fuel.)
For a number of reasons, most diesel engines will emit visible hydrocarbons (i.e.: smoke) if run near or over their stoichiometric fuel ratio. Diesel engines thus always need to be run lean of stoichiometric.
On some gasoline engines, certain cylinders have a habit of running leaner than others, due to something called fuel dropout in the intake manifold. A condition can therefore develop where one cylinder is too lean to fire properly at all, resulting in something that people term a "lean misfire". Because diesel engines are designed to run lean of stoichiometric, such an event can NOT happen in a diesel engine.
 
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IndigoBlueWagon

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I saw this same article, and I know it's 14.7 for gasoline engines. But I couldn't see a number for diesels. Of course the heavier fueling tune is only over-fueled at lower revs when under full fuel request. At 4K plus it looks like it's pretty much spot on if 14.5 is the number.
 

johnnloki

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If I had to guess (and that's all that I'm doing here) I'd say that 14 and up is good, when I look at the curve off your dyno there, your sootless tune vs your sooted tune, you've got that discrepancy between roughly 2k and 2700 that's a lot larger than the rest of the powerband- that's gotta be where the soot's coming from.
 

Drivbiwire

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A TDI is limited by the combustion chamber design as well as the nozzles and fuel pressure.

AFR simply does not correlate well to a diesel as it does with a gasser.

A TDI is considered maxxed out on AFR at around 33:1 (Air to Fuel). This is when the smoke output becomes excessive.

Newer design combustion bowls, higher pressure injection, Piezo injectors with 7 even 9 hole nozzles, 5 or more injections per combustion stroke permit richer fuel air ratios in the upper teens...these are however still in testing and not in use on the roads.

DB
 

dieseleux

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I have o2 wide band install on my down pipe.
On highway sensor read more than 22:1 (limit of my o2 sensor)
When i have heavy foot, a see 10,5:1 !
Lower than 13:1, lot of soot, power is ok
Lower than 12:1, less power.
Lower than 11:1, solid black line of soot on road and less power
Best setting is between 14:1 to 17:1.


Dieseleux
 

IndigoBlueWagon

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Helpful data. DBW, your post explains why searching got me non-answers. But it sounds like I'm in a pretty good range for the engine design I'm working with. More fuel may just make more smoke with marginally more power, it would appear.
 

dieseleux

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IndigoBlueWagon said:
Helpful data. DBW, your post explains why searching got me non-answers. But it sounds like I'm in a pretty good range for the engine design I'm working with. More fuel may just make more smoke with marginally more power, it would appear.
Between 1000 and 1700rpm, you run rich!



Dieseleux
 

TDIMeister

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The stoichiometric AFR for Diesel fuel is very darn close to that for gasoline. It is common to use 14.6:1. However, as previously stated, a Diesel engine will never run at stoichiometric (it can, but it will smoke like a tire fire), but rather well lean of it.

State-of-the-art race Diesel engines like the LeMans Audi and Peugeot V12s would run around a Lambda value of 1.3 at full-load, meaning an AFR of about 19:1. They can still run practically smoke-free at this level, although these are heavily-developed bespoke engines, and not likely to be replicated by tuning street OEM-based motors.

The chart posted above does not give any sensical value for AFR that one would encounter in a Diesel engine; I am curious how they even tap the data to plot out AFR, since there's no O2 sensor, and reported injection quantity are hardly calibrated to anything and are subject to deviate from real IQ when the injectors are swapped out.
 

Got Bearings?

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Drivbiwire said:
AA TDI is considered maxxed out on AFR at around 33:1 (Air to Fuel). This is when the smoke output becomes excessive.

DB
This doesn't make sense. 33 units of air to 1 unit of fuel is rich? That's SUPER LEAN!

On a gasser, 14.7:1 (air to fuel) is stoich (optimal combustion - not optimal power). Numbers above 14.7 is lean (i.e. 17:1) and numbers below 14.7 is rich (i.e. 13:1). On a gasser, optimal power is found between 12.7 - 13.2 AFR. Most of my knowledge is in gassers but I would say that the same principle applies here.

EDIT: I'd say the principle applies - when installing the bigger 502's, the AFR went DOWN. The question still remains: what is optimal for power?

On the graph, since the AFR is going up thru the RPMs, that mean's its leaning out and you need more fuel to fatten it up.

Am I wrong here?
 
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IndigoBlueWagon

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TDIMeister said:
The chart posted above does not give any sensical value for AFR that one would encounter in a Diesel engine; I am curious how they even tap the data to plot out AFR, since there's no O2 sensor, and reported injection quantity are hardly calibrated to anything and are subject to deviate from real IQ when the injectors are swapped out.
I don't know the answer to your question: I will try to find out. I bet it's some kind of interpretation based on power, gearing, etc. Probably only really useful as a relative measure.
 

GoFaster

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Going somewhat richer than what's normally considered a best operating point (18:1 - 19:1 or thereabouts - i.e. lambda around 1.2 - 1.3) WILL make more power, but exponentially more smoke. Ever see the amount of smoke that the diesel drag-race or tractor-pulling trucks are making? That's what happens when lambda is stoichiometric or richer ...

What dieseleaux posted makes sense, best setting 14:1 - 17:1 for max power (i.e. lambda 1.0 - 1.1)

Too rich with a diesel will make less power, same reason as too rich with a gasoline engine will make less power.
 

dieseleux

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GoFaster said:
What dieseleaux posted makes sense, best setting 14:1 - 17:1 for max power (i.e. lambda 1.0 - 1.1)

Too rich with a diesel will make less power, same reason as too rich with a gasoline engine will make less power.
Thanks!

I run with Innovate LC1 with digital display, i see 8:1 to 22:1 range with this equipement.
With o2 sensor a see what happen in my engine in any case.
Most of then are higher than 22:1 but when i need power i see how push throttle because my setup is too heavy, is very helpfull for reduce soot. (good for TDI-M and TD tunning!)
14:1 to 17:1 is good for my engine with my setup but (i hope) ported head with big turbo and high quality injection (common rail or PD) run more power with less soot in 16:1 to 19:1 range. (more power in lower range but lot of soot!)


Dieseleux
 

Typrus

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Ummmm...
I thought stoichiometric was the point of balance in the chemical equation in which all factors of the equation are in balance to react out completely. Yes? Or did paying attention in Chemistry class do nothing for me...?
If this is indeed the case, then think about this-
How is it, that if soot from a diesel is the visible effects of incompletely burned hydrocarbon chains (diesel not being burned) that one can say that a stoich ratio is one that involved bellowing out large quantities of unburned fuel? One would assume that in the situation where there is unburned fuel, that there is an insufficient supply of air. If this is indeed the case, then how can you say the equation is stoich?

Am I missing something? Or am I just an idiot?
 

Got Bearings?

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Typrus said:
Ummmm...
I thought stoichiometric was the point of balance in the chemical equation in which all factors of the equation are in balance to react out completely. Yes? Or did paying attention in Chemistry class do nothing for me...?
If this is indeed the case, then think about this-
How is it, that if soot from a diesel is the visible effects of incompletely burned hydrocarbon chains (diesel not being burned) that one can say that a stoich ratio is one that involved bellowing out large quantities of unburned fuel? One would assume that in the situation where there is unburned fuel, that there is an insufficient supply of air. If this is indeed the case, then how can you say the equation is stoich?

Am I missing something? Or am I just an idiot?
I'm with you Typrus. Stoich is optimal combustion - not lean nor rich. Read my post above.

Ok.. just found this that might shed some light...
In a gasoline engine, the air/fuel mixture always burns at 14.7 pounds of air to one pound of fuel, or nearly so. When the engine is throttled, less of the air fuel mixture is admitted to the combustion chamber, but the ratio of the air to fuel in the mixture is always nearly 14.7:1. In fact, it is impossible by conventional means to burn a air/fuel mixture much different from 14.7:1 in a spark ignition engine.

In a diesel engine, the combustion chamber is always full of pure air before the fuel is injected. When the engine is idling under load, a very tiny amount of fuel is injected. When working hard, say while pulling a big load, a lot of fuel (relatively) is injected. Because the amount (weight) of air in the combustion chamber is relatively constant (I'm not considering boost here...) and the amount of fuel is variable, diesel engines run at varying air/fuel ratios. At idle, with no load, it is not uncommon to have a diesel engine running at an air/fuel ratio of 60 or 100:1. Under full power, most diesel engines need to run lean of stoichiometric. (BTW: the stoichiometric ratio for diesel fuel is NOT 14.7:1 and it varies slightly depending upon the composition of the fuel.)

For a number of reasons, most diesel engines will emit visible hydrocarbons (i.e.: smoke) if run near or over their stoichiometric fuel ratio. Diesel engines thus always need to be run lean of stoichiometric.
http://www.diesel-central.com/News/cackle.htm

After reading that, it's making more sense why people are making power at 17:1 AFR - LEAN of Stoich.
 
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Feinstaub_CH

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Please confirm

Hi Mates

Sorry for digging out this old "fred"

I am going to re-adjust my chip. After reading and studding a lot about AFR, can someone pls confirm my mind?

LEAN means: More air is added than for the Stoichiometric is "needed" - lets say: 17 air to 1 fuel is LEAN? (Stoich is 14.7 : 1) - right?

LEAN means LESS EGT?

But why? Is it because not all air is burned and so the un-used air keeps the EGT down?
If so, why not the opposite way? More unused fuel should IMO better help keep the EGT down?

Under WOT target should be at least AFR of 17:1?

Sorry for my English, Swiss-German is still my mother-language :)


Cheers all
Feinstaub
 

shadowmaker

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With lean you can burn all the fuel at the right moment during diesel combustion. Going rich means that the rest of the fuel will burn piston going down or even in ex manifold.

Gasser won't burn the residual fuel, diesel does. That's why excess fuel is raising EGT/EMP in diesel.
 

TDIMeister

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With lean you can burn all the fuel at the right moment during diesel combustion. Going rich means that the rest of the fuel will burn piston going down or even in ex manifold.

Gasser won't burn the residual fuel, diesel does. That's why excess fuel is raising EGT/EMP in diesel.
This a patently incorrect. Diesel fuel is injected just before combustion is supposed to take place and within that very limited period of time, the atomized liquid jet of fuel must evaporate, diffuse in the air and form a combustible mixture within a local fuel-air ratio not far from stoichiometric. Whether at idle or full-load, combustion in a properly designed engine should be substantially complete within a span of no more than about 45° crank angle from the beginning of noticeable heat release. Combustion duration is more of a function of the engine design and development (swirl, fuel properties, charge temperature at end of compression, bowl/spray geometry, injection pressure/multiple injections (rate shaping) and less about whether the mixture is lean or rich in a global sense.

 

JFettig

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On a ve TDI you can usually get down to 17:1 Afr with little to no smoke but timing plays a big role. You can make more smoke at 20:1 Afr than 17:1 if you have too much timing. CR TDI can do 14.5 with little to no smoke.
 

IndigoBlueWagon

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Funny to see this thread come back. I barely even recall what I was doing with my wagon when I started it in '08.

Last time Jeff tuned my car he told me the timing was too advance (100 in block 2). In the past he liked it set high to make sure the pump had its full timing advancement available. I've moved it back to 75 or so, and based on the low smoke and FE I'm getting I guess it was a good idea.
 

Pat Dolan

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Hi Mates
Sorry for digging out this old "fred"
I am going to re-adjust my chip. After reading and studding a lot about AFR, can someone pls confirm my mind?
LEAN means: More air is added than for the Stoichiometric is "needed" - lets say: 17 air to 1 fuel is LEAN? (Stoich is 14.7 : 1) - right?
LEAN means LESS EGT?
But why? Is it because not all air is burned and so the un-used air keeps the EGT down?
If so, why not the opposite way? More unused fuel should IMO better help keep the EGT down?
Under WOT target should be at least AFR of 17:1?
Sorry for my English, Swiss-German is still my mother-language :)
Cheers all
Feinstaub
I will answer with a combination of what I know and what I am assuming. Counting on TDIMeister and others to crtique my answer - just doing so because it seems nobody else replied to you.

First of all, yes, "lean" mixture means excess air and "rich" means excess fuel beyond stoichiometric.

On a gasoline fired, spark ignition engine, peak EGT happens very close to stoichiometric conditions...with the possible exception that the EGT downstream of lean mixture is seeing a bit of burning still taking place as a lean gasoline/air mixture burns more slowly (thus why lean mixtures burn valves - lot of temp on the exhaust valve seat and excess of oxygen). But, you comment about "unused air" is spot on for gassers - the lack of oxygen to be used to burn fuel means less energy release when lean of stoich - thus lower EGT when lean. For why richer lean of peak - see below.

Air does not burn...well, I guess in a way it does. The oxygen in the air combines with hydrogen and carbon in the fuel in an exothermic reation that in turn heats the other 80% of the air (nitrogen) in the chamber - which will expand to drive the piston. When we use forced induction in a diesel to keep the mixture very "lean", or if we simply deliver less fuel to use the air that is in the cylinder, the ratio of mass of reagents to nitrogen (and some remaining oxygen) means that the greater mass has absorbed the fixed (for reference purposes) energy from combustion, thus gaining less temperature from the same amount of heat (energy). THAT is why the engine makes less power with more air (less temperature to expand the gasses that push on the piston = lower MEP - mean effective pressure - that in turn shows up as a lower EGT when it is released into the exhaust tract.

Oh, the comment about air burning "in a way it does" is because at combustion temperatures, some of that nitrogen combines with some of the available oxygen to make oxides of nitrogen (which is what truly erks me when people in my business routinely refer to nitrogen as "inert"). That is the "burning air" part.

I would assume that the deeper you get into smoke and when you finally DO reach stoichiometry with a diesel, you will also see EGTs decrease - but to get there, you need to keep adding fuel thus an engine seeing very rich mixtures while burning less efficiently could still to a great degree be burning more fuel than another reference point of measurement - thus showing a much higher EGT (to a point).

Remember that the fuel supply in a gasser can only burn over a very narrow range (thus more responsive EGT vs. mixture measurement) whereas a diesel can operate over a VERY wide range of mixtures - thus experience these results.
 

dieseleux

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On a ve TDI you can usually get down to 17:1 Afr with little to no smoke but timing plays a big role. You can make more smoke at 20:1 Afr than 17:1 if you have too much timing. CR TDI can do 14.5 with little to no smoke.
Higher quality of nozzle atomisation and multi injection per cycle are the key of complet combustion and CR are far better on this than VE pump system.




Dieseleux
 

shadowmaker

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This a patently incorrect. Diesel fuel is injected just before combustion is supposed to take place and within that very limited period of time, the atomized liquid jet of fuel must evaporate, diffuse in the air and form a combustible mixture within a local fuel-air ratio not far from stoichiometric. Whether at idle or full-load, combustion in a properly designed engine should be substantially complete within a span of no more than about 45° crank angle from the beginning of noticeable heat release. Combustion duration is more of a function of the engine design and development (swirl, fuel properties, charge temperature at end of compression, bowl/spray geometry, injection pressure/multiple injections (rate shaping) and less about whether the mixture is lean or rich in a global sense.

I wasn't talking about engineering side of diesel combustion. I was talking about AN engine with either lean or rich of its optimal operation point. It doesn't matter IDI, VE, PD or CR, they all behave identical when nothing else in certain engine, but fuel quantity (=injection window, injection pressure stays the same), is changed. Rich means more EGT, lean means less EGT. Optimal operation point can vary between these very different engine types, but I wasn't talking about that as I didn't see any question about that either (from Feinstaub).

And yes, when most of the combustion happens within 45 degrees crank angle, there's always unburned fuel present after that (as can be seen from Meister's chart above). The amount of that residual fuel is the key for EGT temperatures. Bigger amount means that more fuel is still burning after "optimal" crank angle and the heat of that burning can't be converted efficiently to work -> EGT goes up, power doesn't.

And like Meister said there are many ways to get more fuel be burned in that 45 degree crank angle window, but that's a whole other story - even though very important one here in "TDI Power Enhancement"-section.;)
 
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robnitro

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Ideal AFR also depends on RPM and injection window.
For example, in my current tune, .234 inj w 11mm pump calibrated pump voltage to 80 mg/s and smokemap used to limit fuel (not torque limiter like some tunes do above 851 mg/s maf)
RPM AFRsmokefree
5500 18.8
5000 18.65
4500 18.5
4000 18.2
3500 17.9
3000 17.6
2500 17.2
2000 17.1

BUT what I am seeking now is what is the best AFR to set the target boost map, for cruising conditions, aka 10 mg/s 20 mg/s 30 mg/s?
I currently use reduced boost from stock boost map based on advice from ecuconnections, but I hear that higher afr than 17 is more economical to a point, due to better mixing etc...??? (tdimeister- is there an optimal part load afr- or is "enough air" better???)

assuming 1 bar absolute is around 480 mg/s maf (when EMP is not too high)
IQ AFR BOOST@2k BOOST@5.5k
10 48 1000 1092
20 28 1167 1274
30 24 1500 1638
37.5 22 1718 1876
45 21 1968 2150
Higher fueling goes closer to the afr from smoke map due to boost limits of the turbo itself (1749vb-vnt17)

Funny to see this thread come back. I barely even recall what I was doing with my wagon when I started it in '08.

Last time Jeff tuned my car he told me the timing was too advance (100 in block 2). In the past he liked it set high to make sure the pump had its full timing advancement available. I've moved it back to 75 or so, and based on the low smoke and FE I'm getting I guess it was a good idea.
I don't know why the change in opinion.... setting static to 100 only makes it so that the pump can't go below xx BTDC. 75 for example, you see you get 3 btdc at idle irrespective of timing duty cycle. More timing than that is done by the SOI map, which adapts to whatever static you set it to. (example: asks for 10 btdc@ 3000 rpm: with timing at 75, the solenoid would be 60%, but with 100, it would be 30% - example!)

Anyway, with 75, run output tests, the first one does the timing solenoid. Then go to group 04 measuring blocks and see how even at 75, you can get up to 22 or so BDTC at idle! If you rev it to 1200 rpm or so (not too high or it stops the output test) you see it goes to 25 or so!
So this original advice of 100, is kind of overkill in the first place. Maybe it was a fix for the pumps that couldn't get proper case pressure or feed of fuel?
 
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Feinstaub_CH

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Big thanks to your replies!

Thanks for your replies, i can nothing add/write because i need first to understand what i am reading :eek:

Cheers
Feinstaub
 

JFettig

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When i cruise on highway, my AFR are between 28:1 to 40:1, change fast with wind, road angle and throttle.




Dieseleux

Same here, I tuned mine to stick around 30AFR cruising to keep the vanes as open as possible and still efficient burn. It drops down to 25-28 up hills sometimes.
 

penjoseph

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Don't have a TDi but a CRDi.

I have an 'extreme' wideband running in my car which can read AFR upto 230:1. At idle the AF goes as down to 200:1




Video:
https://www.youtube.com/watch?v=_DiM96wgTag&feature=youtu.be


Wideband Controller - Ecotrons ALM
Digital Gauges - Auber Instruments

LED Gauges:
Left green - Intake Manifold Air Temperature
Center orange - AFR (Air Fuel Ratio)
Right red - EGT (Pre-Turbo Exhaust Gas Temperature)

LCD Gauge: Scangauge reading OBD Coolant Temperature, Boost, Intake Temperature & Fuel Rail Pressure (XGauge code)
 
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