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TDI Power Enhancements Discussions about increasing the power of your TDI engine. i.e. chips, injectors, powerboxes, clutches, etc. Handling, suspensions, wheels, type discussion should be put into the "Upgrades (non TDI Engine related)" forum. Non TDI vehicle related postings will be moved or removed. Please note the Performance Disclaimer.

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Old March 9th, 2008, 17:05   #1
IndigoBlueWagon
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Default What's the ideal fuel/air ratio in a TDI?

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.
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Old March 9th, 2008, 18:43   #2
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Is it not 14.5:1, or somewhere thereabouts?

edit
============================
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
=============================
Quote:
Originally Posted by [URL="http://www.diesel-central.com/News/cackle.htm"
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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Old March 9th, 2008, 19:43   #3
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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.
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Old March 9th, 2008, 20:06   #4
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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.
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Old March 9th, 2008, 20:09   #5
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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.

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Old March 9th, 2008, 20:31   #6
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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.


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Old March 10th, 2008, 07:33   #7
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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.
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Old March 10th, 2008, 12:25   #8
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Quote:
Originally Posted by IndigoBlueWagon
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!



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Old March 10th, 2008, 14:43   #9
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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.
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Old March 10th, 2008, 15:18   #10
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Quote:
Originally Posted by Drivbiwire
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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Old March 10th, 2008, 15:41   #11
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Quote:
Originally Posted by TDIMeister
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.
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Old March 10th, 2008, 19:03   #12
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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.
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Old March 10th, 2008, 20:28   #13
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Quote:
Originally Posted by GoFaster
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!)


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Old March 10th, 2008, 22:36   #14
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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?
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Old March 10th, 2008, 22:51   #15
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Quote:
Originally Posted by Typrus
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...
Quote:
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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