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  #11  
Old 28-09-2004, 09:57 PM
SaabTuner SaabTuner is offline
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Indeed. Sorry, I hadn't meant to imply that any amount of injectant had the exact same effect.

I only meant to say that when you do the tuning without WI, you'll need to re-adjust the timing. 5 degrees forward probably is a pretty good rule of thumb. I'm not sure how much water they were injecting in this study as they were just spraying a fine mist at the exposed throttle plates. The study focused on the Ion Sensing feedback controller, rather than the WI itself.

Cyllinder pressures should not change as long as your PPP stays at the same crank angle. WI does not affect the overal pressure curve (Rapid Burn Angle), but rather just delays the flame development angle, and thus delays the combustion.

Apologies for the oversized picture. ops:

Adrian~
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  #12  
Old 28-09-2004, 10:24 PM
Richard L Richard L is offline
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Just summarizing the discussion so far on "effective" ignition timing:

1) lean mixture retards e-ignition
2) rich mixture retard e-ignition
3) injection of water e-retards ignition

MBT will be achieved by trimming (advancing) the static ignition timing according to the three variables. Is everybody in agreement in general?
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  #13  
Old 29-09-2004, 09:37 AM
Richard L Richard L is offline
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Just like to iterate the above with more details:

1) lean mixture retards effective-ignition
2) rich mixture retard e-ignition
3) injection of water e-retards ignition


All above conditions will encourage detonation due to the slowing down of the frame front and the end gas has a greater opportunity to detonate. This effect is more apparent at low engine speed due to longer duration of the burnt. This is to assume the frame speed is pretty constant under boost.


1) lean mixture retards e-ignition

A/f ratio between 15-16+:1 Under these condition, detonation in almost imminent due to slow burn and reduced latent heat absorption from injected liquid. The frame-front temperature is also at its higest due to excess oxygen available to fuel the burnt, this will help igniting the end gas that causes detonation.


2) rich mixture retard e-ignition

A/f ratio of 10-11.9:1 Onset of detonation is partially suppressed by temperature reduction of excess fuel, the end-flame is cooled, but still exhibit the same threat of detonation tendencies.

Also like to add that the this method of suppressing detonation will lean to power loss due to some of the oxgen is used to produce Carbon monoxide (10% or more exiting the exhaust pipe) - full power conversion is not fully harvested inside the combustion chamber. Wasterful exercise, as far as I am concerned.

3) injection of water e-retards ignition
A/f ratio 12-15:1 At this region, the burnt rate is at its highest and power conversion is at maximum but at the same time promotes highest cylinder pressure and temperature. Detonation is again highly likely due to the other side of the burnt rate scale, fast burn effectively advances the set ignition timing. By adding water at this point would off set the early ignition and suppress the onset of detonation by ignition retard (niot as much as extent to excess fuel) and quench any abnormal burnt due to non-uniform distribution of a a/f fuel mixture.

The point I am try to make is- it may not be necessary to touch the ignition timing at all if you are running the ideal a/f ratio for your particular combustion design. Some expenses can be spared on paying out high-end third party "piggy-back" or "full-blown" ECUs to tune your water injection equipped engine. Tuning by water is less complicated than protraited on many SAE papers.

Most of my conclusion is based on some factual statement mentioned on this thread. I hope the discussion will continue and really analyse the myths and mysteries that surrounded the water injection concept for years.
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  #14  
Old 29-09-2004, 06:54 PM
hotrod hotrod is offline
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Default ignition timing

Quote:
The point I am try to make is- it may not be necessary to touch the ignition timing at all if you are running the ideal a/f ratio for your particular combustion design.
I agree entirely. I believe that is one of the reason folks need to fool around with different water/alcohol ratios and injection rates. They are using those variables to arrive at an optimize point of peak combustion pressures, without changing the physical ignition advance.

That in turn leads to maximum mechanical effeciency and free power that would otherwise go out the tail pipe. One of the many ways WI allows both higher fuel economy and power production at the same time.

If you balance the water / alcohol ratio and water injection rate properly for your car, there should be no difference in ignition advance between the injected operation and when the engine is running without injection off boost.

Larry
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  #15  
Old 29-09-2004, 07:48 PM
Richard L Richard L is offline
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SaabTuner,

Does your SAAB uses the same Ionization Gap sensor and are you able to read the ignition time when water is introduced? If it can, it will be great to know what the effect of water in reakl time. The Chart you shown has more or less confirming the effectiveness of the SAAB ECU.

It will be great to study the WI section a bit further as they just use a plabnt mister on the inlet and have no idea what ratio of water to air. Pity that we are not able to relate the chart against the w/a ratio.

So if your SAAB has such an ECU and just happen that you have the readout interface, we have some very accurate data.
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  #16  
Old 29-09-2004, 10:34 PM
SaabTuner SaabTuner is offline
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The method for reading the ignition timing was first invented by the folks in that paper from LinkoPing University. That paper was written in 1999 after T7 (used in my car) was introduced. T8 could have it, but I doubt it.

Mine does have an ionization gap sensor, but it is used to detect knock, mis-fire, and cam phase, rather than as an ignition feedback sensor.

What I want to eventually do is modify it in a similar manner. That would be awesome to get some concrete numbers.

As for water injection slowing the flame rate, I don't believe that's true. I recal reading one of the papers on this forum, I believe it's still posted, which showed that, rather than retarding the burn rate, it retarded the flame development angle.

Slowing down the flame development angle has little effect on detonation because you can adjust the ignition timing to compensate with little or no effect on total cyllinder pressure, or rate of rise in cyllinder pressure.

It's just like a delay between when the flame kernel is ignited, and when it propogates through the combustion chamber generating heat.

The rapid burn angle is what will control detonation. A slow rapid burn angle, as produced with a bad A/F ratio, contributes to detonation.

Adrian~
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  #17  
Old 29-09-2004, 11:30 PM
Richard L Richard L is offline
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Quote:
Originally Posted by SaabTuner

As for water injection slowing the flame rate, I don't believe that's true. I recal reading one of the papers on this forum, I believe it's still posted, which showed that, rather than retarding the burn rate, it retarded the flame development angle.

Slowing down the flame development angle has little effect on detonation because you can adjust the ignition timing to compensate with little or no effect on total cyllinder pressure, or rate of rise in cyllinder pressure.

It's just like a delay between when the flame kernel is ignited, and when it propogates through the combustion chamber generating heat.

The rapid burn angle is what will control detonation. A slow rapid burn angle, as produced with a bad A/F ratio, contributes to detonation.

Adrian~
ppp (peak pressure position) as you referred is a cylinder pressure plot against crank angle arriving at a peak value.

Are you implying that injecting Water will shift this PPP to the right hand side of the chart (delay). If this is the case, retarding the ignition timing would have the same effect as injecting water, I can agree to that to some extent. Torque change further confirmed this assumption.

I am interested on your view on what will happen to the ppp when water is injected after the igniton is advanced to re-align the ppp to the edge of detonation threshold. If this condition can be repeated many times until the there are so much water is being injected and the ingition is so far advanced but in the end the engine has no gain nor torque change? Does this sound correct?
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  #18  
Old 30-09-2004, 04:28 AM
SaabTuner SaabTuner is offline
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------------------------------------------
ppp (peak pressure position) as you referred is a cylinder pressure plot against crank angle arriving at a peak value.

Are you implying that injecting Water will shift this PPP to the right hand side of the chart (delay). If this is the case, retarding the ignition timing would have the same effect as injecting water, I can agree to that to some extent. Torque change further confirmed this assumption.

--------------------------------------------

Yes! To be more precise, what I am referring to has to do with the latter of the two images I posted before. The second one is called the "Mass Fraction Burned" profile.

To completely understand what I mean, two concepts need to be quickly explained:

1. The Rapid Burn Angle: This is the "angle" or "rate" at which 80% of the charge burns. It starts at the crank angle at which 10% has burned, and ends when 90% has burned. Often the units for this are "milligrams/degree". In which case you take 80% of the total milligrams for that combustion, and divide that by the number of degrees it took to complete the "rapid burn".

2. The Flame Development Angle: This is the rate at which the flame begins to form. It is the amount of crank degrees which is needed to burn the first 10% of the charge. 10% of the milligrams/combustion is divided by that number of crank degrees to obtain the angle.

The flame development angle has little effect on detonation assuming you have the same PPP. This is primarily because it occurs next to the spark plug and flame kernel. Detonation usually occurs by the edge of the cyllinder wals at the end of the flame front ... or by the exhaust valves. In either case, the flame development angle has almost no effect on the processes which cause detonation.

Water Injection almost exclusively affects flame development angle. Once the Rapid Burn is taking place, the localised cooling of water is second order to the heat of the flame front. The flame temperature is well in excess of 2,000 degrees. It is the flame development that is sensitive to water ... indeed it is even altered by several degrees between 20% relative humidity and 80% relative humidity. Humidity is probably the largest disturbance to ignition timing on road cars as automobiles are usualy not equipped with humidity sensors, or any kind of feedback ignition system.

-----------------------------------------------
I am interested on your view on what will happen to the ppp when water is injected after the igniton is advanced to re-align the ppp to the edge of detonation threshold. If this condition can be repeated many times until the there are so much water is being injected and the ingition is so far advanced but in the end the engine has no gain nor torque change? Does this sound correct?
-----------------------------------------------

Possibly. But several the NACA studies were done injecting 60% as much water as air at A/F ratios as low as 9:1. So I think it would be difficult to drown the engine. You'd probably run into other problems first.

With water alone, the engine's "max output" seemed to drop richer than about 12:1. With Alcohol/water 70/30 it began to drop if the A/F ratio got richer than about 10.5:1. Remember that's still at a 60% ratio to the fuel.

In this report (812) the engine was non-intercooled, and ignition timing was set at 30 degrees BTDC. Because of the affect water has one Theta Flame, adding water was essentially like pulling back the timing ... so of course more power was available. That's one of the few problems I have with that particular study. Otherwise, the 70/30 Methanol/Water mixture allowed about 65% more torque at 12:1 A/F ratio compared to the maximum torque without any water at about 9:1 A/F ratio.

Also keep in account that the required fuel/water has a LOT to do with the thermodynamic loads of that particular engine. Engines which have well cooled internal parts tend to like leaner A/F ratios. This may be because it's estimated 40% of the cooling effect of surplus fuel is used cooling the combustion chamber, while 60% is used cooling the air-charge.

Apologies for the excessively long post!

Adrian~
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  #19  
Old 30-09-2004, 09:23 AM
Richard L Richard L is offline
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I am begining to understand the published paper a bit more. The paper's main aim was to demonstrate the effectiveness of the controller using the "Ionization Gap Sensor" technique and not a study of the effect of water injection. Nevertheless, it did show the interaction of water in a combution process.

The paper also further demonstrated life after PPP. I think you might like post it here again. I too have the link of the university on can access their papers.

Based on the theory, practice and results of NACA and the above paper, it appears that water injection is a unique substance to controlling detonation without loosing power and torque as in the case of a "rich" a/f ratio.

In you next posting of the "life after ppp", if the chart show no delay/extension on "Mass Fraction Burned", it will clearly proof the water does not delay the burn-rate as a whole compare to rich a/f. Delaying the burn-rate promotes the onset of detonation.

We shall soon get to the point that we can throw some ideas how to tune an engine with water injection - b_boy's original question.
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  #20  
Old 30-09-2004, 08:31 PM
JohnA JohnA is offline
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I think that we need to differentiate between non-intercooled engines and well-intercooled ones.

Ages ago I had a primitive W.I. setup on a non-intercooled turbo bike (before the compressor) and the engine would feel extremely happy no matter how much water I would inject (suck out of the nozzle more like)

I'm not sure that this would be the case in a well-intercooled setup

The 1945 documents all refer to non-intercooled engines, don't they?

PS
This is a very interesting set of threads, a far cry from the usual rubbish and slagging matches on other automotive forums.
Nice one
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