------------------------------------------
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~
|