Competition Timing and cylinder pressures

[NoSeeUm]

OK... .



This thread may not be for everyone. But I am just wondering what effect advancing the timing has on cylinder pressures. There seems to be allot of truth and internet myth spread around on this subject.



What I think I know about advanced timing:

* Increase mechanical efficientcy (More power better MPG)

* Increase cylinder pressure

* Increase cylinder temperature

* Decrease EGT



I am wondering why timing causes the increase in cylinder pressure. Here is a P-V diagram of a diesel engine.



Where:

a - e = Exhaust Stroke

e - a = Intake Stroke

a - b = Compression Stroke

b - c = Fuel Burn

c - d = Power Stroke

d - a = Exhaust



Notice b - c the pressure is constant. As also illustrated here.



Now for gas engines the pressure rises at ignition. Which is illustrated here and here. Steps 3 - 4 are the fuel burn.



The difference is a gas engine is constant volume at ignition and a diesel engine is constant pressure at ignition.



Anyone care to take a stab at this?



Jim

 

[JFaughn]

My guess is that , timing is refuring to when the fuel is added , we are adding high pressure fuel to a contained space that is under pressure , then theres the burning of that fuel add more pressure .
So its not timing directly , its what is being timed .
I had to bookmark that link , thanks .

 

[NoSeeUm]

No other takers?



Is it because maybe the injection occurs BTDC? Then the fuel burn gets compressed?



It does not make sense an aftermarket tuner would set up like that, but who knows.



Jim

 

[DCreed]

from what i understand that IS correct. the injection /ignition MUST take place JUST before tdc and as rpms rise the timing advances even more as the time it takes fuel to burn is a constant, which i believe explains your theory.

 

[GOT-Torque]

Along the same lines, what sort of timing do these engines run? I'm talking the degrees before TDC when fuel injection starts? 20 degrees?



How much does it change based upon rpm,temp,load or performance boxes? Does it change 10 degrees or 2 degrees?



I see people throwin' timing numbers around on 12V trucks, never seen or heard of a number on a 24V truck...

 

[NoSeeUm]

I believe on a 12V the timing number has to do with the timing of the P-Pump cam lift point vice actual engine injection timing. Correct?



I don't believe the range on 24V timing is any where near 20 degrees, more like +- 5. This is due to the mechanical restraints in the VP44. Probably have that wrong.



I get the part about advancing the timing for higher RPM's and the fixed time it takes to inject and burn fuel. Due to the faster velocity of the piston the fuel burn still occurs at about the same point relative to TDC.



Why does advancing the timing at lower RPM's raise cylinder pressures?



Jim

 

[Mhannink]

the reason timing numbers are thrown around the 12v forums is cause its static and so you advance the timing to get the power set where you want the engine to run. i. e. more power up top vs. down lower.



the 24v guys dont worry about it so much cause the vp has dynamic timing. i. e. it can change much like the distributor on a gasser. it can advance the timing according to rpm so you can have a wider power band and smoother running engine.



as far as advancing it. it does raise cyl pressure because if you inject the fuel sooner in the cyl it will expand sooner and make the cylinder pressure higher. and you are right it does everything you have listed above.

 

[NoSeeUm]

Thanks; but I guess that is my point. The fuel burn does not expand in the cylinder and raise pressure, check the P-V diagram comparing points b to c. The pressure is constant for the fuel burn. The maximum cylinder pressure is set by the displacement and stroke length.



I am assuming that during b to c the piston is moving downward hence the constant pressure. The pressure stays constant until all of the fuel is consumed then the pressure drops in the normal adiabatical curve.



Notice that the shape of the power stroke curve is the identical shape off the compresson stroke. Also notice it is not shifted upward as in the gas engine. The curve is shifted to the right, with the area inside the two curves representing the relative Hp output just as it is with any engine.



I am confused sorry... .



I guess it would be nice to see a P-V diagram representing advanced timing.



Jim

 

[GHolcomb]

Ok... can't resist... here is my 1. 5 cents!



You will notice that on the P-V diagram maximum pressure is obtained at TDC or V2. In the diagram the fuel is not injected until TDC. That means that during the compression stroke there is only air in the cylinder. However, if you inject fuel into the chamber before TDC, the fuel squeezes the air even more and the result is higher pressue at TDC because now you have the air plus the fuel in the cylinder. The more fuel you inject before TDC, the more room it takes up, the more it squeezes the air, the higher the pressure at TDC.



Also, I think the higher pressure means higher cylinder temperature as well. The more you squeeze the air the hotter it gets.



There it is... 1. 5 cents worth!



Greg

 

[NoSeeUm]

Yeah OK... . :{



I have been thinking that also. The P-V curve with advanced timing might start to resemble the P-V curve for a gas engine to a limited amount.



Where:

a - e = Exhaust Stroke

e - a = Intake Stroke

a - b = Compression Stroke

b - c = Fuel Burn

c - d = Power Stroke

d - a = Exhaust



Becomes:

a - e = Exhaust Stroke

e - a = Intake Stroke

a - b1 = Compression Stroke

b1 - b2 = Compressing initial part of the fuel charge and incipient stages of the fuel burn

b2 - c = Fuel Burn

c - d = Power Stroke

d - a = Exhaust



The b1 - b2 volume would still decrease as per the normal adiabatic curve, but the pressue could build toward a straight vertical vector. Same as in the gas engine P-V except it would be occur BTDC. From what I can figure out, that could reduce the horizontal offset that occurs during the normal b - c fuel burn duration. So the P-V curve would shift upwards a little and to the right less. At some point I would think that the duration of the fuel burn would have to be increased to obtain the right shift offset other wise the area inside the curve would decrease which is relative to Hp output.



I can see how at lower RPM's with the static timing of a 12V this could really limit power, because b1 - b2 could have a significant "anti-rotation direction force" effect. IE: The b1 - b2 - c component is too vertical and not enough to the right and allot of the energy in the fuel is used pushing the piston the wrong way. At the higher RPM's the P-V would settle into the conventional diesel P-V so b1 - b2 would decrease or cease to exist.



Not sure why an electronic tuner would do this intentionally. There must be a trade off for timing relationship that I am simply to dense to comprehend at this stage of my learning.



There does not seem to be a whole lot of factual information out there that I can easily find and based on the responses in this thread. I would guess there is quite allot of "I believe", which is what I was trying to sort out. Maybe I need to push the "I believe Button" myself.



Thanks;

Jim

 

[NoSeeUm]

Well hmm... .



As I sat here proof reading and editing my previous post something dawned on me. That for some reason makes allot of sense, but probably wrong. If I go back to.



What I think I know about advanced timing:

* Increase mechanical efficientcy (More power better MPG)

* Increase cylinder pressure

* Increase cylinder temperature

* Decrease EGT



It all starts to come together if I consider the length of the c - d curve (Power Stroke) relative to the EGT. Also I must assume that the longer that the c - d line is, the cooler the EGT will be. Just in relative terms.



Now as I think about advancing the timing and how it shifts the P-V curve vertical and less horizontal at b - c (Fuel Burn). To a limited amount increases the area inside the P-V curve which will increase Hp output. Most importantly the curve as part as the same process lengthens for the c - d portion of that curve. A longer c - d means lower EGT for the same quantity of fuel burned.



Make sense?



Jim

 

[NoSeeUm]

I made some drawings today. See what you think. The numbers are not actual, they are for illustration only. I tried to illustrate the drawings so that Heat Input equals Heat Output or Q1 = Q2. This represented by the equal length of the Red and Blue heat transfer lines at the top left and bottom right. All these drawings are shown with no boost. The area inside the curve represents relative Hp.



#ad


P-V with no timing 50% fuel



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P-V with no timing at 100% fuel. Big horsepower and high EGT I would imagine. Needs more boost.



#ad


P-V with -10 timing at 50% fuel. Notice the top part of the curve. This is only what I think it should be.



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P-V with -30 timing at 50% fuel. Notice the top part of the curve with more timing.



#ad


P-V with +10 timing at 50% fuel. Again the focus is at the top of the curve. I am begining to think, this is closer to the OEM P-V curve. When a timing box comes in to play it is closer to my first drawing with no timing.



Opinions?



Jim

 

[GHolcomb]

Ok... you said "opinions" right? It has been way too long since Thermodynamics! I tried to derive a few more equations to help with the temperature side, but gave up.



On your curve showing the compression stroke, I think it might be good to put in where injection happens. That is the point where the curve will turn "near" vertical. I say near vertical, because the piston has not reached TDC, therefore the volume is still decreasing. The pressure will rise more quickly because you are injecting fuel (still incompressible?) in to the chamber with compressible air which will make the pressure rise more quickly.



Now... where does combustion start? Is it at TDC or BTDC? Here goes the opinion part... if the fuel is injected BTDC and combusts at TDC, it combusts at a higher pressure and higher temperature which leads to a longer, more powerful burn. This will lengthen the RED line meaning the piston moves farther down the expansion stroke before the pressure drops.



If the fuel combusts BTDC it seems some of the energy of the combustion will be used up in the compression stroke, leaving less for the expansion stroke.



What I was trying to figure out is why would adding timing lead to lower EGT? If you lengthen the RED line you also have to lengthen the BLUE line which would seem to mean higher EGT.



Also, how does boost affect this diagram?



BTW, I found an article on an SAE website that may explain some of this, but the site is down until tomorrow.



Greg

 

[Gary - K7GLD]

Keep in mind that the *start* of combustion, and FULL combustion do not occur at the same instant - and as RPM (and/or boost level) increases, the need to "light the fire" has to start a bit earlier in order to place full combustion at the most efficient point of the power stroke...



Sorta like bird hunting, where you have to shoot slightly AHEAD of the target to allow for motion...

 

[GHolcomb]

Is TDC the most efficient place in the powerstroke for full combustion?

 

[Gary - K7GLD]

Is TDC the most efficient place in the powerstroke for full combustion?

In a perfect world - probably, but few things are perfect, and I suppose the truth is that in order for best *overall* function and efficiency, there will necessarily be a portion of combustion that must start before TDC - much depends on engine RPM, fuel quality, and piston design in terms of how evenly the air/fuel charge flame front ignites.

 

[DCreed]

probably why many diesel designs use a pre-ignition chamber.

 

[NoSeeUm]

Ok... you said "opinions" right? It has been way too long since Thermodynamics! I tried to derive a few more equations to help with the temperature side, but gave up.



On your curve showing the compression stroke, I think it might be good to put in where injection happens. That is the point where the curve will turn "near" vertical. I say near vertical, because the piston has not reached TDC, therefore the volume is still decreasing. The pressure will rise more quickly because you are injecting fuel (still incompressible?) in to the chamber with compressible air which will make the pressure rise more quickly.



Yeah, I tried to show what "I thought" it should look like for the 3rd and 4th drawings with a bit of minus timing added. Notice the verticle sections of the RED line.



Now... where does combustion start? Is it at TDC or BTDC? Here goes the opinion part... if the fuel is injected BTDC and combusts at TDC, it combusts at a higher pressure and higher temperature which leads to a longer, more powerful burn. This will lengthen the RED line meaning the piston moves farther down the expansion stroke before the pressure drops.



The point where the injection occurs is where the RED line starts. The combustion duration will only continue until all the useable fuel is burned where the length of the RED line represents this duration. It is all relative at any rate, with Q1=Q2 the diagram is trying to describe no net temperature increase or decrease over the cycle. Take another look at the 2nd drawing with 100 fuel, notice how short the "Power Stroke" line gets between the RED and BLUE lines. That would be an example of a longer combustion duration.



If the fuel combusts BTDC it seems some of the energy of the combustion will be used up in the compression stroke, leaving less for the expansion stroke.



What I was trying to figure out is why would adding timing lead to lower EGT? If you lengthen the RED line you also have to lengthen the BLUE line which would seem to mean higher EGT.



I am starting to believe that relative EGT is determined by the length of the "Power Stroke" curve line from the point where combustion stops until the exhaust valve opens. The more you can lengthen this line the cooler the EGT will be. Adding timing extends the pressure of the "Combustion Line" (raises it) and hence extends the length of the "Power Stroke" line.



Also, how does boost affect this diagram?



I'll make a drawing to show you. The reason I did not was because it changes the scale a bit and I was trying to illustrate timing mostly.



BTW, I found an article on an SAE website that may explain some of this, but the site is down until tomorrow.



Greg

There is allot that the P-V does not show concerning burn rates, ignition etc... . But it does show the position of the piston. On the horizontal axis is TDC and BDC or . 06 liters and . 98 liters displacement. The vertical axis is 14. 7 psi (atmos) and 240 psi TDC compression.



Jim

 

[NoSeeUm]

In a perfect world - probably, but few things are perfect, and I suppose the truth is that in order for best *overall* function and efficiency, there will necessarily be a portion of combustion that must start before TDC - much depends on engine RPM, fuel quality, and piston design in terms of how evenly the air/fuel charge flame front ignites.

Yes I know that. The P-V does not describe the whole process. The idea is that it describes (or tries to) the "engine" heat cycle. Another example would be the Rankine cycle for steam turbines or the Otto cycle for gas engines.



FWIW for the life of me I can not find what a diesel cycle is called other than a Deisel cycle.



Jim

 

[fest3er]

Consider:

• it takes a finite amount of time for the initial fuel to begin evaporating, thus producing vapors that will burn
• as the fuel evaporates, it cools, thus reducing available heat, to some degree

The injection of fuel has to be timed so that a portion of the fuel has evaporated and started burning before TDC is reached. As long as the fuel-air mix is swirling around in the cylinder, the fuel will continue to evaporate and burn after TDC, thus maintaining high cylinder pressure even as the piston is dropping.

A 12V's standard injection timing is around 12 degrees BTC, which probably produces optimal operating conditions around 2000 RPM (and, of course, sub-optimal conditions around low idle and governed RPM). Consider if you have your fuel injected at 6 degrees BTC. The engine would run much quieter because ignition likely isn't happening until after TDC, and peak cylinder pressure is considerably lower. Remember, the usual way to reduce a 12V's NOx emissions is to retard the timing, to reduce peak cylinder pressure, which prevents NOx from forming. Of course, power is reduced as well.

Now consider if you have your fuel injected at 20 degrees BTC. If the charge air is cool (such as at little or no boost), the fuel won't evaporate rapidly, and idle operation may be quite similar to that at normal timing. But once boost increases, thus increasing the temperature of the compressing air in the cylinder, fuel will evaporate more quickly and burn sooner. The charge will be heating and expanding while the cylinder is still headed up to TDC.

Now think of what happens in a modified sled puller running 60 degrees of timing. They pretty much have to run reduced compression ratio so that the engine will idle: reduced compression allows fuel to be injected that early without reaching destructive cylinder pressure at idle (that is, ignition so early it tries to turn the engine backward). And as RPM increases on this engine, ignition begins to approach ideal conditions, but is still early, but is OK because the heat of compression is still tolerably low. When the engine is turning fast enough and enough fuel is being burned to light the turbos, ignition is nearly optimal. But the charge air out of the turbos is now too hot, and ignition begins to happen too early. Now they turn on the water injection to cool the charge air and increase the apparent compression ratio. It a combination of RPM, boost, water injection and injection timing that allows a sled pulling engine to generate such large HP without blowing up, because all the elements have been tuned and timed to produce ideal ignition at a specific RPM. Put differently, maximum torque and HP are achieved because all the elements that compression ignition rely on have been carefully balanced. Not enough water? Charge air is too hot and cylinder pressure could become destructively high. And power drops. Charge air is too cool? Ignition may well be delayed. And power drops. Similar things happen when any of the other variables change. Mechanically-injected pulling engines are a wonder of tuning.

This is one reason I still say that common rail engines with piezo injectors will eventually take over. Because it is so easy to vary injection timing on a CR motor, it will be relatively easy to produce a tune for whatever ambient conditions are found. It might even be possible to create a tune that checks all relevant parameters (charge temp, charge pressure, RPM, delta RPM, EGT, requested fueling versus actual RPM, cylinder pressure, block temp, head temp, coolant temp, oil temp, etc. ) and produces optimal torque and HP for the current application. At any RPM. For any injectable fuel.

Huge peak cylinder pressures usually happen because of very early ignition at an RPM. On many Cummins engines, the head gasket gives out first. On a few Duramaxes, rods and pistons have broken throwing the engine into auto preventative maintenace dissassembly mode.

Huge peak cylinder pressures also happen because nitrous is used without considering its effect on ignition timing: nitrous greatly increases the probability that O or O2 will be next to a fuel molecule that is hot enough to burn. This increased probability significantly advances ignition; injection timing must be retarded to compensate, so that ignition still happens at the best time.

As I said, things to ponder.