Injection delay from lines

Leiffi

Leiffi

New member
There was earlier some discussien about injection delay from different lenght injection pipes. Searched an old book that had formula to calculate it.

Injection delay = L / V , where L is lenght of injection pipe in metres and V is speed of sound in used fluid, for diesel it is about 1300 m/s.

Injection delay in crank degrees = 6 x n x (L / V) , where n is engine rpm (r/min)
 
I believe that would only explain how long it would take sound to go from DV to injector. And if you wanted to know that and use miles per hour for the fluid speed of sound, you'd have to compute the injector line in miles. So like .000378 / diesel speed of sound. Then divide by 3600 to get the time units from hours to seconds.

This doesn't really tell you about injection timing though because it's just talking about sound, not the actual compressibility at XXX pressure. To take compressibility into account you need to use the bulk modulus. I believe compressibility is 1/bulk modulus but for it to mean anything you'll have to mess around with the units.
 
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straight 6 roar said:
I believe that would only explain how long it would take sound to go from DV to injector. And if you wanted to know that and use miles per hour for the fluid speed of sound, you'd have to compute the injector line in miles. So like .000378 / diesel speed of sound. Then divide by 3600 to get the time units from hours to seconds.

This doesn't really tell you about injection timing though because it's just talking about sound, not the actual compressibility at XXX pressure. To take compressibility into account you need to use the bulk modulus. I believe compressibility is 1/bulk modulus but for it to mean anything you'll have to mess around with the units.
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Why do you want to use miles per hour ?

Speed of sound is related to density and bulk modulus, that´s why it's easier just to use speed of sound because end result is the same. http://hyperphysics.phy-astr.gsu.edu/hbase/sound/souspe2.html

Do you think those engineers who wrote the book didn't know what they were doing ?

There´s also a delay from delivery valves and needle but their effect is very small compared to that from injector lines.
 
Sorry I misread some stuff in your post when I was using my phone. You are spot on with your units.

A sound wave with very very small pressure passing through diesel fuel, and actually compressing a volume of diesel fuel to a significant pressure (300 bar injector pop pressure for example) are two different things. A sound wave requires very little pressure. In regular conversation, a sound wave is very low pressure. .01 Pa or .0000001 bar. And that's traveling at the speed of sound 340 m/s at sea level. Although this is in air (sound travels slower in gas than liquid), you can see it requires very little pressure.

Actually compressing diesel from 1 bar to 300 is not the same as a sound wave. It requires a much larger displacement of volume (in comparison). Change in volume = (Initial volume x change in pressure) / Bulk Modulus. Using that equation: (1 m^3 x 29900000 Pa) / 1300000000 Pa = .023 = 2.3%. That was using kerosene going from 1 bar to 300 bar, but diesel will be very close.

So the volume in the line will have to be squeezed 2.3% until the injector pops.
 
Did some quick math. Using kerosene, a 22" long line, .084" diameter, and a 215 P7100 plunger lift chart, the delay from when the spill port closes to when the fuel finally hits 300 bar pop off pressure is ~2.5* engine crank rotation. This is neglecting the small amount of fuel in the injector fuel gallery/feed ports and the fuel below the DV.

With ridiculously big lines such as .120 and the same 215 P7100 the delay is around double so close to ~5* engine crank rotation. Longer lines than 22" would also delay the injection further, obviously.
 
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Yes, that formula gives you change of delay at different rpm, delay from fuel compressing in lines stays the same.
 
The formula doesn't exactly work how you think it does though. How are you calculating velocity? If bore stays 12mm, and stroke is constant, but then the DV changes to a larger size, you're increasing velocity. If you put bigger lines on though, you're decreasing velocity.

Then you've got to think about the fact that diesel fluid has air in it and will compress (this means it is compressible before the speed of sound), so now your overall volume plays a big role.
You've got to know your pop pressure (initial when it just starts to leak), and then your exact volume, the bore and stroke, and then find out the amount of compression you'll see before you hit your pop pressure. Then realize that because of the compression being relieved, your injection pressure is going to rapidly decrease to a limit during that event.

Then (assuming parts and lines aren't going to flex and swell, like they actually will do), is to realize that increasing RPM will shift the pressure spike earlier and increase it. The inertia from the injector internals will actually allow it to have a significantly higher pressure spike.


That's why it honestly is better to just play will your timing until it is where you feel it performs the best.
Even if you put together an equation to fine a precise answer, it's not going to take into consideration the tolerances that your fuel system is probably off by anyways.
 
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Leiffi said:
Yes, that formula gives you change of delay at different rpm, delay from fuel compressing in lines stays the same.
Click to expand...

The delay I gave you is in crank degrees regardless of what engine speed. Speaking purely in terms of time, if the engine is turning slower, it will take the plunger a longer amount of time to compress the fuel to 300 bar. If you want it in time, just calculate how long 2.5* of crank rotation takes at your desired rpm. For 3000 rpm I'm getting .0069 seconds.

However ignition delay is something completely different having to do with combustion.

CorneliusRox said:
The formula doesn't exactly work how you think it does though. How are you calculating velocity? If bore stays 12mm, and stroke is constant, but then the DV changes to a larger size, you're increasing velocity. If you put bigger lines on though, you're decreasing velocity.

Then you've got to think about the fact that diesel fluid has air in it and will compress (this means it is compressible before the speed of sound), so now your overall volume plays a big role.
You've got to know your pop pressure (initial when it just starts to leak), and then your exact volume, the bore and stroke, and then find out the amount of compression you'll see before you hit your pop pressure. Then realize that because of the compression being relieved, your injection pressure is going to rapidly decrease to a limit during that event.

Then (assuming parts and lines aren't going to flex and swell, like they actually will do), is to realize that increasing RPM will shift the pressure spike earlier and increase it. The inertia from the injector internals will actually allow it to have a significantly higher pressure spike.


That's why it honestly is better to just play will your timing until it is where you feel it performs the best.
Even if you put together an equation to fine a precise answer, it's not going to take into consideration the tolerances that your fuel system is probably off by anyways.
Click to expand...

Bottom line is that you cannot calculate injection delay on the premise of a sound wave opening the injector. 300 bar is a **** ton of compression, and a subsequent volume decrease that the plunger must sweep, compared to a tiny sound wave. To get my numbers in the previous post I used the line volume, plunger volume, how much the plunger lifts per 1/2* of crank rotation etc.

At the end of the day unless you're working for Cummins it doesn't really matter. An hour on the dyno with an adjustable pump gear and you'll know the best setting regardless of all the variable.
 
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straight 6 roar said:
The delay I gave you is in crank degrees regardless of what engine speed. Speaking purely in terms of time, if the engine is turning slower, it will take the plunger a longer amount of time to compress the fuel to 300 bar. If you want it in time, just calculate how long 2.5* of crank rotation takes at your desired rpm. For 3000 rpm I'm getting .0069 seconds.

However ignition delay is something completely different having to do with combustion.



Bottom line is that you cannot calculate injection delay on the premise of a sound wave opening the injector. 300 bar is a **** ton of compression, and a subsequent volume decrease that the plunger must sweep, compared to a tiny sound wave. To get my numbers in the previous post I used the line volume, plunger volume, how much the plunger lifts per 1/2* of crank rotation etc.

At the end of the day unless you're working for Cummins it doesn't really matter. An hour on the dyno with an adjustable pump gear and you'll know the best setting regardless of all the variable.
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Thinking it again, you are wrong. Sound is pressure, higher pressure is louder sound, and it is pressure wave that is opening injector. People have measured timing close to pump and close to injector and get different timing. Why ? Because it takes some time for a pressure wave to reach injector.

But it's true that you find the optimal timing by testing, that formula is just for those who want to understand what they are doing and why something is happening like it does.
 
Leiffi said:
Thinking it again, you are wrong. Sound is pressure, higher pressure is louder sound, and it is pressure wave that is opening injector. People have measured timing close to pump and close to injector and get different timing. Why ? Because it takes some time for a pressure wave to reach injector.

But it's true that you find the optimal timing by testing, that formula is just for those who want to understand what they are doing and why something is happening like it does.
Click to expand...

Just saw this. No, you are not getting it.

The next time you are able to open a 300 bar pop injector with sound, let me know.


According to your "sound" theory, there could be a 3" diameter injector line between the pump and injector, and as long as it is the same length as factory there will be no difference in timing. Everyone knows this is wrong. I showed you with cold, hard math that there are more variables to consider than bulk modulus. Initial pressure, target pressure, injector line volume, length, plunger diameter, plunger lift per degree of crank advance etc.

Once you understand how much a volume contracts before reaching 300 bar, you will understand how this works.
 
injection delay

I was a solid believer of the math behind line length and injection delay until going to the dyno. We run an abnormal pump and line setup with injection lines about 10" shorter than the typical cummins pump and line setup. I was somewhat surprised to find on the dyno that peak power was found with essentially the same pump timing that others are running with the same injectors and 10" longer lines.
 
I think we need to note two instances. The mathematics prior would be considered a "Ideal" flow even when you incorporate variables and during operation we have "Actual" which half of your test would be done. The other half would be seen at the track to conclude the 'dyno test' hopefully. When testing through your observation it is only as good as the amount of inputs you install to observe or the amount you are capable of using.

Seeing line length mentioned, the shorter will obviously have less overall friction due to length but how much would this affect the overall compared to longer? How would a larger diameter line run compared to a factory sized line? Obviously the total volume would be different but would there be any changes to look closer to as well.
 
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