Injector Reaction Time - Effect on Timing?

[VegasDMAX]

Ok guys, you may want to grab a cold one before you start reading this.

Several months back, Michael Patton aka Killerbee started a discussion on idle timing for the LLY in this thread:

http://forum.efilive.com/showthread.php?t=9692

He emphasized how much fuel is wasted at idle with stock timing. Well, my city mileage is pretty poor and of course in Las Vegas we run the A/C all the time which probably consumes more fuel, so I started looking at changing the idle timing on my LBZ. For the LBZ, we have table B0909, injection timing table B, which apparently serves as base timing down to 300 rpm. 300 rpm is probably for starting, so I only wanted to mess with the 600 and 800 rpm columns.

My first thought was to try the 50/50 rule, where you have 50% of the injection before TDC and 50% after. The famous spreadsheet available on TheDieselPlace.com to calculate this doesn't go below 1400 rpm, so I planned on manually calculating the numbers for 600 and 800 rpm and changing table B0909 accordingly.

First, I looked at B1001, which is fuel pressure for rpm vs. injection quantity, and could see that for idle speeds and typical fuel quantities, the fuel pressure is 30 to 35 MPa. Then looking at table B0720 in the 30 MPa column gives you the microseconds (us) of injection time for the given fuel quantities. Obviously you need the injection time to calculate how many degrees before TDC the injection has to start to give you the 50/50 pulse. However, here is where the problem comes in. In the 30MPa column, you can see that to get 0.1 mm3 of fuel takes 369 us of injection time. But to get 10 times that much fuel, i.e., 1 mm3, takes 382 us, only 13 us longer. So it must be that most of the time is used up just opening and closing the injector. Looking further, to get 10 times THAT much fuel, i.e., 10mm3, takes 615 us, or 233 us longer. Taking the injector reaction time (IRT) out of it, I can't explain why the time-to-deliver verses quantity of fuel is not linear for a constant pressure, but maybe somebody versed in fluid dynamics can.

My point is that, for low fuel quantities, the IRT seems to be significant. Just eyeballing B0720, I think the IRT is around 360 us. If that's true, then the actual injection time for a 1 mm3 pulse of fuel is about 22 us. At 800 rpm, 360 us is about 1.73* of crankshaft rotation, 22 us is only about 0.11*. The standard thinking is that you need a value of (1.73 + .11)/2 or 0.92* BTDC injection timing to get the 50/50 pulse split. I guess that is still true if the actual injection spray occurs in the middle of the 360 us, but I don't think it does. If you consider that the spray goes on until the injector is closed, then the spray probably ends at the end of the 382 us time period. So, if you set your injection timing at .92* BTDC, the actual injection, or the spray, starts at around .81* ATDC, still a very retarded setting.

The only other aspect I can think of right now is that as the injection quantity increases, the injector reacttion time becomes less significant, but as the rpm increases, it becomes more significant again. For example, at 2000 rpm, the 360 us is about 4.3* of crankshaft rotation.

So what do you guys think? Am I way out in left field here? Does any of this matter? Am I too obsessive/compulsive? How do you guys set your timing? Should I just increase it until it sounds funny and then back it off a little? I'm throwing this out there for discussion.

[justin123]

i dont thin injector reaction time is any diffrent the reason i believe that the pulse width can stay the same but inject more fuel is not reaction time its fuel pressure. say take a air compressor and set the regulator at 10 psi and get a air nozzle and put it in a balloon and squeeze the trigger for 1 sec then set the regulator to 50 psi and push it for 1 second. same opening time but more flow b/c of more pressure. not sure if that helps any but someone correct me if im wrong like i said im not the smartest m/f in the world as far as timing goes u need to just test and tune and scan alot. it depends on boost numbers and rpm and pulse width alot and if your running pilots. race tunes u want to use more timing than u would for a MPG tune b/c u need a quick fast burn mpg tunes u want a quick but slightly slower burn but dont want it to burn to slow or inject fuel to far ADTC b/c the fuel will be chasing the piston down wasting fuel and power and create smoke so u have to balence the timing out with what kind of tune ur wanting. increasing boost needs decrease in timng dependin on how much boost b/c of the increase cyl. pressure and increased heat just and example. i have also found tho lower boost gets better MPG on the highway but that was with a tune i made fyi. and i dont much use 50/50 timing i usually use more but i run no pilots. hope that helps some

[killerbee]

There is a really easy experiment to do, to see injector latency.

Reduce pilot injection pulse. When it gets low enough that no fuel is being dispensed, you will get a sharp rise in SPL from the engine.

You can repeat the experiment with various pressures programmed in for idle pressure.

[VegasDMAX]

i dont thin injector reaction time is any diffrent the reason i believe that the pulse width can stay the same but inject more fuel is not reaction time its fuel pressure. say take a air compressor and set the regulator at 10 psi and get a air nozzle and put it in a balloon and squeeze the trigger for 1 sec then set the regulator to 50 psi and push it for 1 second. same opening time but more flow b/c of more pressure.

Yes, but I'm talking about constant pressure here. The numbers I referred to from table B0720 are all in the 30MPa column.

There is a really easy experiment to do, to see injector latency.

Reduce pilot injection pulse. When it gets low enough that no fuel is being dispensed, you will get a sharp rise in SPL from the engine.

You can repeat the experiment with various pressures programmed in for idle pressure.

SPL? Good thought Michael. However it's my understanding that the pulse width table B0720 for an LBZ controls the pulse width on both the main injection pulses and the pilot pulses. Any idea how to reduce the pilot pulse by itself? Also, it would be useful to know if the actual fuel spray occurs in the middle of the latency or near the end. Any ideas on that?

[killerbee]

oh, sorry, I think you are right. Former ECM's had sep tables. You will have to reduce mm3, then log pilot pulse to see where the noise changes.

Edit: spl is noise or sound pressure level. Get's rattly like legacy diesels

[GMPX]

Yeah, that is a bit annoying on the Bosch that there is one PW table for all injections.
Just one other thought, some of you may have already been down this path. There is a number of 'papers' on the internet explaining that with pilot injection you get a more complete 'burn' of the main pulse (plus the reduced noise).

So, what would be the result of reducing the main pulse and increasing the pilot pulse? Lets say you have the pilot currently shut off and are commanding say 12mm3 at 'x' RPM/Load. What if you drop the Main mm3 down to 7mm3 and bump the pilot to 5mm3? Maybe you could get away with less on the main with a larger pilot?

Unfortunately I don't have my own Dmax to play with or I'd try myself.

Cheers,
Ross

[bballer182]

Yeah, that is a bit annoying on the Bosch that there is one PW table for all injections.
Just one other thought, some of you may have already been down this path. There is a number of 'papers' on the internet explaining that with pilot injection you get a more complete 'burn' of the main pulse (plus the reduced noise).

So, what would be the result of reducing the main pulse and increasing the pilot pulse? Lets say you have the pilot currently shut off and are commanding say 12mm3 at 'x' RPM/Load. What if you drop the Main mm3 down to 7mm3 and bump the pilot to 5mm3? Maybe you could get away with less on the main with a larger pilot?

Unfortunately I don't have my own Dmax to play with or I'd try myself.

Cheers,
Ross

Interesting theory...

[VegasDMAX]

So, what would be the result of reducing the main pulse and increasing the pilot pulse? Lets say you have the pilot currently shut off and are commanding say 12mm3 at 'x' RPM/Load. What if you drop the Main mm3 down to 7mm3 and bump the pilot to 5mm3? Maybe you could get away with less on the main with a larger pilot?

I agree that would be a great experiment. I'd like to see somebody with one of Fingers' cylinder pressure sensors try it. I think the risk of getting high pressures is large.

[killerbee]

Maybe you could get away with less on the main with a larger pilot?

exactly what I have found. In some cases noise is reduced also.

[GMPX]

Michael, I figured of all the people you might have already thought about this route
So it's a worthwhile thing then?