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Author Topic: Injector pressure vs. injection timing  (Read 4958 times)

May 15, 2004, 01:52:28 pm

Dropout

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Injector pressure vs. injection timing
« on: May 15, 2004, 01:52:28 pm »
It seems to me that as the injector opening pressure drops due to wear the effect is to advance the timing.

Does anyone have any idea what the correlation is, i.e. 160 bar is OEM spec, but 150 bar is equal to 5 degree (or whatever mm) advance etc?



Reply #1May 16, 2004, 01:58:50 pm

fspGTD

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Injector pressure vs. injection timing
« Reply #1 on: May 16, 2004, 01:58:50 pm »
The rate of pressure rise at the injectors prior to them opening (and even for a little while until they get completely open) is nearly instant.  Edit: this is shown in figure 23 of the 1.5D SAE paper.

Please explain why you believe this to be the case - why would increasing nozzle opening pressure advance injection?  Thanks.
Jake Russell
'81 VW Rabbit GTD Autocrosser 1.6lTD, SCCA FSP Class
Dieselicious Turbocharger Upgrade/Rebuild Kits

Reply #2May 16, 2004, 02:34:03 pm

Dropout

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Injector pressure vs. injection timing
« Reply #2 on: May 16, 2004, 02:34:03 pm »
I must not have been clear. Reducing the injector opening pressure would effectively advance the ignition timing since the fuel would be delivered into the chamber slightly sooner.

I heard a presentation this past week that showed that B20 (20% biodiesel) advanced the ignition timing by .6 to 1.3 degrees due to the difference in viscosity.

Quote from: "fspGTD"
The rate of pressure rise at the injectors prior to them opening (and even for a little while until they get completely open) is nearly instant.  Edit: this is shown in figure 23 of the 1.5D SAE paper.

Please explain why you believe this to be the case - why would increasing nozzle opening pressure advance injection
?  Thanks.

Reply #3May 16, 2004, 04:56:02 pm

VWRacer

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Injector pressure vs. injection timing
« Reply #3 on: May 16, 2004, 04:56:02 pm »
Okay, now that I understand your question, the answer is no. ;)

And Jake is right about the pressure rise. I haven't seen the "figure 23" he refers to, but from basic physics (the Bulk Modulus for Elasticity (E)), we know that a liquid system pressurises at the speed of sound in that medium. For oils, the speed of sound is approximately 1200 mps. Since the pipes from the pump to the injectors are typically less than 1/3 of a meter long, the fuel lines reach full pressure in about 1/4 of one-thousanth of a second.

According to this Dept of Energy article, the combination of the measured values of density, speed of sound, and isentropic bulk modulus together could cause an approximately 1° of injection timing advance when compared with straight diesel. This makes sense, as experiments using gasoline, water, glycerine, alky, etc., all show similar changes in injection behavior as well.

One of our members (Giles) has done experiments shaving small amounts off the back of the injector pintals (less than 1mm) with good results in terms of speeding up the rate of injection. You might search the threads here and at our old old forum.

Hope this helps! :D
Stan
C-Sports Racer