Author Topic: Theory/Principles?  (Read 3207 times)

ronmar

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Re: Theory/Principles?
« Reply #15 on: June 28, 2018, 09:39:27 PM »
It is not the pump, but the injector.  Common rail systems dont use an injector pump as we typically understand it.  On ford powerstrokes, you feed fuel at about 60PSI to the common fuel rail and all the injectors, very similar to a gas EFI system, a simple regulator dumps excess fuel back to the tank to maintain pressure on the rail.

The powerstroke has a high pressure engine oil/hydraulic pump that can push pressure to about 3500 PSI.  It has a solenoid actuated regulator valve that is controlled/modulated by the ECU to regulate this oil pressure between about 500 and 3500 PSI. This high pressure engine oil is fed to all the injectors

The injectors are the key to the high pressure atomization. They are basically a combination of a hydraulic cylinder and a piston pump/syringe.  They have electrically actuated hydraulic spool valves that open when commanded by the ECU to admit the 500-3500 PSI engine oil.  The hydraulic cylinder piston face is 7 times larger than the injector piston face so the oil pressure is amplified into the fuel by a factor of 7. This gets you a variable injection pressure between 3500-24500 PSI.  Injector pressure, firiing duration and timing are all controlled by the ECU.

Interesting system, I think they use the same ECU as the gas engine, but it feeds a second module instead of the injectors directly. The added module is called a fuel injection control module or FICM. It takes the injector firing commands from the ECU and converts them to the 48VDC used by the injector spool valves...  works great, but is dependant on engine oil quality to maintain injector operation...
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dkmc

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Re: Theory/Principles?
« Reply #16 on: June 28, 2018, 10:05:23 PM »

Wow! Interesting.
Even more complicated than I thought!
So I guess the guy on the machine shop forum that says he's not changed the oil in his Powerstroke for 54,000 miles and it's "still going strong" is either pushing his luck, or getting ready for a big repair bill.

ronmar

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Re: Theory/Principles?
« Reply #17 on: June 29, 2018, 12:06:53 AM »
Big repair bill.  The critical part is the spool valves on the injectors, when they stick, no injector firing.  Next would be the oil pump, buried in the top of the engine, lots of labor to reach and replace, then the oil reg solenoid valve.  Couple thou in parts possibly...
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cujet

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Re: Theory/Principles?
« Reply #18 on: July 26, 2018, 01:17:00 AM »
My VW PD (pumpe duse) engine had unit injectors that were self contained. Driven by the roller camshaft, they contained the injector, nozzle and pump, along with a relief solenoid. The injector always makes a full stroke. The pressure was relieved up to 7 times per stroke to control engine speed/power and to manage engine "sounds" (to quiet the idle)

It ran wonderfully, and was trouble free. It was said they operated over 30,000 psi

https://en.wikipedia.org/wiki/Unit_injector
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