Episode 1: [LINK]

Injector Control Pressure (ICP)... everybody talks about it, many know of its importance, but not everybody "gets" it. Our beloved HEUI injector was a marvel in its day - it enabled the injection of fuel through tiny nozzle holes in the 20,000 PSI range. That was an incredible feat at the time, when everybody was working on pressurizing the fuel without having it be a fire hazard - and somebody came up with the bright idea to pressurize the oil and have it push the fuel out with an intensifier piston. Genius.

"OK Tug, it's clear you're impressed with the HEUI injector in its day, but what does fuel at 20,000 PSI do for us?" Easy - it atomizes. Atomization is the whole ball game on a diesel, and higher pressure through a tighter nozzle means a better/more efficient burn. Read that as more power from the fuel with fewer emissions. For those not entirely sold on the importance of atomization with diesels, I'll share something with you:





Now... our ICP doesn't get us all the way to 20,000 PSI - but the ICP is amplified by an intensifier piston in the HEUI injector. The Electrical signal to the injector solenoid opens a valve, this lets in the ICP, and the ICP pushes against something akin to a syringe in the injector (intensifier piston). Like the syringe... there is a limited amount of fluid in our injector, and a little bit of pressure on the big face of one side makes the fluid shoot vigorously out of the tiny end. Once the injector is done firing, the pressurized fuel from the 45-plus PSI fuel pump refills the "syringe". The capacity of the injector is measured in CCs per 1000 cycles of the injector (like the AD 140 or the AC 160) - so we are talking tiny quantities of fuel per an injection event.

That's it for the injector (for now) - this is really about ICP. In order for the PCM to even get in the mood to fire a single injector, one of its requirements (there are others) is a least 500 PSI ICP. The 7:1 intensifier piston in the common AD or AC injectors brings the ultimate fuel pressure out the nozzle to 3500 PSI - that's just to idle. Wanna give it throttle? Well... that ICP's gunna need to come up. So.... Who does the PCM call on to make the ICP climb? It ain't Ghostbusters, but this bad boy has many aliases:

Injector Pressure Regulator, IPR, Injector Control Pressure Duty Cycle, or ICP DC. If I were to describe the IPR in basic terms - its a thumb over a garden hose. There is an electromagnet that pulls on a plunger that closes a valve. The harder the valve is closed, the more ICP pressure builds in the High Pressure Oil (HPO) system. The harder you press your thumb on a garden hose, the higher the pressure gets in the hose. You can see the effects of the elevated pressure in the hose when you slide your thumb to the side a little and the water shoots out instead of gurgling out.

Now... a 12 volt solenoid has pretty much two levels of force - all or none. Unless... you "cycle" a solenoid. Think of it as tapping on the valve quickly. The IPR "taps" fast pulses that have a "duty cycle", and this is how a simple solenoid adjusts the pressure on the valve. Think of it as a tap and hold on your touch screen or a tap and hold on most keys on your keyboard vs. a simple tap. A short duty cycle would have the voltage applied to the IPR about 10% of the time - say 2 ms on and 18 ms off as an example (10% of 20 ms total). That's how we get the idle ICP pressure of 500 PSI. On average, an IPR duty cycle of about 40% gets us about 2800 PSI on a stock truck with everything working as it should - that's 19,600 PSI of fuel out the nozzle. Here is a log of a stock tune on a stock truck, but with air in/out mods:

 

Thanks for the Schooling,You make that easy to understand.

 

Thanks Rich for the lesson.

The real hard working duo in our trucks are the IPC and the IPR. Replacing both at advanced age (+200K miles)would probably save fuel money and often results in a smoother running truck.
Larry