common rail

How exactly does common rail work? Is all the pressure that is needed in the rail? or does the rail just feed it the fuel and something mechanical "pumps" the injector? just wondering if someone can enlighten me

I wasnt sure if it operated just like multiport efi on a gasoline nmotor.

Yes just like multiport only super high pressure and powerful injectors and solenoids to open them.

there are two different types.

First pressurizes the rail to the full amount. The cam and electronics determine which and when an injector opens. And closes.

Second type, like the VW, is not fully pressurized. The injectors themselves do the work.

thanks for the info.. i was thinking man thats a lot of pressure to be in those rails all the time! would think its prone to leaks.

There is unit injection like the PD TDI's & then there is common rail, which is a different deal. The cam does not perform part of the injection process in a common rail system.

Bosch's common rail system works more or less like this:
There is a fuel delivery pump that supplies the fuel rail, or "accumulator" with fuel at full delivery pressure. The Bosch pump they show in their manual is a three plunger radial type pump. The fuel rail then holds the fuel at full delivery pressure to be used by the injectors. The rail holds enough fuel that when an injector uses some fuel for injection the pressure of the rail does not change. The rail also has a type of pressure relief valve that regulates maximum system pressure. If the pressure goes too high this opens & some of the fuel bleeds off into the fuel return system. The injectors are connected to the rail and are seeing full delivery pressure all the time. They are balanced so they stay closed even with that much pressure on them. The injectors have solenoid valves at the top & are all connected to the ECM. The ECM sends an electrical signal to a solenoid to tell it when to open & for how long. The magnet force of the solenoid is enough to change the balance in the injector & allow the needle to lift off the seat & then fuel injects into the cylinder. The neat thing is that this opening & closing is all controlled by electrical signals rather than fluid pulses from a pump like our TDI's have. This allows the system to have things like pre-injection, main injection & then in some applictions post-injection. Some of these Bosch systems are rated up to 29,000 psi of injection pressure! /images/graemlins/eek.gif


There are other companies like Siemens that make common rail systems & I'm not sure how different they are. If you're a tech weenie & want a good read try this:



It's $16 or so at Robert Bentley Publishing.

if im not mistaken isnt this the system my dads duramax uses?

yes

If I'm not mistaken, isn't pressure in a CR system limited to a lower pressure than PD because of the rails themselves? I believe that PD runs at 2050 Bar which is 30125 PSI!! I remember shimming injectors up to their max cracking pressure on 6.9 IH engines. That and setting the pump timing where it was supposed to be always made for happy customers.

I was aware of CR systems that ran at 1650 bar (24255 PSI) but not ~1975 bar (29000 PSI). What engine is it used on?