Automotive Common rail high pressure pump

[Charly Segaert]

Hello everyone,

I have a question about a common rail high pressure pump.

Most common rail pumps make use of a pressure control-valve which controls the amount of fuel that goes to the plunger. This way it is possible to control how much fuel goes to the rail, so it controls the pressure.

But... If you are going downhill without giving any throttle (no injector opens for a while), the engine still spins and therefore the pump spins as well. When the pump spins, but the pressure control-valve is fully closed, what happens? Is there a minimum amount of fuel that always goes to the plunger? And doesn't this keep on pushing more fuel in the rail, thus increasing pressure? Or doesn't the plunger get any fuel at all, creating a kind of vacuüm?

It's a small thing I don't yet understand. Thanks in advance
Charly

 

[Work Hard Play Hard]

There are a number of ways used to control pressure. Isn't the most common to return the fuel to the tank?

 

[Charly Segaert]

Well, I heard of 2 systems:
One where a pressure control valve is mounted on the rail to open and close the return line.
Another where an Mprop opens and closes the inlet of the high pressure pump.

I make use of the second system. If the Mprop is fully closed, pressure in the rail should drop because less fuel is moved by the plungerpump.
But if you give no throttle going downhill, no fuel leaves the rail. Yet the high pressure pump still spins because the engine does. So if you want the pressure to drop, the plungerpump shouldn't pump any more fuel till throttle is applied again. But to do this, the plunger shouldn't move any fuel at all, and this would result in a vacuum. Is it possible?

I know it's a strange question, but I need it because i have to get a rail up to pressure without opening any injectors for a long time.

Greetings

 

[Work Hard Play Hard]

Well, I heard of 2 systems:
One where a pressure control valve is mounted on the rail to open and close the return line.
Another where an Mprop opens and closes the inlet of the high pressure pump.

I make use of the second system. If the Mprop is fully closed, pressure in the rail should drop because less fuel is moved by the plungerpump.
But if you give no throttle going downhill, no fuel leaves the rail. Yet the high pressure pump still spins because the engine does. So if you want the pressure to drop, the plungerpump shouldn't pump any more fuel till throttle is applied again. But to do this, the plunger shouldn't move any fuel at all, and this would result in a vacuum. Is it possible?

I know it's a strange question, but I need it because i have to get a rail up to pressure without opening any injectors for a long time.

Greetings

https://www.dieselnet.com/tech/diesel_fi_common-rail_control.php