Residual Fuel Pressure

Topic: Residual Fuel Pressure (Read 2079 times) previous topic - next topic

Residual Fuel Pressure

June 28, 2012, 01:31:49 AM

I got curious about this in a Electrical Tech thread and don't want to hijack that thread off into the weeds.

What causes the fuel delivery system to remain pressurized after the fuel pump stops running?
I realize the fuel is trapped in the system by a check valve at the pump end, the injectors being closed and the regulator valve being closed.
But I don't understand why it stays pressurized.

Residual Fuel Pressure

Reply #1 – June 28, 2012, 04:59:30 AM

My guess would be because if it can't go back to the tank or return line.. there is no way the pressure can bleed out

Residual Fuel Pressure

Reply #2 – June 28, 2012, 09:33:25 AM

It stays pressurized so the car will have a relatively faster start the next time you try to start it.

Residual Fuel Pressure

Reply #3 – June 28, 2012, 09:53:57 AM

I always thought it was the closed injectors and the fuel pressure regulator keeping it from the return line???

Residual Fuel Pressure

Reply #4 – June 28, 2012, 10:17:37 AM

It DOES bleed off....

Residual Fuel Pressure

Reply #5 – June 28, 2012, 10:39:37 AM

Quote from: V8Demon;393358

It DOES bleed off....

This

Residual Fuel Pressure

Reply #6 – June 28, 2012, 06:36:51 PM

Quote from: V8Demon;393358

It DOES bleed off....

Sure, after awhile....

Residual Fuel Pressure

Reply #7 – June 28, 2012, 07:42:34 PM

Quote from: softtouch;393342

I got curious about this in a Electrical Tech thread and don't want to hijack that thread off into the weeds.

What causes the fuel delivery system to remain pressurized after the fuel pump stops running?
I realize the fuel is trapped in the system by a check valve at the pump end, the injectors being closed and the regulator valve being closed.
But I don't understand why it stays pressurized.

The pressure is relative to 0. Since when you shut off the engine the pump was making pressure, (on the order of 30 psi, determined by the regulator) the lines 'trap' the residual pressure in the system at it's last level. The check valve isn't perfect, so the pressure eventually bleeds down to zero over time. The service manual gives an acceptable decay rate, but the exact number eludes me right now.

I'm curious why do you ask?

Residual Fuel Pressure

Reply #8 – June 28, 2012, 11:16:44 PM

Quote from: flylear45;393392

The pressure is relative to 0. Since when you shut off the engine the pump was making pressure, (on the order of 30 psi, determined by the regulator) the lines 'trap' the residual pressure in the system at it's last level.

If we were talking about air that can be compressed I would agree. Since liquids cannot be compressed I don't see how this applies.
Someone suggested maybe the flexible fuel lines expanded under pressure and squeezed on the fuel after the pump stops.

Quote

I'm curious why do you ask?

Just curious.

Residual Fuel Pressure

Reply #9 – June 29, 2012, 10:30:06 PM

Liquids cant be compressed. But they can be made to become pressurized. The reason they build pressure is exactly why they cant be compressed. Example when you have air in your brake lines the brakes dont work. The master cant compress air enough to make pressure enough to operate the calipers for example. But brake fluid cant be compressed so it transfers the high braking pressures to the calipers the master develops. Anyone that has had a caliper brake line collapse will find out first hand what residual pressure can accomplish. Basically fluids cant be compressed. But they can be compressed enough to build high transferable pressures from a device that will build pressures in a particular system. So because a fluid cant be compressed is why it can build high pressures. All the fluid does is transfer whatever the pressure device makes is distributed without loosing efficiency. Example a master cylinder can make 1300 LBS and the brake fluid transfers this pressure to the caliper. And it does it at almost 100% effectively because it cant be compressed.

Residual Fuel Pressure

Reply #10 – July 01, 2012, 11:59:55 AM

Fluids can only be pressurized as long as something is applying pressure. I don't think they can store or somehow remember pressure that was previously applied.

The fuel system retains its pressure until some fuel eventually "bleeds" past the check valve back to the tank.
Why does this relieve the pressure?
Does it allow the fuel to expand?
No. The fuel would only expand if it had been compressed and liquids can't be compressed.

The only explanation I can come up with is, there is some pressure that wants to reduce the volume or size of the area where the liquid is trapped..

Residual Fuel Pressure

Reply #11 – July 01, 2012, 04:30:49 PM

Quote

Basically fluids cant be compressed. But they can be compressed enough to build high transferable pressures from a device that will build pressures in a particular system.

Technically, you can compress a liquid, however the amount of pressure needed to do so is incredible in most cases. The pressures you find in braking system will show a negligible compression.

Quote

there is some pressure that wants to reduce the volume or size of the area where the liquid is trapped

I would replace the word pressure with force. I always viewed it as pressure is the resistance against said force attempting to compress.

Example: A blower car setup making 12 PSI of boost. We change the restrictive exhaust system and now it makes 10 PSI albeit with increased power output. The input force in this system is the same but the restriction on the back end is less.

Now lets say we have a volume of a gas in a sealed container. If said sealed container could be changed to hold twice the volume the pressure would be less.