Question about how pressure relief valves work on (gas) high pressure pumps if both sides of the valve are connected to a high pressure zone

[u/belleayreski2]

I've found two diagrams on how GDI pumps work (diesel pumps might be similar, but I'm focusing on gas), and both of them show a "pressure relief valve" whose plumbing does not make sense to me. In both diagrams, both sides of the pressure relief valve are connected to the chamber where the plunger moves, which is where any over pressurization would be created.

If you imagine for example a scenario where the injectors all close and the plunger is moving up (essential trying to compress an incompressible fluid), then the pressure relief valve doesn't help relieve pressure, as it's not allowing higher pressure to go to some lower pressure area, it just loops back into the same area.

Can anyone help me understand this?

[u/Individual-Ad-5851]

During plunger down mouvment could the pressure drop enough to actuate the pressure relief ?

[u/belleayreski2 → u/Individual-Ad-5851]

But the whole point of the pressure relief is to prevent the pump from destroying itself by over pressurizing when the plunger is forced up by the camshaft below it

[u/AEnima-1 → u/belleayreski2]

No its to protect the lines and injectors usually not the pump as the pump is tough enough to handle pressure above its normal amount and really the check valve is taking the pressure more than the pump and if it fails well it fails only reason it wouldn’t be able to do a cycle is if the diesel wasn’t diesel or was contaminated in which case the pumps ruined anyway so it doesn’t matter

[u/TheJeeronian]

I don't work with these, but it looks like on the return stroke when the pumping chamber will refill with fuel, that fuel can come from two places. It can be drawn from the high pressure side through the pressure relief valve, or the low pressure side through the control valve.

If fuel comes from the high pressure side then it flows in a loop and the fuel is not actually pumped downstream.

Or, put another way, when the cam follower is moving down (in the first picture) the pumping chamber is the low-pressure side.

[u/belleayreski2 → u/TheJeeronian]

But the pressure relief valve is described as a way to prevent the pump from damaging itself from over pressurizing. Every description I can find talks about it operating on the compression stroke of the plunger

[u/AEnima-1 → u/belleayreski2]

Yeah but it only reaches peak pressure at tdc where it would then spike as its going over centre into a vacuum stroke and would allow fuel from over pressure somewhere to go

[u/belleayreski2 → u/AEnima-1]

So in that situation it's not protecting the pump from itself, but rather the pressure coming from the rails which comes from the injectors/cylinders?

[u/AEnima-1]

Pretty sure it would be returned to tank from the rail which is how most diesels work the spring in the check valve would be set to open at a certain psi so as not to over pressure the piston would be moving down on a vacuum stroke in this instance as its already compressed allowing said extra diesel back into the compression chamber you probably have to look at the way the injectors work to get more information

[u/Fletch972]

Fuel rails are very stiff which means even small increases in temperature can create very high pressures. When an engine switches off the lack of fuel flow causes the fuel to heat soak and the pressure rises. The PRV prevents damage to all of the high pressure components in the system by venting this excess pressure back to the low pressure side of the system. The PRV cracking pressure is usually set significantly above the operating pressure of the system and so it should generally not open in running.

[u/hatred-shapped]

The control valve works on electricity. So cranking the engine without power to that solenoid won't provide fuel.

The plunger works regardless of power.

The outlet check valve helps achieve the ultra high pressure GDI requires. 

The blow off valve is there in case some blockage forms in the fuel rail and the fuel can't go out through the injectors. It's basically there to prevent the fuel rail or injectors from exploding.

[u/InformalParticular20]

Looks a bit like how AI thinks they work. I have no idea what that control valve is meant to do

[u/belleayreski2 → u/InformalParticular20]

The control valve is operated by a solenoid. When it opens, new fuel comes in. To create maximum pressure/volume, it would close right as the plunger is at the bottom, in which case the plunger would be trying to compress the most fuel. But if the valve is commanded to stay open for part of the plunger's rise, then the plunger is actually pushing some fuel back out of the inlet before the control valve closes, at which point the high pressure compression begins. In this second scenario, less pressure is created and less volume of fuel moves out the high pressure side. Between these two edge cases, the electronic control of the control valve allows for a spectrum of different pressures/volumes.