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Let me take a stab at answering this. Though someone more knowledgeable can probably do a better job.
As you have pointed out there are two possible Low Pressure lines the injector leak-back fuel can go.
1. Fuel return from High Pressure pump and Pressure Relief Valve. At less than 1 bar of pressure and sometimes even under slight negative pressure. Goes back to the tank through the fuel cooler.
2. Fuel feed to the High Pressure pump. At around 5 bars as you pointed out. Output of Low Pressure pump that goes through the filter.
There are two considerations when deciding where to dump the leak-back fuel.
First, if you think of it in terms of relative pressures and improving overall pumping efficiencies, you have 100s if not thousands of bars of pressure on the inlet side of the injector. So the injector has no problem dumping its excess fuel into a fuel rail that is at a measly 5 bars of pressure. In fact, you just saved some energy by not having to compress that leak-back fluid back up to 5 bars so that it can be ingested again by the High Pressure pump.
Second, and perhaps more important reason, is the piezo fuel injector has a small hydraulic (fuel filled) amplifier in it that couples the stack of piezoelectric ceramic disks to the valve needle in the nozzle. The piezo disk stack has a very small amount of movement (less than 0.1 mm) when energized. This amplifier, also known as the hydraulic ratio transformer, converts that small movement into a larger stroke which then moves the needle. If there is very low or negative pressure (vacuum) on the overflow side of the injector, then the fluid in this hydraulic amplifier drains away. And the amplifier stops working. Causing the whole injector to stop working. This is why having a little +ve pressure on the leak-back side of the injectors is important.
This video at the 4:10 minute mark shows what the amplifier module looks like and how it functions.
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1st Jul 2025 8:07am
