Friday, Jun 21, 2019 at 12:50
There's no need to pressurise fuel tanks with common rail diesels. The electric fuel pump in the tank supplies plenty of fuel as soon as the ignition is on, and the CR systems are pretty much self-bleeding - unless vast quantities of air have been allowed to enter the fuel system.
The CR system, electric fuel tank pumps, have a higher capacity than the old mechanical injection fuel transfer pumps, because a portion of the high pressure fuel delivered to the CR injectors, is bypassed and allowed to return to the tank, to facilitate the removal of built-up heat from the fuel, caused by pressurising it to extremely high pressures.
This bypassed fuel runs through a fuel cooler to remove the heat from the fuel.
As a result, this constant bypassing of fuel is acting like an air bleed, and it will remove air from the system pretty rapidly.
The old mechanical injectors also had a fuel bypass from the injectors, but it bled very little on average.
In fact, the earliest Cat diesels (1930's) ran the fuel injector bypass line down the side of the engine to the open air, and just let it drip!
In addition, with current diesel fuel systems, you will find ECU's will not allow CR fuel systems to run dry, to protect the CR fuel components from damage.
With CR pressures running at 25,000 to 30,000psi (170 to 200Mpa), and with CR injectors running at half the clearance of mechanical injectors, it's imperative that there's adequate fuel in the high pressure pump and injectors at all times, or serious damage will result very quickly.
Cheer, Ron.
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