CDI Purge Cycles

[GregMN]

Short purge cycles on a high pressure common rail system will leave VO in the rail.
Running on cold VO (less then 160 degrees) can cause injector malfunction and poor spray patterns.
This leads to unburnt VO in the cylinder that can turn to carbon and impact the piston rings in their grooves.
Impacted rings allow fuel past the cylinder piston to the crankcase.
That can cause washing down the oil on the cylinder walls and then wear on the unlubricated walls.
Fuel in the crankcase will dilute the oil. VO in the crank case oil... I can only assume is worse than diesel in the crankcase.
Water injection can help remove the carbon and free the rings, but it cannot repair damage to the cylinder walls or engine bearing surfaces.

On an older diesel, say a 603, the unused fuel is returned from the IP and the injectors at about the same rate and a single purge of the combined returned fuel system can purge the system of VO in preparation for the next cold start.
On a high pressure common rail system, the fuel is returned from the high pressure pump and the common rail at very different rates.
Fuel in the common rail is purged via the fuel rail pressure relief valve, and the injectors return fuel lines.
The pump purges relatively quickly in about 12 - 15 seconds.
The common rail purges via the pressure relief valve, and the injectors return fuel lines and takes about 40 seconds.
The fuel enters the rail in the middle. The pressure relief valve is on one end.
The fuel on the opposite end of the rail from the pressure relief valve can only exit via the injectors, either into the combustion chamber, or via the returned fuel line.
It takes a lot longer to purge the common rail. It takes about 40 seconds and then some run time.
If you set up a common purge for the pump and the rail, you will be returning 1/4 to 1/2 gallon of diesel to the VO tank each 40 second purge. You would not be saving much fuel doing that all the time.
So, for a common rail system there should be 2 separate purges, one for the high pressure pump and one for the rail.
I believe that there will always be a small amount of VO at the far end of the rail beyond the last injector after the purge cycle.

If a common purge is used for the pump and the rail and it is too short, you will purge the pump, but not the rail. The next cold start, you will be injecting cold VO. The high pressure injectors will not like that in cold weather. The one farthest back on the rail will fail first. It will fail by having the bleed circuit stick open under the lower pressure of the starting sequence. That will bleed off the pressure from the rail to below the firing point of the injectors (about 3000psi).

To determine purge cycle timing:
Install 2 separate clear sight glasses in the fuel return lines for the high pressure pump and for the rail.
Put a couple of quarts of ATF in a gallon container and run hose from it to the input side of the VO fuel filter.
Run a fully warmed up engine on VO until the fuel in the sight glasses are extremely red with ATF. At idle, this will take a long time because our engines are so dam efficient at idle, but do it. It will be worth the time.
Start your purge cycle and time how long it takes for the fuel in each sight glass to become completely clear of all red ATF.
Understand that the purge cycle will be shorter if you purge while driving, but how much shorter will vary a lot depending on engine speed and load. So to be safe I go with the time to purge at idle.
I use adjustable time delay relays to manage my 2 separate purge valves. They start when the fuel source is switched from VO to diesel.

[funola]

Good info. How much diesel is consumed in your 2 separate purges for the high pressure pump and rail respectively?

[moon161]

Is there a way to change the flow rate through the common rail?

Change characteristic of PRV?
Change flow rate of lift pump?

[GregMN]

I do not remember how much diesel the purge cycle consumes. It was more than a 606 or 603.

To change the flow rates through the pump and rail it would take some computer programing. If you open up the rail pressure relief valve even the slightest bit, the pressure drops and the engine quits. Changing the flow rate (if it could be done) will only change the amount of time it takes to purge the device, not the amount of fuel needed to do it. And what would you save ? The total purge time is 40 seconds. You could put a lot of effort and money into it and only save 10 seconds.

Diesel is about 1/2 the cost that it was when I got the ML. So I have been running it on diesel since last fall. I have not even converted the E320. If fuel heads for $5 again I will probably do something about it.