View Single Post
  #1  
Old 09-07-2015, 11:30 AM
GregMN GregMN is offline
Registered User
 
Join Date: Aug 2009
Location: MN
Posts: 1,253
CDI Purge Cycles

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.
__________________
Greg
2012 S350 BlueTEC 4Matic
2007 ML 320 CDI
2007 Leisure Travel Serenity
2006 Sprinter 432k
2005 E320 CDI
1998 SLK230 (teal)
1998 SLK230 (silver)
1996 E300D 99k, 30k on WVO
Previous:
1983 240D, on WVO
1982 300D, on WVO
1983 300CD, on WVO
1986 300SDL 237k, 25k on WVO (Deerslayer)
1991 350SDL 249k, 56k on WVO - Retired to a car spa in Phoenix
1983 380 SEC w/603 diesel, 8k on WVO
1996 E300D 351k, 177k on WVO

Last edited by GregMN; 09-07-2015 at 03:45 PM.
Reply With Quote