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Idle thoughts
http://www.lostjeeps.com/forum/phpBB3/viewtopic.php?f=5&t=26844		 Page 1 of 1
Author:  nursecosmo [ Sat Nov 24, 2007 3:09 am ]
Post subject:  Idle thoughts
One of the major reasons that common rail systems are more efficient than the older diesels apart from the infinitely variable timing is that with the super high injection pressures almost perfect atomization of the fuel can be achieved. If that is so then Why do they run at <23000 psi when under light load and<5000 psi at idle, supposedly the time which requires the best burn for emissions requirements. I have researched the theory behind the system and cant find the reason for this. Any thoughts?
Author:  Tinman [ Sat Nov 24, 2007 3:46 am ]
Post subject: 
Part of it has to do with the cp3's ability to make pressure at the rpm it is running. I don't think it could make 23000psi at idle because it isn't turning fast enough. I have a analog rail pressure gauge on my 06 Cummins. While coasting down hill with no throttle, the engine is being turned faster than idle and rail pressure is 10-15K depending on rpms.
Author:  gmctd [ Sat Nov 24, 2007 1:16 pm ]
Post subject: 
Your Jeep runs @ 17-19000psi @65mph - I've seen pressure spike to ~22000 just nailing it in the driveway, but 3500rpm is not normal operational condition, being at a standstill - 23300 is absolute max pressure outta any CP3, but is not necessary and is not used - ECM controls rail pressure at all rpm to give best power and economy under all loading demand, and to reduce NOx, which is a big problem in Diesel engines because of all the oxygen we have left over after the combustion event, even moreso with a turbocharger.

A larger engine, with more displacement but with the same CP3 system, may require a little higher rail pressure, but none runs at 23300, unless reworked for full WOT loading - they usually run two CP3's in tandem to get the added volume, rather than depending on max rail pressure of one IP.
Author:  nursecosmo [ Mon Nov 26, 2007 1:16 am ]
Post subject: 
I realize what the min and max pressures are for the cp3 thus the<. In fact mine runs ~ 16k psi @ 65mph. The pump can put out close to max psi at <1000rpm because the pressure is controlled by the metering valve in combination with the frequency valve. The point of my question is that the optimum atomization is achieved at the highest pressure, so why doesn't the system run at a higher pressure all the time? Incidentally NOx is not caused by excessive O2, it is caused by the very high temps of turbo-diesel combustion creating CO and NOx and a lower amount of of CO2 as opposed to the higher amount of CO2 and lower NOx produced at lower combustion temps.
Author:  gmctd [ Mon Nov 26, 2007 2:29 am ]
Post subject: 
Boost is not selective - you get equally more N with the added O2 - idle does not require 22kpsi for optimum idle, nor does cruising unloaded - rail pressure is increased at high loading to also keep the injected volume steady per cylinder at high system flow rates - rail fluctuation can result in unequal power output - ECM gives correct pressure for any loading to provide max btu from the fuel charge - the combustion event can take a relatively long time to occur at low rpm - doesn't have that luxury at high rpm, where the window is very narrow - takes higher pressure to get the fuel into the cylinder on-time at 5000rpm at greater timing advance, and at the retard required to lower NOx
Author:  msilbernagel [ Mon Nov 26, 2007 2:45 am ]
Post subject: 
Just guessing, but I suspect there are electro-mechanical reasons for not running max pressure all the time. The goal is to deliver a certain amount of fuel with fairly precise timing. I believe most injectors are on/off, so the only two variables are length of the injection event and the pressure available while the injectors are flowing. If there isn't much demand, say - near idle or low engine loads, the pulses might be Ok with 9k psi but too short to be reliable at 20k psi. With the the CR Dodge Cummins, there is a pre, main, and post injection event. Imagine how short those pulses might get at max psi if there isn't much need for fuel at the moment. Given variations in the injectors (and system) ability to reliably open and close with precise timing, any errors would be magnified with shorter pulses also - so some 'stretching' of injection pulse lengths could be deliberate to minimize erratic timing and poor(er) control of the amount of fuel injected?

Mark
==

nursecosmo wrote:
I realize what the min and max pressures are for the cp3 thus the<. In fact mine runs ~ 16k psi @ 65mph. The pump can put out close to max psi at <1000rpm because the pressure is controlled by the metering valve in combination with the frequency valve. The point of my question is that the optimum atomization is achieved at the highest pressure, so why doesn't the system run at a higher pressure all the time? Incidentally NOx is not caused by excessive O2, it is caused by the very high temps of turbo-diesel combustion creating CO and NOx and a lower amount of of CO2 as opposed to the higher amount of CO2 and lower NOx produced at lower combustion temps.
Author:  nursecosmo [ Mon Nov 26, 2007 11:37 am ]
Post subject: 
Ok. That actually makes sense. Thanks msilbernagel :)
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