Common Rail Timing and Durability

[rbattelle]

What has Cummins done to address the durability issues with common rail injection as opposed to a jerk pump arrangement? Specifically, sensitivity to deposit formation and wear at the nozzle orifice (which decreases flow coefficient, eventually leading to increases in delay angle from less-than-desireable mixing rates).



Furthermore, I suspect injection timing is fixed, so how did they address the fact that fuel quantity delivered decreases with increasing engine speed? I believe the new injectors are controlled by pushrods; is the injection timing variable (which would require a variable injector cam)?

 

[Steve St.Laurent]

The timing is completely controlled by the ECM which is the primary reason for the common rail injection setup - better emissions control. The injectors are electronically controlled which allows that control. As a matter of fact there are up to 5 injection events during a single ignition cycle depending on conditions - 2 pre injection, primary injection, 2 post injection. Can't answer your other question.

 

[rbattelle]

Steve,



I'm not sure what is implied by "electronically controlled" injector timing? You seem to imply that a pushrod/cam arrangement is not in use here, but rather some sort of electromechanical injector actuation. If this is the case, then this is one very advanced engine indeed, and very significant gains could be made by reprogramming the injector timing algorithm!



If a common-rail injector is controlled by a pushrod/cam arrangement, then isn't the only option for variable injection timing a mechanism which varies the cam position (like Honda VTEC or similar)?



Or perhaps there is some other way to vary injection timing on a pushrod-actuated injector that I'm missing?



Bottom line: exactly what mechanism is used to open the injectors?

 

[Steve St.Laurent]

You are correct, it does not use a pushrod/camshaft arrangement. The injectors are an electromechanical design. As I said above there are up to 5 injection events during a single injection cycle. I don't know what the exact parameters are but sometimes there are less than 5 and sometimes the full 5 are used. The only way to make that work is through an electronically controlled system. There is a small amount of information on it at http://www.boschusa.com/AutoOrigEquip/Diesel/HDRailSystem/

 

[rbattelle]

No cam system for injector actuation is great! This means we can completely change the injection timing if we want. Man, this is one very sophisticated setup. Oo.



There is still some question about injector durability and susceptibility to deposit formation. I suppose only time will tell.

 

[rbattelle]

Ok, I was reading the Bosch site Steve suggested, and it says these injectors are "plezo-actuated". Anybody know what the heck that means?

 

[nickleinonen]

these new injectors work almost like a gasoline injector... in a gas engine, there is a rail pressure of up to 60 psi [more or less. ] and electronically the injector is fired at what ever fuel injector timing the pcm wants to use. the new common rail diesel works very similarly. the fuel rail pressure is up to 25,000 psi [or more] and the injector is opened electronically [piezo-actuated - think electromagnetically] and the high pressure fuel is forced through the same type of nozzle spray pattern that current diesel designs have. the injectors are higher voltage [96v i think these are]



the injectors should be just as durrable mechanically as current injectors. they probably will not be any more prone to carbon deposits than current ones are... .



but you are right in saying " I suppose only time will tell.

"

 

[Badunit]

Piezo-actuated probably means it uses piezoelectric material to fire the injector. Basically, when a voltage is applied across a piezoelectric material it changes the dimension of the material slightly. An electric signal therefore is turned into mechanical motion. Quartz and ceramics are common piezoelectric materials.

 

[jimnance]

I believe that the new injection setup is similar to that used in the N-14 (14 liter) Cummins engines for a few years. They have proved to be reliable, and the sophisticated electronics which support them continue to yield improved fuel economy.



Imagine, a 14 liter engine pulling 80,000 lbs down the road at better than 7. 0 MPG. This is reality.



The only downside to the electronic injector design is that they are expensive to overhaul or replace.



I'm not a mechanic by trade, just a fleet owner, but the injector design sounds similar.

 

[jwilliams3]

rbattelle,

The position of the engine is communicated via engine position sensor (cam sensor) to the ECM, the ECM decides electronically when to fire the injector.



The Common Rail on the 03 is always at a constant pressure (rail pressure sensor communicates with ECM also) of fuel provided from the Fuel pump (which is not timed), no decrease in fuel is expected at higher rpms.



The 03 fuel system is very simple.

Pump is mechanically powered (gear train), fuel is provided to the Common rail from fuel pump, rail lines lead from rail to individual injectors, injectors are electronically activated, injection timing is totally controlled electronically.



--Justin

 

[ladyjaine]

Do I get a Brownie point because I already knew what pizeo activated meant? I'm still trying to wrap my brain around the whole diesel four stroke system with the high compression. I understood gas combustion, I even understood hemis and controled vertex combustion. Diesel is just escaping me right now. I can't picture the process. Is it still really suck-squeeze-bang-blow?



Jean

 

[Diesel Freak]

Originally posted by ladyjaine

Is it still really suck-squeeze-bang-blow?



Jean

yep... . just add a little squirt!



suck-squeeze-squirtbang-blow!

 

[Badunit]

There are a few differences between gas and diesel engines. The main one is that the gas engine needs a spark to ignite it. So with the gas engine you inject the fuel into the air stream, let it get sucked into the cylinders and compressed along with the air, then you spark it off at just the right time. With diesel, the heat from the compression of the air in the cylinder is enough to ignite the fuel. So a diesel sucks air into the cylinders (no fuel), it compresses the air (which also makes it hot), then injects the fuel into the cylinder with the hot compressed air at just the right time and it lights off. Another difference between the two is that gas engines need a specific ratio of fuel to air to burn properly while diesels don't.

 

[nickleinonen]

4 cycles are pretty simple. now, throw a 2 cycle diesel into the mix and it gets interesting. scavanging, compression, power and repeat...

 

[Task125]

I love it when you talk diesel!

 

[Casey Balvert]

Originally posted by Task125

I love it when you talk diesel!

Yeah, I was going to say something like that too.



Casey

 

[Animal]

Originally posted by Badunit

Another difference between the two is that gas engines need a specific ratio of fuel to air to burn properly while diesels don't.

Nice job, but I'd add one thing to this. In a gas engine EGT's are controlled by fuel being added. In a diesel EGT's are controlled by adding air. This was the hardest thing to understand going from a gas engine to a diesel. The sticky thing here is gas engines have a higher safe EGT (about 1600) where a diesel max safe EGT is 1300. Can someone explain this.



Thanks

 

[rbattelle]

Ok, I know what piezoelectrics are so there must be a typo on the Bosch website. It specifically says "pLezo-actuated".



To answer the question about max EGT limits on diesels, the lower EGT limit may not be a materials limitation but rather an indicator of malfunction. What I mean is, if your EGT is above 1300 it doesn't necessarily mean you're going to start burning or melting parts but rather that something else is wrong somewhere because a diesel under "normal" operating conditions should never see more than 1300 degrees.



Or perhaps the 1300 limit prevents excessive detonation?

 

[ladyjaine]

I get it!

Poof! I get it! Now I really understand the need to replace air filters more frequently and the absence of spark/glow plugs. Now I can picture what's happening inside the cylinder. Is anyone looking into controlling the vertex inside a diesel cylinder? Do we know? I'm sure we care! Was there ever a 'hemi diesel' ? What advantage would it have over a non hemi?



Jean

 

[Prairie Dog]

I think the 1300 figure is due to the forged aluminum pistons that melt at 1340 degrees (I think).