I have a friend with a Dodge Cummins who recently attended the Dodge Turbo Diesel Register May Madness event in Las Vegas. He had a conversation with some of the Dodge/Cummins engineering types about the new (January, 2004) Cummins 600 series engine that now has a catalytic converter. Not being a diesel tech, I hope my explanation of the conversation makes some sense...

I believe he was told that the fuel injection system now has an additional firing event compared to the injection system on the prior version of the engine. The event takes place just before the exhaust valves open and the purpose is to generate more heat for proper operation of the catalytic converter. This indicates to me that one of the reasons the newer Cummins has a decrease in fuel mileage is because the energy from the fuel on this firing is used mainly to heat the converter rather than power the truck.

Since the Duramax and Cummins have similar fuel injection systems, and since all Duramax LLY engines also have catalytic converters, I was wondering if GM and Isuzu are taking an approach similar to the Cummins folks and are using addtional fuel to heat the converter?

However, DMaxalliTech posted a very informative note last week in a thread about turbo lag that said the following:

"the variable turbo has absolutely nothing to do with boosting performance, its used for emissions mainly. The egr system on a gasser needs crankcase vacuum to suck the gasses into the engine, diesel dont make that.... so.... the turbo vanes are manipulated to put a negative pressure in the crankcase and thus suck in egr gasses...
its also used to load the engine during high idle to put some heat in it..... "

http://dieselplace.com/forum/forum_posts.asp?TID=9434&PN=2

So, is GM taking a different approach, such as using the variable turbo, or whatever, to satisfy the converter requirements? If so, is it more efficient than what Dodge/Cummins is doing?

It would be great if some of the knowledge diesel techs on this forum would give an explanation of the injector firing sequence for the Duramax, and if there is any difference between the LB7 and LLY. (This may give me a better understanding of the apparent increased fuel consumption of my LLY truck compared to my brother's 2003 LB7 truck.)

Thanks all,
Bruce H.

That would be pilot injection........


It is an initial shot of fuel prior to the main shot on the injector cycle......it is supposed to quiet the motors diesel clatter.


I do not know if it causes a reduction in emissions or not...the Tech's will answer I am sure and correct a layman's explanation.


Thttp://www.dieselplace.com/forum/smileys/Clown.gifNY

The main reason of the pilot injection, as described above, is to tone the "diesel" noise down a bit. I believe the big 3 all use it now and that is largly a reason for the overall noise reduction in the big 3 engines. I dont know if it has any to do with emissions, but it may... I dont think it effects mileage in any measureable amount.

Well, some of what you are describing sounds different from pilot injection.

With pilot injection, there is an early shot of fuel (aka the "pilot" shot) that gets the combustion started. Then the main shot of fuel comes in after that, for the rest of the combustion cycle. Sort of an attempt to slow the burn slightly - helps ease the BANG.

It's *possible* that, at least in Cummins' case, that they are issuing a 2nd/3rd shot, AFTER the main shot, to get some heat into the exhaust. (It's probably just a 2nd shot... those Cummins are so loud I doubt they utilize pilot injection.) Makes sense, as typical diesel exhaust isn't hot enough to light off your typical off-the-shelf catalytic converter.Edited by: OmniGLH

It's my understanding that the 600 Cummins uses a triple pulse injection from the Bosch system. Part of that to decrease noise and IIRC, it doesn't have EGR since it passes without it.

What BH is describing is not pilot injection, not if it occurs just before the exhaust valve opens. Would be interesting to see some EGT figures for this system compared to Duramax if this be the case.

OmniGLH and Dirty Old Man: You are right, I am not refering to the pre/pilot injection.

I attended a GM dog and pony show when the Duramax truck was being introduced several years ago. The engineer explained to me at that time that there were two firings of the injectors. One was a pre firing (pilot) with a small amount of fuel to reduce noise followed by the main firing for producing most of the power.

I am wondering if one of the diesel techs can tell us if the new LLY engine still has only two injector events, or is more going on now, as what appears to be happening with the Cummins.

(As I recall, I think my buddy was told the new Cummins 600 engine now has four events. The prior (pre January) engine had three events, but they added the fourth for heating the converter. Since I have not spent much time reading about the Cummins I could be wrong on the number of events, but maybe Hoot or one of the other Cummins folks can verify. It sure sounds like a lot is going on in a short period of time.)

As far as I know the 04.5 Cummins (325hp 600lb ft) has a three stage injection sequence.


1. Pilot Injection Event - Small Shot


2. Main Injection Event - Large Shot


3. Post Injection Event - Small Shot (to help burn remaining fuel in the cylinder hence lower emissions)

I'm trying to make a complicated topic simple here.


Oxides of Nitrogen (NOx) is the emissions component that caused alll 3 makers to release the latest versions of there engines on or before 1/1/04.


There are two ways to reduce NOx emissions in a diesel . . . lower cumbustion temperatures to prevent NOx in the first place or eliminate them in a NOx reducing catalyst. In addition to quieting the clatter pilot injection reduces peak cumbustion temps and pressures . . . so all three makers use it to reduce formation of NOx. This is also part of why they all use big intercoolers.


EGR also reduces cumbustion temps. Ford and GM use variable geometry turbos to make pressure in the exhaust manifold exceed pressure in the intake manifold to make EGR work. They also both cool the exhaust gasses before adding them to the intake stream.


The NOx reducing catalyst which Dodge uses instead of EGR requires raw fuel to do it's chemical magic. The post shot provides the unburned fuel. Early designs for NOx reducing catalysts used fuel pumps and lines to deliver the extra fuel to an injector in the exhaust just upstream of the converter . . . but the EPA decided that method was too easy to defeat in the intersest of better fuel economy.


The EPA estimates that the extra backpressure to make cooled EGR work cost about 1% in fuel efficiency, which should be too little to notice. (The difference in 19.8 MPG vs 20 MPG), while the raw fuel needed for the reducing catalyst increases consumption by about 5%. (19 MPG instead of 20 MPG). Histroy says that the EPA is probably too optimistic in both cases, but the relative impacts may be about right.


None of the companies are saying what they think the impact on fuel efficiency is.


The advantage to EGR is better fuel economy. The advantages to NOx reducing catalyst are that it is mechanically simpler there there is no concerns about additional sooting, exhaust gasses condensing in the intercooler or additional coooling system loads imposed by EGR.





Both systems, being controlled by the ECMs, can probably be disabled by aftermarket tuners.





The catlytic converters on Fords and GMs are there to catch (and theoretically burn off) soot only, while the one the Dodge reduces soot, NOx and unburned hydocarbons (LOTS of unburned hydrocarbons!).





Class dismissed.http://www.dieselplace.com/forum/smileys/Geek.gif

Great class Kid...http://www.dieselplace.com/forum/smileys/Clap.gif


Thanks for the intro to diesel emission control.