Diesel Theory

[Matt400]

Hello,

I am new here and also a first time Diesel owner. I need some education on Diesel theory, specifically the new 24V motor.

I come from a strong automotive back ground and know 2 and 4 stroke engine theory well including EFI theory and operation.



What I don't know is the fuel/ignition management and combustion process for Diesels. I don't plan on any tear downs on the new truck just yet but feel odd not knowing whats going on under the hood. In addition it will help me to make sound decisions for modifications being able to understand how those gains are achieved.



Can anyone help me with this or point me to a particular page that I can study?



An example of what I am looking for, but for 2-stroke engines can be found here:

http://w3.one.net/~jschust/animations.html

 

[David Muench]

Does this help?



http://auto.howstuffworks.com/diesel.htm

 

[Vaughn MacKenzie]

Matt, to put it simply:



--Ignition is started by the heat of compression in the cylinder.

--The fuel is injected directly into the cylinder (instead of in the intake).

--Ignition occurs almost the instant the injection starts, and so injection occurs a few degrees before TDC. Injection timing = ignition timing.

--There is no throttle plate, so even at idle the engine is drawing in a full volume of air, restricted only by the turbo, air filter and intercooler.

--Air/Fuel ratio is not an issue on diesels compared to gassers. At idle diesels are around 80:1 (yeah, SUPER lean) and roughly 12:1 at WOT.

--Engine speed and power are controlled by the amount of fuel injected into the cylinder.



Hope this helps



Vaughn

 

[Matt400]

Originally posted by Vaughn MacKenzie

Matt, to put it simply:



Injection timing = ignition timing.

Air/Fuel ratio is not an issue on diesels compared to gassers. At idle diesels are around 80:1 (yeah, SUPER lean) and roughly 12:1 at WOT.

--Engine speed and power are controlled by the amount of fuel injected into the cylinder.



Hope this helps

Yes it helps! good info, now how is injection timing controlled? Do they use Crank, Cam sensors and a computer same as the gas engines for ignition timing?

Must be when you shut off the key that a solenoid cuts fuel flow???

 

[Matt400]

Originally posted by David Muench

Does this help?



http://auto.howstuffworks.com/diesel.htm

Hey David... .

That was some interesting reading along with a neat animation, thanks a bunch for the link! I knew I would find what I was looking for by joining TDR.

I did not know the injectors were placed in the combustion chamber. That makes em subject to a harsh environment for sure.



Anyone know what the compression ratios are on the CTD, PSD and Duramax, or do I dare type those words here?



Ok... . what am I doing wrong here, how come only one of my posts shows my sig when I have checked "show signature"?

 

[Blakers]

Originally posted by Matt400

Yes it helps! good info, now how is injection timing controlled? Do they use Crank, Cam sensors and a computer same as the gas engines for ignition timing?

Must be when you shut off the key that a solenoid cuts fuel flow???

Some diesels vary the injection timing (such as the 24 valve Cummins) and some have a set timing (such as the 12 valve Cummins) that is physically set by the position of the timing gear on the injection pump. If you look in the 12 valve forum you'll see where guys are increasing power and mileage by increasing their timing. The 24 valve engines shuold do it by themselves.



Yes, to shut off the engine, a solenoid (or hand-operated cable) shuts off the fuel supply.



Hope that helps.



Blake

 

[Blakers]

Originally posted by Matt400

Ok... . what am I doing wrong here, how come only one of my posts shows my sig when I have checked "show signature"?

It is set up that way to save space.

 

[David Muench]

Originally posted by Matt400

Anyone know what the compression ratios are on the CTD, PSD and Duramax, or do I dare type those words here?

I believe the 03 Cummins is 18:1 approximately, and the current PSD and Duramaxes are about the same. The old Chevy diesels (6. 2 and such) that weren't turboed were even higher, like 22:1. Same goes for the old non turbo 6. 9 and 7. 3 ford diesels. Those numbers might not be exact, but they're close.

 

[DBVZ]

Matt400 Sig shows once per page.

 

[Big_Daddy_T]

I think the s/o 24v has a 16. 5 to 1 and the h/o has 17 to 1. Ive read of the non turbo'd having 24 to 1 before.

 

[Matt400]

Originally posted by Blakers

Some diesels vary the injection timing (such as the 24 valve Cummins) and some have a set timing (such as the 12 valve Cummins) that is physically set by the position of the timing gear on the injection pump. Blake

So then are the 12V motors injection pumps mechanically timed and driven where as the 24V is computer controlled and electrically driven?

 

[Matt400]

DBVZ and Blakers,

Thanks for the sig info, its interesting how different boards across the net configure things, makes good sense not to repeat it every-time.

 

[David Muench]

Originally posted by Big_Daddy_T

I think the s/o 24v has a 16. 5 to 1 and the h/o has 17 to 1. Ive read of the non turbo'd having 24 to 1 before.

That was true on the 2nd gens, but on the 3rd gens the compression ratio is the same HO vs. SO. It's somewhere in the 17:1 - 18:1 area.

 

[Blakers]

Originally posted by Matt400

So then are the 12V motors injection pumps mechanically timed and driven where as the 24V is computer controlled and electrically driven?

Yes Matt, you are right. The nice thing about the 12 valve engines is they will run with no battery whatsoever. You'd have to wire up the shutdown lever but they'd never miss electricity.

 

[rbattelle]

The injection pump is still mechanically driven, but the injectors use solenoid valves.

 

[rbattelle]

Also

For a good book in IC theory, including diesels, I strongly recommend the 2-volume series by Charles Fayette Taylor, "The Internal Combustion Engine in Theory and Practice". Volume 2, in particular, has some great stuff about diesel combustion characteristics.

 

[Matt400]

Originally posted by rbattelle

The injection pump is still mechanically driven, but the injectors use solenoid valves.

So then the new injectors operate much the same as an EFI Gas engine with a mechanically driven pressure pump behind them?

If so then why not an electric in tank pump?



On these new 5. 9s is the injection pump simply a fuel pump or is there more to it?

 

[Blakers]

Matt, I'm just learning the details of this new breed of diesels, but I think you are basically right in your comparison to gasoline EFI engines. The this diesel runs I believe 20-something thousand psi in fuel pressure (instead of about 50 psi in the gas engine) and the injectors have to be tough enough to spray fuel into combustion chambers. A big task compared to a gasoline engine.

 

[rbattelle]

Well, the mechanical pump driven from the engine is inherently more reliable than an electric version. Besides, I think the power required to drive the pump would require an excessively-bulky electric motor with large power requirements (which would require larger batteries and larger alternator).



The injectors themselves are very much similar to gasoline injectors, except they're built to handle much more significant pressures and temperatures.

 

[A Johnson]

Matt your 03 has basically 3 fuel pumps. First is a low pressure (15PSI priming)pump to draw fuel from the tank and supply the mid pressure pump (150PSI), the midpressure pump is a basic gear pump, it supplys the high pressure pump with fuel, the high pressure pump builds up to 25000 psi in the common rail for injection. The high pressure pump uses three oposing pistons pumping into a common chamber, much like the internals of the VP-44 pump. The electronic injectors only control injection beginning and ending (duration and timing). Quantity of fuel injected is determined by the duration of the injector and pressure in the common rail system. This is a very basic explanation.



I think you make a good point on putting the low pressure pump in the tank, seems to me the in tank pumps a pretty reliable these days. As far as that goes I think you can throw the priming pump away, as long as you don't lose the prime in the fuel system. For years Cummins just used a gear pump to pull fuel from the tank and supply the injectors with pressure on the PT system, it worked very well but you need to run at least a 1/2" supply line and sometimes when they ran out of fuel it took a bit to start them, also the PT system was a lot more tolerant about starting with some air in the system.



A Johnson