Diesel is a different beast than a gas engine yet shares a similar architecture...
On a gasser, you control engine speed with restriction in the intake (throttle plates) which produces a vacuum signal when the piston performs it's intake stroke. This limits the amount of atmosphere that can enter the cylinder, which becomes the limiting factor for power production. You then inject the fuel before the intake valve to mix it with the atmosphere. The mixture is pulled into the cylinder and then compressed. The mixture resists ignition. This is a function of the octane rating, higher octane more resistant to pre-ignition, which would manifest itself as Knock (VERY BAD!). This is why gas engines have spark plugs, to ignite the mixture that was resistant to Knock when the timing is right. Hense, the term "ignition timing" on a gasser. Once compressed, the spark plug ignites the mixture and the work is extracted by the piston. The exhaust is read by the O2 sensor and the Management system adjusts the next mixture to attempt to meet stoich and a lean/hot cycle.
A Diesel is fuel throttled. That is; no throttle plates, no (or little) vacuum. Its done purely on the quantity of fuel injected, but it goes about it differently than a gasser. The cylinders are free to take in as much atmosphere as they can, dependent on valve position. So it goes like this: Air is drawn in intake stroke, the intake valve closes, then it's just atmosphere that's compressed. Fuel is then injected directly into the cylinder which ignites (cetane rating) due to the high combustion chamber temperatures caused in the compression cycle. So we have "Injection timing" on a diesel as opposed to "ignition timing" on a gasser. You will often here this timing referred to as "spark timing' or "spark tables" on a diesel, but it's pretty much a hold over from guys working on gassers.The piston then extracts the work and the exhaust is expelled. Since there's no need to deal with a stoich mixture on a fuel throttled engine, there's no O2 sensor and no feed back to the Management system.
Cetane number (diesel fuel) and octane number (gasoline) both measure the tendency of the fuel to ignite spontaneously. In the cetane number scale, high values represent fuels that ignite readily and, therefore, perform better in a diesel engine. In the octane number scale, high values represent fuels that resist spontaneous ignition and, therefore, have less tendency to knock in a gasoline engine. Because both scales were developed so that higher numbers represent higher quality for the respective use, high cetane number fuels have low octane numbers, and vice versa.
So, Cetane rating is the ignition rating of the fuel. Since it's fuel injected directly into the cylinder when appropriate, pre-ignition is not a concern. A diesel just injects its fuel and goes about it's business. More fuel = more power.
There's a finite end to that though. Once you reach a point where you don't have enough air in the cylinder to burn all the injected fuel, power production stops, you start seeing that big ol' black cloud diesels have that nasty reputation for and EGT's soar.
What you need now is more air. That's why guys cram bigger turbo's on a diesel, to get in more air to get in more fuel. But there's a limit to that too. The engine is only going to take so much boost before it starts popping head gaskets, cracking blocks and doing all other sorts of nastiness (IOW - $$$$$$).
Now, to throw one more curve at you; lean/rich - hot/cold are backasswards (for a gas head guy) when comparing a diesel and gasser engine.
When a gasser goes lean (from stoich), it gets hot and starts toasting valves, melting pistons, etc. When it goes rich, it gets cold.
When a diesel goes "rich" (ie: lots of fuel) it runs hot and can melt things down. When it goes "lean", it runs cold. It all has to do with how a diesel is throttled compared to gas. remember: More fuel in a diesel means more power, but more fuel means more heat. Diesel fuel does have a "stocih" (pretty close to what it is for Gas) but it's just not important in the operation of a diesel engine. Diesels can run anywhere from 3:1 to 42:1. The "lean" number being idle, "rich" being perhaps a transient WOT condition.
The thing about diesels is just keep pouring the fuel and air to them, watch the EGT's and ECT's so the engine doesn't melt down (mechanical limitations of the materials) and run them hard until they explode.
Now, since we're talking a GM 6.2/5 forums here, you're looking at a high compression engine that can't tolerate a lot of boost and has some very real (IE: low) mechanical limitations.
As a "rule of thumb":
Sustained ECT's below 210
Sustained EGT's below 1000-1100
Max boost below 15-20 Psi.
Keep in mind, those are just rules of thumb, many have exceeded those number s before and have reported no problems.
Others?
Well, scattered parts are a pretty depressing sight.....
There are what is essentially a MAP and TPS sensor, but they don't serve teh same purpose as a gasser.
The MAP sensor is a boost sensor and feeds that info to the PCM for scaling fuel to boost. IOW, more boost, more available air in cylinder, more fuel injected according to the programs scalars.
The TPS is actually the throttle pedal (termed an APP-Accelerator Pedal Postion) in these trucks. These trucks are "fly by wire" throttles, unless you're running a DB2 injector pump. The throttle is essentially a rheostat (or is it potentiometer? I always forget) which tells the PCM where you have set the pedal position with your foot. The PCM then commands the PMD to open the Fuel Solenoid to open and injects the corresponding amount of fuel into the cylinders.
There's a fair bit of technical trickery going on for these being such old trucks....
Cheers.
THXalot!!!!!!
I do that when I eat to much broccoli! :hihi:
uttahere:
