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How is the A/F ratio controlled in 6.5td?

Ama83

Ama83

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I have done quite a lot of work with gasser turbo engines, but diesel is somewhat new to me. So I´m intrested about how mixture is controlled in these engs.
Is the amount of fuel based on MAP or TPS? Can these things go lean?
If I increase boost and put #9+ res in, is the engine running with correct mix?
With chip it´s possible to control boost (if OEM setting) and fuel charts, correct? What about when I have removed vacuum control and installed TM with max boost 1,2 kg/cm? It´s little bit too high, but I VERY rarely give so muck throttle that it rises over 1,0 kg/cm. With this setup, will it go lean?

Please explain to me the "basics" of forced intake at 6.5TD!!! :) THXalot!!!!!!
 
diesels can't go to lean and harm the engine like gassers can. You can go to rich and get black smoke high egts
 
You can JAM as much air into the chamber as possible without any negative effect, except for the power taxed and heat generated to create the charged air. (Turbo Boost)

If you have too much fuel, you belch black smoke.

Kind of why TURBO's found their way into diesel engines mainstream faster than Gasoline.

You can basically take a non turbo engine, slap a turbo on it, and crank up the fuel and have it go fast, or in the 6.5's case, go faster. :)
 
The fuel rate is completely dependent on pedal position. The fuel rate and turbo drive pressure determine the amount of air mass that passes. Like said, no strict A/F ratio. The only detriment is that too much boost is inefficient and too little boost is inefficient :) Youll also get higher IATs or EGTs or ECTs from one or the other. So thats why most turbo diesel towing types drive by EGT and ECT gauges to determine their efficiency and power limits.

When using a mechanical wastegate actuator the drive pressure is then completely regulated by fuel rate and RPM, more air passing and more fuel equals more drive pressure that spins the turbo. That and how tight the actuator is to bypass some of the exhaust gasses.
 
Bypass exhaust gasses,,, :D I do that when I eat to much broccoli! :hihi:

Bet you didn't think you be reading about Broccoli in this thread did you?

1 question,, and 4 of my favorite posters all in 1 thread! ;)
 
Bypass gasses and Broccoli? Only Dave would bring little green trees and farts into a thread about fuel/air explosions in confined places ...

I love this place.

Some days, I don't understand it, but I still love it.

:D
 
And my #1 poster chimes in,,, Hi Jim,, Long see, No time! :lol:

Aren't you glad you found this place,, Ama? :lol: :hurray: I gotta go,, :eek:uttahere:
 
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.
 
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great white said:
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.

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
Click to expand...

The MAP/boost sensor really does not play a role in fuel delivery like it does in a gasser, at least not in the OBD1 vehicles, and I doubt it does in the OBD2 6.5s. It serves as feedback for the PWM vacuum control, because boost is regulated as a function of fuel rate and RPM. Fuel rate determined by RPM and TPS, not MAP. So the more fuel the more boost in general, because you can generate higher drive pressures to the turbocharger. The boost sensor allows the PCM to control the vacuum controller to adjust the wastegate until it achieves the boost it was expecting per the predefined curve. However, if you lose boost or overboost per the defined curve and get a code for it then the PCM can scale back fueling, but it does not play a role in normal fuel delivery.

The main reason to use a larger turbocharger is to get a more efficient one. If we have a small turbocharger and boost to 15psi and have to open the wastegate that means we have more drive pressure that could be used. But 15psi of boost is 15psi of boost, the same flow of air from any turbocharger. The difference is a larger turbocharger can achieve the boost at lower driver pressures and just not have or use the wastegate. This typically sacrifices spooling speed to some extent. The larger more efficient turbocharger will be less parasitic of engine power, and likely reduce heat transfer to the compressed air of the intake, which can indeed then allow more air mass, which would be slightly more CFM from the ambiant air, with the same CFM through the engine. So a larger turbocharger is not needed for higher boost, which can be achieved with the GM turbos, and small turbos alike. Higher boost is what achieves more air, and by using a larger one you can accomplish it more efficiently.
 
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The valve opens, 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 ignition- otherwise known as Knock. This is why gas engines have spark plugs, to ignite the mixture that was resistant to Knock unitl 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
Click to expand...


Diesels use the higher compression to ignite the lower octane/cetane fuel, hence no spark plugs.

Ever run a diesel on gas? you'll hear the pre-ignition before you here the BOOM!!!!

How many 9.5 to1 diesels have you seen.
 
I ponder these things often while driving -

Boost, is almost like planning for the future, you Spend energy NOW to prepare for the future fuel injection cycles being more.

However, To create MORE boost at any given point you are taking energy (power) from from the engine at the PRESENT moment.

You want to create JUST enough Boost to Completely burn all the fuel you plan on injecting in the FUTURE injection cycles.

Anything more may help cool things down a tad, but robs the engine of power in the PRESENT moment.

Anything less and you start to belch black smoke, and waste fuel.

The more in - The better.

The easier the air out- the better.

Diesels don't need much backpressure. (Any?) The turbo creates more than enough backpressure, so Muffler and Cat's aren't necessary, or obnoxiously loud without.

I always liked how simple the older diesels were. So simple you could actually fire them up Backwards, meaning they would suck air through the exhaust, and after combustion blow it out the air cleaner! They would run forever, without any power assist what-so-ever. Dead alternator? Drive it home without lights.... Not quite so anymore.
 
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