Diesel turbo operation question. Boost reference?

[SandBar]

So I have built several gas turbo engines, and all requires a boost reference to allow the ecu to determine how much more fuel to add to maintain the requested AFR.

On a 6.2/6.5 turbo diesel, I don't see how this works. I don't see a boost reference on the injector pump and no mechanical linkage. Someone please enlighten me.

This whole question came about as I found my box of turbo parts to convert my 6.2.

 

[LT67]

So I have built several gas turbo engines, and all requires a boost reference to allow the ecu to determine how much more fuel to add to maintain the requested AFR.

On a 6.2/6.5 turbo diesel, I don't see how this works. I don't see a boost reference on the injector pump and no mechanical linkage. Someone please enlighten me.

This whole question came about as I found my box of turbo parts to convert my 6.2.

If I'm correct it's within the DB2 injection pump itself. The 1994 and up 6.5's used an electronic injection pump which could be reprogrammed with a chip

 

[SandBar]

For example when one adds a turbo to a 6.2, what mechanism requests more fuel based on boost levels?

 

[Keith_J]

The skinny pedal. Throttle in a diesel controls only fuel. The injection pump is calibrated to provide only as much fuel as possible, the limiting factor is exhaust gas temperature. More air with a turbo means that limit is higher. More can be injected with a turbo so there is an adjustment which increases the max plunger stroke.

 

[Keith_J]

To further this topic, diesels nearly always run lean. Far lean, at idle, this engine injects 6 to 10 milligrams of diesel each combustion stroke. 379 cubic inches of air gets under 50 milligrams of diesel. That is 7000 milligrams of air, basically a 140:1 air to fuel ratio.

Full throttle is 50 milligrams per stroke or 400 milligrams to 7000 milligrams of air or an air to fuel ratio of 17.5 to 1. Still lean compared to a gasoline engine but the above numbers assume 100percent volumetric efficiency. It's under 70 percent at 3600 RPM.
Now how can this happen? The flame in a gasoline engine is homogeneous, burn rate is dependent on compression ratio, gasoline quality and combustion chamber shape/size.. The flame in a diesel is more like a blowtorch. It ignites shortly after injection starts, then progresses over the duration of injection window. In the indirect injection diesel, it progresses out of the chamber at higher throttle positions as it consumes all the oxygen in this chamber (30 to 40% of the total squish volume), eventually bearing a jet of flame directly on the piston.

So, what does this mean when you add a turbo to this engine? You have more air so you can add more fuel. A turbo without more fuel means no power increase. Most injection pumps have a bit more capacity built into the mechanism for calibration. This also has the benefit of bumping the timing a bit more advanced.

 

[LT67]

For example when one adds a turbo to a 6.2, what mechanism requests more fuel based on boost levels?

Stomp on the pedal, more fuel. Lightly press the pedal less fuel....

 

[SandBar]

Thanks for the explanation, in my mind I was imagining a mechanism to add fuel based on boost level, which is clearly not the case.

So people turn the pump up a 1/4 turn for a little extra power NA, how much is it turned up for a turbo swap?

 

[LT67]

Thanks for the explanation, in my mind I was imagining a mechanism to add fuel based on boost level, which is clearly not the case.

So people turn the pump up a 1/4 turn for a little extra power NA, how much is it turned up for a turbo swap?

I think(?) the Banks system says half a turn, but don't take my word for it lol

 

[SandBar]

Ok it's all coming together now. So in gas turbo setups each atmosphere of boost roughly adds the amount of HP the engine has NA. Does that generalization not ring true for turbo diesels?

 

[Barrman]

It isn't a linear power curve or relationship between fuel increase and power increase. Mainly because of how slow diesel fuel burns. At least with the indirect injection 6.2/6.5 engines. I don't know about the direct injection engines.

Theoretically a 110 hp NA 6.2 could make 200 hp with a turbo. Or more. But, the IDI engines are governed by heat and air flow. More heat inside the pre combustion chamber means more heat in the head which means coolant temperature will rise very quickly as boost gets added. How long can the cooling system remove the heat before it starts loosing the battle?

A stock 6.2 really shouldn't see more than 14 psi unless the head gaskets have been changed and studs have replaced the head bolts. Even then, 18-20 is considered the absolute boost limit. A Banks Sidewinder or any of the GM turbos except the 8 won't make more than 12-14 psi anyway so it doesn't matter. What does matter is Exhaust Gas Temperature. I can be cruising along at 75 mph with 2 psi of boost and 650° of EGT and 198° coolant temp with a 195° thermostat. All good. Then comes a hill. Boost moves first. Then the EGT will start to climb followed by the engine coolant. Short hill and EGT will stay below 1050° and the engine coolant will stop climbing around 203°. Longer grade or a grade every few minutes for hours and hours will have the coolant pushing 210° and the EGTs getting high enough backing out of the throttle is needed.

I am currently playing with my timing to lower EGTs and increase boost. More boost means more air to cool things inside the engine which will lower EGTs. But, these engines were designed to get great mpg, not earth shaking power. Don't expect to pull a trailer and keep up with a new PowerStroke. You won't do it. Just adding a turbo and not touching the fuel setting in the Injection Pump makes it drive like a different truck. Definitely worth it. Change the fuel rate and it is even better. Also remember the square body Chevy is well, square. They attack the wind with a parachute attached. The happy speed of the truck and turbo is probably around 62 mph. The wind just kills above that speed. Which is where the turbo makes you smile because even a fraction of a psi is better than stalling out not being able to maintain speed on a hill like a NA truck.

To answer your exact question. Diesels are just different. You are wanting to build torque with boost, not horsepower. 2400-2800 rpm is where the turbo can't keep pushing more air in depending on what turbo you have. Because the engine can't efficiently burn it that fast. Yes, the 6.2 has a mechanical governor that limits rpm around 3500 rpm. But, you won't get that high with a heavy load on the engine. 1800-2200 is the peak efficiency range.

 

[LT67]

With turbos added to a 6.2 and 6.5, the biggest issue will be heat. Keeping the engine adequately and evenly cooled along with getting exhaust gasses out as quickly as possible is the main concern. The 6.5 turbo diesels from the factory had a myriad of issues all due to heat. The 6.5 turbo diesels can be made into reliable engines that are capable of throwing out low end torque.

 

[Keith_J]

The key is volumetric efficiency. Forced induction improves mass flow but the volumetric efficiency stays approximately the same, that is between 1800 and 2200 RPM.

When the engine is operated outside of the peak efficiency, there is more waste heat. Higher speeds mean more waste heat to the pre combustion chambers, low throttle positions make only a tiny flame which barely touches the piston.

Yes, peak efficiency tracks peak torque. Pressure is efficiency and pressure makes torque. The graph of torque and power as a function of engine speed is done at full throttle, partial throttle efficiency is always lower. Now you know why a stock 1008 gets 10 MPG at 65 MPH.

 

[SandBar]

Thank you, been reading diesel turbo theory literature and it is coming together.

Also glad my "m1009" came with a 700r4 from the Navy.

 

[Barrman]

I'm glad this thread came back up. It has been hovering in the back of my mind since the other day. I get the feeling we are speaking different languages and not answering what you are asking.

Atmospheres to me is a term used in aviation to talk about manifold pressure. Standard pressure is 29.92 Inches of Mercury. You loose about 1 Inhg per thousand feet of altitude due to less oxygen in the air the higher you go. Piston powered planes with constant speed propellers set their power by rpm and manifold pressure. A non turbo plane will probably be happy with 2500 rpm and 25 InHg. That is called running "square" settings.

Add a turbo and it might be intended to just keep you "square" at say 15000 feet when without the turbo the engine couldn't do more than 15 InHg. WWII fighters could make up to 60-70 InHg. They were all about performance. The planes that race in Reno every fall make over 100 InHg. They didn't measure boost in PSI or really at all. They measure manifold pressure. The British even had a saying to help people remember to run more Inhg than rpm when going for long range: "Reduces the revs and boost the boost so you can make it home to roost." Running way over square with more InHg than rpm is considered not good for engine longevity. But, LIndbergh crossed the Atlantic running over square the entire way.

Is that the kind of atmospheres you were asking about? Because diesel engines since they don't have a throttle plate don't really measure out that way.