Turbo/Compression/Timing

#1

I am new to the board, so forgive me if this subject has already been covered. I noticed some talk of adding a turbo to an LD28 and think, like many of you, that it could be very beneficial. However, after giving some thought to this subject, a couple of ?s have popped up. I am not a diesel mechanic but, I understand the principals of how they work. The first ? is: If you increase the amount of air entering the combustion chamber, won't that increase the compression inside the cylinders of an engine that was not built for a turbo quite a bit? I'm not really talking about the PSI increase that the turbo is creating. I'm talking about the compression that that added volume of air would create at TDC. Secondly, wouldn't the air (and fuel) inside the cylinder reach the required compression needed for combustion sooner than normal if you increase the volume of air entering the cylinder with a turbo? For instance, if the pressure needed for combustion = X, and you compress the air entering the cylinder to X minus 5-PSI, the piston would only have to move slightly (up the cylinder) to reach combustion pressure. I realize that a turbo doesn't increase the inflowing air pressure to X minus 5-PSI but, any increase in air volume would have to make the fuel/air mixture inside the cylinders detonate sooner than normal. These factors may be very minimal and have very minimal effects on the life of the engine or not. I would love to put a turbo on my LD28 but, like I said, I'm not a diesel mechanic and don't want to blow it up. I know that some of you guys have lots of diesel experience and would like to here what you think about this.

Frank

#2

The other page talking about adding huge turbos was just ribbing eachother about what one could wish for. Not enough hot air to run one big turbo between all of us.

If you increase the amount of air entering the combustion chamber, won't that increase the compression inside the cylinders of an engine that was not built for a turbo quite a bit?

Yes, and unless the engine is beefy enough to handle it something can let loose under extreme RPM (to create enough boost).

Secondly, wouldn't the air (and fuel) inside the cylinder reach the required compression needed for combustion sooner than normal if you increase the volume of air entering the cylinder with a turbo?

This would be true in a gasoline engine (detonation), but a diesel would have more heat at the same point of injection, therefore timing would remain the same.

An engine is an air pump, but a diesel is more so because there is no throttle. A diesel pulls in all the air it can and fuel regulates the speed. A gasoline engine has a throttle which regulates speed. (this is why a diesel is more efficient, it can run on a lower fuel/air ratio than a gas, and also there are more BTU's in a gallon of diesel than a gallon of gas.) A turbo helps a diesel push in more air, adding more fuel (injected) creating more exhuast, turning the turbo faster, more air, more fuel, leading to more heat and on a long hill can lead to a hole burned in a piston (aluminum melts at about? 1600 degrees) Some time spent over at turbodieselregister.com should give you some ideas about what it takes to make a lot of horsepower, and how to manage it without destruction.

Actually I just read the best definition the other day: Torque is the force that gets the work done and horsepower is what determines the speed at which it is accomplished.

#3

Shagbark wrote:If you increase the amount of air entering the combustion chamber, won't that increase the compression inside the cylinders of an engine that was not built for a turbo quite a bit? I'm not really talking about the PSI increase that the turbo is creating. I'm talking about the compression that that added volume of air would create at TDC.

Excellent question. Yes, the compression pressure (without fuel; that is, before the fuel is squirted) is higher. Whether the compression pressure after fuel is squirted is higher depends upon the amount of fuel squirted and when it is squirted.

wouldn't the air (and fuel) inside the cylinder reach the required compression needed for combustion sooner than normal if you increase the volume of air entering the cylinder with a turbo?

Yes. Which would be really great if we had boost while cranking the engine on a cold engine! Alas, that's not the case.

. . . any increase in air volume would have to make the fuel/air mixture inside the cylinders detonate sooner than normal.

With diesel engines, fuel is squirted when you want combustion to occur (I'm simplifying and ignoring several related factors here). If the air (without fuel yet) is hotter than it would have been, due to the turbo packing in more air before the valves shut, this aids in efficient combustion of the fuel that is squirted -- when it gets squirted.

I would love to put a turbo on my LD28 but, like I said, I'm not a diesel mechanic and don't want to blow it up.

The IP on the LD28s isn't designed to add more fuel based upon intake manifold pressure (ie a turbo when it's generating boost). Therefore, with an unmodified LD28 IP (1) no more fuel is injected while boosting than would have been before adding a turbo to the system. There are a couple of factors here to consider:

Smoke (Particulate Matter, PM). My LD28s both have visible black smoke when I push the accel past a certain point. To my thinking, that's overfuelling: injecting too much fuel for the available air to oxidize. I could cure this smoking by reducing the maximum injection volume that the IP delivers. Alternatively, my hope is that adding a turbo (adding more air) will instead allow more complete combustion of the fuel that my LD28s are already being fed.
Exhaust Gas Temperature (EGT). Overfuelling leads to high EGT. High EGT is non-linearly related to overall engine life: lower is better, a little higher might be a little shorter life, and above a threshold temperature, engine life goes down precipitously; the exact point of this magic dangerous EGT threshold is a matter of opinion, and is also affected by the EGT sensor (pyrometer sensor) placement in the exhaust flow. My thought is that adding more air and not adding more fuel will (should?) lead to lower EGT. However, EGT is also affected by the temperature of the incoming air, which is raised when compressing it, as in the case of a turbocharger.

Intercoolers come in at this point: they dump some of the heat of compression from the incoming air (to the surrounding ambient air), to keep temps inside the combustion chamber lower (which, come to think of it, should also inhibit NOx formation). This reduces thermal load on the piston crown and valves.

I hope I have answered your questions, at least partially, and maybe raised a couple more. I, too, am not a diesel mechanic, and these days I make my living as a locksmith, but my background is in this area.

(1) There may be ways of making the fuel delivery on the LD28's VE IP be altered based upon intake manifold pressure. One way that seems especially sano is to use the boost sensing doodads from a similar Bosch VE IP, such as the Six cylinder Volvo engine, whose IP is very similar. It was offered for at least three years with an optional turbocharger and has the requisite gear. But I, personally, do not plan on adding more fuel to my LD28s, at least not in the first iteration.

#4

I appreciate you guys clearing this up for me. As you both noticed, I thought that the injectors were loading the cylinder as the piston was on the down stroke. I had been wondering how the timing stayed true as the engine temperature changed (ha-ha). But, like Al said, this raises other questions for me. I'm no physicist but, common sense tells me that the injectors can't "instantaneously" put fuel in the cylinder. They may squirt the fuel very fast, but not instantaneously. So, if combustion starts at the moment the injectors start squirting fuel, and it takes time for them to finish squirting fuel, then that would explain why diesels seem a little sluggish but produce a lot of power. More like a "controlled burn" than an explosion. Am I on the right track here, or somewhere out in left field? I'm just amazed that the pump and injectors operate with that type of speed and accuracy. What type of pressure (ballpark) is this system operating on? Also, any idea how far the piston moves while the fuel is being injected? Are the injectors still spraying as it gets close to the bottom of the combustion stroke, or is it over way before that point?
See, I just thought I understood the principals of how it worked

. Looks like a trip to the library is in my near future.

Frank

#5

The inline Bosch pump operates at fairly low pressure. IIRC about 1400-1500 psi. Modern common rail systems on DI motors seem to run way higher than this.

Yes, the injection process takes some time. The fuel also does not burn as fast as gasoline, which is much more volatile.

#6

Shagbark wrote: if combustion starts at the moment the injectors start squirting fuel, and it takes time for them to finish squirting fuel, then that would explain why diesels seem a little sluggish but produce a lot of power. More like a "controlled burn" than an explosion.

Gasoline and diesel burn the same way, and in much the same period of time.

Diesels have higher reciprocating mass, which requires more time to accelerate. If you're comparing reving engines in neutral, that alone explains the slower rev time of a diesel vs gasoline engine.

Diesel fuel contains more BTUs (more energy) per unit than does gasoline. Diesel engines produce more torque (on average) than an equivalent gasoline engine primarily because they're built to take advantage of the diesel method of ignition, which requires heavier components to withstand the higher cylinder pressure involved, which in turn limits how fast the parts can reciprocate (a function of weight and piston acceleration limits), which means since you can't spin it fast, make the stroke long and get greater mechanical advantage: torque.

Again: diesel fuel burns much like gasoline. The higher compression ratios of diesel engines makes them more efficient heat engines in a thermodynamic sense; gasoline can't use those high/efficient compression ratios because it won't burn slowly enough at that pressure to push the piston down to where the connecting rod has best mechanical advantage to the crankpin. Diesel fuel can.

I'm just amazed that the pump and injectors operate with that type of speed and accuracy.

They do. Which is why they want very clean fuel and no backyard mechanic fiddling.

What type of pressure (ballpark) is this system operating on?

A new, in-spec LD28 nozzle will open at 1,920 to 2033 PSI. This is middle-of-the-road "pop-off" pressure for Indirect Injection diesels of that era. As Byron mentioned, modern Direct Injection diesels with common-rail injection systems push 25,000 PSI now, and feature multiple injection events per cycle, to control noise, peak combustion pressure, and NOx emission formation.

Do you have a Factory Service Manual yet? If not, do get one, it's absolutely necessary, and it has those pesky numbers in it too

Also, any idea how far the piston moves while the fuel is being injected? Are the injectors still spraying as it gets close to the bottom of the combustion stroke, or is it over way before that point?

Injection is probably ended by the time the piston moves down in the bore 1/2". The fuel's gases are still expanding though long beyond that. It takes a while to wring out all the energy contained in each squirt.

#7

the turbo thing has been done successfully on the LD28.

If you have ever read the "fryer to fuel tank" book on WVO...the cover is a first generation Z with a turbo diesel LD28. The guy put an LD28 engine in and bolted on the turbo set up from the 280ZX. Claimed to achieve 40 mpg and 120 mph.

Since the engines are twins to a great extent, many parts inter-change and bolt up. The entire turbo system from the 280ZX supposedly bolts to the LD28...as far as other modifications to make it run right that I don't know. Probably IP adjustments or higher flow injectors or what not.

#8

odie wrote:the turbo thing has been done successfully on the LD28.

If you have ever read the "fryer to fuel tank" book on WVO . . .

Arrgh . . . don't quote Joshua Tickell's efforts, he has written a lot of things that have been proved wrong over the years.

Better, read about member Dslsmoke's experience doing the turbo install years ago.

#9

most of his book is fluff and not technical info...but it's a decent intro to running WVO and such.

But he did supposedly make the turbo thing work...maybe not a good as it could have been, but he did do it.

anyway, this whole field of WVO is all garage guys...and I don't see many others stepping forward to write books about it. Most are lying low and you will not see Detroit or Exxon stepping up to the plate either...

Don't "bash" the book...it's only one little part of the whole WVO puzzle.

and I was only referencing the book in this case due to the Turbo LD-28 powered car on the cover to illustrate that it is possible.

#10

odie wrote:But he did supposedly make the turbo thing work...maybe not a good as it could have been, but he did do it.

AFAICT, Joshua didn't do the turbo addition, rather he paid someone else to add a turbo. That person posted about that about the time that the car went up for auction on eBay the first time -- it's been sold at least one other time since then.

#11

paid someone else? ain't that cheatin'

I saw the car go on ebay about a year-ish ago. over ten thousand as I recall, I think 12K.

So the car sold again? guess it wasn't as good as the buyer thought it would be....or maybe trying to flip it for a quick buck...

#12

odie wrote:I saw the car go on ebay about a year-ish ago. over ten thousand as I recall, I think 12K.

$11.2k

#13

My thought is that adding more air and not adding more fuel will (should?) lead to lower EGT.

my hope is that adding a turbo (adding more air) will instead allow more complete combustion of the fuel that my LD28s are already being fed.

I may be missing something here. From where I'm sitting (the short bus

) these two statements appear to to contradict one another. What I mean is, if you burn the fuel more efficiently, that should create more heat. More heat would create higher EGTs. If all that black smoke swirling around in the rear view mirror is unburned fuel, that should be helping to keep EGTs down, simply because it wasn't burned. It would appear that adding a turbo to provide enough air to burn the fuel that is being wasted, would in turn create higher EGTs, again, because the fuel was burned inside the engine instead of wasted as black smoke. Like you said, the fuel is already there, as evidenced by all the smoke. I guess what I'm saying is, it wouldn't appear to matter that you are only adding more air and aren't adding more fuel because, it's already there in excess. There may be something that I'm overlooking or am unaware of but, it appears that adding a turbo could increase EGTs because it allows for more complete combustion which would create more heat. I'm not trying to be argumentative or anything, I'm just looking for answers and trying to learn something.

Frank

#14

My take on smoke in the rearview: This is excess fuel, as you state, but it continues to burn (to some degree) after the power stroke (ie. into the exhaust stroke in extreme cases). This can really cook exhaust valves etc. More air (via turbo) will allow the combustion to complete within the duration of the power stroke, thus reducing EGT, as there is less burning fuel exiting the cylinder. With a turbo, you should see higher peak combustion pressures (it's doing more work), but w/o adding fuel, the EGT should go down. Diesels aren't air/fuel ratio sensitive in the way gassers are.

#15

Is this why turbos are more efficient? meaning they get better power, economy and less noise?

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