Compound and Twin Charging Discussion

[HowlerMonkey]

When you have two stages of supercharging, names should be used that distinguish the coolers of each stage from each other.

 

This is important for the parts guys and technicians so the wrong part does not get ordered or replaced.

Edited September 23, 2011 by HowlerMonkey

[Tony D]

"When you have two stages of supercharging, names should be used that distinguish the coolers of each stage from each other."

 

Absolutely, that is why they are usually called "Low Pressure Intercooler" "Medium Pressure Intercooler" and "High Pressure Intercooler"....or more mundanely and specific "Intercooler 1, Intercooler 2, Intercooler 3" Or "First Stage Intercooler, Second Stage Intercooler, etc..."

[HowlerMonkey]

I am considering the above post just a pathetic attempt at trolling or an attempt to get the last word in.

 

Every diesel and aero catalog as well as many compressed air equipment catalogs list them as intercoolers and aftercoolers.

 

Try to order "intercooler #2" from any diesel or aero engine parts supplier and you will be asked to clarify if you mean the intercooler or the aftercooler.

 

The only time they number intercoolers is in system with three or more stages of compressing.

 

TonyD you are a smart and knowledgable man but your willingness to jump down people's throat needs to be tempered a bit.

Edited September 27, 2011 by HowlerMonkey

[SleeperZ]

But the parts themselves are similar; there is nothing intrinsically different between an intercooler and an aftercooler, it's just the location in the plumbing scheme that creates the difference. So if a replacement is needed, use the manufacturer's part number or the dimensions to specify the part. Tony is not off base here.

Edited September 27, 2011 by SleeperZ

[Tony D]

I'm not jumping down anybody's throat here. This is a specific terminology based on application. This is what SleeperZ is reinforcing, and is what I've said all along.

 

The basic crux of it is that the engine IS a final stage of compression, and therefore intercoolers are used throughout, what you would call an 'aftercooler' from a laymans point of view may be what you think is correct, but from an engineering standpoint it is not---it's simply another form of INTERcooling to another subsequent stage of compression (the engine at X:1 Compression Ratio at a minimum.) That has been my stance from the beginning. The design criteria for an aftercooler are quite different from an intercooler...

 

There is no trolling here on my part. I was unaware that we were restricting our discussions to only TWO stages as well. It's not uncommon on Diesels to have four turbos to make 200+ manifold boost. So what then? Intercooler aftercooler 1, aftercooler 2, aftercooler 3? No. Intercooler 1, 2, 3, 4...

 

Aftercoolers deal with air going to free field, not being compressed further, but used. An engine is a compressor, same as any supercharging device.

 

I deal with this daily, our equipment has TWO intercoolers and ONE aftercooler. Their construction is identical---the ONLY difference is the one off the third stage is not going to another compression stage, it's going to be used like bleed air to power accessories or perform work.

 

NOW, on the exact same machine but packaged to a BOOSTER COMPRESSOR for hiking the pressure from 8.6 bar to 40+ bar in two more stages of subsequent compression in another compressor downstream...want to know what all the MANUALS and PARTS listings call that third cavity?

 

It AIN'T "AFTERCOOLER".... Viewed as a PACKAGE it has now simply become another intercooler as it's no longer final compression stage, it's now just another in the train.

 

Same part number, same position on the machine, different package.

 

I've said all along, the difference (and this is mainly a problem with American Terminology) is use of the air after it's been cooled. If it goes to another compression stage: Intercooler. If it goes to do work: Aftercooler.

 

This is another one of those things where in America only, the terms become an issue. I'm not trolling to 'get the last word in'---this is my job and it bugs me when people slaughter the terminology haphazardly and don't even admit the intellectual argument for use of the proper terminology.

 

I'll meet you half way Howler Monkey: the day an engine runs on a O:1 Compression ratio, I will concede that the second (or last) cooler in the train can be called an 'aftercooler' but until such time as engines are operating on no compression ratio internal to their mechanical configuration...they are properly called 'intercoolers'! Pedantic? Yeah. Because it's the right thing to do.

[Leon]

I'll meet you half way Howler Monkey: the day an engine runs on a O:1 Compression ratio, I will concede that the second (or last) cooler in the train can be called an 'aftercooler' but until such time as engines are operating on no compression ratio internal to their mechanical configuration...they are properly called 'intercoolers'! Pedantic? Yeah. Because it's the right thing to do.

 

Therefore, when an intercooled turbo-diesel truck is using a Jake Brake then the intercooler effectively becomes an aftercooler?

 

[Gollum]

So, getting back on topic... Does anyone have some good calculations on how to factor compressor sizes when compound boosting?...

[Tony D]

Therefore, when an intercooled turbo-diesel truck is using a Jake Brake then the intercooler effectively becomes an aftercooler?

 

 

If it's cooling the exhaust after coming out of the jake brake---the jake brake turns the engine from simple power piston to PURE COMPRESSOR (thanks for making my point for me, original stirrer of the pot!)

 

The engine IS a compressor! The diesel is a point of compression before injection of the fuel---the ONLY function that differs on Jake-Brake is that fuel is stopped, and the outlet of the exhaust is restricted to let the engine build to a set pressure before the brake allows it to blow off. The MECHANICAL function of the pistons doesn't change with the removal of fuel (which has been my point all along)--the engine isn't point of use! It's just subsequent compression.

 

As for stage sizing, it's all in the relevant engineering manuals. It's mass flow in and pressure ratio starting at point of power production with desired horsepower goal, and working backwards so as not to exceed roughly (in the old days) 2.5:1 CR per stage. Remember, with proper staging in the 2.5 range, it doesn't take much to get 8.6 bar out of two or three stages. and 40 bar out of four!

Edited September 30, 2011 by Tony D

[Gollum]

Sweet, thanks for the input Tony. I'll have to run some numbers and calculate a setup and have you grade my paper, oh teacher of boost.

 

Here's another question that's probably not as big of a deal, but still just hit me like a baseball in the head. In a compound turbo setup, I can't see any reason to use more than one BOV, but if you were to recirculate it, should it recirculate to the last state of compression before the engine, in order to keep your most immediate compressor spooled, or do you recirculate it to the FIRST state of compression in order to balance the pressure in the system as a WHOLE?

[Leon]

If it's cooling the exhaust after coming out of the jake brake---the jake brake turns the engine from simple power piston to PURE COMPRESSOR (thanks for making my point for me, original stirrer of the pot!)

 

The engine IS a compressor! The diesel is a point of compression before injection of the fuel---the ONLY function that differs on Jake-Brake is that fuel is stopped, and the outlet of the exhaust is restricted to let the engine build to a set pressure before the brake allows it to blow off. The MECHANICAL function of the pistons doesn't change with the removal of fuel (which has been my point all along)--the engine isn't point of use! It's just subsequent compression.

Tony, I understand what you mean by the engine acting as a subsequent stage of compression, but your explanation shows that you don't understand how the Jake Brake functions. The Jake Brake does not turn the engine into a pure compressor. What you're thinking of is a simple exhaust brake which slows the engine by increasing exhaust pressure (pumping losses).

 

The Jake Brake operates on a completely different principle. It opens a separate "exhaust" valve during the compression stroke in order to bleed off cylinder pressure. If you draw up a p-V diagram of what's going on, the engine is making "negative horsepower" (roadwheels must drive the engine) thus slowing the truck down. Therefore, the mechanical function of the engine DOES change.

 

 

the day an engine runs on a O:1 Compression ratio, I will concede that the second (or last) cooler in the train can be called an 'aftercooler'

My point being, while the Jake Brake is engaged, the engine is effectively running a 0:1 compression ratio. Will you concede now?

 

 

I did originally stir this pot, but it was because I had learned this from a former Honeywell engineer, although he is into planes as well and some of his terminology may have come from the aero world. The Jake Brake info comes from a long-time Mack Trucks engineer.

[Tony D]

It is not running 0:1, the Jake uses compression bled off to slow the engine-watching the PV curve will show the rise and bleed off of pressure---if no pressure is built, no negative horsepower can be put to the drivetrain. It has to do work to functionally slow the engine through compression braking. The Jake brake allows 'flow' which raises the compressive load more than simply running a against a closed exhaust valve, or other such device. The same function as an exhaust brake, but more controllable and with variable volume comes the ability to modulate when the exhaust brake doesn't really have.

 

In these cases (exhaust and jake brake), BTW, the Turbos are not producing any boost, the intercoolers are doing nothing but recovering from heat soak...

[Leon]

Well, if we're being pedantic...

 

Tony, there are instants where there is compression of the air during Jake Brake operation and there are instants were the comressed air is being bled by an open exhaust valve. Compression ratio is defined as encapsulated volume in the cylinder at BDC divided by encapsulated volume at TDC. At TDC, the brake exhaust valve is open and the volume in the cylinder is essentially infinite. Therefore, at that point in time, the compression ratio is ZERO. I don't care what the turbos are doing, by your definition the intercooler becomes an aftercooler.

[Tony D]

If the turbos aren't doing anything, then they aren't inter OR aftercoolers, they are inlet ducting...

Without multiple stages of compression, there is no inter, or after-cooling occuring!

 

Being pedantic...

[Leon]

Didactically speaking, the turbo will be doing something, just not much of it...

[Tony D]

You said 'Didactically'!!!

[Leon]

I did, I did!

 

I also accidentally took this thread waaaay off it's tracks! At least we all know what to call heat-exchangers based on their locations now.

 

Resume: compound charging

[kiwi303]

Since this is going so esterotic... could you clear up a point for me... know of using a Supercharger (mechanical engine driven displacement compressor) for low rev boost with a Turbocharger (exhaust gas powered turbine compressor) taking over for later higher rev power above and beyond that the supercharger can give.

 

My question is, what layout is the most efficient?

 

I have seen writeups of superchargers pulling air through effectively stalled larger turbochargers which later as the ehaust gas volumes increase, the cavity between the turbocharger and the supercharger is pressurised so the supercharger is compressing already compressed air into the engine.

 

I have also see writeups where the supercharger pushes air through the stalled large turbocharger at lower revs, and at higher revs a clutch disengages the supercharger while a valve opens an alternate air intake into the turbocharger, removing the supercharger from the air feed chain.

[Tony D]

One of those is not what is being discussed, what you describe with clutch disengagement is SEQUENTIAL Supercharging, where one goes into bypass at a point where the other takes over. This discussion is about putting the discharge of one sueprcharger into the other for COMPOUND Supercharging, meaning Turbo 1 produces 5psi which goes into Turbo 2 which boosts it from 5 to 15psi. That would be your first example, but generally it's not a 'stalled' turbo it's drawing air through, it's merely one that is not at boost threshold.

 

 

In the that scenario the Supercharger is blowing into engine being fed by turbocharger (the most common compound action in this scenario) as this allows HUMONGOUS compressor wheels to be used with realitively high slip from the turbine to compressor meaning the supercharger is geared to give X psi at just off-idle, and progressively goes upwards, but as it's efficiency drops off as the CR on the mechanical devices raises (meaning excessive heat generation) the turbo is starting to 'overfeed' the mechanical supercharger meaning the CR on the second stage drops, as does its' temperature and a compounding effect is noted where a mechanical supercharger giving 5psi (@ 100C IAT) at top rpm now is giving you 45psi at the SAME IAT because the turbo is pumping in 15psi above X000rpms keeping the CR constant and preventing the temp from rising uncontrollably (thanks to that 1st Stage INTERcooler... )(In this application only a first stage intercooler may be neccary to run because the effect of the turbo into the mechanical supercharger is such that the CR being generated by the mechanical unit is not sufficient to require it. If it was employed, that second intercooler under the mechanical Supercharger can be sized for a lower heat rejection as CR remains relatively constant.)

 

This is particularly noticeable in industrial compressors where 1st and 2nd Stage discharge temperatures are in the mid-high 200 F range, while the last stage of compression is discharging air in the high 100's. It means you use larger intercoolers on the 1st and 2nd Stages, and the last stage can be pumped directly into the engine (as is the case with many commercial diesel applications now) the density loss at that temperature many times is not worth the weight or complexity/cost penalty of adding a third intercooler to the stack.

 

Another way to do it is to pump the first turbo uncooled into the second and then cool it, using the mass calculations from the hotter air to size the turbo appropriately to feed it and then just cool it all down afterwards. Knowing 4:1 CR will net you something in the region of 455F you really want to shy away from that kind of approach as it's the kindling point of paper...and when you pump 455F into an aluminum cooler they tend to crack in short order from the extreme thermal cycling.(experience talking here...) I'd shy from that setup.

Edited October 3, 2011 by Tony D