Compound and Twin Charging Discussion

[Gollum]

It seems we haven't had a decent thread on this topic in a good long time and I was recently re-interested in it via a book I was reading on EMS tuning called "how to tune and modify engine management systems" by Jeff Hartman. It was a decent read, but I'm not here to review this book. I'm here because they go through covering an interesting project twin charging a MR2. What got me thinking (and frustrated) is that they had predictions on how things would go down on the dyno and the results surprised them all. They had it mocked up to run supercharger only, turbocharger only, and combined as well to study the way they interacted together. What was frustrating was the lack of resolve to the project coverage and how things ended up in the end.

 

So let's talk shop. What experience does everyone have on the subject. How should the terms twin charge versus compound charge be used? (it seems to me that a system with a bypass that switches from one charger to the other shouldn't be called "compound"). Why would you go with a setup like this? What would you have done differently (if you've done it before or seen it done first hand)?

 

I'll start off with some interesting talking points.

 

1. It seems that when a positive displacement supercharger is used with a turbo, it's fairly safe to size the turbo according to the total displacement of the engine+supercharger. I've seen some suggest multiplying engine displacement by supercharger pressure ratio, but that could lead you to some absurd sized turbos if you're wringing every last ounce of performance from your supercharger.

 

2. Air temps will always be an issue with these setups. I personally feel with how available E85 is you can just use that in a twin-fueled setup spraying it under boost, or using E85 exclusively. Something else to look into is the fact that water injection systems have come a long way regarding dependable information combined with tried and true setups with concrete results.

 

3. I'd personally like to try compound charging someday, and think a great test bed would be a VQ33ER. We all know those bottom ends will handle 400+hp without modifications and it all comes down to fuel and spark control with lots of boost. What really interests me is that the heads don't flow extremely well in the higher RPM range and as such the high HP turbo dynographs look wonderfully flat and huge. These aren't "peaky" motors, even at 500hp. I'd love to see how throwing a supercharger into the setup changes things up. Can you get the benefit of faster spool (from supercharger) combined with the torque spike + drop off that leads to flat HP curves? Or is it just going to be impossible to size a turbo to do both?

 

 

If this thread totally flops I'll only be mildly disappointed, but if we even get just a little bit of good information flowing it'll be worth the effort many times over as this isn't a well covered subject out there on the internet and I know we have the minds around here to contribute good info.

[Tony D]

A 'compound' charged setup is multi-stages of compression. Turbo 1 feeds Turbo 2 feeds Turbo_n...

Of course it could be a crankshaft mounted supercharger, feeding a higher speed belt-overdriven supercharger, with a turbo feeding the inlet of the whole schebang... or an integrally geared double supercharger one feeding the other before going to the engine...

 

A 'twin' charged is more marketing to differentiate it, but isn't normally compounded (one feeding into another) rather two different styles of superchargers working at different times. Supercharger runs from idle to X,000rpms while boost from Turbo comes up to a point where a bypass is opened and the turbo takes over from X,000 to redline. Or perhaps a gear driven supercharger for lower rpms, while a centrifugal belt driven supercharger takes over at higher rpms and the lower speed unit is bypassed.

 

I would stay away from 'Twins' simply due to complexities related to the bypass.

 

As for compounded? Muahahaha!

[Gollum]

As for compounded? Muahahaha!

 

Exactly why I started this topic!

 

Please, share some info wiki-D! What's your experience been?

[Chaparral2f]

One might contact Streetfighter over at Z31 Performance.

[Nelsonian]

This was the first time I became vaguely familiar with the term twin charged:

http://www.groupbrally.com/lancia.shtml

 

Then someone here introduced this crazy rear tire fryer:

 

 

This is one thread I will just sit in the back ground and soak up the contributed information.

[Cannonball89]

I would think that the thermal efficiency of compound compressors would be much higher than that of a single compressor at the same boost.

 

That is just my speculation, I have no experience with them, but I would imagine that you could achieve very high manifold pressure without drastic increases in temperature, especially if you were to implement an intercooler both between the compressors and between the final compressor and the manifold.

[Leon]

...especially if you were to implement an intercooler both between the compressors and between the final compressor and the manifold.

 

That would be an intercooler and aftercooler!

 

The word intercooler is improperly used most of the time. Technically, most "intercoolers" people talk about are actually aftercoolers.

 

And that's my fun fact of the day!

[johnc]

Pratt and Whitney R-4360-43 Wasp Major.

 

http://www.youtube.com/watch?v=3wvEzhyY9F4

[Tony D]

That would be an intercooler and aftercooler!

 

The word intercooler is improperly used most of the time. Technically, most "intercoolers" people talk about are actually aftercoolers.

 

And that's my fun fact of the day!

 

We went through this before, and as someone who works in the multi-staged compressor business cooling BETWEEN stages is INTERCOOLING, and cooling after the final stage to point of use is AFTERCOOLING.

 

Since a single, or multiple compressor stages are simply preparing the air for the FINAL STAGE of compression (in the ENGINE) they all are INTERCOOLERS.

 

 

As to Nelsonian's thought on multiples being more efficient: ONLY if they are INTERCOOLED. The thermal dynamics of compression can not be changed without intercooling. If you cool the charge BETWEEN stages, you then get better efficiency in steps.

 

BUT, if the compression ratio you wish is less than about 2 or 3:1 it gets VERY hard to justify multiple stage compression since the efficency of one-stage compression will be upwards of 82% in your typical turbocompressor of today. And that will be EXTREMELY hard to beat with two compressors running at 1.5:1 and intercooled unless you have residual cooling from a 'bank' like an ice chest to give you less than ambient temperature into the second stage of compression, and then again before the final stage of compression in the engine. Lots of work for marginal payback.

 

Generally compound charging is used in things like diesel tractors to get 200+ PSI of manifold pressure. They used water into the stages as coolant to take advantage of high latent heat and state change to cool somewhat and keep efficiencies up.

 

In Air Compressors for Industrial usage, the breakpoint generally is around 55psi for 1 and 2 stage compressors. You CAN get 55psi out of one stage of compression, but two gives better power consumption. Next breakpoint is around 100psi, where you CAN get away with two stages of compression, but three gives better power consumption. I've run PET Centrifugals on surge tests at over 700psi(g) on a four stage turbocompressor, with stage pressures of 30, 120, 320, and 700psi ultimately, with a working pressure on the 4th stage of 580PSI all day long, 24/7. The last stage having a different compression ratio depending on what they chose to operate at, but generally this is about 3:1 compression ratio per stage.

 

So if you want to consider multiple stages of compression, and lower pressures...knowing a single stage can get great efficiency at 3:1 (and automotive turbochargers are a bit more radical now, having ratios over 4:1!!!) you would do well to consider closely the mechanical complexity you are adding for minimal gains in efficiency.

 

But again, I digress.

 

That 'aftercooler/intercooler' thing gets me every time. They're intercoolers, the final stage of compression is the engine.

[johnc]

 

[Tony D]

As has been pointed out to me on occasion 'aircraft are different than automobiles'...

 

From the source I hate to use, but at 2AM it comes in handy:

 

"The inter prefix in the device name originates from historic compressor designs. In the past, aircraft engines were built with charge air coolers that were installed between multiple stages of supercharging, thus the designation of inter."

 

Going further, the differentiate the 'end of the supercharging train...

"Modern automobile designs are technically designated aftercoolers because of their placement at the end of supercharging chain. This term is now considered archaic in modern automobile terminology since most forced induction vehicles have single-stage superchargers or turbochargers although "aftercooler" is still in common use in the piston engined aircraft industry."

 

When looking at the engineering aspects of designing the compression ratios in the engine system, the engine is simply another stage of compression, and final compression ratio will be determined by intercooling efficiency and final inlet air conditions to the engine (final compression stage) to prevent (or aid) in self-ignition of the fuel in the combustion chamber.

 

When you view it this way (as you should for design purposes) they're all intercoolers, as they all are performing the same efficency enhancing job on the next stage of compression.

 

It also comes from this 'American English' crap... Changing names just to do it...like inventing 'Series 1' 240's...

[(goldfish)]

Never seen a wasp major with the superchargers in place. They are quite daunting on their own!

 

But more on topic, If you have a two stage design, with both stages at say 3:1. If the final stage puts out say 100 cfm it would need to ingest ~300cfm. So wouldn't the first stage need to put out 300 cfm, or ingest 900 cfm, requiring it to be a larger unit?

[Tony D]

Yes, each stage will either spin progressively faster, or be physically larger to accommodate the flow required to boost teh pressure to the next higher level: PV=nRT

[HowlerMonkey]

The term intercooler is similar to the term "NOS".

 

It has been used for a long time and is now accepted whether it is right or wrong.

 

I come from the aviation and heavy diesel end of things where there are both intercoolers and aftercoolers on the same engine.

 

In a situation where you have both, you cannot call both of them "intercoolers".

 

That said, I've done a couple of mustangs and ford GT with both supercharger and turbocharger together but I'm not a big fan of most installations I have seen because most compound turbo or supercharer/turocharger seem to lack a cooler at some point.

 

Our mustang ran an aftercooler between the blower and the engine capable of flowing 2000 horsepower's worth of air and an intercooler between the turbo and the blower.

 

We've been much more successful with a twin turbo setup and are getting north of 2000hp from a 5.4 ford modular engine that features almost a 5000rpm powerband at which full boost is obtained.

 

It's also driven on the street with a/c and all factory comfort options retained.

 

You run out of room pretty quickly for placement of both of the heat exchangers tasked with dissapating the heat.

Edited September 19, 2011 by HowlerMonkey

[Gollum]

Didn't Tony just lecture you in another thread about intercooler/aftercooler terminology? I don't pretend to think anyone is right or wrong, I just find the argument almost hilarious because in the end it really doesn't matter for our application what we call it.

 

Personally, if I was to do a compound setup, I'd use E85 or similar heat soaking fuel to help cut out some of the heat. But I do agree that cooling the charge can become incredibly important if both your air compressors are only running at a 50% efficiency.

 

Air to water coolers can definitely help regarding packaging for cooling capability.

[HowlerMonkey]

Didn't Tony just lecture you in another thread about intercooler/aftercooler terminology? I don't pretend to think anyone is right or wrong, I just find the argument almost hilarious because in the end it really doesn't matter for our application what we call it.

 

Personally, if I was to do a compound setup, I'd use E85 or similar heat soaking fuel to help cut out some of the heat. But I do agree that cooling the charge can become incredibly important if both your air compressors are only running at a 50% efficiency.

 

Air to water coolers can definitely help regarding packaging for cooling capability.

 

 

No lecture required.

 

I am a FAA certified airframe and powerplant mechanic and a certified heavy diesel technician having worked reciprocating aircraft engines with two stage supercharging and turbo compound as well as heavy diesel marine engines for hatteras yachts that feature both superchargers, turbochargers, aftercoolers, and intercoolers on each engine.

 

Tony is not wrong and neither am I.

 

You, however, are wrong in your scolding.

 

My point is that you really don't need a supercharger in the mix when turbo technology has come far enough to make it not necessary for any non-diesel application.

 

I think the biggest reason you even see any cars with both superchargers and turbochargers is because the car of choice was factory equipped with a supercharger already.

Edited September 21, 2011 by HowlerMonkey

[jgkurz]

I first learned about compound charging when this car came onto the hot rod scene.

 

 

 

 

[Gollum]

You, however, are wrong in your scolding.

 

Sorry you thought I had tar and feathers waiting, that was not my intent. Maybe I needed some smileys to show my lightheartedness?

 

If anything I was trying to hear what you had to say on it, and why but didn't want to force you if it was a sore subject that we didn't need to beat to death.

 

Again, as I said before I could care less about nomenclature. It changes constantly through history and I personally believe I'm only required to learn enough to communicate effectively with those I interact with, and since I'm not in frequent communication with engineers in the field on a regular basis I haven't had the need to make a discrepancy. I'm not promoting the continuation of ignorance because that'd just be backwards. But I'm saying we can't all know everything in the world and you have to choose what to go after.

[Tony D]

"Again, as I said before I could care less about nomenclature."

 

Then bow out when nomenclature is discussed. Not to be pedantic, but application does dictate the terminology. As I mentioned above, long ago I got the talk "Cars aren't Aircraft".

 

"Definition of Terms" is the first section of any paper. If you don't care what you call it, then anybody reading what you write won't have any applicable basis from which to judge or draw conclusions, and the effort is wasted through the confusion in terminology.

 

Words mean things, if you don't care what term is used, then let's call them radiators. Radiators dissipate heat. The terminology is correct from your reasoning.

 

We came to this point the last time this 'fact' was pointed out---to the end it came that in the field the mechanics and what they call them may not have a clue the engineering terminology used or why they are referred to in the particular way they are. An axial flow turbine providing air for subsequent mechanical compression would have an intercooler for that next stage of compression, but the air supplied off that discharge (bleed air) would have an aftercooler before going into cabin air management system. One is subsequent compression, one is point of final use...

 

Edited September 22, 2011 by Tony D

[Gollum]

Then bow out when nomenclature is discussed.

 

I do believe that I have yet to argue either way, which I would think in a sense is "bowing out" so to speak. I've never intended to question anyone on the subject and don't wish to argue on it, which is a rarity for me.

 

Trust me, I have a deep passion about words and how they're used. I also have little patience for arguments when everyone in the discussion understands what everyone meant without confusion from the onset. Now, if there needed to be clarification because someone didn't understand what was being communicated then there's a good reason to have a discussion as to why a word is used for something.

 

But in order to maybe take this minor conflict in a positive direction. Let's bring it into something useful that takes us somewhere.

 

So if we use a cooling device after a compressor outlet, but before the engine we might call that an intercooler. If we take that charge and then cool it before sending it somewhere outside the system it's an "aftercooler", but what about cooling a charge at the inlet of the first compressor in the system? Obviously something like a traditional air to air cooler wouldn't be used, but something more like a liquid injection cooling, or a water-air cooler. Would that be called a.... precooler?

 

Reason I even bring it up, is that if you've got a compound setup, controlling inlet temps could be quite important if you live somewhere that outside temps can get over 110 degrees on a regular basis.