philly
solid fixture here in the forum
so i am gonna do a thought dump on here and revisit it every so often like other turbo thread and tweak and update it as new information becomes available or new discoveries are made. since i got on the subject with brian and john in a totally unrelated thread i thought i would put out what i understand about sequential or series or staged turbocharging... first some visual aids for your mental masturbatory pleasure... obviously there are many ways to skin a cat... the system i would design for racing use looks mostly like this one from mazda's diesel lemans project...
lets start with how i would work the hotside... a fast response, torquey, typically undersized (for our purposes) turbo (or supercharger) is used for low engine speed off the line power build up. this gives you instant on throttle response and traditionally would run out of breath in the upper RPM range.... so a SECOND (actually referred to as primary but dont mind that confusing shit) A SECOND larger turbocharger is setup further down the exhaust stream from the first... as in this diagram here, you can see even the wastegate discharge from the little turbo pours into the big turbo...
meaning that the lag you would experience with the much larger turbo isnt felt because the little turbo is turning out power down low in the RPMs... and if you sized the turbos appropriately somewhere around the time the smaller turbo reaches its preset pressure ratio on its own, the larger turbo will be "lit" and carry you through the higher RPM's
never mind whatever garbage wiring diagrams are in this picture.... but it could look something like this if you wanted to run a low boost setup (under 30 psi)
and after reading that last sentence im sure you chuckled and said "damn phil 30psi sounds like a lot of MF boost to me..." and you're right, to me too, from one big turbo an efficient 28-30 psi is significant. but realize that 29.4 psi is just a pressure ratio of 3:1 the pressure ratio is an important factor as thats whats usually on the left hand vertical axis of a compressor map
briefly, and you can refer to the compressor maps thread for more detail here...
viewtopic.php?f=86&t=1215
pressure ratio is the PSIa or actual (all the pressure in the manifold including the atmospheric) divided by the atmospheric... so at sea level for us thats 14.7 atmospheric plus 29.4 PSIg (gauge) makes 45.1 PSIa (actual) divided by the atmospheric 14.7 makes for 3:1 so in essence there are 3 atmospheres or 3 bar in that manifold.
http://www.engineeringtoolbox.com/air-a ... d_462.html
but what would happen if we CHANGED what the compressor thinks atmospheric pressure is? after all, it just takes the mass of air and pressurizes it based on a pressure ratio determined by the wastegate... for our purposes at sea level an atmosphere is 14.7 psi but at 50,000 ft above sea level the atmospheric is only 1.61 psi... at that elevation a 3:1 pressure ratio gives you 3.22psi on the gauge... but the turbo is working equally hard and in the same exact efficiency on its compressor map. all it knows is whatever goes in its INLET will be turned into X pressure ratio (in our example 3:1) on the OUTLET.
so lets say we took the OUTLET from the big turbo and stuffed it in the INLET of the smaler one... (as pictured above in the lemans mazda motor but i edited this opel diagram because it illustrates it better...)
from this diagram i think you get a pretty good idea where im going with this... if we stuff higher than atmospheric pressure in the INLET of the smaller turbo, it will do what its supposed to and triple the pressure ratio accordingly. so in our example if we stuff 29.4 psi (thats our new atmosphere we created between the outlet of the big turbo and the inlet of the smaller) its gonna convert that 3:1 and give us 58.8 PSIg on the OUTLET of the small turbo and thus in our intake manifold... now THATS HIGH BOOST
sizing the turbos... heres where im just speculating and i think everyone else is too.
on a small displacement motor i would use the squirrelpf calc from the other turbo thread
http://www.squirrelpf.com/turbocalc/
viewtopic.php?f=86&t=1215
however i would size the turbo based on about 80% of redline just to make sure theres absolutely no lag in the bottom end. now to size the larger turbo... i would base that partially on the smaller one...
if we stuff 14.7 PSIg thru a small turbo in a 2liter motor... we effectively double its output to that comparable to a 4 liter... so we can use our motors displacement and our small turbos pressure ratio to determine how big we should get a big turbo. in the event that its impractical to get one massive ass turbo (say a 3:1pressure ratio on a 496-540" big block) either because one big enough doesnt exist, wont fit in the package space, or we cant afford it... there is another option... figuratively cut the motor in half and use two of these setups combined...
something like this:
now as far as efficiency is concerned, you can see above in the lemans mazda motor that after the outlet of the big turbo, compressed air runs through a water to air inter-stage-cooler to cool off and get more dense before it goes to the inlet of the smaller turbo... then obviously you run that thru whatever intercooler setup your heart desires after the small turbo and before the intake manifold.
and heres how volkswagen wants to do it with a supercharger and a turbocharger staged up for your viewing pleasure.
the end. for now.
lets start with how i would work the hotside... a fast response, torquey, typically undersized (for our purposes) turbo (or supercharger) is used for low engine speed off the line power build up. this gives you instant on throttle response and traditionally would run out of breath in the upper RPM range.... so a SECOND (actually referred to as primary but dont mind that confusing shit) A SECOND larger turbocharger is setup further down the exhaust stream from the first... as in this diagram here, you can see even the wastegate discharge from the little turbo pours into the big turbo...
meaning that the lag you would experience with the much larger turbo isnt felt because the little turbo is turning out power down low in the RPMs... and if you sized the turbos appropriately somewhere around the time the smaller turbo reaches its preset pressure ratio on its own, the larger turbo will be "lit" and carry you through the higher RPM's
never mind whatever garbage wiring diagrams are in this picture.... but it could look something like this if you wanted to run a low boost setup (under 30 psi)
and after reading that last sentence im sure you chuckled and said "damn phil 30psi sounds like a lot of MF boost to me..." and you're right, to me too, from one big turbo an efficient 28-30 psi is significant. but realize that 29.4 psi is just a pressure ratio of 3:1 the pressure ratio is an important factor as thats whats usually on the left hand vertical axis of a compressor map
briefly, and you can refer to the compressor maps thread for more detail here...
viewtopic.php?f=86&t=1215
pressure ratio is the PSIa or actual (all the pressure in the manifold including the atmospheric) divided by the atmospheric... so at sea level for us thats 14.7 atmospheric plus 29.4 PSIg (gauge) makes 45.1 PSIa (actual) divided by the atmospheric 14.7 makes for 3:1 so in essence there are 3 atmospheres or 3 bar in that manifold.
http://www.engineeringtoolbox.com/air-a ... d_462.html
but what would happen if we CHANGED what the compressor thinks atmospheric pressure is? after all, it just takes the mass of air and pressurizes it based on a pressure ratio determined by the wastegate... for our purposes at sea level an atmosphere is 14.7 psi but at 50,000 ft above sea level the atmospheric is only 1.61 psi... at that elevation a 3:1 pressure ratio gives you 3.22psi on the gauge... but the turbo is working equally hard and in the same exact efficiency on its compressor map. all it knows is whatever goes in its INLET will be turned into X pressure ratio (in our example 3:1) on the OUTLET.
so lets say we took the OUTLET from the big turbo and stuffed it in the INLET of the smaler one... (as pictured above in the lemans mazda motor but i edited this opel diagram because it illustrates it better...)
from this diagram i think you get a pretty good idea where im going with this... if we stuff higher than atmospheric pressure in the INLET of the smaller turbo, it will do what its supposed to and triple the pressure ratio accordingly. so in our example if we stuff 29.4 psi (thats our new atmosphere we created between the outlet of the big turbo and the inlet of the smaller) its gonna convert that 3:1 and give us 58.8 PSIg on the OUTLET of the small turbo and thus in our intake manifold... now THATS HIGH BOOST
sizing the turbos... heres where im just speculating and i think everyone else is too.
on a small displacement motor i would use the squirrelpf calc from the other turbo thread
http://www.squirrelpf.com/turbocalc/
viewtopic.php?f=86&t=1215
however i would size the turbo based on about 80% of redline just to make sure theres absolutely no lag in the bottom end. now to size the larger turbo... i would base that partially on the smaller one...
if we stuff 14.7 PSIg thru a small turbo in a 2liter motor... we effectively double its output to that comparable to a 4 liter... so we can use our motors displacement and our small turbos pressure ratio to determine how big we should get a big turbo. in the event that its impractical to get one massive ass turbo (say a 3:1pressure ratio on a 496-540" big block) either because one big enough doesnt exist, wont fit in the package space, or we cant afford it... there is another option... figuratively cut the motor in half and use two of these setups combined...
something like this:
now as far as efficiency is concerned, you can see above in the lemans mazda motor that after the outlet of the big turbo, compressed air runs through a water to air inter-stage-cooler to cool off and get more dense before it goes to the inlet of the smaller turbo... then obviously you run that thru whatever intercooler setup your heart desires after the small turbo and before the intake manifold.
and heres how volkswagen wants to do it with a supercharger and a turbocharger staged up for your viewing pleasure.
the end. for now.