two small turbos better than one big turbo
09-18-2006, 04:23 PM
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Really good thread you guys. Keep it up.
Just a question to throw into the mix. How come multistage turbines and or compressors have not been historically used in turbo chargers?
Would seem to me you could increase the efficientcy overall, but obviously increase the cost and complexity.
Jim
Just a question to throw into the mix. How come multistage turbines and or compressors have not been historically used in turbo chargers?
Would seem to me you could increase the efficientcy overall, but obviously increase the cost and complexity.
Jim
09-19-2006, 01:15 AM
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I can email the thesis study to anyone who's interested. Better yet, if anyone knows where/how to host it on the net, I'll do that.
For 'subscription' ,drop an email address here under this thread, or alternatively private-message me.
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As it comes to avenues what manufacturers employ in deciding on turbocharger systems employed, it's a cost driven world. Less buck tied onto turbo, the more profit it'll bring in. Period.
Top end sports cars have the privilige to contain costly (= better) technical solutions.
For 'subscription' ,drop an email address here under this thread, or alternatively private-message me.
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As it comes to avenues what manufacturers employ in deciding on turbocharger systems employed, it's a cost driven world. Less buck tied onto turbo, the more profit it'll bring in. Period.
Top end sports cars have the privilige to contain costly (= better) technical solutions.
09-19-2006, 07:59 AM
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09-19-2006, 02:44 PM
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I asked the same question on another board just quite recently. Engineering folks who are into motorsports and engines visit that forum frequently. Below is a quote from that forum.
Here is a link to all of you people who are unfamiliar with SAE Formula competition class mentioned below: http://students.sae.org/competitions/formulaseries/
Here's the quote from that other forum, after I posted the same quote from the thesis as I did on this forum. (BTW - he says 2 ports on each turbo because it's a mere 4 cylinder engine we're talking about.)
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innovator (Automotive) 19 Sep 06 13:01
Interesting Artsi that you quote Fredrik Westin as I have spoken to him (I judged his car at Formula student and a year later he was in my judging team)about twin turbos. He said that 2 small turbos were the way to go for general all round use. He was talking about having one turbo on 2 ports and the other on the other 2 ports. They would spin up and get on boost a lot quicker yet still give reasonable top end power.
I did not really get into any more technical details at the time.
John
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Here is a link to all of you people who are unfamiliar with SAE Formula competition class mentioned below: http://students.sae.org/competitions/formulaseries/
Here's the quote from that other forum, after I posted the same quote from the thesis as I did on this forum. (BTW - he says 2 ports on each turbo because it's a mere 4 cylinder engine we're talking about.)
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innovator (Automotive) 19 Sep 06 13:01
Interesting Artsi that you quote Fredrik Westin as I have spoken to him (I judged his car at Formula student and a year later he was in my judging team)about twin turbos. He said that 2 small turbos were the way to go for general all round use. He was talking about having one turbo on 2 ports and the other on the other 2 ports. They would spin up and get on boost a lot quicker yet still give reasonable top end power.
I did not really get into any more technical details at the time.
John
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09-19-2006, 03:06 PM
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jumping into this a little late, just saw it. why would you want to run parallels? yes, with smaller turbos you do have less inertia but the driving heat energy is also divided. you pretty much have the same thing as a single charger 5.9, but now it's a single charger 3.0. then you have more efficiency loss, as awesome as turbos are, they suck a lot of energy to drive.
if you look at turbine aircraft engines they're not rocket science, it's a turbocharger with a combustion chamber instead of a piston engine to drive it. on both the Allison 250 and Pratt&Whitney PT-6A when they go to bump up power they don't add more compressor and turbine wheels, they make the current ones bigger. especially the allison, used to have about a 7 stage axial compressor in front of the centrifugal compressor for about 300hp; now they're around 700hp and have a single stage centrifugal that's way bigger. the BIG PT6A-65 and -67 engines have two power turbines because they had to go well over 1,000hp and keep the physical outside size of the engine small; otherwise they would've just put a larger single wheel in there...even at that though, they two power turbine wheels are different sizes, the second stage is larger. the difference in power output on the big PT6 vs the small PT6 engines is about the same as running a single or sequential setup on our trucks.
now, the sequential setups that bypass the small turbo when the engine rpms are up are pretty cool, mercedes has been doing that for a while, I remember reading a very in depth tech article on a setup they were running in europe, that was about 2 or 3 years ago. complex? kinda, just another thing for the computer to monitor/adjust but not very difficult.
main reason parallels are run on cars are because of plumbing routing, especially with a V or opposed(pancake/porsche). it's easier to mount a turbo right on the exh manifold log and there's very minimal heat loss there as opposed to running a pipe across the engine compartment from one bank. plus, it's way easier to run two 3" exit pipes instead of one 5" exit pipe under a car.
if you look at most of the guys that upgrade the turbo'd supras(which are dang fast stock), they get rid of the bypassing sequential in favor of a single and they have no problem with a streetable 550-650hp range.
there's no practical reason to take a single charger off to replace it with smaller parallels. the efficiency loss wins. now, 'twins' when they're sequential and different sizes are great, like already mentioned, the big charger isn't for the engine, it's just to provide a higher baseline atmosphere for the little guy to start multiplying from. a large single charger is still more efficient than this setup BUT is less practical for street.
if you look at turbine aircraft engines they're not rocket science, it's a turbocharger with a combustion chamber instead of a piston engine to drive it. on both the Allison 250 and Pratt&Whitney PT-6A when they go to bump up power they don't add more compressor and turbine wheels, they make the current ones bigger. especially the allison, used to have about a 7 stage axial compressor in front of the centrifugal compressor for about 300hp; now they're around 700hp and have a single stage centrifugal that's way bigger. the BIG PT6A-65 and -67 engines have two power turbines because they had to go well over 1,000hp and keep the physical outside size of the engine small; otherwise they would've just put a larger single wheel in there...even at that though, they two power turbine wheels are different sizes, the second stage is larger. the difference in power output on the big PT6 vs the small PT6 engines is about the same as running a single or sequential setup on our trucks.
now, the sequential setups that bypass the small turbo when the engine rpms are up are pretty cool, mercedes has been doing that for a while, I remember reading a very in depth tech article on a setup they were running in europe, that was about 2 or 3 years ago. complex? kinda, just another thing for the computer to monitor/adjust but not very difficult.
main reason parallels are run on cars are because of plumbing routing, especially with a V or opposed(pancake/porsche). it's easier to mount a turbo right on the exh manifold log and there's very minimal heat loss there as opposed to running a pipe across the engine compartment from one bank. plus, it's way easier to run two 3" exit pipes instead of one 5" exit pipe under a car.
if you look at most of the guys that upgrade the turbo'd supras(which are dang fast stock), they get rid of the bypassing sequential in favor of a single and they have no problem with a streetable 550-650hp range.
there's no practical reason to take a single charger off to replace it with smaller parallels. the efficiency loss wins. now, 'twins' when they're sequential and different sizes are great, like already mentioned, the big charger isn't for the engine, it's just to provide a higher baseline atmosphere for the little guy to start multiplying from. a large single charger is still more efficient than this setup BUT is less practical for street.
Your example is not relevant to our discussion, because turbine lag in a jet engine is not a primary concern. So you can just make the existing design bigger and get more thrust (all but turboshaft) or drive hp (turboshaft)-- not a problem. Moreover, the jet engine provides its own drive energy, which scales with the turbine size. Thus, a linear translation in most cases (though generally, a larger slower turbine is more efficient than a smaller, higher RPM one is).
But with a turbocharger, this is all different. First, the drive energy available to the turbo is NOT increased-- it's bolted to the same engine. Hence, a larger turbo brings lag, because the engine takes more RPM to produce the drive energy necessary to power the compressor past the boost threshold.
You are comparing a steady-state jet engine with a mostly-variable turbocharger-- not a good idea, imo. NOw if you are just talking about one given set of conditions, then it's valid. But how can you drive a truck at constant RPM, constant Boost, and Constant load? Hard to do, right?
The reason that larger turbos are not a major penalty on a Supra is that a Supra has a LOT of RPM range to deal with. Thus, giving up a little lag of a couple hundred rpm is not that big a deal. So they can exploit the efficiency of the large single.
But on a Cummins with only 2200rpm of useable rev range, lag is CRITICALLY important.
JH
09-19-2006, 04:55 PM
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actually I don't believe I said anything about a jet, the rocket science thing is the only thing I can think of that you should be able to derive 'jet' from; and that was just a term that meant 'really complex'. last time I split a PT-6 a couple days ago it was a turbo-shaft engine; last time I worked on an Allison 250 series engine in a helicopter a few weeks ago, it was also a turbo-shaft engine.
and a turbo-shaft engine IS very very similar to a turbo diesel engine in basic theory. on both, the throttle is controlled by fuel control. rpm is a result of how efficiently the engine is using the burned fuel. it is not primarily the rpm of the engine but the amount of heat energy that turns the turbine. and both have a turbo(turbine and compressor). the only difference is that a diesel's pistons/cylinders are replaced with a combustion chamber. the aircraft I deal with see quite a bit of variation in throttle, they don't run constant like an APU turbine does. there's a reason there are condition levers that the pilot's left hand are on for a majority of flying. they do not stay at constant load or constant torque. they crank and bank, up and down and have a constantly varying load.
I'm not comparing a single steady state jet to a mostly variable turbocharger. there's nothing variable about our turbos, they have a very specific efficiency range no matter what size. the variation comes from the fuel control. the fuel control is affected by more than throttle position....BOMBing, specifically. add more fuel isn't necessarily for higher rpm but for more heat energy at the same rpm to spool a bigger turbo. these turbo shaft engines see quite a bit of variation in speed, they change several thousand rpm throughout a flight.
reason we don't BOMB the turbine aircraft is the FAA doesn't allow it. add more fuel, change the turbine/compressor wheels, some of the $80k parts take out the $500k engine and crash the $1million plane carrying 800 gallons of whatever that should not be put on 1 acre of anything(which results in fines and a lawsuit) plus whatever the aircraft crashes into and destroys(field, equipment, vehicle or house....) plus the injury/pain/suffering/death of the pilot and that should explain why we don't mod the aircraft engines but just put the next size bigger in. liability. not because we can't or it'll mess up the "steady state" turbine, they are amazingly variable.
and a turbo-shaft engine IS very very similar to a turbo diesel engine in basic theory. on both, the throttle is controlled by fuel control. rpm is a result of how efficiently the engine is using the burned fuel. it is not primarily the rpm of the engine but the amount of heat energy that turns the turbine. and both have a turbo(turbine and compressor). the only difference is that a diesel's pistons/cylinders are replaced with a combustion chamber. the aircraft I deal with see quite a bit of variation in throttle, they don't run constant like an APU turbine does. there's a reason there are condition levers that the pilot's left hand are on for a majority of flying. they do not stay at constant load or constant torque. they crank and bank, up and down and have a constantly varying load.
I'm not comparing a single steady state jet to a mostly variable turbocharger. there's nothing variable about our turbos, they have a very specific efficiency range no matter what size. the variation comes from the fuel control. the fuel control is affected by more than throttle position....BOMBing, specifically. add more fuel isn't necessarily for higher rpm but for more heat energy at the same rpm to spool a bigger turbo. these turbo shaft engines see quite a bit of variation in speed, they change several thousand rpm throughout a flight.
reason we don't BOMB the turbine aircraft is the FAA doesn't allow it. add more fuel, change the turbine/compressor wheels, some of the $80k parts take out the $500k engine and crash the $1million plane carrying 800 gallons of whatever that should not be put on 1 acre of anything(which results in fines and a lawsuit) plus whatever the aircraft crashes into and destroys(field, equipment, vehicle or house....) plus the injury/pain/suffering/death of the pilot and that should explain why we don't mod the aircraft engines but just put the next size bigger in. liability. not because we can't or it'll mess up the "steady state" turbine, they are amazingly variable.
09-19-2006, 09:15 PM
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Chrysler ME4-12
Does anybody rember the concept car Chrysler and MB let out in 04. It was a v 12 with 4 parallel turbos each one being fed by 3 cylinders. I don't know how effeciently it ran but it was supposed to have 800 hp.
http://www.exoticcarsite.com/pages/c...12_concept.htm
http://www.exoticcarsite.com/pages/c...12_concept.htm
09-19-2006, 09:51 PM
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ok guys my knowledge on this subject is very limited but wouldent it make more sense in a compound system to feed the larger turbo with the smaller one. i am thinking that the smaller turbo would spool faster building boost for the larger turbo to feed off of causing to have a higher atmospheric pressure to start off with. and if the larger turbo has a higher pressure to start with it would make more boost with less work. But at a certain RPM bypass the smaller turbo because once the larger turbo got going the smaller one would just be holding it back.you could bypass with with those things they use on some street legal drag cars that when on the street they flip a switch and the exhaust runs though a muffler and when on the strip they flip the switch the other way and it just dumps
09-20-2006, 01:14 AM
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the ME-412 is the chrysler concept car, yes, quad turbo V12. mostly done for routing and also for wow factor. again, on a v or opposed engine that is in a little tiny engine compartment, you lose too much heat energy and it takes up too much room to route all the exhaust into one. much more feasable to run multiples on a V/opposed when there's limited room. anyone ever seen the turbo routing on a newer 911 turbo? a friend has an '05, holy wow, they greased a shoe-horn to fit that package in there. and even with the supposedly all amazing twin turbos, it has a pretty high boost threshold(what most people mistake for lag). I've driven it a few times and I've ridden in it a LOT. and the low boost threshold doesn't mean I'm saying it's slow, that thing pulls like a raped ape up in the high performance powerband it was meant to pull.
--a bypass style big n little works pretty good, it's just complex and expensive to make it work well. I've seen a few different manufacturers do that at one point or another(pretty much european). basically has doors/valves to direct exhaust gas through the small turbine, which feeds the compressor inlet of the big turbo and then into the engine, then at a certain load/rpm it switches the exhaust into the big turbine and closes the valve coming from the small compressor.
pretty complicated and it doesn't seem like it'd be a very cost effective or long lasting solution for the gain it would provide.
a single turbo is mechanically/physically more efficient than two parallels, there's no way around it. there's nothing wrong with two twins, especially on an opposed/V where it's much more feasable to negotiate the plumbing. on an inline I see no reason to run parallels.
--a bypass style big n little works pretty good, it's just complex and expensive to make it work well. I've seen a few different manufacturers do that at one point or another(pretty much european). basically has doors/valves to direct exhaust gas through the small turbine, which feeds the compressor inlet of the big turbo and then into the engine, then at a certain load/rpm it switches the exhaust into the big turbine and closes the valve coming from the small compressor.
pretty complicated and it doesn't seem like it'd be a very cost effective or long lasting solution for the gain it would provide.
a single turbo is mechanically/physically more efficient than two parallels, there's no way around it. there's nothing wrong with two twins, especially on an opposed/V where it's much more feasable to negotiate the plumbing. on an inline I see no reason to run parallels.
09-21-2006, 09:00 PM
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The benfit of running a large primary is that you can increase the amount of mass flow that enters into your engine. Yes you can match that with a single but you have to have a wheel with the ablity to push large amount of flow and the ablity to make boost but it just take awhile to happen. With twins (compounded twins) your able to collect abit more of the exhaust energy to push more boost/flow into the cylinder.
04-20-2007, 10:56 AM
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I know this is kinda of irrelevant now due to the age of this post.. but its not exactly an unknown fact that a sequential (2 medium same size or 1 standard and 1 slightly larger) turbo setup is at least twice as effective as a single large unit when properly plumbed. and if any difference in power in the same rpm range and same boost range, a twin sequential would have more power than a single large unit, especially in the lower rpm ranges when your exhaust is struggling to spool up that larger heavier set of vanes. I'm not real sure if maybe somebody else said the same thing as I did basically because I only read the first page of posts, I just figured I would throw it out there what I know, because I have a similar idea for my ford later on down the road.
05-21-2007, 09:31 PM
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a single turbo is mechanically/physically more efficient than two parallels, there's no way around it. there's nothing wrong with two twins, especially on an opposed/V where it's much more feasable to negotiate the plumbing. on an inline I see no reason to run parallels.
It looks like we've come full circle. Apparently, the twin parallel turbos are superior from a spoolup standpoint, but the single larger turbo is superior from a pure efficiency standpoint?
This would stand to reason. Since there's no free lunch, you're giving up something to get the spoolup of parallel turbos (efficiency) or the efficiency of a large single (spoolup).
Or is this debatable? I thought I had this figured out, but Jon and some others have convinced me that I don't what I thought I did!
Justin
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