Sequential Turbo
Sequential Turbo
(OP)
Hi all,
I've been looking for a forum and place on sharing ideas about sequential turbo charging. Many forums I've seen on the Internet about turbocharging is mainly focusing on horse power. Sequential twin (or multiple) turbo setup goal is not just for high power but also taking care about low end grunt of an engine.
To make it sort, I will only talk about twin turbo setup to simplify the introduction of sequential turbo . The 1990's Mazda RX7 and Toyota Supra are well known setup of sequential turbo charging system. Tuners often change to twin parallel turbo or even one large turbo to replace the stock setup and achieve a high output engine. This doesn't mean the stock setup is no good, it is because tuner's goal is aim for high HP numbers with simplicity. Everyone who experience with turbochargers know that using a large turbo can achieve high horse power with sacrifice of low end torque. Sequential turbo charging is a way to optimize both low and top end power of an engine.
Take RX7 as an example, two turbo chargers exists in the system. These two turbo are the same size. The primary turbo works in all engine rev range. The secondary turbo only works in mid-high rev range. Since the 13B rotary engine is an engine having wide range rpm, it is an ideal motor to having a sequential setup to maximize the torque on each rpm range and deliver the most out of it. In low engine speed, both rotor's exhaust gas are pumped to the primary turbo, making the primary turbo spools up quickly so that the engine can have boost as soon as possible. When the engine speed goes higher, the primary turbo efficiency decreases and requires two turbo to work in parallel to generate enough boost for the engine. At this time, a flap in the exhaust manifold open wide and allow the exhaust gas of the second rotor to pump into the secondary turbo . At this stage, the engine just work as a parallel twin turbo system.
The disadvantage of sequential turbo setup is complication of piping work, control and known how of it. Nowadays we have a much better technology base then in 1990s. Does anyone know anybody in the world is doing some similar projects using a better design then the RX7 and Supra? I have been in this topic for many years, but I didn't have enough resources and time to build a prototype of it. I've done lots of design and research work in this area, but not seeing many of others is working on it. At the time I was studying, forum and blogs are not that common at all. Recently, I bought a R34-GTR, the engine revs from 1000 to 8500, with little modification it can go to 10000rpm. This engine is very tough. Currently, it is a parallel twin turbo setup and is missing low end grunt . I am looking to modify it to a sequential setup . I was trying to locate the Garret VNT (Variable Nozzle Turbine) turbochargers to achieve a similar goal, but can find very little information about it .
If anyone is in the field, please share.
I've been looking for a forum and place on sharing ideas about sequential turbo charging. Many forums I've seen on the Internet about turbocharging is mainly focusing on horse power. Sequential twin (or multiple) turbo setup goal is not just for high power but also taking care about low end grunt of an engine.
To make it sort, I will only talk about twin turbo setup to simplify the introduction of sequential turbo . The 1990's Mazda RX7 and Toyota Supra are well known setup of sequential turbo charging system. Tuners often change to twin parallel turbo or even one large turbo to replace the stock setup and achieve a high output engine. This doesn't mean the stock setup is no good, it is because tuner's goal is aim for high HP numbers with simplicity. Everyone who experience with turbochargers know that using a large turbo can achieve high horse power with sacrifice of low end torque. Sequential turbo charging is a way to optimize both low and top end power of an engine.
Take RX7 as an example, two turbo chargers exists in the system. These two turbo are the same size. The primary turbo works in all engine rev range. The secondary turbo only works in mid-high rev range. Since the 13B rotary engine is an engine having wide range rpm, it is an ideal motor to having a sequential setup to maximize the torque on each rpm range and deliver the most out of it. In low engine speed, both rotor's exhaust gas are pumped to the primary turbo, making the primary turbo spools up quickly so that the engine can have boost as soon as possible. When the engine speed goes higher, the primary turbo efficiency decreases and requires two turbo to work in parallel to generate enough boost for the engine. At this time, a flap in the exhaust manifold open wide and allow the exhaust gas of the second rotor to pump into the secondary turbo . At this stage, the engine just work as a parallel twin turbo system.
The disadvantage of sequential turbo setup is complication of piping work, control and known how of it. Nowadays we have a much better technology base then in 1990s. Does anyone know anybody in the world is doing some similar projects using a better design then the RX7 and Supra? I have been in this topic for many years, but I didn't have enough resources and time to build a prototype of it. I've done lots of design and research work in this area, but not seeing many of others is working on it. At the time I was studying, forum and blogs are not that common at all. Recently, I bought a R34-GTR, the engine revs from 1000 to 8500, with little modification it can go to 10000rpm. This engine is very tough. Currently, it is a parallel twin turbo setup and is missing low end grunt . I am looking to modify it to a sequential setup . I was trying to locate the Garret VNT (Variable Nozzle Turbine) turbochargers to achieve a similar goal, but can find very little information about it .
If anyone is in the field, please share.





RE: Sequential Turbo
Study the turbo system on the BMW 335d, which is a current production vehicle with a turbodiesel 6 cylinder engine with a small and large turbo.
RE: Sequential Turbo
It seems to me most car companies are doing away with their flagship power monsters that would even use this. At least the Japanese companies and US car companies seem usually to prefer al motor or occasionally super chargers.
I have always loved Toyota and I am a little POd at them for ditching all their cool cars. The Supra, the Celica all-track and the MR2 were their cars that would make people drool and get them in the door. Now it's all bland eco cars and behemoth SUVs and trucks.
Mazda went NA when they went to the RX8 and went from having a badass full blown sports car to a sporty compromise car.
I am curious what your HP goals are and how much low end it's missing.
I always figure you should be able to find a quick spooling turbo that will get you to 300 HP on a smaller motor. Even more on a larger motor.
If you are going for more than that then you could be sacrificing some spool time. I also look at the overall RPM curve. Even if it has nothing from idle to 4k but spins to 10k RPM then you still have a 6k RPM powerband which from a racing or performance standpoint is more than enough. It could be a little doggy around town or for daily driving but then do you really want a 500 hp highly tuned car for that purpose?
RE: Sequential Turbo
Forget VNT turbos on a gasoline engine. They too don't work very well, only Porsche have been game to try. VNT technolgy has been around for over thirty years, where are all the VNT production cars and race winners ?
If you want a very broad power band, turbocharging an already supercharged engine (twincharging) works well, and is quite easy to get going.
RE: Sequential Turbo
Believe it or not. Car makers doesn't do much on automotive electronic or technology developments, most of them are done by other companies like Bosch/Mahle . If these companies doesn't have an off-the-shelf solution, then they won't use it. Porsche 997 turbo setup is done by BorgWarners . Every year I visit those automotive parts engineering exhibitions, these companies are always trying to sell their complete solutions to car makers. I've also seen VNT on gasoline engine solutions. Here is a picture of it.
I am a developer, always not a follower, if someone has done something, we take it as reference, which is good. If someone never did something, it doesn't mean that we can't do it right. Engineering is application of science. First we have to have an idea, then know the science, and collect real life figures, turn them into applications. Let's keep it up and find out the other unknown variables before someone or me to proceed for a prototype.
Its not a matter of how much HP I want on my car. I am not an individual trying to achieve high HP numbers on my car. I am from a company and group of people. We are distributor/importer and sell high performance parts to tuners. I am the engineering manager of the company. I personally own a Nissan R34-GTR it has lot of HKS/Nismo stuff in it, it has more power than I need. Our company and group of people own so many sports cars and high power cars. Porsche Cayene turbo, Porsche 996, R32 GTR, R33 GTR, Audi A4 DTM, Audi RS4, Golf GTI 2.0 Turbo.. Not to mention customer's cars . If we develop a solution, there are so many test platforms to test with . Achieving a smooth torque curve with high power output is always the goal, its an engineering problem of cost/efficiency/performance ratio.
Superchargers efficiency is too low, its not in consideration. Turbochargers is much more flexible. Think of supplying a supercharger kit, and the difference of supply a turbocharger+valves+electronics . The second can be easily to install in many types of cars.
I don't use to write forums and blog. Its even my first post. I am just trying to share my idea here and to see if anyone has done some engineering work about this recently. Its nice to hear some replies. Thanks.
RE: Sequential Turbo
Nissan diesels have run VNT turbos for decades, the Nissan R&D engineers have full access to the full range Garrett VNT technology.
Why do you suppose Nissan decided to avoid the VNT altogether for the Skylines and stay with ball bearing turbos ?
The level of computer sophistication on the 35 GTR is truly incredible, but still no VNT turbo. There is a lot more to getting one to work on a petrol engine than just writing a few lines of code.
RE: Sequential Turbo
I'll use my MR2 as a example. It has a little 1.6 liter engine. I am running a DSM T25 which is good for close to 250 hp. The most boost I ran on my old motor was 12 PSI and it would make that somewhere between 2500 and 3000 RPM. You will notice I am most familiar with MHI turbos and will use them as reference.
a 14B is good for something like 300 hp and would still make full boost by 3500 or so. This would be a great setup for the street. There is really no need for a lot of power below this. Also 300 hp in a 2500 pound car and out of a 1.6 liter motor should be plenty for that kind of use.
A 16G will hit something like 20 psi by 4500 and is still making 12 at 3500. This means down low it will pull hard and above 4500 it will pull real damn hard. my motor will spin to 8k so that's still 3500 RPM of butt puckering power with minimal lag. That's plenty to use the gears to stay in the powerband. I wish I could pull up that dyno sheet (not my car) but the website is down.
As far as racing goes it's just not worth the extra weight, complication and extra parts that can fail when you can just use the gears to stay in the powerband.
As far as DD I would say the same thing plus do you really need that much power? Even if you do how long do you expect to get a motor to last pushing 200+ hp per liter?
For the most part I think twincharge is silly and I would put sequential in the same category.
There are only a couple instances where I have really felt the concept was practical. The one I can remember is a 600+ hp hill climb car where they needed a huge turbo to make the power and used a SC to keep some low end and to help spool the turbo.
If you are making less than 200 hp per liter you should be able to size a single turbo that met your power goals and still gave you good spool and drivability.
RE: Sequential Turbo
But what about driving in traffic at legal road speeds between 2,000 and 3,000 rpm ? For more sensible driving, a very broad flat torque curve with instant throttle response makes for a far more relaxing drive.
Big capacity American V8 pushrod engines are still very popular for that simple reason, they are so darned easy to drive, and still deliver the goods with plenty of easy power when required.
RE: Sequential Turbo
If you need to pass or for whatever reason need to accelerate then you drop a gear and put it in the powerband.
Also if you are looking for a relaxing drive you shouldn't be driving a skyline or a supra. You are right for the driving you describe a motor targeted for lower RPM power may be better.
That said though I used a 1.6 as an example because for one I have one, for another it's a very common size for commuter cars. If the 300 hp turbo seems slow in the lower RPM then don't even think about the NA MR2 or econo civics. You have to rack those things to the moon just to get them out of their own way.
I also think it's funny that you put 3500rpm and flat out in the same sentence. With a car that revs to 8k 3500 is a bump above idle. I'll hit that in fifth gear at about 75 and I have the taller ratio transmission.
I'm also not bashing displacement or the many other options to increase and broaden your powerband. In fact that's sort of my point. Unless limited by a class of racing or something why would I twin charge my 1.6 4A-GE and try to push it to such limits when I could drop in a 1GR or 2GRfe and run just a few pounds of boost, have nearly instant spool, gobs of power and not be stressing the motor nearly as hard.
RE: Sequential Turbo
Before you say to much more about what car manufacturers actually do, a lot of people who post here work as OEM engineers or as engineers for OEM suppliers.
Oh, and Warpspeed is right.
Regards
Pat
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RE: Sequential Turbo
Go back and read your post that Warpspeed was replying to. It was you that spoke of full boost at 3,500 rpm, of 300hp and of exciting rides. What he is talking about is a combination that will do both - an exciting ride when that's what you want or an easy drive when you don't feel like stirring the gearbox every time you want a bit of grunt. There are plenty of people who want the excitement available in a Skyline or Supra, but also want to drive according to the road rules and with courtesy for fellow motorists, which means they can can only use the available performance less than 10% of the time. Foe the other 90% they would like a car that is easy to drive!
I agree with Warpspeed's advice. To anyone considering sequential turbocharging, twincharging is a much better proposition offering better performance and numerous additional benefits for less effort.
Engineering is the art of creating things you need, from things you can get.
RE: Sequential Turbo
I've built/modified many turbo assemblies into hybrids from different compressor/turbine (manufactures) and impeller combinations. I also devised an adjustable device that works only in non-split housing turbines that help with spool up speed without limiting higher end boost performance. The mechanical systems input variables determine it's design and application.
As the norm, most requires the proper mathematical applications to formulate a proper and more accurate starting place. Sometimes finding the mechanical devices and or turbo combinations to achieve a lower torque gain becomes difficult. Short of having your own foundry to build the perfect turbo. Yet I've found some we've modified almost perfect per our applications and perform with a strong linear torque curve throughout the rpm range. The more important qualifier will depend on the mechanical configurations of the engine.
Good luck with your pursuit.
RE: Sequential Turbo
I don't believe exhaust gas temperature issues. How difficult is to make the vanes out of ceramics or even cast iron with some machining work? We have ceramic turbines in Skyline GTR since 1988 in R32. 1988 Shelby CSX used a Garrett VNT turbo in gasoline with electronic boost control. Look at http://www.aerocharger.com , Cocky Bell is the guy who wrote the book "Maximum Boost" and introduced the VATN Aerocharger more than a decade with gasoline (turbo charge a Honda NSX). I read the book in the 90s and it is almost twenty years from now. If you look closely the design of the vanes of the Aerocharger, the Garrett VNT vanes are even more rigid than the Aerocharger . The vanes in the Aerocharger just stamped with sheet meter with little spot weld into shape. If this thing can withstand gasoline EGT for the last 2 decades, why not Garrett VNT?
I don't want to mention Porsche 997 Turbo/GT2 because they use a slide vane BorgWarner VGT which is different. But even for VNT or VGT, I just can't see how EGT come into play, unless one of you guys have experience on it with gasoline which fail a VNT turbo due to EGT.
I just know a guy from NDperformance who use a Garrett GT35V on his Ford Mustang and have success. He is an electrnical engineer and developed his own microcontroller based VNT boost controller. He done his project a few years ago. It looks like aftermarket tuners to get away of VNT is due to lack of a decent off-the-shelf electronic or microcontroller based
controller for it. If you do a search about VNT turbo modification, almost every modification of VNT turbo throw the electronics away and change to a traditional vacum or wastegate style actuator mechanism.
I also have contact with the manufacturer of VNT vanes for Honeywell/Garrett, they are the major supplier of the Garrett factory in China . EGT problem is a myth, I think control is the truth. If the same factory is supplying turbine wheels and vanes, I cannot see the difference of casting the vanes out of the turbine material (they are steel, compare the stresses cause by thermal and centrifugal acceleration of the turbine and movement of the vanes).
After I post and started the thread, I got many replies. But none of the reply did supply any concrete technical reason or failure cases for VNT on gasoline. I know about Nissan, we own S14, R32, R33, R34, and R35 GTRs . We also own a bunch of VAG cars, Audi/Porsche/VW turbos . Our group distribute and supply automotive parts to tuning shops and motorsport teams. Its a pitty that I am the only engineer in the company, while other people are all business man, and administration . After talking to the guy how succeed in his VNT Ford Mustang conversion, my decision is to use two of the cars, one Audi and one GTR as a project car, so that we have one single turbo and one twin setup as experimental project for VNT in gasoline. If the project go success, then will persuade others to convert all the cars to VNT to gain more experience. Last step should convert these results to a product and market it.
I didn't answer the question of how many hp going to make. There is no answer for that. How many hp is going to make depends on size of turbo and compressor you choose, and how would you like in which RPM the engine run to. To me, it is totally worthless to build an engine over 1000hp and boost the RB26 til 10000rpm. This is a test for your deep pocket and making zero achievement. For the turbo size, I will choose the same compressor size as some existing turbo kits with a known torque curve. Because this will proof the improvement of the turbo lag on the torque curve.
We also have another project working on an electronic diverter valves (similar to blow-off-valve or recirculation valve, a common setup on stock VAG turbo cars). These can help to prevent overboost your engine by release the charged air by an electronic decision, and can take part of the work in a sequential secondary turbo spooling stage. Setting up a sequential VNT turbo is just a software achievement with an additional throttle body. Provided the VNT vanes of the secondary turbo can block all exhaust gas effectively.
We are sourcing the GTB2260VK (BMW/VAG OEM) turbos to arrive, and still need some support by Garrett about the compressor and turbine maps, also the electronic specifications of the actuator.
Honeywell is a very large company, I am still waiting for them to get the correct contact person to answer my questions. If I got any update from them, I will keep it posting. If anyone of you knowing anyone did any kind of VNT gasoline work, please bring something up.
I promote forward engineering, not reverse engineering. We studied all sorts of science and engineering, we use scientific method to solve technical problem. I don't know why Nissan don't go to VNT, but Porsche did . How a product comes out of a company have many business reasons behind. Nissan spent 20 years of development of the Toroidal CVT transmission. At the year it completed and released the Gloria (USA have a different name, I can't remember). The company got major share sold to Rennault. Where is the Toroidal CVT now? Some journalist and reporters even told that R35 GTR will use the toroidal CVT transmission. Why they don't put the best of the best in the top of the line of the car? Maybe they want to hide this secret weapon until someday it is necessary to use it. In Japan, some people has converted their Nissans to use the Toroidal CVT transmissions, these things fly. There are also many video in you tube about it too. The legend is real... I hope Nissan engineers can give me an answer of why not VNT? Maybe the proper answer for this is from their business management.
Let's keep focus on engineering rather than looking at the matter as a backyard hobby or DIY car modification. Questions like how I want to drive the car and how much hp really doesn't matter. The GTR has more than enough power to drive. My daily car is an Audi A4 DTM Quattro (2.0L turbo) and another family car, a Nissan Serena . So I already have too many cars that satisfy my need.
Just talk about driving a Nissan GTR ...
My driving experience about the GTR is a nightmare, slower than a bus when under 4000rpm, pulls like mad when over 5000rpm. There is no throttle control at all with six throttle butterflies, it just too sensitive at high engine speed. All the electronic LSD is only good when cornering or accelerating at the traction limit. The car has a guage to display the front wheel torque in terms of percentage. It appears it never go more than 10% or even zero at all time. I bought this GTR for experimentation. Audi is the car I prefer to drive with medium performance and very good driveability.
Apology to post such a long message. But I have to clarify the necessity of creating this thread. Otherwise, it will be another google result of VNT turbo myths.
RE: Sequential Turbo
RE: Sequential Turbo
1. Control & Safety
Gasoline engines tend to have a higher rev range and volumetric efficiency. When achieving a good low end boost with VNT turbo, the turbine size will get maxed out in high RPMs. During this maxed out situation, the turbine speed becomes uncontrollable because VNT designs forces all exhaust gas to go through the turbine (no wastegate). By using VNT in gasoline and high rev engines, it is desirable to install an additional external wastegate as a safety device, or an electronic diverter valve (Like Audi/VW/Porsche). When overboost, the ECU will open up the diverter valve (like blow off valves, but recircuilating back to the compressor inlet) this way the system makes no boost.
2. Maintenance issues due to carbon build ups
Gasoline engines causes carbon build up on the vanes, eventually these vanes will cake and get seized or difficult to move. This also lead to a control issue because when carbon builds up (before it got seized), the vane controller need to be adaptive. If the vane fails to open, then it will cause safety issues as well. Diesel engine carbon tends to be powder, which is different. Frequent service and clean up together with an adaptive controller will still able to use VNT wtih gasoline.
The above 2 issues have nothing to do with exhaust gas temperature. I should start another thread for VNT turbocharger to focus on this topic. The Aerocharger company is not responsive, they will face the same issue as above, their front page have a CFM calculator (air flow rate calculator) based on your engine size, so I guess the maxed out situation can be avoided by selecting the correct turbo charger that will never get maxed out in the desired RPM range. For carbon issues, it will be the same. Unelss Aerocharger invented some special material or coating to prevent this sisutation. I am still waiting for a reply from them. Porsche 997 turbo, they have electronic diverter valve, and how did they solve carbon deposit problem? Does anyone know? The turbochargers are from BorgWarners.
RE: Sequential Turbo
Garret know this only too well, they would be marketing their VNT turbos instead of their range of excellent ball bearing turbos if they had any potential advantage on a gasoline engine.
Nissan have used VNTs on their diesels for decades, but never on any of their range of sporty turbo cars.
Honda used VNT on thier race winning Formula one engine, but never in their passenger car range. And they spent a vast fortune trying to make it work on the Honda Legend before completely giving up the idea.
http://dwolsten.tripod.com/articles/jan89a.html
Perhaps you can do better than the engineers at Garrett, Nissan, and Honda?
RE: Sequential Turbo
I am not saying I can do better. I am still focusing on the technical issues of VNT setup in gasoline. I know many people tried it. Even some videos in youtube demonstrate some VNT in gasoline applications. What are the reasons behind it is not widely used? Nissan and Garrett don't do VNT on gasoline, Porsche+BorgWarner did.. If it is technically impossible, then why there are so many success stories out there? Price of Porsche cannot compare with Nissan, it looks to me its a cost reason more than technical. Complexity means cost, we have not yet come down to a point to talk about cost. Thanks for your input on the Honda page, but if you can point out why Honda didn't continue to produce these turbos, it will be nice.
RE: Sequential Turbo
The vanes instantly slam almost completely shut, exhaust back pressure suddenly spikes up from 5psi to 85 psi.
How do you think the engine is going to respond to that sort of back pressure while under load?
Boost may actually rise fairly quickly, but not as fast or as far as the exhaust back pressure rises when you close the vanes up. Instead of the massive surge of torque you were probably expecting, the engine just completely dies in the bum.
Explain to me your proposed control strategy to control both the variable vanes and an exhaust turbine wastegate over all modes of operation.
I would really be interested to know.
Until you actually try to make a VNT work on a real gasoline engine, it all looks so terribly simple. It is not simple.
It is far easier to get both response and power from a properly matched ball bearing turbo. That is why Garrett and the other turbo manufacturers still sell vastly more fixed geometry ball bearing turbos than VNT turbos.
RE: Sequential Turbo
I am from a Mechatronics engineering background, where computer controlled machinary is my main expertise . Vane control is not an ON/Off mode. Garrett VNT3 actuators have vane position feedback and a PWM input for vane actuation. The actuation power comes from engine oil pressure. Intake pressure, rpm, throttle position, vane position feedback, atmospheric pressure are all input variables to the controller. As I already said before, implementing these complex control by using pure mechanical/neumatic actuators is likely to fail.
I will not explain any control theory here. But to gain the best control over the vanes, it is desirable to read the ex-manifold pressure and take it into account so that we can control the pressure ratio of ex-back pressure. Turbine theory is well known, it is a matter of pressure ratio, and you will end up with a simple transfer function that describes the system's model. As soon as we know the mechanical limits of all variables, then it should be able to implement properly. The system is a microprocessor based control, it is not an electronic control base on passive components.
There exists an external wastegate, but we should not rely on this device to control. It exists in the experiment only for safety issues. If this device end up become a must, then it will add to the cost of the setup. So far, its still unknown until we run the system.
By the way, how much experience do you have in microcontroller? The situation that you mention is just a back pressure limit control, and this number should be a known preset value . So the controller is able to handle this situation. MAP+RPM will tell you the gas flow with a fixed VE engine, even you have a variable intake camshaft system, the required flow rate can be mapped.
The guy who did his project a few years ago also implement his own controller. He is an electrical engineer . The difficult part he mentioned is the adaptive of vane movement resistance due to carbon deposits (as this is an unknown variable). We are still in conversation about this matter and hopefully can come down a feasible solution.
Thanks for input to make this thread more interesting. I do think we should start another VNT turbo thread though.
RE: Sequential Turbo
Are you an engineer. Is this in anyway work related.
Regards
Pat
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RE: Sequential Turbo
It looks like setting up the project doesn't cost too much, two piece of turbo, some custom pipe work etc.. So the investment is minimal. If I work something out, it is possible to market this as a product through the company . But I am taking my own risk and investment for all additional parts and resources.
RE: Sequential Turbo
Regards
Pat
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RE: Sequential Turbo
RE: Sequential Turbo
There are a lot of people who feel that they know better than the large manufacturers. Most times, their "revolutionary" widget dies a slow drawn out death while they go broke trying to support it. But, every once in a while, the small guy does one-up the large manufacturers.
Lots of people go off and build something they are happy using on their own vehicle. However, it's a large leap to take the step from a personal project to a successful commercial product.
RE: Sequential Turbo
I don't feel this is revolutionary. This is not even a big project at all. We already know how other people did it. Its a matter of cost and maintenance issue on these setups. If you never setup one, you will never know. Tell people to go off have no contribution. So far, only a few people in this forum has asked some useful questions.
This is my first thread in this forum. I never post any development work in any forum before. I thought this forum really focused on engineering. You can see what other people says and how useful are these replies. 9 out of 10 of it have nothing about technical, including yours.
9.9 out of 10 mechanical engineers I met in the field lack of electronics and computer control know how?. The guy who made a controller for VNT turbo in his experiment is an electrical engineer. You are from electrical side, how difficult is to build a microcontroller (hardware+software+control) by a mechanical engineer who knows only little electronics? It can be understood why Robert Bosch design most of the control in early EFI systems out of mechanical components. We can still see air flow meters in the 1990s build by Bosch using a mechanical flap+dial design. Even in 1990s semiconductor components are so cheap already.
It is not possible for a person to know so much and even more than all other automotive engineers. That's why I am here in the forum. If there exists many automotive engineers having experience about VNT turbo setup in gasoline applications, love to see them turn up and point out something that I don't know.
It is now clear that VNT gasoline setup is a cost/maintenance/safety issue. At least, I was told by a person who actually did it. If he didn't start his own experiment, I would have never known. The point is Garrett never tell you the truth why don't they use VNT in gasoline. I am trying to contact BorgWarners who did they successfully use a VNT turbo in the latest Porsche 997 turbo but so far have no reply yet.
RE: Sequential Turbo
I agree, controlling a VNT turbo on a gasoline engine is solvable, in principle.
As somebody mentioned, the turn-down ratio of existing mass produced designs is insufficient for a passenger car engine (assuming boost response is desired at cruise rpm), since the existing designs are for diesel engines (which have a narrower operating range). Even diesel VNT applications encounter the turn down issue. It was also mentioned that this can be solved with a waste gate, which is quite true.
To avoid the additional expense and complexity of wastegate, Cummins chooses to add an orifice downstream of the turbine of their VNT applications, which makes the turbine less efficient at higher flow rates, thereby replacing the function of a wastegate. There is in fact an efficiency penalty across the board, but as it is exponential with flow, it is an acceptable trade-off.
Holset restricts the application of their VNT turbos to a temperature that is 150F (IIRC) lower than the limit for their wastegated turbos. I can tell you that this temperature is hundreds of degrees lower than the peak exhaust temperatures of a boosted gasoline engine.
RE: Sequential Turbo
You also keep posting that you want details from factory engineers who have done the R&D work. I hate to break it to you, but these folks might not find it's in their best interest to give away all their hard erned R&D knowledge. It's also possible they are reading your basic open ended "hey give me all the details about topic X" post and just don't want to write an essay for your benifit. You already got the basic answers to your "why not" question - high EGT and hard to tune.
So, why don't you come back with some specific application questions?
RE: Sequential Turbo
1)If the vanes stick or fail in either position, it won't be catastrophic, since you will have a wastegate.
2) Actual testing will let you know if it is a feasible setup. Shift, or expand, the range of A/R's that are available with the mechanical stop on the actuator.
3) Even if you end up with just a small range of usable A/R's (between 0.68 - 0.96 for example) you can still say "it works."
4) Will the results justify the extra costs and complication?
RE: Sequential Turbo
- Steve
RE: Sequential Turbo
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Pat
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RE: Sequential Turbo
There is also a mechanical limit stop for the vanes that can be set to restrict the closed vane position.
The number one problem with the VNT is that there is just insufficient mass flow at very light throttle, and the high exhaust restriction needed to rapidly spool the turbo just chokes down the engine.
Diesels are not throttled, and the Honda formula one engine only worked over a very high and narrow speed range. For a practical everyday road car, it just does not work as well as it might at first intuitively seem.
A lot of people have had a go at this over the last thirty years, home tuners, racers, hot rodders, professional engineers, large multinational companies.
Honda tried and totally gave up, so did SAAB. The problems are not easily solvable. The biggest step forward with a VNT turbo is to toss it into the dumpster, and fit one of the latest generation ball bearing turbos in place of it.
RE: Sequential Turbo
You can be sure that the Bugatti engineers know all about VNT turbos, and if there had been any performance advantage, even a slight one, that is what they would have used.
Think about it.........
RE: Sequential Turbo
It looks like there are people using WG actuators controled by MS.
http:
MS can also run stepper motors and do many other things so there should be a number of options.
I don't think I can link to the search I pulled up but search for VNT on that site and there are a number of threads on the subject.
If you were serious about making a production setup you could either incorporate MS to run the system or figure out how MS does what it does and try to make a simpler system that does what you need it to.
This seems like a fun project and I would love to play with the idea on a project car (given the time and money) but I do agree with others that it seems like a long shot to make a production system that meets all the requirements it would need to meet.
I'm not saying you shouldn't try it, just saying a lot of good points have been brought up on the cons in this thread.
RE: Sequential Turbo
- The issue of matching control lever force and VNT resistance is a non issue really. The control lever should have its own position control cct with position feedback. The main controller sends the position setpoint to that loop.
- Are you suggesting that Robert Bosch were using mechanical/electrical sensors in the 1990s due to lack of electronics expertise? I hope not. If Bosch were still using flap style AFMs in the 1990s it was because that was the best engineering solution to the client's brief at the time.
hemi.
You say VNT's typically lack turn down ratio. Are you referring to A/R range? Does anyone know what A/R range is common on VNTs. I would think somthing like 3:1 would be sufficient to control maximim boost (with some margin) and still extend the torque (boost) peak to significantly lower RPM.
This may be a stupid question, but for performance (race) applications, why not operate at the minimum A/R at all times when boost is below the desired setting? Assuming of course that the minimum A/R is not so low as to be unsuited to the engine, in which case mechanically limit the minimum A/R as suggested by 1gibson.
Engineering is the art of creating things you need, from things you can get.
RE: Sequential Turbo
If we are talking about race appications, we would have to go for max power output setup. Selecting gear ratios to pickup largest area under the torque curve. Every product i ls driven by market, most people who pay to modify their cars ask for more poer. Once going for a turbo upgrade, main hesitation is driveability, fuel consumption. Majority of the market is from street cars. Race and motorsport customers are easy to handle, they just ask tor the part they want. Street customers requires heavy aftersales support and service.
Take Nissan GTR as an example. How many GTRs were sold for past models? Its a decreasing number from R32 to R34. How many R35 GTRs were sold even the price tag is the highest? We even see lots of female owners around. Driveability is really the key to make sales. VNT may not be the best available technology to the solution, but is current best available technology that can be implemented.
RE: Sequential Turbo
Gear ratios should be selected to maximise average power.
Engineering is the art of creating things you need, from things you can get.
RE: Sequential Turbo
They way that VNTs are applied to diesel engines, from my observation, the operating range turn-down issue shows up at the top end; see my previous post for details.
BTW, the Cummins-Westport L Gas Plus engine that was introduced in 2004 uses a Holset VNT. It is a lean burn engine with peak EGT below 1200F; and an operating range like a diesel engine. This application uses an orifice post-turbine for the reason given above.
RE: Sequential Turbo
Changing gear ratios, only happens in motorsport/race applications. How often a street car customer will do that?
Going back to the control, the point of varying engine volumetric efficiency is a good question. Unless have direct control of the ECU, otherwise this will be a significant limitation of the VNT turbo controller. I expect the VNT turbocharger installation will cause map trace to walk further (higher pressure) in the low rpm side, which is untouchable due to turbo lag by using traditional turbo. The whole ECU maps needs to be remapped anyway (because of turbocharger change).
However, one advantage for the VNT is that it can wide open its vanes under cruise to improve fuel economy by reducing pumping loss. If the VNT controller is going to implement this wide open mode, this variation in VE is likely to cause a lean mixture. This is one of the drawback I can see by using an ECU that doesn't recognize VE variation.
RE: Sequential Turbo
RE: Sequential Turbo
...VE versus RPM on one axis, and a variable representing load on another axis, e.g. MAP. Values typically fall between 0.6 and 1.0, though a highly tuned performance application may slightly exceed 1.0 VE in a narrow rpm range.
RE: Sequential Turbo
I have and will again
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Pat
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RE: Sequential Turbo
Everyone except me - perhaps you should educate me? I would particularly like to know about "Selecting gear ratios to pickup largest area under the torque curve"
Engineering is the art of creating things you need, from things you can get.
RE: Sequential Turbo
RE: Sequential Turbo
While I have my doubts about the validity of this thread and the coherence of the OP, he is obviously English second language and I think we can cut some slack with regard to his use of terms that do not quite fit if we can clearly see what he means to say.
I think if this thread denigrates into yet another torque vs horsepower it will only detract from the thread and the participants and the forum.
Regards
Pat
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RE: Sequential Turbo
RE: Sequential Turbo
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Pat
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RE: Sequential Turbo
If you use a wider ratio gearbox the two vertical lines will be further apart and the area will be greater, so the widest possible ratios will yield the greatest area and there for the greatest acceleration? There is clearly something wrong with your method. So I repeat.
Gear ratios should be selected to maximise average power. ie move your two lines so they are either side of the power peak and as close to the power peak as possible.
Engineering is the art of creating things you need, from things you can get.
RE: Sequential Turbo
RE: Sequential Turbo
RE: Sequential Turbo
http://www.aerocharger.com/aerochargers.php
Looks pretty interesting but some of their marketing is kind of hard to believe.
It is self contained, no engine oil and it looks like no coolant.
It has a small oil resivoir that mists the turbo and the oil needs to be checked once a year.
One thing that bothered me was in the FAQ.
"Does the Aerocharger require a turbo timer?
A:
No turbo timer is required with the Aerocharger. Since the Aerocharger does not use hot engine oil, hot shutdowns are not an issue. The drag on the bearings is so little in fact, that the Aerocharger will slowly spool down on it's own when the engine is shut off. Whereas conventional turbos suffer from engine oil heat and are stopped almost abruptly after the engine is shut off due to a flooded bearing design. This is very damaging over time and is exactly why the Aerocharger does not use and oil source from the engine."
Seems funny since the coking problems conventional turbos have is not from the hot engine oil but from the even hotter turbo. How does their turbo not have this problem with no water or outside oil cooling?
It is still interesting though. I could see playing with something like this at some point.
RE: Sequential Turbo
I had almost forgotten Aerodyne. They made a kit for something (I think it was a Mercedes diesel) with the turbo shaft vertical and the turbine housing integral to the exhaust manifold, with the runners feeding directly to the periphery of the turbine nozzle. Very innovative I thought.
Engineering is the art of creating things you need, from things you can get.
RE: Sequential Turbo
It could be more of an issue with marketing though. I see that way more often than I'd like.
RE: Sequential Turbo
Or put another way, 15 years to build a reputation, 15 seconds to blow it.
Regards
Pat
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RE: Sequential Turbo
1) The wheels being out on cantilevered shafts greatly decreases the strength and stability and makes it a lot more sensitive to out of balance especially on the hot wheel.
2) Wear or slack in the bearing causes exagerated clearance problems due to the extra distance from the bearing to the wheel/housing interface area.
3) You still need shaft seals between the hot and cold sides and these should be cooled.
4) The air inlet to the compressor wheel has to have a shaft across it and this disrupts airflow.
Regards
Pat
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RE: Sequential Turbo
Engineering is the art of creating things you need, from things you can get.
RE: Sequential Turbo
RE: Sequential Turbo
1. It can be done and already been done. OEM's do not like it as it's costly. Power-train developers had those systems drawn out and tested decade ago, mainly for racing applications and high power staff like tracks / generators engines.
2. EGT are NOT a problem, even if one want to use two VGT's / VNT's. If you worry so much about EGT - run E85, it runs 200C cooler. Very close to diesel temperature. You will also get more HP from E85 due to high octane rating. Will use more fuel, but it cost
3. One may want to have variable boost pressure delivery across the RPM range. When engine VE (volumetric efficency) peak (usually 3000-4000RPM) you will need to reduce boost to keep torque curve flat. This will save engine and maximize utilization of strength of internals. They are rated by max load.
4. Good set up will have one smaller turbo to spool from idle and larger to take care of the rest. You can and want to have more boost after 4000RPM, therefore larger turbo.
5. There was the point mentioned about opening the vanes of VNT to aid engine efficiency at cruise: same can be done by keeping the wastegate open. I do not think it will be much help, exhaust gas flow is minimal under cruise, there will be very little restriction from the turbine.
6. One may wonder if two sequential VGT's will be an ultimate answer.The problem is that those turbos are made in the limited range and may prove hard to find two suitable petrol units. Diesel models may not be optimal due to different flow/PR configurations. Maybe units scavenged from Porsche can be used.
Thoughts?
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RE: Sequential Turbo
Being done: http://forums.tdiclub.com/showthread.php?t=270527
RE: Sequential Turbo
Manufacturing parts in China www.pa-international.com.au
RE: Sequential Turbo
Have you thought about making a twin turbo system where a bigger feeds charged air to a smaller turbo? The system will go like this
Hot side: Exhaust Manifold>Big Turbo>Small Turbo>Exhaust
Charge side: Air Filter>Big Turbo>Small Turbo>Cooler>Throttle Body>Intake Manifold
The small turbo is basically compound-charging whatever the big turbo spits out to it. The boost pressure multiplies. The small turbo picks up the low to mid-end while the big turbo shines in top-end.
It is essentially the same as twincharging only change the supercharger into a smaller turbo.. Comparing to a twin charge system, this might still lose a tiny bit of low-end.. However, it will not have the hassle of dealing with the bypass for SC when part throttle or idle..
And I believe some cummins diesel engines use this setup
RE: Sequential Turbo
I forgot what I was going to say
RE: Sequential Turbo
You should read through it if interested
RE: Sequential Turbo
That won't work. Needs to be "Hot side: Exhaust Manifold>Small Turbo>Big Turbo>Exhaust"
Engineering is the art of creating things you need, from things you can get.