Sequential Turbo 3

[R34ztune]

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.

 

[R34ztune]

Yes or no. I work with a company in auto parts business, carry lots of brand names, HKS, Apexi, K&N... Project initiation is totally on my own, but I have access to company related resources, a full line up of GTR's, from R32 to R35, Porsche/VW/Audi cars etc.

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.

 

[patprimmer]

So in fact you are a spare parts salesman?

Regards
Pat
See FAQ731-376 for tips on use of eng-tips by professional engineers &

http://eng-tips.com/market.cfm

for site rules

 

[R34ztune]

I am an engineer. Yes or no related to its work related or not. Our boss focus in trade/sales, wouldn't want to invest in development work. So I am doing this on my own now. Hopefully, I can come down a solution that is not too expensive so that the result can be commercialized. Or it will be a pure money burning experiment. However, the experience gain in the project should be quite valuable.

 

[LionelHutz]

You will have to go off and just try it yourself. Obviously, you feel that you know more or you're more capable than all the other automotive engineers who have tried this and you will never be convinced that it can't work unless you fail yourself.

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.

 

[R34ztune]

LionelHutz,

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.

 

[Lou Scannon]

R34ztune,

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.

 

[LionelHutz]

Oops, I didn't mean to imply, in any way or form, that your idea is revolutionary.

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?

 

[1gibson]

Use a wastegate. Set up your VNT controller (electronic or mechanical) and physically block the travel of the vanes between two reasonable A/R's. Think "throttle stop" with set screws for adjustability. Low A/R for faster spool, limited by excessive backpressure, high A/R for top end. Might only need to limit the low A/R.

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?

 

[SomptingGuy]

I've seen presentations (from Ricardo Martinez-Botas) before, concerning actuating a VNT through the cycle to extract the optimum energy from the pulsating exhaust gases. When asked how practical it was, his reply: "Not my problem, I am a university researcher. It's your problem."

- Steve

 

[patprimmer]

A damper on the vane closing process might also help to reduce spikes.

Regards
Pat
See FAQ731-376 for tips on use of eng-tips by professional engineers &

http://eng-tips.com/market.cfm

for site rules

 

[Warpspeed]

Yes Pat, if you slow down the rate of closing sufficiently, with a bit of effort it may even be made almost as good as a fixed geometry 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.

 

[Warpspeed]

Probably the most fearsome turbo car yet built, without regard for cost or complexity, the Bugatti Veyron uses four fixed geometry turbos.

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.........

 

[yoshimitsuspeed]

It looks like megasquirt should be able to do what you want.
It looks like there are people using WG actuators controled by MS.

http://www.msextra.com/forums/viewtopic.php?f=91&t=30255&p=199690&hilit=vnt#p199690

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.

 

[gruntguru]

R34Ztune.
- 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.

 

[R34ztune]

The VNT3 actuator does have position feedback. And the actuation is by a continuos variable PWM signal. It does provide total close loop control.

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.

 

[gruntguru]

Selecting gear ratios to pickup largest area under the torque curve.

Gear ratios should be selected to maximise average power.

Engineering is the art of creating things you need, from things you can get.

 

[Lou Scannon]

gruntguru, I have not personally done a VNT turbo match, so I'm not acquainted with the physical A/R turndown of a VNT.
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.

 

[R34ztune]

I believe everyone in this forum know that the usable area under the torque curve means deliverable power output.

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.

 

[Lou Scannon]

In all of my experience with electronic engine controls, VE mapping is done from the intake manifold to the exhaust manifold, so throttling, supercharging, and exhaust restrictions downstream of the exhaust manifold (e.g. turbine) do not figure into the VE map. VE maps typically are 3d tables of VE

 

[Lou Scannon]

crazy touchpad sent the post before I was finished!

...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.