blau928

Thomas,

I am going to say that you are asking questions using terminology that is quite confusing.

Anyway, you are using a BSFC chart and discussing power at certain RPM, when the correlation is the best BSFC correlated to peak torque, and HP is a resultant function of TQ and RPM..

In addition here are a few points you need to consider to understand what is happening in the combustion cycle...... :

PRESSURE (in your case 10psi) is not what makes the increasd power. Density and air mass does... (MORE air + fuel + spark)

Pressure is resistance to flow..

Turbos are spun from waste heat in the exhaust charge, and not only from exhaust gas mass airflow...

The heat of the turbocharged compressed air charge (or any compressed air charge) comes from compressing the air (increasing the pressure in the manifold and or cylinder)...

Air fuel ratio at idle is ususlly leaner than stoich.. It can be higher than 14.7:1

Anyway, about your concept..

It is theoretically possible to use variable geometry turbos (like the old Aerocharger etc. and some newer ones) to broaden the RPM range of operation... However, you run into many issues, of which plumbing is the least of them..

The other point of running a motor at lower RPM and getting more power, is done everyday with Diesel engines, which usually make more torque across a narrower RPM range than a gasoline motor, and use a transmission to apply the torque effectively to move the vehicle..

You could also do it with a CVT, or constantly variable transmission..

One of the concepts that shows promising light is the Electric turbocharger.. It is instant on/off, and is not a function of engine RPM.. It's driven by an electric motor.. However, it still needs to be powered by another motor, and that needs energy from something, and it also needs to be packaged properly, and,...and,..... you get the idea..

Your concept is not impossible, but what works in an economy car, is not neccessariy good for supercar territory.. The only things that do go together, are that an engine with a lower BSFC will make more resultant power and use less fuel to do it than one with a higher BSFC at the same RPM point.. The reason is that the fuel is being consumed effectively in the combustion process..

You need to look at the entire process, and find out why things work, not just the data points on the chart.. I could explain more, but then I would be writing a book... It's easire for you to get a good turbocharger book, and have a go...

However, like I mentioned earlier, Diesels do this everyday with a good gearbox.. (All those trucks... some with 16 speed gearboxes)..

Cheers, hope the comments helped..!

tveltman

Ahh, at last I see what everyone is getting at. Clearly my poor grasp of terminology is getting in my way, but I'm with the program now. My controller should work and not damage the engine. I'll set about building it and see what I can come up with. Thanks for everyone's help =)

Lizard928

if your manifold is always under pressure then your engine will be revving to the redline the entire time.

That is unless you use the new technology that Fiat has which removes the throttle body and just limits how much air is let into the engine by limiting the valve openings. Which in that case you could get away with a constant pressure.

tveltman

Yes, exactly. I have a bit more experience with diesels, and in my excitement, I forgot that gasoline engines are controlled via air intake, whereas diesels are controlled by fuel. Now that I am less of an idiot [though I still have plenty of idiot to go around, if anyone needs some ], things are more clear

dr bob

Thomas--

This is an intersting concept. It would of course take a hell of a wide-capability turbine to make it work. At idle, the total mass of gas flowing into the turbine (the 'drive' side of a turbocharger) is low,a dn the relative heat energy is also low. But you want to have 10PSIG upstream of the throttle, likely so that you can pop the throttle open and have that 10PSIG available with the throttle open, lifting the intake manifold pressure to 10PSIG from idle pressure of about -13PSIG. This is where the fun starts. You really need enough energy from the exhaust at idle with the throttle almost completely closed to also make a 23 PSI change in intake pressure with the throttle wide open. albeit still at idle or barely off-idle speed. While idling, all that energy converted in the turbine will need to go somewhere, and that's usually a bypass valve from compressor outlet back to its inlet or to atmosphere, or the exhaust is bled off by a wastegate ahead of the turbine. The wategate is a poor choice for you because you really are looking to have the turbine and compressor at idle already spinning fast enough to make that +10PSIG into the intake at idle with the throttles wide open.

After all that, your turbine and compressor need to spool up fast enough to maintain that +10PSIG of intake pressure all the way to redline with throttles wide open, all without exceeding the max aloowable turbine blade and bearing speed. Limits are bearing lubrication and sonic velocitiy of the air in the compressor.

Getting the wide range of flows from the turbine and compressor system is the challenge, and that range is likely not available with just one unit. You can perhaps understand why there are sometimes dual-sequential units fitted to gas motors where a wider performance range is needed.

From a practical driveability standpoint, getting the EFI and ignition to follow the idle to just off-idle positions with high upstream throttle pressures will be fun. Training the driver's foot may also be a challenge. If you look at the stock S4 fuel and ignition maps for example, the tip-in point includes an ignition retard for a short period while the fuel system 'catches up', and an artificial fuel enrichment at the same time to help mask the lag in MAF response. The ignition retard eliminates the initial detonation while the airflow sensor responds to the changes in airflow as the throttle is opened, but at the same time it seems really soggy on the pedal for a second. This may be one area where CIS has an advantage, and why there are throttle position (TPS) sensors and manifold absolute pressure (MAP) sensors used on many modern EFI cars. It's easier and faster to calculate airflow from throttle plate position and DP across the throttle area than it is to wait for the hot wire current requirements to change. Anyway, I'm rambling off into another area.

Roy928tt

May I draw your attention to the relationship between the torque curve and fuel consumption in the Volkswagen TDI graph, combine that with a CVT and you will create the most fuel efficient car.

Dennis Wilson

If you are wanting faster spoolups, why not go with the KKK turbos like are used on the 951's. Instead of a BOV they use a recycle valve than puts excess boost back into the intake side of the turbo. This keeps the turbine spinning and reduces the spoolup time (lag) you normally see when using a blow off valve system.

Dennis

Imo000

Chrysler has used VNT turbos in the early 90's, on the Daytona Iroc with the 4cyl Turbo III engine. They were not fuel efficient. Diesel is much denser than gaoline and this is mainly where the fuel mileage comes from and not from the VNT turbo. My DD is a '00 TDI.

djurek

Dumb Question but isn a N/A car running at 14.7 PSI at sea level? Constant pressure induction. Is boost just supplemental pressure?

tveltman

yes, the original idea was to have the turbo spooled at very low rpms, and by using a VGT, eliminate the need for a wastegate (except maybe as an absolute fail-safe) and use the variable geometry to continuously resize the turbo depending on engine load such that the boost pipe is always pressurized to some arbitrary pressure, 10 psi in the above example. I forgot to account for the butterfly valve inside the throttle body, and I was confusing the pressure of the boost pipe with the pressure of the manifold itself. They are independent, and indeed, had better be, or otherwise you are always running the engine at redline, as Colin pointed out. The concept is still quite sound, however, and is in fact mentioned at the end of Corky Bell's book as the way forward for new turbo technology.

Also, atmospheric pressure is 14.7 psi, so when I say 10 psi, i really mean 10 psi OVER atmospheric, or 24.7 psi absolute. In any gasoline engine, the pressure inside the intake manifold is the control mechanism for engine speed, and so unless at WOT, the manifold pressure will not equal atmospheric pressure.

Imo000

Tell me why is the concept is still quiet sound? So far everything suggested has been unobainable.

tveltman

The concept of controlling the vane actuator, not any of the other stuff. That was all I cared about to begin with. It would operate essentially as a standard wastegate actuator would, except instead of opening a relief valve, it varies the turbo geometry. End of discussion.

Imo000

What you've described is how many VNT (variable geometry) turbo operate today. Every '99.5 VW TDI has this, this is not a new concept.