Although somewhat out of the ordinary, I wouldn't like to speculate on the plumbing, just from that pic.Quote:
Originally Posted by Dougal
For those who are unaware, I'm replying about the Borg Warner R2S (Regulated 2 Stage) turbocharger systems.
AFAIK BW supplies a lot of these, in different sizes, for many of today's diesel engines, including IIRC the latest Disco and Rangie diesels, and more than likely the 3 litre Iveco above.
IMHO they make a lot more sense than pure compound turbos for small diesels and tick all of the boxes unless you are a dedicated racer (or dyno queen) wanting very high boost pressure.
- Small manifold turbo on its own creates boost early
- Large atmospheric charger starts to get the exhaust flow it needs to create boost
- During this stage the boost pressure from the atmospheric charger is compounded by the small manifold turbo, so boost pressure and flow increase rapidly
- When boost has reached the required pressure, the exhaust flow is Regulated around the small manifold turbo and the large atmospheric turbo takes over practically all of the turbocharging
- At this stage the small atmospheric charger is still up to speed if it is called upon, but not creating a resistance to exhaust flow, or adding much heat to the charge air
In simple terms:
Radial flow compressors, used on small turbochargers, create dynamic pressure, which is a function of the velocity of the air leaving the tips of the wheel. For this reason (mainly), small compressor wheels are not capable of creating high boost pressure efficiently, i.e. they waste a lot of power, which creates heat in the charge air at moderate boost pressure.
At best adiabatic efficiency of 0.75 a Garrett GT2052 turbo (52 mm exducer) compressor wheel typically would at best have a pressure ratio of approximately 2.2 (17.5 psi) with a 52 trim wheel or less than 2 (less than 14.5 psi) with a 48 or 50 trim. These wheels are off the map by approximately 2.5 PR
By comparison a Garrett compressor wheel with 71 or 76 mm exducer and largish trim can have better efficiency beyond a PR of 3.0 (over 30 psi).
When compounded the adiabatic efficiencies also compound, and for example if both were 0.75, the overall would be 0.56 (0.75 x 0.75).
The R2S takes advantage of the high efficiency afforded by the large wheel by changing from compound to sequential turbocharging at whatever PR the regulation starts.
The regulation is normally controlled by the ECU to optimise performance, but could easily be controlled manually with far more advantages and none of the disadvantages of manual control of VNT turbos.
