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Catégorie: Porsche - 911 - 997 - Turbo
Langue: English
Région: Worldwide
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Re: TURBINES.
Quote:
xandi911 said:
sorry if its a stupid question... the turbines of 997TT have the same size?
The distinguishing characteristic of the variable turbine geometry is that the turbine blades' angle changes (relative to the turbine wheel) with engine rpm's (controlled via guide vanes) to facilitate quicker spool-up time at low rpm's, with the angle or "pitch" adjusted to stay open longer at higher rpm's. But the turbine blades aren't necessarily different sizes. By changing the turbine inflow angle and thus inflow speed at the turbine wheel inlet, the VTG can combine the benefits of small and large exhaust gas turbochargers in one unit. With the guide vanes set in the "closed" position, the turbo creates a steep enthalpy gradient equating to a high charging pressure-and thus high boost at low rpm's. With the guide vanes "open," the turbo has a high flow rate and centripetal force, enabling higher flow at high rpm's. Thus the "flat" torque curve in the 997TT.
I believe BW has another serial twin turbo project with two turbochargers in sequence, what they have trademarked as "R2S." Given the conflict between desiring transient response with maximal torque (requiring small turbine and compressor) and high rated nominal power output (requiring a large turbocharger), the ideal setup for a sports car would be a combination of small and large.
The "R2S" turbocharger uses two turbochargers connected in series with "bypass regulation" with a bypass valve controlling flow between high- and low-pressure turbines. At low rpm's, at low exhaust gas flow rates, the bypass can stay closed so the entire exhaust flow can be expanded by the high-pressure turbine with fast and high boost pressure increase. At higher rpm's, exhaust gas expansion progressively shifts to the low-pressure turbine as the bypass valve progressively opens-possibly via pneumatic actuation.
BW states that in the "R2S" system, the entire fresh air flow can be first compressed by the low-pressure stage, then compressed further in the high-pressure stage, then cooled. With "pre-compression," the smaller high-pressure compressor can reach high pressures, forcing requisite air through the system.
Does anyone know if Porsche has any interest in regulated two-stage turbocharging?
Re: TURBINES.
Quote:
trip said:
I think the 959 used sequential turbos.
As I recall reading about it, I believe the system operated BOTH sequentially and in parallel. The 959 engine was based on the 935/936/956/962 rather than 911road powerplant, it was a boxer-6/2.85L, with liquid-cooled cylinder heads. The turbocharger unit used two turbines of EQUAL size, hooked up serially, intially with only one turbine operating alone up to 4,300rpm's, but at high engine speeds over 4,300rpm's operating in parallel, cooled by two intercoolers placed at the sides of the engine. To minimize turbo-lag, Bott and his engineers ingeniously arranged the turbo's regulated with valves so that up to 4,300rpm's, the first turbine operated under constant pressure with continuous boost; above 4,300rpm's, a second upstream valve opened, with pressure equalized by an electronically controlled bypass valve, so that the second turbine operated in parallel with the first, yielding up to 2.0BAR maximal boost below 4,300rpm's.
This was good for 0-100kph launches of 3.7seconds. Does anyone know of dyno maps of the 959's HP and torque curves?
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