Turbocharger w/Variable Turbine Geometry
Turbocharger With Variable Turbine Geometry
The exceptional driving performance is due above all to the newly developed turbocharger technology of the six-cylinder engine. By using extremely high-temperature resistant materials, it was possible to develop an exhaust turbocharger with variable turbine geometry which is capable of withstanding the high exhaust temperatures produced by gasoline fueled engines of up to 1832 °F (1000 °C).A turbocharger with variable turbine geometry combines the respective advantages of small and large turbochargers, and permits optimum utilization of the exhaust energy for charging at any engine operating point; in addition, there is no longer any need for wastegate valves.
The result of the new technology is a significant increase in torque and performance: the opposed cylinder engine delivers 480 bhp (353 kW) at 6,000 rpm, 60 bhp (44 kW) more than the engine of the previous model, and this with an unchanged displacement of 3.6 l. At the same time, the nominal torque increases from 415 ft lbs (560 Nm) to 460 ft lbs (620 Nm) in a much wider rpm range: On the new Turbo, the maximum value is now in the range between 1,950 and 5,000 rpm.
Variable Turbine Geometry
The greatest development potential for exhaust turbochargers is solving the conflict between good response at low engine speeds and high specific performance values at high engine speeds. Variable turbine geometry with variable vane adjustment in front of the turbine wheel has shown itself to be the optimum solution for further improving turbocharger response and thereby the response of the turbocharged engine.
The new 911 Turbo with variable turbine geometry again sets new standards for Otto engines with exhaust turbocharging.
Technical Challenge
The design of variable turbine geometry is based on adjustable vanes which guide the exhaust mass flow from the engine onto the turbine of the turbocharger in a variable and targeted manner. The use of variable turbine geometry for Otto engines is made more difficult by the significantly higher exhaust temperatures. Compared with temperatures of approx. 1472 °F (800 °C) in the case of diesel engines, the maximum exhaust temperatures at the turbine inlet on Otto engines with exhaust turbocharger are significantly higher at approx. 1832 °F (1,000 °C). This leads to considerable additional stressing of the material and high demands on design realization. The delicate adjusting elements of the vanes in the hot exhaust stream are particularly critical. In addition to the high-temperature resistance of individual components, it is also necessary to take into account high temperature fluctuations in design.
In view of a possible temperature range from cold starting at -20 F (-30 °C) up to a maximum regulated exhaust temperature (at the turbine inlet) of approx. 1832 °F (1,000 °C), it is necessary to take into account the different material expansion factors and safeguard the functioning of the entire adjustment system, including the many individual components. This is guaranteed, by selection of suitable materials, oil cooling, as well as by additional water cooling of the bearing housing. This can be activated via the Motronic system by an electric coolant pump both at low speed (less than 2,000 rpm) combined with high coolant temperature (greater than 208 °F/98 °C) as well as after the engine is switched off.
Technical Principle
On the new 911 Turbo, the boost pressure is controlled only by adjusting the vanes (without bypass valve). This is done by way of an adjusting ring, which is actuated by an electric servo motor via a coupling rod (one "boost pressure adjuster" per turbocharger).
Small turbochargers have good response characteristics (small "turbo lag") due to the small acceleration mass of the turbine wheel and the high flow momentum of the exhaust gas. This momentum is generated in the turbine housing in the transition to the turbine wheel by way of small flow cross-sections with high flow speeds. However, the small flow cross-sections in both the turbine housing and turbine wheel increase the flow resistance for high air throughputs and therefore high engine speeds, and also produce high exhaust backpressures ("choking"). As a result, the maximum engine power is limited.
Large turbochargers have poor response characteristics (large "turbo lag") due to the high acceleration mass of the turbine wheel and the low flow momentum of the exhaust gas. In contrast to small turbochargers, however, the exhaust backpressures are lower for high air throughputs due to the larger flow cross-sections in the turbine housing and turbine wheel. This results in less exhaust work for the pistons, as well as an improved charge cycle with a lower residual gas amount in the cylinder and better cylinder filling, for example. This results in a higher maximum engine power.
Vane Adjustment System
The principle of variable turbine geometry is essentially based on the following two physical characteristics:
- Variable vane gap
- Variable air impact angle
The adjustment system with adjusting ring and movable vanes is the mechanical heart of an exhaust turbocharger with variable turbine geometry. It consists of 11 adjustable vanes which are interconnected by the adjusting ring. The adjusting ring is connected in turn via a coupling rod with the electric servo motor which is responsible for controlling adjustment of the vanes.
With variable turbine geometry, small turbochargers are simulated by closed vanes (small vane gap) and large turbochargers by open vanes (large vane gap). With the respective advantages, variable turbine geometry permits both very good response with high torque values even at low speeds, as well as high output values at high speeds. The high torque is therefore available for a significantly larger rpm range.
The variable vane gap is achieved by turning the adjusting ring and thus turning the vanes. A small vane gap reduces the flow cross-section. The resultant higher gas speeds mean that the exhaust gas is directed onto the turbine vanes with high momentum. The turbine wheel therefore rotates more quickly and drives the compressor wheel located on the same shaft. This in turn compresses the air which is supplied to the engine for combustion. As a result, the engine receives more air more quickly and accelerates more dynamically.
Vane Adjustment
Adjustment of the vane system and of the vane gap allows the exhaust mass flow to be directed onto the turbine wheel with maximum effectiveness for every operating point throughout the whole rpm range, thereby allowing the boost pressure to be adjusted to the corresponding setpoint value. This control technology combined with selection of a suitable turbine size makes it possible to dispense with the bypass valve (wastegate) usually required for engines with exhaust gas turbocharging. Adjustment of the turbine vanes does not just change the vane gap, it also changes the impact angle of the exhaust gas on the vanes. This variable impact angle assists the dynamic response of turbocharging using variable turbine geometry.
Vanes Closed
Small vane gaps do not just result in higher gas speeds in the vane gap. In this vane position, the impact angle of the exhaust gas on the turbine vanes is more direct and therefore produces higher angular momentum of the turbine wheel. If the vanes are closed at low engine speeds, the exhaust gas is accelerated in the small air gap and impacts on the turbine wheel radially with high energy. As a result, the compressor wheel located on the same shaft is accelerated quickly and increases the boost pressure. This in turn leads to good response characteristics of the turbocharger and thus high dynamic engine and vehicle acceleration.
Vanes Open
If the exhaust mass flow increases (increasing engine speed and load), the vanes are opened by the DME control unit according to a control map when the desired (maximum) boost pressure is reached. The adjustment duration for the vanes from open to closed and vice versa is only approx. 100 milliseconds.