Current turbocharged diesel engines use exhaust gas recirculation (EGR) to effectively meet emission standards. With exhaust gas recirculation it is possible to keep the nitrogen oxide (NOx) emissions to a minimum, largely by lowering the local peak temperatures in the combustion chamber. Exhaust gas transportation from the exhaust side to the air side can be realized in different ways. All have in common that, a drop of pressure from the exhaust to the air is needed. In this paper the high pressure exhaust gas recirculation concept will be discussed, where the exhaust gases are transported from the upstream side of the turbocharger turbine to the downstream side of the charge air cooler. In this concept a negative pressure difference between turbine inlet and engine intake is needed, leading to inefficient gas exchange and, in the end, increasing fuel consumption. In order to keep the overall fuel consumption increase as low as possible, some of the current 6-cylinder Mercedes-Benz truck engines, that have EGR, are equipped with the so-called asymmetric twin scroll turbine to provide the most efficient exhaust gas transportation. In this design concept the negative pressure difference between engine intake and turbine inlet is generated in just one of the two exhaust branches. Thus, whilst some cylinders are operated with a high exhaust gas backpressure, others are operated with a fuel-saving low exhaust gas back-pressure. The different back-pressures in the two exhaust branches are created by designing each flow path of the twin scroll turbine differently. The exhaust branch with the higher back-pressure needs a turbine scroll with a much smaller flow parameter than the exhaust branch with the lower back-pressure. As both flow paths are coupled to the same turbine wheel, the flow parameter is modified using the design parameters of the scrolls. This produces two totally different turbine concepts in one turbine housing. The turbine path with the higher flow parameter has a classical radial turbine reaction value of 0.5. This flow path can thus be optimized for maximum efficiency in comparison with other radial turbines. In contrast, the turbine path with the lower flow parameter combined with the turbine wheel is operated with a reaction value approaching zero. This flow path tends to need an axial turbine with a high flow direction change like an impulse turbine, even if a radial turbine wheel is used. Operating a radial turbine wheel under this boundary condition needs new development steps to improve the turbine with regard to mechanical feasibility and thermodynamic efficiency. This paper describes the principle mechanism of the asymmetric twin scroll turbine. Detailed engine cycle simulations give a brief introduction into the main advantages of asymmetric turbines in combination with exhaust gas recirculation. Hot gas test stand studies show the principle characteristics of this turbine type and the numerical flow simulations give a detailed insight into the flow phenomena in the turbine. The key design values will be discussed and the future outlook indicates the next development steps that will be required.
The current study investigates the flow conditions of a twin scroll asymmetric turbine. This is motivated by the operating conditions of the turbine at a heavy-duty reciprocating internal combustion engine with exhaust gas recirculation. The flow conditions of the turbine at the engine can be described best with the turbine scroll interaction map. Standard hot gas measurements of a turbocharger turbine are presented and discussed. Due to the strong interaction of the turbine scrolls, further hot gas measurements are performed at partial admission conditions. The turbine inlet conditions are analyzed experimentally, in order to characterize the turbine performance. The turbine scroll pressure ratio is varied, leading to unequal twin turbine admission conditions. The flow behavior is analyzed regarding its ability for further extrapolation. Beyond scroll pressure ratio variations, unequal temperature admission conditions were studied. A way of characterizing the representative turbine inlet temperature, regarding the reduced turbine speed, is presented. The different scroll parameter ratios are evaluated regarding their capability of describing flow similarity under different unequal turbine admission conditions. In this content, turbine scroll Mach number ratio, velocity ratio and mass flow ratio are assessed. Furthermore, a generic representation of the turbine flow conditions at the engine is presented, based on standard turbine performance maps.
The current study investigates the flow conditions of a twin scroll asymmetric turbine. This is motivated by the operating conditions of the turbine at a heavy-duty reciprocating internal combustion engine with exhaust gas recirculation. The flow conditions of the turbine at the engine can be described best with the turbine scroll interaction map. Standard hot gas measurements of a turbocharger turbine are presented and discussed. Due to the strong interaction of the turbine scrolls, further hot gas measurements are performed at partial admission conditions. The turbine inlet conditions are analysed experimentally, in order to characterize the turbine performance. The turbine scroll pressure ratio is varied, leading to unequal twin turbine admission conditions. The flow behaviour is analysed regarding its ability for further extrapolation. Beyond scroll pressure ratio variations, unequal temperature admission conditions were studied. A way of characterizing the representative turbine inlet temperature, regarding the reduced turbine speed, is presented. The different scroll parameter ratios are evaluated regarding their capability of describing flow similarity under different unequal turbine admission conditions. In this content, turbine scroll Mach number ratio, velocity ratio and mass flow ratio are assessed. Furthermore, a generic representation of the turbine flow conditions at the engine is presented, based on standard turbine performance maps.
A variable geometry concept for advanced turbocharger (TC) systems is presented. The variability of the device is based on outlet area changes as opposed to the more common systems that are based on inlet turbine geometry changes. In addition to the conventional variable turbine geometry (VTG), the new variable turbine type is termed variable outlet turbine (VOT). The flow variability is achieved by variation of the flow cross section at the turbine outlet using an axial displacement of a sliding sleeve over the exducer and provides a simple solution for flow variability. In order to predict the aerodynamic performance and to analyze the loss mechanisms of this new turbine, the flow field of the VOT is calculated by means of steady state 3D-CFD (computational fluid dynamics) simulations. The VOT design is optimized by finding a good balance between clearance and outlet losses. Furthermore, experimental results of the VOT are presented and compared to a turbine equipped with a waste gate (WG) that demonstrates an efficiency advantage of 5%. Additionally, engine performance measurements were carried out to investigate the influence of the VOT on fuel consumption and to asses the functionality of the new pneumatic actuating system. The VOT engine tests show also performance advantage in comparison to a WG turbine especially toward high engine loads. It is found that the use of the VOT at this condition shows a turbine efficiency advantage of 6% related to a reduction in engine fuel consumption of 1.4%. The behavior at part load is neutral and the peak turbine efficiency of the VOT is comparable with a fix turbine geometry.
Turbochargers with variable turbine geometry (VGT) are established in diesel engines for passenger cars because of the beneficial effect on transient operation. The variability permits the reduction of exhaust back pressure, resulting in lower fuel consumption. There are only a few applications in heavy duty truck engines due to increased mechanical complexity and vulnerability to failure. This paper presents a turbine concept with a simple variability developed for a heavy duty engine. The variability is achieved upstream of the rotor by changing the sectional area of the volute. This can be done through a rotationally movable ring which shifts the circumferential position of the volute tongues. These separate both scrolls of a double segment turbine and can be rotated by an electric actuator. The performance maps measured at the hot gas test stand show the large variability of the flow parameter and the high efficiency levels over the operating range of the variable asymmetric turbine (VAT). The flow field is computed by the use of 3D-CFD simulations in order to analyze the loss-generating mechanisms that occur within the machine. Test runs on an engine test stand demonstrate the high potential of the concept concerning reduction of fuel consumption and a wide scope of realizable EGR rates in order to reduce NOx emissions in a cost-effective way. The resultant large mass flow variability allows the deletion of the waste gate and enables efficiency improvements.
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