Variable Geometry Turbocharger

Content Introduction Turbocharging History Problems with simple turbocharging Working of VGT Benefits Designing of VGT Summary and Conclusion References Common questions

Introduction Variable-geometry turbochargers (VGTs) also known as variable nozzle turbines/VNTs), are a family of turbochargers Designed to allow the effective aspect ratio (A:R) of the turbo to be altered as conditions change It produces variations in the flow of inlet air mass by geometry change of inlet conditions of turbine blades according to requirement.

Turbocharger A turbocharger consist of four parts: Turbine Compressor Common axle Sub assemblies Air cooler WORKING The turbine inlet receives exhaust gases from the engine exhaust manifold causing the turbine wheel to rotate This rotation drives the compressor, compressing ambient air After it delivers it to the air intake manifold of the engine at higher pressure, resulting in a greater amount of the air and fuel entering the cylinder

Problems with simple Turbochargers Difference in the optimum aspect ratio of nozzle at different running conditions Low boost at slow running conditions Choking at high speed Variation in the inlet air mass according to engine performance Starting lag Do internet search and add more

Turbocharger configuration

Turbocharger integration Linking of engine performance with turbocharger performance

Variable geometry turbocharger In variable geometry turbocharger the variation in inlet air mass is brought by compressor speed Changes is done on the turbine Nozzle angle is changed by stator vane, operated by ECU unit of vehicle For low speed, the vane is brought in closed position so that boosting can be done by compressor At high speed ,the vane comes at open condition allowing most of the exhaust to impact on the blades ,and the pressure decreases of exhaust gases and choking is avoided

Working of VGT Low speed run condition Engine running parameters is taken by ECU as input and send direction to vane to close High speed run condition

Benefits No throttling loss of the waste gate valve Higher air–fuel ratio and higher peak torque at low engine speeds Improved vehicle accelerations without the need to resort to turbines with high pumping loss at high engine speeds Potential for lower engine ΔP (the difference between exhaust manifold and intake manifold pressures) Control over engine ΔP that can be used to drive EGR flow in diesel engines with high pressure loop (HPL) EGR systems A better ability to cover a wider region of low BSFC in the engine speed–load domain Ability to provide engine braking Ability to raise exhaust temperature for after treatment system management

Designing of the VGT Three parts Electronic control unit Compressor designing Turbine designing

Electronic control unit The opening of the guide vanes is commanded by the electronic control unit (ECU) of the car Aspects of engine performance that ECU consider for the varying the nozzle angle 1) the instant and quantity of injected fuel 2) the opening and closing of the intake and exhaust valves 3) pressures, temperatures, flow rates Mass flow rate is given by Add diagrams and notes about ecu

Compressor design Flow coefficient Circumferential Mach number Isentropic work coefficient

Turbine design Parameters: Pressure ratio Corrected mass flow rate Isentropic efficiency Corrected speed VNT positions

Conclusion An VGT is designed to position the nozzle to attain the optimum air mass flow inside the engine and improving the break specific fuel consumption. An VGT, under low running speed, provides closing of vane blades to allow more impact of exhaust gases on it improving intake air mass.. Designing of the compressor and turbine include the study of there design parameter on the maps which allow us to decide standards for the components. Pumping losses , pressure energy loss, engine noise and heat energy loss is recovered by VGT. These problems commonly occur on vehicle with conventional turbocharging system or in case of no turbocharging. If there occurs VGT failure, the system will revert to normal turbocharging operation. Normally the ECU unit will turn on the light in case of any fault.

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