Twin Turbocharger System Operation

Low Load Operation - Exhaust Gas Flow 

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In low engine load conditions the turbine bypass is closed. All exhaust gas leaving the engine is diverted into the turbine of the VGT turbocharger. The vane position of the VGT is determined by the Mass Air Flow (MAF) and the boost pressure sensor values. This provides the required energy to the compressor to achieve the boost pressure required. Exhaust gas exits the VGT and is passed over the fixed geometry turbocharger turbine. This serves 3 purposes for the system:

1. Any energy not extracted by the VGT is available to be extracted by the fixed geometry turbocharger increasing the efficiency of the boosting system.
2. It enables a smooth switching event between turbochargers as the fixed geometry turbocharger does not have to be accelerated from stationary
3. To prevent an otherwise stationery compressor from restricting intake air flow to the VGT.

Low Load Operation - Intake Air Flow 

Courtesy of JAGUAR LAND ROVER NORTH AMERICA, LLC

Courtesy of JAGUAR LAND ROVER NORTH AMERICA, LLC

Engine Speed/Load Increases - Mid Range 

Courtesy of JAGUAR LAND ROVER NORTH AMERICA, LLC

Courtesy of JAGUAR LAND ROVER NORTH AMERICA, LLC

As the engine speed and load increases the turbine shut-off valve starts to open allowing a small amount of exhaust gas to enter the fixed geometry turbocharger. The fixed geometry turbocharger starts to produce a pressure which matches the pressure produced by the Variable Geometry Turbine (VGT). The pressure produced at the fixed geometry turbocharger is not enough to create a pressure difference across the passive compressor bypass valve so it remains closed.

High Engine Speed/Load Operation 

Courtesy of JAGUAR LAND ROVER NORTH AMERICA, LLC

Courtesy of JAGUAR LAND ROVER NORTH AMERICA, LLC

At high engine speed or load condition, the fixed geometry turbocharger takes over as the main supplier of charge air to the engine. The turbine intake shut-off valve is fully open, directing exhaust gas to the fixed geometry turbocharger. The charge air pressure produced is enough to open the passive compressor bypass valve so the pressurised air goes directly to the charge air cooler. This allows the fixed geometry turbocharger to become the main supplier of charged air.

The Variable Geometry Turbine (VGT) turbocharger is kept spinning at its maximum capacity. The difference in pipe diameter allows the fixed geometry turbocharger to supply most of the charge air and receive most of the exhaust gas flow.

If required, the Turbine Bypass Valve (TBV) is opened to control boost pressure. This may happen if the boost pressure supplied by the fixed geometry turbocharger becomes too high (overboost).

Fixed Geometry Wastegate 

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The fixed geometry turbocharger utilizes a wastegate to control the exhaust gases entering the turbine section of the turbocharger. When the wastegate is actuated the exhaust gases are able to bypass the turbine section and exit directly into the exhaust system (post turbocharger). This reduces the turbine speed and ultimately the intake air pressure. The control diaphragm contains an internal spring which holds the wastegate in the open position when there is no vacuum. This creates a 'fail-safe' situation where the wastegate will remain in the open position if there is an electrical or vacuum issue.

Turbine Bypass Valve 

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The Turbine Bypass Valve (TBV) is used to control the flow of exhaust gases leaving the engine and entering the turbocharger turbines. When closed the exhaust gases leaving the engine will act directly onto the Variable Geometry Turbine (VGT) before passing through the fixed geometry turbocharger and exiting into the exhaust. When fully open the exhaust gases can access both the fixed geometry turbocharger turbine and the VGT turbine.

The position of the TBV valve is controlled by the Powertrain Control Module (PCM) via an electrical solenoid and vacuum actuator. The position of the TBV valve is not fed back to the PCM and is merely calculated by a pre-determined algorithm held within the PCM.