To improve power and efficiency, the TITAN XD diesel engine uses forced induction through a two-stage turbocharger system:

There are two sides to a turbocharger: the exhaust side (turbine side) and the intake side (compressor side). We will cover both sides, starting with the intake components and moving to the exhaust components. Each component will be listed with a description of its operation. Some sensors that may seem unrelated to the turbochargers, but are part of the intake system, are also listed. By the end of this article you will have a good working knowledge of this two-stage turbocharger system.

SCR catalyst (3 sections)

Air Filter – The air filter filters the air entering the engine, but there are some unique details for the diesel engine:

Turbocharger Compressor Intake Pressure Temperature Sensor

  • This sensor is located on top of the air filter housing.
  • It monitors the temperature and pressure of air in the intake air duct.
  • The sensor information is used by the ECM to determine air filter restriction, changes in altitude, help determine air volume entering the engine, and manage engine operation.

    • Mass Air Flow (MAF) Sensor

  • The MAF sensor is located on the intake air duct just after the air filter housing.
  • The ECM uses data from the MAF sensor and other air intake sensors to determine the quantity of air entering the engine.
  • In addition, the MAF sensor has a major impact on Exhaust Gas Recirculation (EGR) calculations and Selective Catalytic Reduction (SCR) dosing with Diesel Exhaust Fluid (DEF).

    • Turbocharger Speed Sensor

  • Located on the top of the high-pressure turbocharger, on the compressor side of the housing.
  • This sensor monitors the rotational speed of the high-pressure turbocharger to protect against over-speed conditions.

    • Low-pressure Turbocharger Boost Pressure Sensor

  • This sensor is located on the compressor side near the outlet of the low-pressure turbocharger.
  • It monitors air pressure (compressed air) created from the low-pressure turbocharger.

    • Compressor Bypass Valve

  • The compressor bypass valve is located in the low-pressure turbocharger compressor outlet tube.

    • Compressor Bypass Valve Operation

  • All fresh air entering the engine goes through the low-pressure turbocharger. After passing through the low-pressure turbocharger, intake air can be directed through the high-pressure turbocharger, or it can be directed to bypass the high-pressure turbocharger.
  • When the bypass valve is closed, intake air is directed through the high-pressure turbocharger; this occurs at lower rpm.
  • At higher rpm, the bypass valve is open, allowing intake air to bypass the high-pressure turbocharger.
  • The bypass valve is not a variable position valve; it is either fully open or fully closed.

    • The following components are used to actuate the compressor bypass valve:

  • Vacuum Pump – The vacuum pump is driven by the fan belt. It provides vacuum to the compressor bypass solenoid.
  • Compressor Bypass Solenoid – Located on the front left side of the engine. It is an ON/OFF solenoid that is controlled by the ECM. When energized by the ECM, the compressor bypass solenoid directs vacuum to the compressor bypass valve actuator to open the compressor bypass valve.
  • Compressor Bypass Actuator – Located below the low-pressure turbocharger compressor outlet tube. When vacuum from the compressor bypass solenoid is applied to the actuator, the bypass valve is opened. When no vacuum is applied, the bypass valve is closed.

    • Charge Air Cooler

  • Located between the radiator and the A/C condenser.
  • The charge air cooler cools the compressed air before it enters the air intake connector.

    • Air Intake Connector

  • Located on the top of the engine.
  • Air from the charge air cooler is combined with cooled and/or uncooled EGR in the air intake connector before entering the intake manifold.

    • Charge Air Cooler Outlet Pressure/Temperature Sensor

  • Located on the top front of the air intake connector.
  • This sensor measures the intake air pressure and temperature after it exits the charge air cooler and before it is mixed with EGR.

    • Intake Air Temperature Sensor

  • Located on the left side of the air intake connector, just before the air intake manifold.
  • This sensor measures the temperature of the combined intake air (air from the EGR system and air from the charge air cooler) before it enters the intake manifold.
  • This is the last air temperature reading the ECM receives before intake air enters the combustion chambers.

    • Exhaust

    Exhaust Gas Pressure Sensor

  • This sensor is located at the back left corner of the intake manifold.
  • To maximize longevity of this sensor, it is mounted on the side of the intake manifold with a tube that connects it from the exhaust manifold to the intake manifold passage. This keeps the sensor away from high exhaust heat that could damage it.
  • The ECM uses input from this sensor to help determine the correct position for the rotary turbine control valve. Its input is used mainly during cold starts, thermal management for the aftertreatment system, and one of the inputs to determine the quantity of EGR flow.

    • Rotary Turbine Control Valve

  • This valve is located within the low-pressure turbocharger housing.
  • It directs/regulates exhaust gases through three possible paths to support the various turbocharger modes.
  • This variable position valve is controlled by a linkage connected to the rotary turbine control valve actuator.

    • Rotary Turbine Control Valve Actuator

  • Located at the top rear portion of the intake manifold.
  • This actuator controls the position of the rotary turbine control valve based on commands from the ECM.
  • The actuator assembly also includes a sensor that monitors the position of the valve, giving feedback to the ECM.

    • NOTE: If the rotary turbine control valve actuator or linkage is removed or replaced, the actuator must be calibrated using CONSULT-III plus.

    Turbocharger Modes of Operation

    The rotary turbine control valve and actuator can be in any position within their range of operation; they are a variable position design. The ECM is constantly moving the actuator to correctly position the valve for various engine operating conditions.

    Although the ECM controls the rotary turbine control valve to any position within its range of operation, there are five basic positions (operation modes) described here:

    1. Exhaust Throttle
    2. Two Stage
    3. Two Stage Modulated
    4. Single Stage
    5. Wastegate 

    Exhaust Throttle Mode

    (Other names for this mode are Regeneration Mode and Thermal Management Mode)

    Exhaust Throttle Mode is used to increase exhaust system (aftertreatment system) temperatures.

    Exhaust Side – The rotary turbine control valve is positioned to restrict exhaust flow. This causes the engine to work harder. In this position, more exhaust is driven through the EGR system and into the intake manifold (higher intake temperature creates higher combustion and exhaust temperature.

    Intake Side – The compressor bypass valve is closed, so intake air comes through the low-pressure turbocharger and is then directed through the high-pressure turbocharger.

    Exhaust Throttle Mode is used to increase exhaust system (aftertreatment system) temperatures. At engine start, this may be used to heat the engine and aftertreatment system quickly. During active regeneration events, this mode is used to help bring the aftertreatment system up to very high temperatures to oxidize soot in the Diesel Particulate Filter (DPF).

    Two Stage Mode

    Two Stage operation is used when boost pressure is needed quickly, such as high load conditions (acceleration just off idle; low engine rpm).

    Exhaust Side –  The rotary turbine control valve directs all exhaust flow to the high-pressure turbocharger first. After passing through the high-pressure turbocharger, the exhaust flow then passes through the low-pressure turbocharger.

    Intake Side – The compressor bypass valve is closed, so intake air comes through the low-pressure turbocharger and then is directed through the high-pressure turbocharger. The high-pressure turbocharger produces boost quickly to eliminate turbocharger lag.

    At low engine rpm, a lower volume of air is moving through the engine, compared to high engine rpm. The smaller turbocharger – the high-pressure turbocharger – spins at very high rpm with the relative low exhaust flow that is available at lower engine rpm. This very high rpm of the high-pressure turbocharger creates higher pressure in the intake to satisfy the forced air induction demands of the engine at lower engine rpm.

    Two Stage Modulated Mode

    Two Stage Modulated operation occurs when transitioning between Two Stage and Single Stage operation.

    Exhaust Side –  The rotary turbine control valve directs exhaust flow in varying proportions to both high and low-pressure turbochargers.

    Intake Side – The compressor bypass valve opens from the closed position. With this valve open, the path of least resistance becomes the passage from the low-pressure turbocharger to the charge air cooler. The high-pressure turbocharger is bypassed.

    As the engine rpm increases, the much higher volume of air from the larger turbocharger – the low-pressure turbocharger – gradually takes over, delivering a smooth transition between low engine rpm and high engine rpm, almost eliminating turbocharger-lag.

    Single Stage Mode

    Single Stage operation occurs when maximum boost is needed for higher speeds or high loads. The larger turbocharger (low-pressure turbocharger) is needed to supply the larger volume of air required for higher engine rpm.

    Exhaust Side –  The rotary turbine control valve directs full exhaust flow to the low-pressure turbocharger. Although the passage is open to the high-pressure turbocharger, the exhaust flow takes the path of least resistance flowing to the low-pressure turbocharger. Only small amounts of residual exhaust flows to the high-pressure turbocharger.

    Intake Side – During Single Stage operation, the compressor bypass valve remains open allowing intake air to bypass the high-pressure turbocharger.

    During single stage operation, the compressor bypass valve remains open, allowing unrestricted airflow from the low-pressure turbocharger to the charge air cooler. The rotary turbine control valve directs full exhaust flow to the low-pressure turbocharger.

    At higher engine rpm, the low-pressure turbocharger supplies all of the intake boost pressure.

    Wastegate Mode

    Wastegate operation is used to limit boost pressure.

    Exhaust Side –  The rotary turbine control valve is positioned so that exhaust can flow directly to the aftertreatment system, bypassing both turbines. The passages are open to both turbines, however exhaust flow takes the path of least resistance to the wastegate with only some residual exhaust flowing through the turbines.

    Intake Side – The compressor bypass valve remains open. The high-pressure turbocharger is bypassed.

    During wastegate operation the compressor bypass valve remains open and the rotary turbine control valve directs exhaust flow to the aftertreatment system.