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Definition:
A Temperature Vacuum Switch (TVS) is a device used in automotive applications to control the vacuum supplied to the Exhaust Gas Recirculation (EGR) valve and/or the canister purge valve based on the temperature of the engine coolant or intake air.

Detailed Description:

  • Purpose:
  • Regulate Emissions: Ensures that the EGR and canister purge systems operate only when the engine is at the appropriate temperature, thereby optimizing emissions control.
  • Engine Performance: Prevents EGR operation when the engine is cold, which can affect drivability and engine performance.
  • Components:
  • Temperature Sensing Element: Typically a wax pellet or bimetal element that responds to changes in temperature.
  • Vacuum Ports: Inlet and outlet ports for connecting to the vacuum lines.
  • Valve Mechanism: Opens or closes the vacuum pathway based on temperature changes sensed by the temperature element.
  • Operation:
  • Cold Engine:
    • When the engine is cold, the TVS remains closed, preventing vacuum from reaching the EGR valve and/or canister purge valve.
    • This ensures that the EGR system is inactive and the canister purge does not occur, preventing drivability issues and allowing the engine to warm up efficiently.
  • Warm Engine:
    • As the engine reaches its operating temperature, the temperature element in the TVS reacts, causing the valve to open.
    • This allows vacuum to reach the EGR valve, enabling the recirculation of exhaust gases into the intake manifold to reduce NOx emissions.
    • Similarly, it allows vacuum to the canister purge valve, permitting the purge of fuel vapors from the charcoal canister into the intake manifold for combustion.
  • Applications:
  • EGR Systems: Controls the operation of the EGR valve to reduce NOx emissions by recirculating a portion of the exhaust gases back into the engine’s intake.
  • Evaporative Emission Control (EVAP) Systems: Manages the purging of fuel vapors stored in the charcoal canister, preventing them from being released into the atmosphere.
  • Advantages:
  • Emission Control: Enhances the efficiency of emission control systems by ensuring they operate only when beneficial and necessary.
  • Engine Protection: Prevents EGR operation when the engine is cold, avoiding potential drivability issues and protecting the engine from undue stress.
  • Fuel Economy: Improves overall fuel economy by optimizing the conditions under which the EGR and canister purge systems operate.
  • Example:
  • Automotive Application: In a typical passenger vehicle, the TVS is mounted on the intake manifold or a coolant passage. When the engine is cold, the TVS blocks the vacuum, keeping the EGR valve closed. As the engine warms up, the TVS opens, allowing vacuum to reach the EGR valve and enabling its operation, which helps reduce NOx emissions during cruising conditions.

Illustration:

  1. Cold Start:
  • Coolant Temperature: Below operating temperature (e.g., 60°C)
  • TVS Status: Closed
  • EGR Valve: Closed
  • Canister Purge Valve: Closed
  1. Normal Operating Temperature:
  • Coolant Temperature: At or above operating temperature (e.g., 90°C)
  • TVS Status: Open
  • EGR Valve: Open
  • Canister Purge Valve: Open

By understanding the function and importance of the Temperature Vacuum Switch (TVS), it becomes clear how this component contributes to effective emissions control and engine performance, ensuring that emission control systems operate optimally only when the engine is at the correct temperature.

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