Hydrocarbon Adsorptive Catalyst System: Prius
The Toyota HCAC (Hydrocarbon Adsorptive Catalyst) system has been adopted to improve clean emission performance requirements of exhaust gases when the temperature of the TWC is low.
HCAC system consists of HCAC Vacuum Switching Valve (VSV), actuator, by-pass valve and Three-Way Catalytic Converter (TWC) with Hydrocarbon (HC) adsorber. Actuator and by-pass valve are located at front of TWC on exhaust pipe. See Fig 1. HCAC VSV is located on bracket bolted to timing chain end of engine and uses a Brown electrical connector with Black and Black/White wires. See Fig 2.
Before engine is started, by-pass valve is open. When engine starts, Engine Control Module (ECM) delivers an output signal to HCAC VSV. HCAC VSV opens, allowing vacuum to be applied to actuator which causes the by-pass valve to close. When by-pass valve closes, exhaust gases pass into the HC adsorber, where the gases are stored until the temperature of front TWC increases. This prevents hydrocarbons from being delivered from vehicle when TWC temperature is low. After TWC becomes warm, the HCAC VSV closes and vacuum to by-pass valve is shut off, causing by-pass valve to open. When rear TWC becomes warm, exhaust emissions are cleaned by the TWC. The HCAC VSV may also be operated during a deceleration condition when HC adsorber is warm in order to scavenge the hydrocarbons that remain in the HC adsorber.