To achieve low emission levels, the handicap of the TWC is its light-off characteristic. It achieves a maximum hot efficiency of nearly 100 %, but this requires a temperature in the range of 300 to 450 °C. To improve this time lag after cold start, the TWC needs additional help to reach the targets of future low (LEV) or ultra low emission (ULEV) levels.
This paper describes the work on additional devices to reach the ULEV-levels such as:
Electrical Heated Catalyst (EHC).
Burner Heated Catalyst (BHC)
Hydrocarbon Trap (HCT) as external device.
Adsorber Coated Substrate (ACS) in the usual converter box.
The comparison of these systems was done with a concept car.
The low mileage exhaust results demonstrated the principal suitability of all these devices, but there is still much work to be done to meet the ULEV levels with the guaranted durability. The advantages and disadvantages of the systems are discussed, including estimated weight and cost.