8. Thermal Management of the DPF, DOC, and SCR Processes by Heat Recovery

The after-treatment systems function well when there is a combination of city, rural, and highway operations with the latter two helping in maintaining high operating temperatures above light off. However, when the vehicles operate only under urban conditions the engine-out temperatures in the exhaust rarely reach values above 150 °C for sufficient time. Injecting fuel late in the cycle to produce heat is wasteful since the problem prolongs throughout the city drive. The apparent difficulty to reach the temperature without heating is resolved by various schemes to recover the exhaust heat from the gases after the exothermic reactions in DOC and DPF. Ingenious schemes like the STARFILTER that integrate the catalysts in a compact enclosure, achieve both energy recuperation and reduction in heat losses to the surroundings. The endothermic reactions in an SCR require a small intermediary heat source within the recuperation loop to keep the temperature level sufficiently high.