As emissions regulations continue to tighten, both from lower imposed limits of pollutants, such as nitrous oxides (NOx), and in-use and real-world testing, the importance of quickly heating the aftertreatment to operating temperature during a cold start, as well as maintaining this temperature during periods of low engine load, is of increasing importance. Perhaps the best method of providing the necessary heating of the aftertreatment is to direct hot exhaust gasses to it directly from the engine. For heavy-duty diesel engines that utilize turbochargers, this is achieved by fully bypassing the exhaust flow around the turbine directly to the aftertreatment. However, this disables a conventional turbocharger, limiting engine operation to near-idle conditions during the bypass period. The addition of a driven turbocharger, a mechanically or electrically driven turbocharger, allows for supercharging power to be delivered to the compressor to maintain boosting abilities to allow the engine to operate at higher loads when the turbine bypass is utilized. This results in rapid heating of the aftertreatment during cold start and periods of prolonged low-load engine operation, greatly reducing the amount of time to when the aftertreatment becomes functional, as well as limiting the amount of time that high fuel consumption thermal management strategies are used. It can also reduce the cost and complexity of future aftertreatment architectures, including Light-Off Selective Catalytic Reduction (LO-SCR) and Electrically Heated Catalyst (EHC). This article will show data from the latest engine tests and build upon the results from California Air Resources Board (CARB) Phase III low NOx program. It also explores the possibility of combining engine stop-start with the driven turbocharger for thermal maintenance of the aftertreatment while simultaneously reducing fuel consumption.
