The benefits of a mid-route exhaust gas recirculation system for two-stage boosted engines

Exhaust gas recirculation is a widely known technique applied in internal combustion engines for controlling the combustion process and harmful emissions. The recirculation of gases can be achieved either by delivering burnt gases from upstream of the turbine to downstream of the compressor (short-route) or by taking the exhaust gas from downstream of the turbine and deliver to upstream of the compressor (long-route). Although long-route system is preferred for highly boosted engines due to the higher exhaust gas recirculation availability at low engine speeds, it lacks a fast response time during transient performance compared to the short-route system. This article examines the potentials of introducing an alternative exhaust gas recirculation route which can be applied in two-stage boosted engines. The proposed mid-route exhaust gas recirculation system, applied in a gasoline engine, combines the benefits of the long routes and short routes. The system provides high exhaust gas recirculation rates at all engine speeds while the transport delay in the case of transient operation is relatively short. The potential of a hybrid exhaust gas recirculation system combining mid-route and long-route exhaust gas recirculation is examined and various components’ (i.e. compressor, turbine and coolers) sizing and transient performance studies are performed to understand the trade-offs of the system. It was demonstrated that mid-route could provide high exhaust gas recirculation particularly at high- and low engine speeds. A combination of mid-route and long-route exhaust gas recirculation can provide maximum exhaust gas recirculation rates at all speeds with a maximum fuel consumption penalty of 1.4% at engine speeds below 2500 r/min. The reduction in exhaust gas recirculation response time was of the magnitude of 50%, while the faster exhaust gas recirculation purging time combined with the smaller turbine implemented dropped the load tip-in response time by 25%. The coolers’ sizing study revealed that a long-route exhaust gas recirculation cooler is unnecessary, whereas the mid-route exhaust gas recirculation cooler can also be omitted when the flow is delivered prior an intercooler with a 25% larger cooling capacity than of the baseline engine.