Divided Exhaust Period (DEP) has previously been studied on SI engines while results fromHD diesels are scarcer. In this paper the DEP concept has been numerically simulated on two HD dieselengines; one without EGR and one with high rates of short route EGR. The aim is to reduce fuelconsumption, residual gas content and to improve boost control, while current EGR rates are maintained.

The central idea of the DEP concept is to let the initial high energy blow-down pulse feed theturbocharger, but bypass the turbine during the latter part of the exhaust stroke when back pressuredominates the pumping work. The exhaust flow from the cylinder is divided between two exhaust manifoldsof which one is connected to the turbine, and one bypasses the turbine. The flow split betweenthe manifolds is controlled with a variable valve train system.

Results show a reduction of pumping losses for both engine configurations. In the non-EGRcase, the DEP concept offers the possibility to control the mass flow and pressure ratio over the turbine.This allows the turbocharger to operate in a high efficiency mode for a wide range of engine loadpoints. For the EGR case, there is less freedom in control of turbine mass flow, since the blow-downphase is used for both turbine work and EGR flow. Therefore the fuel consumption benefit is reduced.

The conclusion of this paper is that the simulations of the DEP concept show improvements toengine performance and efficiency. In the case of high EGR rates it is shown that the EGR flow shouldnot be deducted from the blow-down phase.

In a previous paper we showed the effects of applying the Divided Exhaust Period (DEP) concept on two heavy-duty diesel engines, with and without Exhaust Gas Recirculation (EGR). Main findings were improved fuel consumption due to increased pumping work, improved boost control and reduced residual gas content. However, some limitations to the concept were discovered.  In the case of high rates of short route EGR, it was apparent that deducting the EGR flow from the turbine manifold impaired optimal valve timing strategies. Furthermore, for both of the studied engines it was clear that the size and ratio of blow-down to scavenging valve area is of paramount importance for engine fuel efficiency.

In this paper, the DEP concept has been studied together with a long route EGR system. As expected it gave more freedom to valve timing strategies when driving pressure for EGR is no longer controlled with the valve timing, as in the short route case. However, when evaluating different combinations of intake, blow-down and scavenging valve area, the optimal relation proves to be strongly dependent on the current EGR system and EGR rates. Hence, for different engine setups the trade-off between total intake and total exhaust area needs to be re-evaluated for optimal engine fuel efficiency. This paper also presents general trends in how different valve timing strategies and EGR rates affect both pumping work and boost pressure.