A Simulation Study of the Impact of LP EGR on a Two-Stage Turbocharged Diesel Engine

Current engine development projects have to face crucial challenges: raw pollutant emission reduction while maintaining or even reducing the engine fuel consumption. To solve this issue, several technologies have been studied and some can offer part of the solution. Among these technologies, turbochargers provide one of the best trade off. Different turbocharging technologies can be used as well as different air path layouts, but one particularly promising technology relies in the Two-Stage series turbocharging. Coupled to turbocharging devices, and despite the recent development in NO after-treatment systems, EGR remains a cost-effective solution to respect current and future emissions standards. Thanks to the widespread use of Diesel Particulate Filters, High Pressure EGR is no longer the only efficient solution to provide exhaust gas to the cylinders, in so far as one of the main drawbacks of Low Pressure EGR – compressor and air intake cooler reliability – is reduced. In this context, it is difficult to assess intuitively the impact of the different possible air system configurations on the engine performance, and the consequences on the associated control strategies. It is therefore necessary to use advanced tools, such as simulation in general and engine system simulation in particular. In this paper, we present the way engine system simulation can help in the understanding of technological issues as well as control issues. The approach is illustrated by the study of the impact of Low Pressure EGR in the specific case of a Two-Stage Turbocharged Diesel Engine. Indeed, the impact on the air path layout and on the engine performances are presented, and comparisons between simulation results of the same engine with a High Pressure or a Low Pressure EGR circuit are analyzed. In parallel, some specificities of the control strategy of a Two-Stage Turbocharged Diesel engine are discussed. The impact of a LP EGR circuit on the control laws is analyzed and a solution to adapt the strategies is finally proposed.

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