Numerical Analysis on the Potential of Different Variable Valve Actuation Strategies on a Light Duty Diesel Engine for Improving Exhaust System Warm Up

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The need for achieving a fast warm up of the exhaust system has raised in the recent years a growing interest in the adoption of Variable Valve Actuation (VVA) technology for automotive diesel engines. As a matter of fact, different measures can be adopted through VVA to accelerate the warm up of the exhaust system, such as using hot internal Exhaust Gas Recirculation (iEGR) to heat the intake charge, especially at part load, or adopting early Exhaust Valve Opening (eEVO) timing during the expansion stroke, so to increase the exhaust gas temperature during blowdown.In this paper a simulation study is presented evaluating the impact of VVA on the exhaust temperature of a modern light duty 4-cylinder diesel engine, 1.6 liters, equipped with a Variable Geometry Turbine (VGT). Numerical simulations were carried out by means of a commercially available 1D-CFD software (GT-SUITE) and a predictive combustion model (DIPulse) was adopted in order to properly evaluate the impact of different VVA strategies on the combustion process. The analysis was focused on the assessment of the potential of 3 different VVA strategies for managing the exhaust temperature: Early Exhaust Valve Opening (EEVO), obtained by means of valve lift modifications, Exhaust Phasing, by changing the valve timing, and Exhaust Valve ReOpening (EVrO) during the intake stroke for iEGR. Moreover, for the EVrO strategy, two different EGR combinations (iEGR-only and low pressure EGR with iEGR, respectively) were evaluated to identify the best trade-off between the exhaust temperature increase and the Brake Specific Fuel Consumption (BSFC) penalty.Thanks to the abovementioned VVA strategies, in steady state conditions increases in the exhaust temperature up to 70 K with BSFC penalties below 8% at low engine loads were achieved. Finally, the impact of VVA strategies was evaluated under transient conditions over the WLTC (Worldwide harmonized Light vehicles Test Cycle), highlighting a temperature increase of 30 K of the Diesel Oxidation Catalyst after the first 300 s with a total fuel consumption penalty lower than 1%.