Assessment of the Time Step in the Modeling of Cold Flow in a Motored Diesel Engine

The work investigates the effects of the time step in the modeling of the intake and in-cylinder systems of a diesel engine, under the motored condition. The engine has a bore of 79.5 mm and a stroke of 86 mm. The valves and piston movements are included. The equations are numerically solved, including a transient analysis of the intake stroke, for an engine speed of 1500 rpm, using a commercial Finite Volumes CFD code. For the purpose of examining the in-cylinder turbulence characteristics two parameters are observed: the discharge coefficient and swirl ratio. Regarding the turbulence, computations are performed with the Reynolds-Averaged Navier-Stokes, Eddy Viscosity Model k-ω SST, and also the k-ε standard cubic model (usual in the automotive industry), with standard near wall treatment. A moving hexahedral mesh independence study is presented. In the same way many convergence tests are performed, and a secure criterion established. The enthalpy equation is also solved, and the air compressibility is considered, being treated as perfect gas. A steady state flow-bench intake calculation is also presented, and the results are compared against experimental data. Thought the results it is possible to note divergences between the turbulence models employed and time steps.