Performance Prediction, Optimisation and Validation of a CNG Engine Intake Manifold of a Commercial Vehicle Using Transient CFD Analysis

Abstract

Developing countries like India have large consumer markets driven by huge demands. Commercial vehicles play a critical role in full filing these demands. Commercial vehicles increasingly face stringent emission norms criteria and hence designing an ICE-powertrain with optimum operating efficiency becomes paramount. Intake manifold is the critical part of an internal combustion engine that supplies fuel/ air mixture to all the cylinders combustion chambers. It ensures a uniform mixture at cylinder inlet for better mixing inside the cylinders for better volumetric efficiency. Uneven distribution of fuel/air mixture causes unstable torque and unburnt fuel which fails to meet the emission norms. It also results in uneven temperatures in each cylinder because of cylinder misfiring. In current paper, 3D Computational Fluid Dynamics (CFD) simulations are carried out to investigate the variance and uniformity of CNG/air mixture at the outlet of intake manifold. Commercial CFD tool Ansys Fluent is used to study the flow distribution of mixture inside the manifold and runners. Initial estimation of flow pattern is done by performing a steady state simulation to predict the uniformity index of CNG at cylinder inlet. For detailed investigation, transient simulation is performed by taking fresh air and CNG mass flow rate as a function of crank angle. In this paper, mesh dependence study was done initially to achieve an optimum cell count with good accuracy. A detailed transient analysis using multi-species modelling for air & CNG was done using automated scripts with time steps as small as 1 degree crank angle rotation coupled with injection pressure and injection timing study. This helped to identify critical areas and optimise the design to improve the mass flow rate variance from 15-20% for baseline case to 6-7% for final design, and also improve the uniformity index. It also helped reduce the CNG engine mis-firing issue. The results have been well validated with Laboratory Test Results.