Numerical Comparative Analysis of In-Cylinder Tumble Flow Structures in Small PFI Engines Equipped by Heads Having Different Shapes and Squish Areas

Abstract

For increasing the thermal engine efficiency, faster combustion and low cycle-to-cycle variation are required. In PFI engines the organization of in-cylinder flow structure is thus mandatory for achieving increased efficiency. In particular the formation of a coherent tumble vortex with dimensions comparable to engine stroke largely promotes proper turbulence production extending the engine tolerance to dilute/lean mixture. For motorbike and scooter applications, tumble has been considered as an effective way to further improve combustion system efficiency and to achieve emission reduction since layout and weight constraints limit the adoption of more advanced concepts. In literature chamber geometry was found to have a significant influence on bulk motion and turbulence levels at ignition time, while intake system influences mainly the formation of tumble vortices during suction phase. The most common engine parameters believed to affect in-cylinder flow structure are: 1. Intake duct angle; 2. Inlet valve shape and lift; 3. Piston shape; 4. Pent-roof angle. The present paper deals with the computational analysis of three different head shapes equipping a scooter/motorcycle engine and their influence on the tumble flow formation and breakdown, up to the final turbulent kinetic energy distribution at spark plug. The engine in analysis is a 3-valves pent-roof motorcycle engine. The three dimensional CFD simulations were run at 6500 rpm with AVL FIRE code on the three engines characterised by the same piston, valve lift, pent-roof angle and compression ratio. They differ only in head shape and squish areas. The aim of the present paper is to demonstrate the influence of different head shapes on in-cylinder flow motion, with particular care to tumble motion and turbulence level at ignition time. Moreover, an analysis of the mutual influence between tumble motion and squish motion was carried out in order to assess the role of both these motions in promoting a proper level of turbulence at ignition time close to spark plug in small 3-valves engines.

3D CFD Analysis of the Influence of Some Geometrical Engine Parameters on Small PFI Engine Performances - The Effects on Tumble Motion and Mean Turbulent Intensity Distribution

“…In [1] the authors carried out an analysis of a small 3-valves PFI engine characterized by a small bore and a high stroke-to-bore ratio. They compared three geometries differing only in the squish area distribution and values.…”

“…As in the papers [1,2], the present paper deals with PFI motorcycle engines characterized by having high stroke-to-bore ratio (C/D) and small bore (D), as summarized in Table 1. In [1] simulations on three different engine industrial configurations (Table 2) were ran by means a 3D CFD code: the simulation results showed that the configuration having the highest percentage squish area and the intake duct most capable of generating high tumble ratio at IVC (namely C3_original) was characterized by the lowest combustion velocity. This was the evidence of a not coherent tumble structure related to an increase of the squish area which in turn results in a weaker tumble breakdown during the compression stroke.…”

“…Starting from this C1 geometry and once assessed the effect of the squish area on these type of engines, in the present paper the further step toward a fast engine design was to investigate the effect on combustion speed of each single parameter of the multivariable problem not investigated in paper [1]: the piston shape, the compression ratio and the intake duct.…”

“…In Table 1 were summarized the engine data. The engine is the 3-valves PFI pentroof engine in the configuration C1, which was found to generate the more structured and coherent in-cylinder charge motion in the previous paper [1]. The geometries under analysis were the following, as summarized in Table 3:…”

“…In order to assess the different behavior of the proposed engine geometries the simulation of the intake and the compression phases with the 3D code FIRE v. 2010.1 by AVL was performed. Boundary conditions for the intake process was set on the basis of the data from a proper 1D engine model [1]. Simulations were ran at full load and 6500 rpm with a computational domain composed by exhaust system, intake system and cylinder.…”