CASE STUDY USING PISDYN OF THE EFFECT ON ... - Ricardo

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INTRODUCTION A key asset of any simulation software is its ability to be easily used for parametric studies and optimisation. At ACL Piston Products in Australia, Ricardo’s PISDYN software is used to help design pistons. The key criteria used to evaluate piston skirt designs performed using PISDYN are noise, wear, friction and fatigue strength. It is known that small variations in pin offset, skirt profile (the small variations in diameter of the skirt with height along the skirt) and skirt contour (the ovality in the skirt) can have a big influence on how well a piston performs on each of these criteria. PISDYN can be used to determine the most suitable combination of pin offset, skirt profile, skirt contour and in some cases land diameters for a particular piston application. Piston noise is caused by the slapping of the piston against the cylinder. This occurs as a result of the changing interplay of forces on the piston - due to cylinder pressure, inertia and the supporting force of the connecting rod - as it moves up and down the bore during a cycle. This noise is greatest when the piston is cold, as this leads to a larger clearance between the skirt and cylinder, allowing for greater kinetic energy, from lateral and rotational motion to be gathered by the piston before it impacts the cylinder. Piston noise tends to also be largest during engine start up when the oil film between the skirt and cylinder may be very thin. Piston noise problems are typically one (or both) of two types - "croaking" and "rattling". "Croaking" problems result from excessively strong impacts of the thrust side of the piston against the cylinder at a crank angle of around 10-40 degrees after TDC (top dead centre) at the start of the expansion stroke, while the engine is running at low speed and low load. "Rattling" noise problems result from excessively strong impacts of the top of the anti-thrust side of the skirt or the top land on the anti-thrust side against the cylinder around TDC at the end of the compression stroke, while the engine is running at about 2500-3500 RPM, with moderate load. 2 of 13
Skirt-liner friction is greatest and wear problems most likely when the engine is running at high power output conditions, when the piston is hottest and therefore the skirt-cylinder clearance is least. The most important wear problem to avoid is skirt scuffing, which is caused by an excessive instantaneous wear load at a particular point on the skirt. A suitable wear pattern on the skirt is also necessary. Wear on the very top or side edges (i.e. at the pin cut-out edge) of the skirt, where the skirt is at its stiffest, can cause wear damage to the cylinder liner. In simulating the fatigue strength of pistons, ACL uses PISDYN to determine the loads on the piston, that are then input as boundary conditions into a finite element stress analysis computation performed in IDEAS. The loads on the piston used in the stress analysis are gathered from the PISDYN simulations for particular crank angles, at particular engine operating conditions, that are expected to be worst-case in terms of the stress levels they generate in the piston. The loads on the piston needed for the finite element stress analysis are the loading due to cylinder pressure, the pin force on the piston, the hydrodynamic and asperity contact forces of the cylinder liner on the piston skirt and the inertial forces of the piston. The cylinder pressure at the relevant crank angle (an input for PISDYN simulations) is obtained from engine test data. The pin lateral and axial forces and the hydrodynamic and asperity contact forces on the piston at the relevant crank angle are simply read off the results of the PISDYN simulations - from the RPLOT graphs and the *.lnr files respectively. The inputs needed for the finite element analysis to account for the inertial loads of the piston are the piston lateral and axial accelerations at its centre of gravity and its angular acceleration. The piston angular acceleration is simply read off the results of the PISDYN simulations. The lateral and axial accelerations of the piston centre of gravity involve adding respectively to the lateral and axial piston accelerations at the pin bearing, given by the results of the PISDYN simulations, the components due to angular acceleration of the piston. The above techniques were used by ACL to optimise the skirt profile and contour for a piston to be produced for use in a four cylinder, petrol engine. The results of engine tests were compatible 3 of 13
Page 1: CASE STUDY USING PISDYN OF THE EFFE
Page 5 and 6: 3) To get the greatest bore-skirt c
Page 7 and 8: SIMULATION OF EFFECT OF USING LONGE
Page 9 and 10: 2. Once a PISDYN simulation model h
Page 11 and 12: Comparison of Skirt-Liner Impact Po
Page 13: Figure 7. Predicted wear pattern on

CASE STUDY USING PISDYN OF THE EFFECT ON ... - Ricardo

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