CASE STUDY USING PISDYN OF THE EFFECT ON PISTON PERFORMANCE OF USING A LONGER CONROD

Transcription

1 CASE STUDY USING PISDYN OF THE EFFECT ON PISTON PERFORMANCE OF USING A LONGER CONROD By: Prabakaran Naganathan and Adrian Martin, ACL Piston Products, Australia. ABSTRACT PISDYN simulation software is used by ACL Piston Products to optimise in the design of pistons, skirt profile, skirt contour, pin-offset and in some cases land diameters. The most suitable combination of these parameters is required to meet design requirements including quietness, wear resistance, low friction and durability. PISDYN was used to successfully design a skirt profile and contour for a piston to be used in four-cylinder, petrol engines to be produced by a major car manufacturer. When the manufacturer wanted a feasibility study of the possibility, in terms of piston performance, of using a longer connecting rod, this was easily achieved by merely varying a few parameters in the input deck of the simulations for the original engine. The study showed that increasing the connecting rod length in this case would not adversely affect piston performance, apart from causing a small possibility of a rattle noise problem. 1 of 13

2 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 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 RPM, with moderate load. 2 of 13

3 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

4 with the PISDYN simulations used in designing the skirt profile and contour. Consequently, when the major car manufacturer that the piston was to be produced for, asked ACL to determine the feasibility (in terms of piston performance) of using a 7.9% longer connecting rod in the engine, while maintaining the same piston design, all that was required for an initial study was to change the connecting rod geometry and inertial data in the PISDYN input deck and to repeat the simulations. OPTIMISATION OF PISTON SKIRT TO BE USED IN ORIGINAL ENGINE To optimise the skirt profile and contour, PISDYN simulations were performed to model three engine operating conditions: 1) At 1000 RPM and 2.5% of maximum engine power to simulate conditions when "Croaking" noise would be approximately at its greatest. 2) At 2800 RPM and 13% of maximum engine power to simulate conditions when "Rattling" noise would be approximately at its greatest. 3) At maximum power operating conditions to simulate conditions when skirt wear and skirt-liner friction would be expected to be greatest. Noise minimisation To simulate worst case conditions in the "Croak" and "Rattle" simulations: 1) The simulations are performed using piston thermal expansion data (from computations performed by TRIBFE) assuming a cold engine operating in a -20 C environment. 2) Another simulation parameter was modified to simulate start-up conditions. 4 of 13

5 3) To get the greatest bore-skirt clearance, the bore diameter was set at the maximum of its tolerance range and the skirt was set at slightly lower than the minimum of its tolerance range to allow for reduction in effective piston skirt diameter during use of a piston. The quantity output by PISDYN used for examining piston slap noise is the Skirt-Bore Oil Film Instantaneous Power/Unit Area which gives the negative of the impact power per unit area between the skirt and bore. The maximum of the negative of this quantity identifies the greatest slap. However, this quantity has some limitations. It does not account for the greater noise created by impacts of the same maximum impact power per unit area but with greater impact duration and greater impact area. Also it does not distinguish between power transfer through direct asperity contact between skirt and bore and that passed through the oil film, where induced vibrations will be more damped. Hence, it is necessary to examine other quantities output by PISDYN such as the Piston Secondary Kinetic Energy. The final skirt design was predicted to be very quiet under both "Croak" and "Rattle" conditions and in both cases land-liner impact was predicted not to occur. This was confirmed by both dynamometer and in-car engine tests, including on engines chilled to -20 C. Wear and friction simulation The wear characteristics of piston skirts are sensitive to the bore distortion due to assembly and thermal loads. Since, this is dependent on the characteristics of the block, cooling system etc of each particular engine and is hard to estimate, the bore distortion used in the simulation was calibrated with engine test results. In simulations of initial piston designs, the bore distortion used was taken from that, used in simulations of a similar engine previously studied. Following a hot scuff engine test using an initial piston design, the skirt and bore wear patterns were noted and the bore distortion in the simulation model was modified such that the simulation predicted the actual wear pattern. There was then confidence (which was confirmed later by testing) that 5 of 13

6 the PISDYN model would be able to satisfactorily predict the wear patterns on subsequent design iterations of the piston. Consequently, only two prototype versions of the piston needed to be actually produced and tested - an initial design and the final design. Two separate simulations of each design were performed at maximum power operating conditions. In simulation A, where roughness parameters of the skirt and liner were set in PISDYN to replicate actual values, the purpose was to determine the skirt-liner friction and to determine the maximum wear loads in order to ascertain whether scuffing could occur. In simulation B, the roughness parameters were altered such that the simulation would better predict the wear pattern on the piston. It was necessary to make special adjustments to predict the wear pattern in order to account for the fact that during actual use the wear pattern will be wider than predicted. This is because in actual use, the region of the skirt most prone to wear will be first to experience a reduction in thickness (and therefore an increase in clearance with the liner), leading to surrounding regions of the skirt beginning to wear against the bore. In both simulations, worst case conditions were assumed by setting skirt-bore clearance to the minimum of the tolerance range. Simulation A of the final design predicted that scuffing would not be a problem. This was confirmed by a hot scuff engine test at maximum power output conditions. Simulation B of the final design predicted that the wear pattern on the skirt was suitable. This was again confirmed by the engine test. Figures 1 and 2 compare the wear patterns from the PISDYN simulation and from the hot scuff engine test respectively. The two patterns are quite similar apart from a wear region in the lower part of the skirt around the thrust plane, which was not predicted by the simulation. 6 of 13

7 SIMULATION OF EFFECT OF USING LONGER CONNECTING ROD To simulate how using the proposed longer connecting rod would affect the performance of the designed piston required changing only four parameters in the input deck of the existing PISDYN models: the connecting rod length in the ENGINE section of the input deck and three geometric and inertial parameters in the ROD section of the input deck. Quietness evaluation As seen by figures 3 and 4, the simulations predicted that using the longer connecting rod would have little effect on noise during operating conditions that could cause croaking. The simulation showed however, that increasing the connecting rod length is likely to increase rattle noise. As illustrated by Figure 5, the maximum impact power per unit area of the skirt against the liner the main quantity used to examine noise - was 230% times higher if the longer connecting rod was used. The kinetic energy transfer from the skirt to the cylinder during the slap causing rattle noise (shown by the drop from the peak in Figure 6) was predicted to be 30% times higher if the longer connecting is used. Note, that the simulation showed that land impact with the bore would not occur during rattle conditions even if the longer connecting rod is used. If the maximum skirt-liner contact power per unit area is assumed to be representative of noise power, then the simulations predict that using the longer connecting rod would approximately increase rattle noise by 5.2 decibels. Such an increase in decibel level will be noticeable by the human ear (a 10 decibel increase in noise is felt by the human ear as about twice as loud). However, since the original engine was very quiet this may not be a problem. 7 of 13

8 Wear and friction simulation Simulation type A showed that increasing the connecting rod length would reduce skirt friction loss by 1.0%. The maximum instantaneous wear load and maximum cumulative wear load were also predicted to be lower if the longer connecting rod is used, so scuffing or excessive wear should not occur if the longer conrod is used. The wear pattern predicted by simulation type B if the longer connecting rod is used is shown in Figure 7. This is clearly suitable and as seen by comparison with Figures 1 and 2, quite comparable to that produced when the original connecting rod was used. CONCLUSIONS The conclusions of the feasibility study using PISDYN of the effect on piston performance of increasing the connecting rod length by 7.9% in the particular four-cylinder petrol engine are: 1. There will not be a danger of piston scuffing and the wear pattern will have little change. 2. There will be little change in skirt frictional power loss. 3. There will be little change in croak noise. 4. There will be a significant increase in rattle noise amounting to about five decibels. Given the piston was found to be very quiet under rattle conditions when the original conrod was used, it is possible that the rattle noise with the new connecting rod will still be acceptable. The broader conclusions of this work are that: 1. The PISDYN simulation package can be used to successfully optimise skirt profile, skirt contour, pin offset and other piston specifications to meet quietness, wear resistance, friction and durability design requirements. 8 of 13

9 2. Once a PISDYN simulation model has been validated by an engine test, modifications to piston and engine specifications can easily be accounted for in the model and new simulations performed. ACKNOWLEDGEMENTS The authors would like to thank Nigel Tait for his assistance in writing this paper and Hung Nguyen for performing the initial simulation work for the piston design. 9 of 13

10 Figure 1. Predicted wear pattern on thrust side of skirt in original engine. Regions of greatest wear are in red. Figure 2. Wear pattern of thrust side of skirt in original engine following hot scuff test. 10 of 13

11 Comparison of Skirt-Liner Impact Power during "Croak" Condition Non-dimensional Power/Area Units Compression TDC Expansion Crank Angle Existing conrod Longer conrod Figure 3. Simulated effect of increasing connecting rod length on power of skirt impacts with liner during operating conditions conducive to croak noise. Comparison of Piston Secondary Kinetic Energy during "Croak" Condition Non-dimensional energy units Compression TDC Expansion Crank Angle existing conrod longer conrod Figure 4. Simulated effect of increasing connecting rod length on kinetic energy of skirt impacts with liner during operating conditions conducive to croak noise. 11 of 13

12 C o m p ariso n S kirt-l in er Im p act P o w er d u rin g "R attle" C o n d itio n Non-dimensional Power/Area Units Compression TDC Crank Angle Existing conrod Longer conrod Figure 5. Simulated effect of increasing connecting rod length on power of skirt impacts with liner during operating conditions conducive to rattle noise. Comparison of Piston Secondary Kinetic Energy during "Rattle Condition" Non-dimensional energy units 1.50E E E E Compression TDC Crank Angle Existing conrod Longer conrod Figure 6. Simulated effect of increasing connecting rod length on kinetic energy of skirt impacts with liner during operating conditions conducive to rattle noise. 12 of 13

13 Figure 7. Predicted wear pattern on thrust side of skirt if longer connecting rod was used. Regions of greatest wear are in red. 13 of 13