Multi Body Dynamic Analysis of Slider Crank Mechanism to Study the effect of Cylinder Offset Vikas Kumar Agarwal Deputy Manager Mahindra Two Wheelers Ltd. MIDC Chinchwad Pune 411019 India Abbreviations: FHP: Frictional Horse Power 3D: Three Dimensional MBD: Multi Body Dynamic FMEP: Friction Mean Effective Pressure TS: Thrust Side ATS: Anti Thrust Side RPM: Revolution per Minute Keywords: Crankshaft, Friction, Thrust Force, Piston, Offset Abstract To improve the mechanical efficiency of a single cylinder motor cycle engine, cylinder offset is introduced. The offset leads to lesser piston-liner interface forces and subsequently lesser frictional power. The crank train dynamics are evaluated for crank offset. A linear 3D MBD model with rigid bodies is set up in MotionView and solved using MotionSolve. The combustion pressure with the rotational crank motion is imposed on the mechanism. The simulation for important crank angular velocities viz. maximum power and torque are carried out. HyperGraph is used to derive the frictional force & frictional power at piston liner interface assuming the primary rigid behavior of the piston and a constant friction coefficient. An important decrease in the liner thrust force is observed particularly during expansion stroke. The frictional power is evaluated from velocity integral of thrust force, which shows sizeable improvement during expansion stroke. Therefore, based on the study the initial assumption regarding cylinder offset is justified and finds its place in the final engine design. Introduction In recent years, with increased demand for low fuel consumption vehicles, the efficient consumption of the fuel has become one of the most important factors for automobile industry during engine development stage. A number of techniques are used for better fuel efficiency such as gasoline direct injection, variable valve timing, variable valve lift etc. resulting in improved thermal efficiency. Reducing friction loss, which contributes around 5 percent of total power loss in the engine, is another area to focus on with an intention of improving fuel economy [1-3]. Reducing friction would improve mechanical efficiency of the engine considerably. It has been seen that a 10 percent reduction in engine friction improves fuel economy by 1-1.5 percent at full load [4]. In an engine, the major frictional loss happens at valve train, piston ring assembly, crankshaft & other moving parts. The piston ring assembly alone contributes 40-65 percent of the total friction loss [5], of which the loss in piston skirt is about 40-50 percent. A crank offset methodology was proposed to reduce friction loss in the piston assembly [6]. The effect of crank offset has been studied in more detail by Shinichi et al. Simulate to Innovate 1
[7], which reports that, when a crank offset is kept, fuel economy improves by 3 percent at low engine speed & part load, and there is an optimum point to maximize the offset effect. This effect is largely due to the piston side force and sliding speed. This study sets up a useful model to analyze the offset with regard to the side thrust force and cumulative effect with the piston sliding motion to examine the effect of crank offset on reduction in friction, which will help in determining the amount of offset for a given engine based on its operating conditions. Process Methodology Equations of Motion Figure 1 shows a schematic diagram of the crank offset engine. In such arrangement the crankshaft center is positioned towards the piston thrust side w.r.t. cylinder bore. The amount of offset chosen is within a range ensuring that the rotation of the crankshaft is not disturbed by the cylinder block. With application of the offset, it is also necessary to adjust the connecting rod length and the crank radius to fit the combustion chamber volume and compression ratio. Figure 1: Schematic diagram of piston crankshaft assembly with crank offset Simulate to Innovate 2
The kinematics of such a system is quite different from traditional arrangement as discussed by A Ghosh, A K Mallik [8]. The piston movement, speed and acceleration with an offset are calculated as follows. s t r cos 2πωt l r sin 2πωt e (1) v t 2rπω sin 2πωt (2) a t 4rπ ω cos 2πωt r r sin 2πωt e l r sin 2πωt e Where, r = crank radius, l = connecting rod length, e = crank offset ω = Crank Angular Velocity s(t) = Piston Displacement, v(t) = Piston Velocity & a(t) = Piston Acceleration r sin 2πωt e sin 2πωt r cos 2πωt l r sin 2πωt e (3) The change in piston kinematics affects the piston dynamic inertial force and the load on the piston pin & crank pin. The changes in the loads and speeds result in the changed value of frictional power loss. Results & Discussions Figure 2 & 3 shows the piston speed & acceleration plots respectively during a power stroke and its variation with offset present. The zero crank angle in all the plots denote TDC. Figure 4 shows the side force working upon the piston as a result of the crank offset under full engine load at 8000 engine rpm. Considering that the piston acts on the liner with this side force, it is expected that the side force reduction by offset would be effective in reducing friction in the piston skirt. It is observed that the maximum side load occurs during compression stroke known as thrust side (TS), and relatively less peaks occurs during other strokes known as anti thrust side (ATS). This happens due to high combustion pressure contributing to the side force. Figure 4 also shows that side force working in the piston thrust direction decreases when crank offset is applied to an extent that increases with increase in offset magnitude. Because of the crank offset, however, the side force to the antithrust side increases. When the absolute maximum force during on power stroke is taken and such value is plotted against crank offset for various engine speed, it is found to attain minimum values for specific offset value. It is shown in Figure 6. In addition, at each engine speed it is seen that there is an offset magnitude that minimizes the maximum side forces. However, when the engine speed changes, the offset magnitude at which the side force is minimal tends to differ. For this engine the optimal offset seems to lie between 4mm to 12 mm. The piston movement on the liner interface with the evaluated side force generates the frictional loss. The cumulative effect of the two under full engine load at 8000 engine rpm is shown in the figure 7. The plot shows reduction in the frictional loss in the expansion stroke, an increase concurrently occurs during the compression stroke. The net frictional power loss is evaluated for a complete power cycle. Figure 8 shows the variation of frictional loss with respect to offset. It shows minimum frictional loss at 10 mm crank offset. Figure 9 shows the results at other working engine rpm values. It is observed that the optimum value lies between 8mm to 12mm. Therefore, based on the this study we can conclude that crank offset shall be decided after careful deliberation on the engine working conditions, since a single offset value may not yield same advantage in all the engine running conditions. Simulate to Innovate 3
Figure 2: Piston speed variation at various crank offset Figure 4: Piston Side Force variation at various crank offset Figure 3: Piston Acceleration variation at various crank offset Figure 5: Piston Maximum Side (TS & ATS) Force variation with crank offset Simulate to Innovate 4
Figure 6: Piston Maximum Side Force variation with crank offset Figure 8: Piston Frictional loss variation with crank offset Figure 7: Piston Frictional loss variation at various crank offset Figure 9: Piston Frictional loss variation with crank offset at various engine rpm at full load condition Benefits Summary The study shows a considerable improvement in the mechanical efficiency of the engine due to introduction of crank offset. The advantage is 6% at 8000rpm and 8.2% at 7000 rpm. The overall reduction in the frictional loss varies between 4% to10%. The effective improvement in the brake horse power of the engine is about 0.5 percent. Challenges An accurate combustion pressure acting on the piston is the most important input in the determination of the piston side force and subsequent friction power. Moreover, since combustion at part load engine running conditions is very difficult to ascertain. Therefore, an accurate test data would improve the results Simulate to Innovate 5
significantly. However, for this study an estimated pressure values have been taken in absence of the experimental values. Future Plans The piston liner interface hydrodynamic study is planned for further analysis of the side force and its distribution over the piston skirt along the thrust side and anti thrust side. Considering that the secondary motion of the piston skirt is affected by the side force working upon the piston, which plays an important role in the distribution of the friction, lubrication. The other important factors viz. Asperity contact, oil viscosity analysis, temperature also need to be looked into depth. Other factors must come into play therefore, in order to realize a net gain in friction with an offset crankshaft architecture. Conclusions The study elaborates a plan to determine the optimum offset for a given engine and its operating conditions. It will help engine designer to understand the effect of offset and decide the most feasible offset based on the engine layout, packaging and friction advantage. ACKNOWLEDGEMENTS The author would like to thank R&D, Mahindra Two Wheelers Ltd. & Altair India for their continued help & support. REFERENCES [1] Kovach, J. T., Tsakris, E. A. and Wang, L. T. Engine friction reduction or improved fuel economy. SAE paper 820085, 1985. [2] Pohlmann, J. D. and Kuck, H. A. The influence of design parameters on engine friction. In Proceedings of Conference on Combustion Engines Reduction of Friction and Wear, 1985, C73/85, pp. 67 73. [3] Hoshi, M. Reducing friction losses in automobile engines. Tribology Int., 1984, 17(4), 185 189. [4] Parker, D. A., Adams, D. R. and Barrett, D. J. S. The reduction of friction in the internal combustion engine. AE Technical Symposium, 1982, paper 29. [5] Rao, V. D. N., Kabat, D. M., Yeager, D. and Lizotte, B. Engine studies of solid film lubrication coated pistons. SAE paper 970009, 1997. [6] Nakayama, K., Tamaki, S., Miki, H. and Takiguchi, M. The effect of crankshaft offset on piston friction force in a gasoline engine. SAE paper 2000-02-0922, 2000. [7] Shinichi, S., Eiichi, K. and Tatehito, U. Improvement of thermal efficiency by offsetting the crankshaft center to the cylinder bore center. JSAE paper 9638770, 1996. [8] A Ghosh, A K Mallik, "Theory of Mechanism & Machines" Simulate to Innovate 6