i am working on project in which we will be converting a gasoline powered engine to E100.To increase the compression ratio we have decided to change the piston as we have decided to increase the diameter of the cylinder.So what changes should we make to the connecting rod so that it withstands the increase in force,torque etc.We don 't want to change the diameter of the pin as it would require us to make changes in the crankshaft as well.
First, check current design: static / fatigue safety factors, with help of FEM identify the weakest zones and then increase wall thickness, change radiuses etc. Also you could improve the material grade in order to reach higher allowable stress.
Best regards
Due to changes in the diameter of piston as well as the compression ratio, I would recommend you to figure out how much difference in terms of force and stress or in this case I would say the compression stress would be.
If the difference is not significant, in my opinion it would not since the instant inertia would around the same number since ethanol works a little different from Gasoline.
It would be better to use a lighter material for the piston so the rod wouldn't need much changes.
Best Regards,
Gilbert
There are two types of forces acting at the connection rod:
1. Pressure induced force Fp from combustion. Fp is a function of the pressure characteristics in cylinder and the acting surface, in this case the piston upper section surface (as a function of the piston diameter). Fp acts on the connecting rod as compressive-force, leading to compressive stress. in addition, the piston pin and the crank pin are stressed, mainly by shear stress and bending stress. An increase of the piston diameter and of the compression will lead to increased cylinder pressure, which increases Fp.
2. Inertia forces that stem from the movement of the cranktrain components. For your question, the translational inertia force Ft might play a major role. Ft leads to tensile stress in the connecting rod as well as shear stress and bending stress in the piston pin and crank pin. Increased piston diameter might lead to increased moved mass that rises Ft.
For dimensioning the connection rod (but also piston pin and crank pin) the forces mentioned above should be considered.
Kind regards,
Mario
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