MMM314 High Speed Internal Combustion Engines
Materials and Manufacturing Tutorial Sheet
1. Show that the hoop stress in a cylinder liner under internal pressure, P, is given by
where r is the liner radius and t its thickness if the liner can be considered as a thin shell. A grey cast iron liner (diameter 120mm, thickness 10mm) is used in an engine with a compression ratio of 8.5. Determine the hoop stress and hence comment on whether this size is likely to be adequate.[Assume that atmospheric pressure is 1x105Nm-2]
2. The hoop and radial stress in a thick walled tube are given by
where r1 is the inner surface radius, r2 is the outer surface radius and r is the radial position between them. Plot the variation of hoop and radial stress with r for the cylinder liner in question 1 given that the ID is 120mm and the OD140mm. Hence determine where the stresses are maximum and how much the thin wall cylinder expression underestimates the maximum operating stress.
3. The Figure below shows a simple representation of piston, connecting rod and crank assembly.
It can be shown that the acceleration of the piston, ap, depends on the angular position of the crank, theta, and is given by
where r is the radius of rotation of the crank and L is the length of the connecting rod. Derive an expression for the net force transmitted during the acceleration of a piston in a vertical engine. A vertical engine has a cylinder of diameter 120mm and a stroke of 140mm. The length of the connecting rod is 240mm. The reciprocating parts have a mass of 1.2kg and the crank rotates at 20rev/s. When the crank is at theta=30o the net piston pressure is 0.8MPa. What is the resultant load on the gudgeon pin and the thrust on the cylinder liner at this instant?
4. If the cylinder liner in question 3 has a yield strength of 160MPa and a thickness of 10mm would it be expected to survive these service stresses?
5. Describe the squeeze casting process. Why does this improve the reliability of cast pistons?
6. What is the difference between white and grey cast iron? Which material is preferred for cylinder liners?
7. What are the requirements for the material for an exhaust valve?