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MSE 5090: Case Studies in Material Selection

Week 3 - Material Selection Process

 Weighted Property Index Methods - Problem 5

The idea of using filamentary composite materials to manufacture the moving parts of an automobile engine has long been attractive to automotive engine designers.

Discuss the choice of these materials for one such part, the connecting rod in a four-stroke spark-ignition internal combustion engine.

This component (Fig. Q38. 1) is subjected to particularly demanding operating conditions (Table Q38.l) and has therefore proved to be the most difficult engine component to design successfully in filamentary composite materials.
 

Figure Q 38.l Pictorial view of some parts of an internal combustion engine

TABLE Q38.1 OPERATIONAL PARAMETERS OF A CONNECTING ROD OF A
FOUR-STROKE SPARK-IGNITION INTERNAL COMBUSTION ENGINE
(Length = 100 mm)
 

Operational Parameter	Value
Maximum load Maximum extension Maximum distortion of big- and small-end eyes Minimum durability Minimum number of load cycles Minimum temperature range	40 kN (compressive), 25 kN (tensile) 160 mm (compressive), 100 mm (tensile) 10 mm 3000 h 3 x 108 -30°C to 180°C (small-end eye). -30°C to 140°C (big-end eye)

TABLE 038.2 CANDIDATE MATERIALS FOR A CONNECTING ROD OF A FOUR-STROKE SPARK-IGNITION INTERNAL COMBUSTION ENGINE

Material	Tensile Modulus, E (GPa)	Maximum Compressive Strength, s (MPa)	Density, p (kg m-3)	Maximum Service Temperature (°C)	Unit Cost ($ t-1 )	Specific Stiffness, E/p (106 m)	Specific Compressive Strength, sc/p (103 m)
Steel Aluminum alloy Silicon carbide ceramic S glass fiber reinforced   polymer laminatea Carbon fiber reinforced   polymer laminatea	210 71 310 56 138	1000 280 490 540 600	7890 2700 3200 1997 1661	800 350 1000 220 220	880 1,200 5,700 5,000 130,000	2.71 2.68 9.88 2.86 8.47	12.92 10.57 15.61 27.56 36.82

These service requirements, together with the need to withstand hot engine fluids (engine oil, engine oil additives, etc.), place severe restrictions on the types of materials that can be used for connecting rods. Some of these materials are compared in Table Q38.2, which shows that only steel, the ceramic, and the continuous carbon fiber reinforced polymer (CFRP) are suitable. Kevlar 49 fiber reinforced polymer is not considered because of its poor compressive strength and creep properties.

A typical CFRP connecting rod is shown in Fig. Q38.2. The rod components are divided into a compression part and a tension part mainly to allow the carbon fibers to be placed in the direction of the greatest load. Compression molding is the most suitable manufacturing process for the big- and small-end eyes, pultrusion is suitable for the part to bear compressive loading, and filament winding is best for the parts to be in tension. All fibers are to run axially. The force acting on the piston, due to combustion chamber pressure, is transmitted through the gudgeon pin, small end, and compression part to the crankshaft via the big-end assembly. The tensile forces, due to inertia loading, are transmitted to the tensile part of the rod through the loops of the wound fiber at the small end of the rod.

The problem of obtaining the correct fiber orientation in the eyes has yet to be solved. In one of the first CFRP connecting rods (designed and built by engineers at the Institute of Plastics Research in Aachen, West Germany, and tested for Volkswagen as part of the Composites for Highly Stressed Components in the Internal Combustion Engine research project, which is promoted by the West German Ministry for Research and Technology), the big- and small-end eyes are made of steel.
 

Figure Q 38.2 Composite-material connecting rod for an automobile.

REFERENCE 1.Lewis , G. Selection of Engineering Materials, Prentice Hall , 1990, pp373-375. Another selection from Dr. Gladius Lewis's book. Prof. Lewis was using case studies long before they were recognized as a significant contribution to engineering education., a distinction he shares with a limited number of engineering educators.


Last update 9-12-98