Construction Materials

PRIN. OF INTERNAL COMBUSTION ENGINES - OD1619 LESSON 1/TASK 1

(a) As the piston moves from the top of the cylinder to the bottom (or vise

versa), it accelerates from a stop to a speed of approximately 50 miles per hour

(mph) (80 kilometers per hour [kph]) at midpoint, and then decelerates to a stop

again. It does this approximately 80 times per second.

(b) The head of the piston is subjected to pressures in excess of 1000 pounds

per square inch (psi) (6895 kPa).

(c) The piston head is subjected to temperatures well over 600€ F (316€ C).

(2) Construction Materials.

When designing pistons, weight is a major

consideration.

This is because of the tremendous inertial forces created by the

rapid change in piston direction. For this reason, it has been found that aluminum

is the best material for piston construction.

It has a very high strength-to-

weight ratio and, in addition to being lightweight, aluminum is an excellent

conductor of heat and is machined easily. Pistons are also manufactured from cast

iron. Cast iron is an excellent material for pistons in low-speed engines. It is

not suitable for high speeds because it is a very heavy material.

(3) Controlling Expansion (figure 14 on the following page).

Pistons must have

built-in features to help them control expansion. Without these features, pistons

would fit loosely in the cylinders when cold, then bind in the cylinders as they

warm up.

This is a problem with aluminum because it expands so readily.

To

control expansion, pistons may be designed with the following features:

(a) It is obvious that the crown of the piston will get hotter than the rest

of the piston. To prevent it from expanding to a larger size than the rest of the

piston, it is machined to a diameter that is approximately 0.03 to 0.04 in. (0.762

to 1.106 mm) smaller than the skirt area.

(b) One of the ways to control expansion in the skirt area is to cut a slot up

the side of the skirt. As a split-skirt piston warms up, the split will close up,

thereby keeping the skirt from expanding outward and binding the piston in the

cylinder.

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