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 strengthto
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 lowspeed 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
builtin 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 splitskirt 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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