Pistons, Rings, and Connecting Rods. Pistons. The piston's primary responsibility is to take thermal energy created by the ignition of fuel and air, and transform it into linear motion. Linear motion acts on the crankshaft journal and becomes rotary motion.
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A low-friction moly coating on the skirt of this piston helps prevent piston scuffing when the engine is cold.
Notice the temperature difference between a forged piston and a cast piston.
Pistons are often cam-ground to produce the elliptical shape when the piston is at room temperature.
Cross-sectional piston pins.
Most piston pins are hollow to reduce weight and have a straight bore.
Some pins use a tapered bore to add strength.
Engine rotation and rod angle during the power stroke causes the engine to press harder against one side of the cylinder, creating a major thrust surface.
In this clockwise-rotating engine, as viewed from the front of the engine, the major thrust surface is on the left side.
Circlips or snap rings hold full-floating piston pins in place.
The preferred material for compression rings is a low-alloyed, heat-treatednodular cast iron (KV1/GOE 52). This material is characterized by a high bending strength of min. 1300 MPa and a high modulus of elasticity attributable to a martensitic microstructure and spherulitic graphite structure.
In the 2nd groove, alloyed grey cast irons are used in a heat-treated condition.
Unalloyed grey cast iron is used for 2-piece oil rings in the 3rd groove. These ring materials (STD / GOE 12, GOE 13) are characterized by a fine-lamellargraphite structure in a pearlitic matrix and have good conformability due to a relatively low modulus of elasticity.
Reduced width piston rings in gasoline engines to match reductions in the overall height of pistons, and increasing combustion pressures in diesel engines call for materials with increased strength characteristics.
The wear resistance derives from finely distributed chromium carbides of the type M23C6 and M7C3 embedded in the tempered martensite matrix.
Chromium facing can be seen on the right side of the sectional view of the piston ring.
Molybdenum facing can be seen on the right side of the sectional view of the piston ring.
This typical three-piece oil control ring uses a hump-type stainless steel spacer-expander.
The expander separates the two steel rails and presses them against the cylinder wall.
The gapless ring overlaps, while the conventional ring design uses a gap.
Combustion chamber pressure forces the ring against the cylinder wall and the bottom of the ring groove.
These are the two sealing surfaces that the top ring must be able to seal for maximum engine power.
The piston rings must have the specified side and back clearance.
The rectangular and the barrel face are the most commonly used top compression rings because they provide the best seal.
The taper face ring provides good oil control by scraping the cylinder wall.
If this design ring were accidentally installed upside down, the tapered face would pump oil into the combustion chamber.
Torsional twist rings provide better compression sealing and oil control than regular taper face rings.
Some designs utilize spit holes or bleed holes
A typical connecting rod and related engine parts. The connecting rod is probably the most highly stressed part in the engine.
Combustion forces try to compress it and when the piston stops at the top of the cylinder, inertia forces try to pull it apart.
Some connecting rods have balancing bosses (pads) on each end of the rod.
Rod caps are unidirectional and must be reinstalled in the same rod position.
The rod bearing bores normally stretch from top to bottom causing the rod bearing to wear most near the parting line.