LO 7-2 Classify pistons according to their application, design, construction features, and material

1. Pistons

Pistons One of the most important components in a reciprocating engine is the piston. Tremendous heat, pressure, and inertia forces are endured by the pistons as they absorb combustion energy and transmit it to the other reciprocating and rotating parts of the engine Figure 4. In order to operate reliably for many hours under extreme conditions, in high horsepower and high-speed engines, pistons require some important construction features. Piston design is a major factor in determining a variety of engine characteristics, such as maximum power output, emissions characteristics, operating speed, oil consumption, and durability.

FIGURE 07-4: High combustion pressures and temperatures require pistons to be capable of withstanding high thermal loads and cylinder pressures.

1.1. Video: Pistons

1.2. Piston Design

Piston Design Pistons have three main parts: The crown, or top of the piston, which is subjected to cylinder pressure and heat. The ring belt, which is where piston ring grooves are located. The skirt, which is the surface that slides directly against the cylinder wall to stabilize piston movement Figure 5. In the uppermost part of the skirt is the piston pin. Additional piston material reinforces the area around the pin and is called the pin boss Figure 6. Steel struts are often cast into the pin boss for improved strength. Their shape, construction method, and materials today categorize piston designs. Several basic piston designs used in today’s diesels are made from either aluminum or steel alloy material, including slipper skirt, cam ground, trunk, articulating, and newer single piece mono steel alloy pistons Figure 7.

FIGURE 07-5: The main sections of a piston.

FIGURE 07-6: Piston nomenclature. The connecting rod and piston are forged.

FIGURE 07-7: Major types of piston designs used in today’s HD diesel engines include steel alloy, trunk, and articulating.

1.3. Video: Controlling Piston Expansion

1.4. Cam Ground Pistons

Cam Ground Pistons

A piston will expand when heated. How much it expands depends on the temperature and mass of the area that is heated. Hotter and thicker areas of a piston will expand more than thinner, cooler dimensions. If a piston’s operating temperature is excessive, or operating clearances between the piston and the cylinder wall are too small, the piston will begin to scuff and score and eventually could seize in the cylinder Figure 8. A number of construction features are used to help minimize the problem of piston expansion. The most common technique for piston construction is to use a shape that is carefully machined to allow for a controlled expansion of selected portions of the piston in the cylinder. The piston shape that is almost universally used to enable expansion without seizing is an elliptical or oval shape. A cam ground piston is made with the smaller diameter measured across the piston pin, giving it a slightly oval shape. When the piston heats up to operating temperature, it will then be round Figure 9. This piston configuration is commonly known as a cam ground piston due to its egg or elliptical shape.

FIGURE 07-8: A heat-resistant, graphite-based antiscuff coating is applied to the piston skirt to reduce friction and blowby and to create a tighter fit between irregularly shaped cylinder walls.

FIGURE 07-9: To permit piston expansion while reducing the looseness of the piston in the cylinder, pistons are machined elliptically and become round when warmed up.

Most piston expansion takes place in the dimensions where there is the most metal and heat. This means the crown will expand the greatest amount when heated. Because the wrist pin bore is reinforced, it will also expand the most. Areas that expand the least are the bottom of the skirt and the thinner skirt areas located at about 90 degrees from the wrist pin. This area corresponds to the major and minor thrust surfaces of the piston. Knowing this helps explain why measuring the piston diameter should be done at 90 degrees opposite to the piston pin bore; it is the widest cross section of the piston.

Aluminum pistons also possess a barrel or taper shape. The crown and ring belt parts of the piston are significantly smaller than the bottom circumference. A slight bulge in the middle section to reduce piston-to-cylinder clearance is built in to minimize noise and piston slap (piston contact with the cylinder wall) and to enhance sealing capabilities. Because the cam ground piston is irregularly shaped, rings (not the piston) are responsible for sealing most of the clearance between the cylinder walls when the engine is operated cold. This explains why piston rings on aluminum pistons will generally be thicker and require greater ring tension to ensure a gas-tight seal.

1.5. Slipper Skirt Pistons

Slipper Skirt Pistons

In the design of a slipper skirt piston, a portion of the skirt is removed on both non-thrust sides of the piston. These sides of the piston are not heavily loaded by compression of combustion forces. When combined with the angle of the connecting rod, this forces the piston against the cylinder wall Figure 10. When sections of the piston skirt are removed, more clearance is available for the turning radius of the crankshaft counterweight when the piston approaches bottom dead center (BDC) Figure 11. Slipper skirt pistons enable the use of shorter connecting rods, which permits the manufacture of vertically smaller, lighter, more compact engines. This is important when the vertical profile of the engine must be kept short in order to fit into a smaller vehicle, such as in a pickup, van, or automobile. More compact engines also translate into lighter block designs for better vehicle dynamics. Slipper skirt pistons are often used in smaller V-block diesels where compact engine design is vital.

FIGURE 07-10: Major and minor thrust forces are created by a combination of an angle between the connecting rod and the piston plus the cylinder pressures during the compression (minor thrust) and power (major thrust) strokes.

FIGURE 07-11: This piston uses a slipper skirt piston design.

1.6. Trunk Pistons

Trunk Pistons

Trunk pistons have a full skirt and are dimensionally longer than they are in diameter Figure 12. The full skirt design is stronger than a slipper skirt piston. This enables the cylinders to support higher cylinder pressures and accompanying side thrust. Side thrust is the force of a piston against the side of a cylinder wall produced by the angle of the connecting rod, which pushes the piston sideways when cylinder pressures are high. Cylinder pressure during the power stroke produces major side thrust while compression pressure produces minor side thrust. Trunk pistons are used in more rugged, high horsepower diesel engine applications and are almost always made from aluminum alloys. As trunk piston design has developed, pistons are being shortened to minimize cylinder wear, noise, mass, and friction as well as the likelihood that oil can be pumped into the combustion chamber Figure 13.

FIGURE 07-12: A trunk-type piston is longer than its diameter and has a full piston skirt.

FIGURE 07-13: The evolution of trunk pistons.

1.7. Articulating Pistons

Articulating Pistons

Another innovation to piston technology is the use of articulating piston articulating pistons. This two-piece piston has an aluminum skirt but a steel crown. An important feature of the steel crown is to reduce the clearance between the crown and the cylinder wall Figure 14. This region, between the piston crown and the cylinder wall above the top compression ring, is called the crevice volume. The strength of a steel crown enables the top ring to be placed closer to the crown without causing a fracture between the top ring groove and the crown (a fracture that would occur if a thin aluminum piston crown was used). Another name for a piston using this feature is low crevice volume piston. Low crevice volume (LCV) pistons place the compression ring closer to the top of the piston crown. Because fuel can hide in this area and cool, or quench due to low heat, manufacturers’ studies have found that reducing crevice volume improves fuel economy as much as 5%. Eliminating the dead air space in the crevice volume area lowers hydrocarbon (HC) exhaust emissions by as much as 50% Figure 15, compared with older, trunk-type aluminum pistons with high crevice volume. Trunk pistons are also made with reduced crevice volume, but aluminum pistons cannot reduce the crevice volume to the same extent as pistons with steel crowns.

FIGURE 07-14: Articulating pistons use steel crowns to withstand high cylinder temperatures and pressures. These pistons have low crevice volume crowns.

FIGURE 07-15: Crevice volume is the area between the crown and the cylinder wall. A larger crevice volume area quenches fuel combustion and produces more noxious emissions.