LO 7-3 Identify and explain the purpose and functions of major piston construction features

1. Piston Construction

Piston Construction A piston can be made using either a casting or a forging process. In a casting process, molten metal is injected into a mold from the bottom up, minimizing the likelihood of trapped air pockets forming in the casting. Various components can be fixed in the piston during the casting process, including reinforcing steel struts in the pin boss area or hardened nickel alloy inserts for ring grooves. When cooled, the piston features are machined to the correct dimensions and to accept rings and wrist pins. Cast pistons have a limited ability to withstand a sudden rapid rise in cylinder pressure that shock loads the piston in a diesel engine. This explains why most diesel pistons are more commonly made by an alternative process called forging . As the name suggests, forging involves heating and hammering the piston into a final shape. When forged pistons are made from aluminum, aluminum alloy billets are stamped into shape by forging dies. In the forging process, the billet is heated to a plastic-like state, placed into a mold, and then hammered or squeezed under high pressure into a finished shape Figure 16. The forging process realigns the grain structure of the metal and compacts the metal structure to form a denser, heavier piston. Forged pistons are able to withstand higher cylinder pressures and temperatures than cast pistons that have a more crystalline metal grain structure; however, the denser structure of a forged piston will not conduct heat away as fast as a cast piston; a forged piston will run 20% hotter. Forged aluminum pistons also have a greater expansion coefficient than cast pistons and therefore require larger running clearances in cold engines.

FIGURE 07-16: Forged pistons have parting lines where the halves of the stamping dies are separated.

FIGURE 07-17: The finely machined lines on this piston skirt are used to retain oil for lubrication.

A few pistons may have the crown and skirt manufactured in two separate processes. The skirt may be forged and the crown cast, but both will be bonded together by welding or by simply casting the crown onto the forged skirt. After shaping through forging, the skirts of aluminum pistons are anodized to resist wear. Anodizing is a process used to harden aluminum by electrochemically reacting oxygen with aluminum to form an aluminum oxide outer layer. To get an idea of how tough anodized aluminum skirts are, remember that aluminum oxide is used to make grinding wheels and sandpaper grit. A micro-finish etching, or machining of fine lines, is commonly machined into the piston skirt to retain oil Figure 17. Oil trapped on the skirt is used to help lubricate the piston as it glides over the cylinder wall.

1.1. Piston Construction Materials

Piston Construction Materials

Due to the high speed of a reciprocating engine and the tremendous inertial forces of the piston at the end of each stroke, easily machined aluminum alloys are traditionally the material of choice for pistons. The lower inertial force of aluminum alloy pistons translates into less stress on other engine components, including connecting rods and crankshafts, which extend engine life. Engine vibration and harshness are also minimized with the use of aluminum alloy pistons. In addition to being one-third lighter than steel or iron, aluminum alloy conducts heat approximately four times faster than iron. This means more heat will be transferred from the piston to the cylinder wall and engine oil. While aluminum pistons appear to have all the important advantages, alloyed steel pistons are now the most common piston material used in medium-duty (MD) and heavy-duty (HD) diesels. These pistons can withstand higher cylinder temperatures and pressures, and steel pistons can support features that resist the formation of noxious and GHG emissions. Before examining the reasons for the widespread adoption of steel alloy pistons, it is important to first understand a few disadvantages of aluminum pistons.

Disadvantages of Aluminum Pistons
Advances in material and manufacturing technology have enabled the use of more rugged alloyed steel pistons, which were once unthinkable to use. Alloyed steel pistons have not completely displaced the use of aluminum but are more widely used now due to several disadvantages of aluminum. These disadvantages include:

• Low melting point: Aluminum has a melting temperature of approximately 1250° F (677° C), but the flame temperature of diesel fuel is much higher at 3900° F (2149° C). High compression pressures and combustion temperatures would easily melt aluminum pistons if fuel was burned at stoichiometric ratio and did not have excess combustion air Figure 18. To overcome this problem, aluminum is alloyed with approximately 10% of other materials, including silicon, copper, zinc, or chromium. Silicon, which can be found in quartz or beach sand, gives the alloy higher temperature resistance and minimizes thermal expansion. A hypereutectic piston, used by almost all diesel engines with aluminum pistons, has high silicon content (approximately 16–20% silicon). Other silicon pistons are 8–11% silicon.
• Poor wear characteristics: Because aluminum is soft, it does not stand up well to wear. This problem can be partially overcome by alloying aluminum. One of the areas on a piston that encounters the greatest wear is the top ring groove. The high combustion temperatures, combined with ring movement in the top ring groove, can cause rapid wear to this area. To prevent this, a band of special stainless steel–nickel alloy is cast into the top ring groove or even the top two ring grooves. The band is machined with a groove to accept the piston ring. This band is commonly known as a Ni-Resist insert. A Ni-Resist insert further reinforces the top ring piston land, which will prevent the top of the weaker aluminum of the ring land from fracturing and breaking away Figure 19. Installing this nickel insert has the disadvantage of adding complexity and cost to the manufacturing process.
• High expansion coefficient: Aluminum, when heated, will expand four to seven times more than ferrous metal. If this expansion is not controlled, the piston will expand and score the cylinder and piston or even seize in the cylinder. Using excessive clearances is not a good alternative because this will result in accelerated piston wear and piston slap, a noise most often heard when the engine is cold. This noise is the result of large operating clearances between a piston and a cylinder wall. Collapsed skirts, poor sealing, excessive cylinder and piston wear, and poor oil control are other undesirable results of running excessive clearances.

FIGURE 07-18: This aluminum piston crown melted due to the failure of a common rail injector. After the crown overheated, cylinder pressure pushed a hole through the piston.

FIGURE 07-19: A. A nickel insert is located within the compression ring grooves of this trunk piston. B. The steel crown of this articulating piston does not need ring groove inserts.

1.2. Aluminum Pistons and Emissions

Aluminum Pistons and Emissions

Aluminum trunk-type pistons were satisfactory for many years and continue to be used today in many engines. However, other disadvantages of aluminum pistons have come to light as manufacturers strive to reduce GHG and noxious engine emissions while increasing engine longevity and horsepower. Because aluminum pistons cannot withstand the high cylinder pressures and temperatures produced by smaller, power-dense engines with increased torque and horsepower, new materials have been sought as replacements. Ceramic-coated pistons and ceramic alloy crowns are among the replacement materials. These pistons not only withstand greater thermal loading but also help reduce heat loss from the combustion chamber. Reducing heat transfer to the coolant or oil means more fuel energy is converted into mechanical energy.

1.3. Mono Steel Pistons

Mono Steel Pistons

A majority of newer diesel engines now commonly use mono steel pistons. Brand names for these from Mahle are MONOTHERM® piston, also known as Ecotherm pistons , and are the newest innovation in diesel piston design Figure 20. The most noticeable feature of the first HD engines using these pistons is the one-piece alloy steel design, which uses less material between the piston crown and the piston pin Figure 21. Shortening the piston achieves nearly a 50% reduction in the distance between the centerline of the pin bore and the top of the piston—a dimension referred to as the compression height. Thinner skirt walls, a large cooling oil passageway, and the lower compression height help make these pistons comparable in weight to aluminum pistons used in similar applications. The smaller compression height has the potential to enable the use of shorter connecting rods and reduced block height to lower engine size and weight.

FIGURE 07-20: Ecotherm pistons from Mahle are single-piece alloyed steel pistons used in MD diesel engines.

FIGURE 07-21: A MONOTHERM ® piston by Mahle is a one-piece alloyed HD steel piston design.

Mono steel pistons are also a type of LCV piston . Mono steel piston design offers the following advantages:

1. Alloyed steel, although heavier than aluminum, is stronger than aluminum. The strength of the alloyed steel allows less material to be used in the construction of the piston, resulting in an alloyed steel piston comparable in weight to an aluminum piston. The fact that mono steel pistons are short also makes them lighter.
2. Steel expands at a lesser rate than aluminum, so operating clearances are much smaller for mono steel pistons. This reduces piston noise at cold-start and low-load conditions when operating clearances are greatest. Tighter operating clearances also reduce blowby and oil consumption.
3. One-piece steel design increases load-carrying capacity. This means higher peak combustion pressures are sustainable. The current maximum cylinder pressure for mono steel pistons is 3770 psi (260 bar). This advantage alone increases power density from an engine, which means more power can be produced from each cubic inch (or cubic cm) of engine displacement.
4. Higher load-bearing capabilities enable the use of larger cylinder bores for higher displacement volume and power output within the same engine block. Originally, these pistons were used in engines with approximately 15 L (915 in 3 ) displacement. However, increasing numbers of small- and mid-bore–size engines are now using these one-piece steel pistons Figure 22.
5. Increased power output and lower fuel consumption. Fuel consumption in test conditions is as much as 2% lower. Reduction of the size of the piston skirt and the corresponding reduction in friction, plus the tighter operating clearances of steel, provides 1% higher power output and a 1% reduction in fuel consumption. This improvement also allows turbocharger speed to decrease by 1000 revolutions per minute (rpm) in some engines, which leads to improved turbocharger durability.
6. Reduced engine friction. A smaller skirt area results in reduced cylinder wear. Much of the improvement in efficiency is achieved through a reduction in friction and tighter operating clearances.
7. The strength of the alloyed steel reduces tear-apart forces during catastrophic failure; this leads to less engine destruction.
8. A one-piece design makes assembly and service simpler. No piston pin bushing is required by the steel construction. Instead, the wrist pin bore is coated with phosphate for reduced friction. A black phosphate coating over the piston crown also reduces the possibility of corrosion due to exhaust gas recirculation (EGR).

FIGURE 07-22: The use of mono steel pistons can reduce a piston’s compression height.