LO 8-3 Identify and describe the construction techniques and composition of cylinder blocks and outline the advantages and disadvantages of each design

1. Cylinder Block Materials

Cylinder Block Materials

The requirement to produce durable, reliable, and lightweight engine blocks is driven partly by GHG regulations. Although a lightweight block enables a vehicle to carry more load, reduced fuel consumption is possible from lighter and stronger engines. Smaller displacement engines can also have smaller internal cylinder wall areas, reducing overall engine friction from rings and pistons. Other friction-generating surfaces, including camshafts and crankshafts, are smaller as well. Many compact block designs are made possible by using specialized block materials. Most cylinder blocks are manufactured from cast iron, but aluminum is popular in light-duty diesels such as General Motors (GM) and Mercedes 3.0 L diesels. Compacted graphite iron (CGI) blocks are also a popular block material in light-duty (LD), MD, and HD applications, not only for their strength and durable wear surfaces but also for their superior noise reduction properties.

1.1. Cast Iron

Cast Iron

Cast iron is metal made from iron ore with a carbon content of 3–5%. The carbon is scattered throughout the metal in flakes, creating a material with a crystalline-like structure. The metal is poured or cast into molds, commonly made from sand or fiberglass. The advantages of cast iron are that it is inexpensive to produce, is easy to manufacture and machine, it absorbs vibration well, is inherently strong, and is corrosion and heat resistant. The high carbon-graphite content of iron makes it a compatible, or low-friction, wear-resistant surface for moving parts, such as pistons and piston rings. To improve strength, hardness, and corrosion resistance, manufacturers may add nickel, molybdenum, and chromium alloys to the casting. Cast iron is still not as strong as steel, and its crystalline structure allows it to fracture easily rather than bend. This means machined threads in the block for cap screws usually have a coarse thread pitch, which have deeper thread engagement to minimize the problem of pulled or stripped threads. Main bearing caps are often made from harder cast alloys in high horsepower applications. These caps are identified by a different color or appearance of metal from the block casting.

Iron Casting and Block Seasoning
Sand casting cylinder blocks requires the removal of sand from inner passageways. Core plugs, sometimes called frost plugs (because they are pushed out by frozen coolant), are used to cover holes made in the block to flush sand out of narrow passageways during casting (Figure 13 and Figure 14). After rough casting, blocks tend to warp when rapidly and unevenly cooled. To prevent this, diesel blocks are often stored at high temperatures for a day or two to heat-treat and “season” them. Heating the blocks while slowly and evenly cooling them not only hardens them but also makes them resistant to stress and minimizes future distortion. Subsequent to seasoning, castings are bored, tapped, and machined to finish. Induction hardening may also be performed to increase the hardness and durability of cast iron cylinder blocks. Induction hardening is a heat treatment process that involves passing alternating electrical current through coils of heavy-gauge wire surrounding the material to be hardened. Through magnetic induction, heat is produced in the metal, which is then quenched with water to produce a hard, wear-resistant, metal surface Figure 15.

FIGURE 08-13: Core plugs are installed in a block only after casting sand, used to form the block’s internal passageways, is cleaned out of the block through vibration and pressure washing.

FIGURE 08-14: Two types of core plugs are domed and dished.

FIGURE 08-15: The arrow shows an area of discoloration where the upper third of this cylinder has been induction hardened. Induction hardening increases engine life by minimizing cylinder wall wear in the ring turnaround area where there is the least amount of cylinder lubrication.

Disadvantages of Cast Iron
Cast iron is heavy, and engines made from cast iron add considerable weight to the vehicle in comparison to lighter materials, such as aluminum or CGI. This can negatively affect vehicle handling and braking characteristics. Heavy engines need larger springs and heavier suspension components, and they tend to dive more in the front end when braking. Cast iron does not transfer heat as readily as aluminum and cannot be as easily repaired as steel or other metals. Whenever repairs to damaged blocks are made, welding techniques involve preheating the block and using a welding rod with high nickel content or a powdered iron welding rod. An original equipment manufacturer (OEM) repair recommendation for block porosity, where oil cools and seeps from a block wall, is to apply epoxy sealant over the affected area. This repair technique can only be performed in areas where mechanical stress is not significant.

1.2. Aluminum

Aluminum

Aluminum is being used more commonly as a block material in high-performance diesels because it improves performance and also saves on weight. When used to make small, six- cylinder diesel engines, aluminum can eliminate more than 77 lb (35 kilogram [kg]) of weight compared to a similar engine block made from cast iron.

Aluminum, however, does have its disadvantages. Until the 1990s, aluminum engine blocks were used only in gasoline-fueled engines because high-performance diesel engines were required to withstand high temperatures and at least three times more cylinder pressure than spark ignition engines. Softer aluminum also has challenges with rigidity, strength, and reliable bolting of the cylinder head and main bearings. Although it transmits heat well, aluminum has an expansion coefficient of about four to seven times the rate of iron. This causes enlargement of critical dimensions, such as the crankshaft bearing bores and cylinder bore diameters. Most importantly, soft aluminum does not tolerate friction between pistons and the cylinder wall and rapidly wears.

Because of aluminium’s significant weight-saving advantage, manufacturing techniques have been developed to overcome these difficulties. One technique is to cast iron alloy cylinder sleeves in place when the aluminum block is cast Figure 16. The iron sleeves are generally not replaceable, but they do not contribute to extra engine weight. This aluminum block configuration is preferable in automobiles and light trucks where engine mass, during cornering and braking, causes undesirable driving characteristics. Cornering and braking dynamics are improved, and lighter suspensions can be used. One construction method for aluminum cylinder blocks can use a warp anchor principle to prevent distortion of the block from cylinder head bolts. This means the cylinder head and the cylinder block are bolted together by tie bolts. Sliding steel sleeves that are locked in the block accept the cylinder head bolt from one side and the tie bolt from the other Figure 17. A similar technique uses tension bolts. Tension bolts use a single bolt to clamp both the cylinder head and the main bearing cap together Figure 18. The most common construction method to minimize cylinder wear are cylinder walls and bearing bores that are made by placing iron liners into the aluminum block structure to provide durable, low-friction wear surfaces.

FIGURE 08-16: This aluminum block uses a cast-in-place, iron alloy sleeve to resist cylinder wear. Some Mercedes Sprinter vans and GM 3.0 L diesels use this block construction technique.

FIGURE 08-17: Warp anchor principle. The cylinder head and the cylinder block are bolted together by tie bolts. Sliding steel sleeves locked in the block accept the cylinder head bolt from one side and the tie bolt from the other, preventing distortion of the block.

FIGURE 08-18: Tension bolt blocks anchor the cylinder head bolts through the main bearing cap or the main bearing girdle on ladder block designs.

Nanoslide Cylinder Wall Coating
Another technique to harden the soft cylinder walls of aluminum blocks is to apply another, harder material to cylinder wall surfaces. Spray welding, also called plasma welding , applies an alloyed iron coating over the cylinder wall. Coatings of tool-grade hardness iron carbon alloys can be achieved by a plasma-spraying technique called Nanoslide technology Figure 19. Using a rotating plasma torch and metal alloy powder, parent bore block walls are sprayed with a coating, a few tenths of a millimeter thick, to give the cylinder walls great wear resistance and durability Figure 20.

FIGURE 08-19: Nanoslide technology uses a plasma-spray welding technique to deposit a very hard, crystalline-like finish onto the aluminum block cylinder wall. Friction is reduced up to 50% compared to cast iron sleeved engines.

FIGURE 08-20: Adding a mirrorlike coating of extremely hard iron alloy is the latest friction-reducing strategy used by today’s engines.

Mercedes-Benz developed Nanoslide technology to coat the cylinder walls of the 3.0 L (183 in 3 ) block with a tool-grade hardness iron alloy. The technique first melts wires of iron–carbon alloy in an electric arc. Pressurized gas then deposits the melted material onto the cylinder walls to only 0.004–0.006" (0.102–0.152 mm) thickness. This ultrafine, nanocrystalline coating is then finished to a smooth, mirrorlike surface using a special laser-honing process. Despite the extreme smoothness, a laser beamed over the cylinder wall melts material and opens pores in the wall surface that can retain oil for lubricating the piston and cylinder wall. The result is a low-friction surface that reduces mechanical losses due to friction by up to 50% in comparison to cast iron cylinder liners. In addition, extremely high wear resistance is imparted to the cylinder walls. Other advantages include lower engine weight, reduced fuel consumption, and lower GHG emissions.

1.3. Open Versus Closed Block Decks

Open Versus Closed Block Decks

Aluminum engine blocks are commonly made using a die cast process. This involves squeezing molten aluminum between two steel halves of a stamping die to produce a finished part. The advantages include improved block dimensional accuracy, smoother sealing surfaces, the ability to use thinner block walls, and eliminating secondary machining in comparison to cast iron or aluminum blocks. Die casting is a faster production method and less expensive than casting blocks using sand or fiberglass molds. Sleeves and steel inserts can also be cast in place. Aluminum used for the casting has a “mushy” consistency when it is injected into dies and pressurized before the dies are separated. Using this method, diesel engine cylinders can be separated with aluminum that is as thin as 0.315" (8 mm) to form a very compact engine that is 30% lighter than cast iron. These blocks are specially heat-treated and particle hardened, followed by a plasma-sprayed coating of hard iron alloy applied to the cylinder walls. Cast iron main bearing caps can be used in these engines. In contrast to cast iron blocks, the use of stamping dies requires die cast blocks to use what is known as an open-deck design Figure 21. Open decks are easily identified because the block deck (the top surface of the block in contact with the cylinder head) has no connecting metal between the block walls and the upper portion of the cylinder wall. Although coolant can better circulate in open-deck blocks, the design has the disadvantage of an absence of reinforcement in the upper cylinder walls. Without a metal bridge between the cylinder walls and the block deck, the weakly supported cylinder walls are capable of moving during service. Closed decks connect the block deck with the cylinder walls to form a stronger, more rigid cylinder structure Figure 22.

FIGURE 08-21: This die cast block for an off-road diesel application uses an open-block deck. There is no support at the top of the cylinder that bridges the cylinder walls with the rest of the block.

FIGURE 08-22: Closed blocks connect the top of the cylinder wall with the block deck by using a metal bridge. This bridge closes the gap between the cylinder and the rest of the block.

1.4. Compacted Graphite Iron

Compacted Graphite Iron Using CGI (or sintered graphite) for a block material is the latest engine block construction technique. The construction method and material provide blocks with greater strength, rigidity, and durability at a reduced weight and brittleness compared to cast iron. Theoretically, a CGI block can be constructed with the strength of steel but with almost the weight of aluminum. The stronger, steel-like-strength material enables the use of thinner block and cylinder walls without sacrificing stability or strength. CGI construction starts with powdered iron and graphite alloys squeezed into molds at high pressures that are then heated to bond the metal particles together. This construction technique yields blocks with at least 80% higher tensile strength, 45% greater stiffness, and approximately double the fatigue strength of conventional gray cast iron and aluminum. The weight of an engine block made from CGI alone can be reduced by 22% through a 15% reduction in the thickness of the block walls. Noise transmission through the block is dramatically reduced as well because of the vermicular (wormlike) grain structure of the material, which has sound-absorbing properties. By reducing the block size and weight while increasing strength, CGI materials allow engine designers to improve power density, performance, fuel economy, and durability while reducing engine weight and noise. Many LD, MD, and HD diesels are now made from CGI, including blocks from Audi, Cummins, International MaxxForce 7.0 L (427 in 3 ), 13.0 L (793 in3), and the latest Navi star A25 12.4 L (757 in3). Other models include John Deere 9.0 L (549 in3), Paccar MX-13 (in which both block and cylinder head are made from CGI), VM Motori (Fiat Chrysler Automobiles [FCA] RAM 3.0 L [183 in 3 ] EcoDiesel), and Ford’s F-150 3.0 L (183 in3 ; introduced in 2018) and 6.7 L (409 in3) Power Stroke (introduced in 2010). Ford claims that the 6.7 L (409 in3) Power Stroke is made from patented SinterCast CGI material that is 40% more rigid and 100% more fatigue resistant than gray cast iron Figure 23. CGI has allowed Ford designers to reduce the engine block wall thickness by 15% in comparison to the thickness that would be needed if it used gray cast iron. A weight reduction of 160 lb (70 kg) is achieved in comparison to the earlier cast iron 6.4 L (391 in3) engine. CGI engine blocks have their own unique service challenges in terms of machining. CGI is steel-like in hardness but also abrasive and “gummy” to cutting tools when machined. Special machine tools with extraordinary hardness are needed to cut the material. Honing cylinders of CGI blocks is much more difficult due to the harder material; a coarse-grit honing stone will produce a smoother finish than the same stone will produce in a cast iron block. CGI blocks cannot be repaired when damaged, so they must be replaced.

FIGURE 08-23: A popular example of a CGI engine block is the Scorpion 6.7 L (409 in3) Power Stroke. CGI block material provides for a quieter, stronger, and lighter design.

1.5. Video: Grey Iron

1.6. Video: Alloys