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

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.