Cylinder block material
Traditionally, the cylinder blocks of trucks and buses were made of gray cast iron, usually alloyed with at least some silicon to reduce the brittleness of the material. As mentioned earlier, most medium and large bore highway diesel engines use cylinder liners, meaning the base material of the cylinder block does not directly bear the engine's cylinder pressure and temperature. Recently, there has been a trend towards significantly increasing cylinder pressure while manufacturing lighter engines. This has led to the emergence of today's low-weight, high-strength cylinder block castings. Comparing the weight-to-power ratio of today's engines with those of the 1980s: the power produced per pound of engine weight in today's engines is often twice that of their predecessors - at least before accounting for the weight of all the emission control hardware on current engines. Nowadays, most diesel engine manufacturers (OEMs) use compacted graphite iron (CGI) in cylinder block manufacturing because it is lightweight and can withstand much higher forces.
Compacted Graphite Iron (CGI) Cylinder Blocks
Compacted Graphite Iron (CGI) cylinder blocks have been a key factor in reducing engine weight over the past decade. Contemporary high-horsepower diesel engines are often lighter than engines of the previous generation that produced only half the power. Almost all original equipment manufacturers (OEMs) use CGI cylinder blocks for their heavy-duty engines. The advantages of CGI include:
• 70% lighter than equivalent gray cast iron cylinder blocks
• Fatigue limit 200% higher than gray cast iron
• 40% more rigid than gray cast iron
Plasma Transferred Wire Arc
In the past, Caterpillar used Plasma Transferred Wire Arc (PTWA) as a method to repair cast iron cylinder liners in diesel engines, but now this technology is used to enable light and medium-duty diesel engines to use aluminum alloy cylinder blocks without cylinder liners. The PTWA process provides a highly wear-resistant and temperature-resistant surface on iron-based or aluminum alloy substrates. The process combines compressed air and electricity to generate a plasma jet with a temperature of up to 35,000°F (19,500°C), into which a wire is fed through a rotating gun. This breaks the wire into droplets about 25 microns in size, which bond at the molecular level to the cylinder liner wall. As the molten wire oxidizes, it forms a crystal structure called wüstite, and the final cladding layer is about 150 microns thick. When PTWA is used on aluminum alloy cylinder blocks, the target surface must be pre-treated. The PTWA process is often referred to as cladding. Without specialized equipment, cladded cylinder blocks cannot be repaired.
Cylinder Block Functions
Although there are some differences among manufacturers, diesel engine cylinder blocks must perform some or all of the following functions:
• Provide cylinder bores for the piston assemblies.
• Provide main bearing bores for the crankshaft.
• Provide coolant passages/water jackets.
• Provide lubricating oil passages/oil galleries.
• Provide mounting locations for other engine components.
Cylinder blocks may contain longitudinal holes for installing camshafts. Figure 9-2 shows the aforementioned holes and passages. Figure 9-3 shows the locations of expansion plugs or cup-shaped plugs in a typical six-cylinder engine block. These are usually installed with a slight interference fit and fixed using a medium-strength adhesive such as Loctite Blue.
Integral Cylinder Bore
Most automotive and small-bore diesel engines use integral or linerless cylinder blocks. They are less likely to be used in commercial diesel engines, especially large-bore ones. At one time, integral cylinder bores were used on medium-bore engines; they were derisively called one-time blocks. This was because during refurbishment, it was difficult to bore the holes to install cylinder liners due to the close proximity of the cylinder bores to each other. However, with the advent of induction hardening of iron-based cylinder blocks and specialized cladding processes such as the aforementioned PTWA, this situation has changed. Older Cat 3208, Cummins ISB and MB-900 series engines (produced until 2010) are examples of engines with cylinder block assemblies without cylinder liners. The initial cost savings achieved by manufacturing such a design of cylinder block may be offset during engine overhauls when the cylinder block needs to be replaced, re-bored and fitted with cylinder liners, or sent back to the factory for repair. The advantages of engines without cylinder liners are as follows:
• Lower initial cost
• No risk of cylinder liner O-ring failure
• No need to comply with cylinder liner protrusion specifications during reassembly
Induction-hardened cylinder bore without cylinder liner
In the late 1990s, Freightliner/Mercedes-Benz introduced an engine without cylinder liners, with the cylinder bores induction-hardened. The MB-900 series four-cylinder and six-cylinder engines featured a unique helical striation induction-hardened feature in the upper ring band scavenge area. Compared to other engines without cylinder liners, this significantly extended the service life of the cylinder bores. If the cylinder bores wore beyond tolerance, they could be re-bored and fitted with cylinder liners. Mercedes-Benz recommended that only the manufacturer perform the re-boring and cylinder liner installation on these engines. This engine was highly successful in both on-road and off-road applications, but was no longer used in on-road applications after the 2010 model year. Nevertheless, a large number of such engines are still in use ten years later.
PTWA cladding without cylinder liner
The PTWA process briefly described earlier has been used for many years as a method to repair cast iron cylinder blocks without cylinder liners, but this technology is also applicable to cladding the cylinder bores of aluminum alloy cylinder blocks. As original equipment manufacturers (OEMs) continue to strive to reduce the weight of diesel engines, aluminum alloy cylinder blocks with PTWA-clad cylinder bores have been used in light-duty diesel engines, and it may only be a matter of time before we see them in heavy-duty diesel engines.
Wet cylinder liner
The design of the cylinder block allows the water jacket to be in direct contact with the cylinder liner in the cylinder bore; therefore, the cylinder liner must have sufficient wall thickness to withstand the peak combustion pressure of the engine. Wet cylinder liners can effectively transfer heat to the coolant and are easy to replace during overhauls. Their main disadvantage is that they must remain sealed throughout their service life, and O-ring failure can lead to coolant contamination of the engine oil.
Figure 9–3 shows the position and installation method of the cup-shaped plugs in a straight-six cylinder block.
Cylinder Block Classification
Cylinder blocks used in commercial diesel engines can be classified by type as follows:
• Integral cylinder bores or no cylinder liners
• Surfacing without cylinder liners
• Wet cylinder liners
• Dry cylinder liners
• Combined wet/dry cylinder liners
Other factors, such as two-stroke cycle and air cooling, are also reflected in the design of the cylinder block.