LO 9-2 Describe the functions, construction, composition, types, and applications of diesel engine cylinder head assemblies
| Site: | CDX Learning Systems |
| Course: | DE Section 2-1: Engine Blocks and Cylinder Heads |
| Book: | LO 9-2 Describe the functions, construction, composition, types, and applications of diesel engine cylinder head assemblies |
| Printed by: | Guest user |
| Date: | Thursday, 5 November 2026, 6:03 PM |
Table of contents
- 1. Classification of Cylinder Heads and Valve Trains
- 1.1. Classification of Cylinder Heads by Materials
- 1.2. Aluminum Versus Cast Iron
- 1.3. Particle Hardening of Aluminum
- 1.4. Compacted Graphite Iron
- 1.5. Classification by Intake and Exhaust Port Arrangement
- 1.6. Reverse Flow Cylinder Heads
- 1.7. Variable Swirl Intake Ports
- 1.8. Classification by Number of Cylinder Heads
- 1.9. Classification by Valve Train Arrangement
- 1.10. Video: Valve Train
1. Classification of Cylinder Heads and Valve Trains
LO 9-02: Describe the functions, construction, composition, types, and applications of diesel engine cylinder head assemblies. |
Classification of Cylinder Heads and Valve Trains
Cylinder heads are classified in several ways, including by the number of cylinders capped by the head, the type of material used, the way exhaust and intake air ports are arranged, and the valve train arrangement. The variety of cylinder head designs is driven by the function of a cylinder head that requires it to operate while withstanding high combustion temperatures and pressures without distortion. It is essential that the head is constructed with a high degree of strength and temperature resistance. Cooling of the head is also critical to keep oxides of nitrogen (NOx) emissions low, and the addition of temperature-sensitive components, such as injectors and electronic sensors, and other temperature regulation strategies has made the cooling of cylinder heads particularly important in the latest engines. Air and exhaust gas flow through the cylinder head is another design concern that must be considered in order to achieve the best engine efficiency with high levels of power and performance without generating excessive emissions. The shape and position of the intake and exhaust ports, the position of the valves, and the location of the camshaft determining the overall geometry of the cylinder head have significant influence on power output, fuel consumption, and emissions. In addition to these considerations, manufacturers must be able to construct cylinder heads that meet EURO IV–VI and the U.S. EPA’s durability requirements, require the least amount of maintenance, are economical to produce, and promote outstanding performance accompanied by low emissions and fuel consumption. To meet the constantly improving benchmarks of engine performance, a wide variety of cylinder head designs, construction techniques, and service procedures are currently in use.
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1.1. Classification of Cylinder Heads by Materials
LO 9-02: Describe the functions, construction, composition, types, and applications of diesel engine cylinder head assemblies. |
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Classification of Cylinder Heads by Materials Material selection for the construction of a cylinder head is dependent on a wide range of factors. Potentially corrosive coolants, flow of viscous and potentially abrasive oils, high horsepower engine applications in which cylinder temperatures and pressures are very high, and the demand for lightweight engine designs are just a few of the factors involved in the selection of cylinder head materials. While traditional materials such as cast iron have provided good strength and wear characteristics for valve train mechanisms, new materials and construction techniques are enabling improved engine performance Figure 4. Lighter-weight designs now have greater durability and lower maintenance requirements while improving fuel economy. |
FIGURE 09-4: This Detroit DD15 uses a weight-saving, aluminum camshaft frame to support the dual overhead camshafts, which are also hollow. |
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1.2. Aluminum Versus Cast Iron
LO 9-02: Describe the functions, construction, composition, types, and applications of diesel engine cylinder head assemblies. |
Aluminum Versus Cast Iron
Cylinder heads are commonly made of cast iron alloy because of its superior strength, ease of manufacture, machinability, and capability to withstand high temperatures. Aluminum is another material now gaining widespread use as a material of choice. At one time, aluminum was not considered for several reasons:
- Tendency to warp and crack, especially when overheated
- Difficulty in tolerating dirty oil, which can quickly wear away softer aluminum friction surfaces
- Extra manufacturing steps required to install cast iron or steel alloy valve guides, valve seats, and bushings for overhead camshaft (if used)
- Greater electrochemical corrosion in engines containing high aluminum content, making aluminum unable to tolerate old, contaminated coolant in the same way as cast iron
- More costly to cast than iron
The higher expansion rate of aluminum at operating temperature compared to iron is another problem. The expansion coefficient of aluminum, which is four to seven times greater than that of cast iron, demands improvements in the selection and replacement of head gasket materials and greater precision in cylinder head fastener tightening procedures. Valve lash can be similarly affected by expansion. Cylinder head bolts must stretch too and require nonreusable torque-to-yield bolts.
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The primary advantage for aluminum, however, is that it is lightweight. Unnecessary engine weight is the enemy of good braking, handling, and quick acceleration, and it displaces the vehicle’s capacity to carry load Figure 5. Because aluminum is 50% lighter than cast iron, aluminum cylinder heads can easily save more than 60–100 lb (27–45 kilogram [kg]) in some larger diesel engines. Aluminum also lends itself well to the manufacture of head casting with larger intake and exhaust ports to create more powerful engines. Another favourable attribute of aluminum is its higher heat conductivity, which improves heat transfer from the combustion chamber to the coolant. This is especially important for emissions. Valve temperatures can be as much as 200° F (93° C) cooler in aluminum cylinder heads. This not only lowers the variations in valve lash clearances but also enhances durability of the valve seats and increases reliability of the engine as a whole. Aluminum carries away excess heat more rapidly, and localized hot spots are eliminated with aluminum cylinder heads, thus higher combustion pressures and temperatures can be sustained. Similarly, because high combustion chamber temperatures cause increases to NOx production, this emission can be reduced with the use of aluminum heads. |
FIGURE 09-5: The cylinder heads on this 6.6 L General Motors Duramax engine are made from hardened aluminum. |
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1.3. Particle Hardening of Aluminum
LO 9-02: Describe the functions, construction, composition, types, and applications of diesel engine cylinder head assemblies. |
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Particle Hardening of Aluminum |
FIGURE 09-6: The popular Duramax diesel uses aluminum cylinder heads. |
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1.4. Compacted Graphite Iron
LO 9-02: Describe the functions, construction, composition, types, and applications of diesel engine cylinder head assemblies. |
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Compacted Graphite Iron A material from which many new diesel cylinder blocks and heads are constructed is compacted graphite iron (CGI). CGI, also called sintered graphite , is a material produced from powdered iron alloys squeezed into molds at high pressures and then heated to bond the metal particles together. When made from CGI, diesel cylinder heads have approximately the same weight as aluminum but also possess higher strength and rigidity than steel and have sound-damping characteristics. Paccar’s MX-13 diesel engine is an example of an HD diesel using a CGI cylinder block and cylinder head. Paccar reports an increase of 75% in both strength and rigidity, a 200% increase in thermal fatigue resistance, and 150 lb (68 kg) of weight reduction compared to conventional cast iron Figure 7. This means when an engine overheats due to a loss of coolant, cast iron cylinder heads that would normally crack are not as likely to if the head is constructed from CGI material. Ford and first-generation Dodge Ram 3.0 L diesels also use CGI materials in diesel engine blocks and cylinder heads. |
FIGURE 09-7: Paccar’s MX-13 diesel engine uses a CGI cylinder block and cylinder head. |
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1.5. Classification by Intake and Exhaust Port Arrangement
LO 9-02: Describe the functions, construction, composition, types, and applications of diesel engine cylinder head assemblies. |
Classification by Intake and Exhaust Port Arrangement
Because intake and exhaust port arrangements strongly influence gas flow and turbulence, or swirl, into the cylinders, configuration of these ports affects engine performance and emissions. The size and number of valve ports and where they are located primarily determines this classification method. For example, when intake and exhaust ports are arranged on either side of the cylinder head, the construction method is called a cross flow cylinder head. Cross flow cylinder head design is the most popular for diesel engines Figure 8. In this configuration, engine breathing characteristics are improved when airflow travels into the intake valve ports and out the exhaust valve ports located on opposite sides of an in-line engine cylinder head. Larger intake and exhaust ports can be used because there is no crowding from a nearby exhaust or intake port. Additionally, this port arrangement avoids the unnecessary heating of intake air as it passes over heated exhaust surfaces. This design provides cooler air for efficient combustion and helps allow for improved fuel economy, maximum efficiency, and reduced emissions. The drawback to this configuration is that exhaust and intake manifolds on opposite sides of the cylinder head create a wider temperature gradient across the head, with the exhaust side having much higher temperatures than the intake side. This makes the head vulnerable to potential cracking, distortion, and head gasket leaks caused by excessive and unequal movement of the cylinder head across the block deck.
A parallel flow head, sometimes referred to as the uniflow design , places the intake and exhaust manifolds on the same side of the engine Figure 9. In this design, joining together shorter, large cylinder head ports provides a more compact engine design with adequate airflow to the cylinders. Less thermal stress of the cylinder head takes place because the cooler intake ports are placed alongside the exhaust ports.
FIGURE 09-8: A cross flow cylinder head arranges the intake and exhaust ports on opposite sides of the cylinder head. |
FIGURE 09-9: A parallel flow head arranges the intake and exhaust ports on the same side of the engine. |
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1.6. Reverse Flow Cylinder Heads
LO 9-02: Describe the functions, construction, composition, types, and applications of diesel engine cylinder head assemblies. |
Reverse Flow Cylinder Heads
A newer engine design for V-type engines is called the reverse flow cylinder head that uses a cross flow valve port design. On a V-block engine using reverse flow heads, the exhaust ports are located inside the engine “Vee” rather than in their traditional location on the outside of the cylinder head. Instead, intake ports—not exhaust ports—are located on the outside surface of the head. This port arrangement shortens the connection distance between the exhaust ports and the turbocharger, or an exhaust aftertreatment system catalyst. The result is a quicker, more powerful turbocharger response. If the exhaust catalyst is closely coupled to the engine and located between the cylinder banks, it will heat up more quickly to enable faster breakdown of noxious emissions. Without outboard exhaust manifolds, engines using reverse flow heads can occupy a smaller footprint beneath the hood of a vehicle where space is limited Figure 10. In fact, when these heads are used in SUVs, pickups, and vans, they take up less under-hood space than the spark ignition engine they replace Figure 11.
FIGURE 09-10: The Ford 6.7 Power Stroke uses reverse flow cylinder heads. |
FIGURE 09-11: This Ford 6.7 L Power Stroke uses CGI cylinder heads and a reverse flow cylinder head manifold arrangement. |
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1.7. Variable Swirl Intake Ports
LO 9-02: Describe the functions, construction, composition, types, and applications of diesel engine cylinder head assemblies. |
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Variable Swirl Intake Ports Using either one or two intake valves and a port for each valve has the advantage of potentially increasing the air turbulence entering the cylinder. The shapes of the intake ports of most modern diesels are tangentially arranged with the cylinder head centerline. This offset of the intake valves where one is slightly behind the other, or placement of intake and exhaust valves in a diagonal pattern, is designed to impart a twisting or spinning motion to the air as it enters the cylinder. The purpose of twisting intake airflow through bends and curves in the port is to increase swirl-type air turbulence. This setup, called tangentially arranged intake valve ports , also uses uneven lengths for the intake ports (or intake runners), which further helps to spin the air entering the cylinders Figure 12. |
FIGURE 09-12: A tangential or star-shaped arrangement of the intake valves turns the valve placement 45 degrees to the centerline of the cylinder head. The placement helps impart more turbulence to intake air entering the cylinders because it enters at an angle. |
At low-speed operation, increasing intake air swirl enhances mixing of air and fuel to produce cleaner, less sooty combustion. At high-speed operation, airflow is maximized when the air can take a straighter and more direct pathway into the cylinder. To further improve air turbulence characteristics to match engine operating conditions, variable intake swirl valves, or air motion systems, are installed in the intake ports of most light- and medium-duty (MD) diesel engines. These devices operate to vary the amount of cylinder air swirl during low- and high-speed engine operation. On engines equipped with variable intake swirl systems, one of the two intake air ports have a flap- or butterfly-type throttle plate valve installed in the port opening Figure 13. At low speeds, the flap is closed or partially closed to allow air into a cylinder only through a port having a sharper bend or curve. The port shape will bend or impart a twisting airflow to most of the air entering the cylinder to increase air swirl. At higher speeds, the plate is deactivated, and airflow can enter through both intake ports and valves for maximum airflow.
| Technicians Tip | ||
| Variable intake swirl valve faults are a common occurrence on engines operating in stop-and-go urban driving conditions. In these driving conditions, excess soot can accumulate from high exhaust gas recirculation (EGR) flow through the intake system. Actuator motors operating the swirl valves eventually require too much torque to rotate the valves and can burn out Figure 14. |
FIGURE 09-13: A throttle plate in one of the intake runners will vary between open and closed to change the degree of air turbulence entering the cylinder. |
FIGURE 09-14: The intake runners and swirl valves of this V6 engine are fouled with coolant from an EGR cooler leak that mixed with the normal soot loading in the intake system left by EGR gas. |
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1.8. Classification by Number of Cylinder Heads
LO 9-02: Describe the functions, construction, composition, types, and applications of diesel engine cylinder head assemblies. |
Classification by Number of Cylinder Heads
The number of cylinder heads used by a diesel engine is influenced by a variety of factors. Using a single-piece casting enhances strength and rigidity of the engine, but this type of cylinder head does not have the flexibility to adapt to multiple engine platforms. Modular engine construction where fewer components can be assembled into multiple engine configurations of 2, 3, 4, or 6 cylinders, V engines, or in-line engines favour the use of individual cylinder head castings capable of being interchangeable for each cylinder. Engine application, gas flow, serviceability, and the location of the camshaft are other considerations that need to be factored in when selecting the type of cylinder head casting.
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Single-Piece Cylinder Heads
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FIGURE 09-15: A steel spacer plate between the block deck and the cylinder head on this Caterpillar C15 engine minimizes cylinder head clamping forces applied to the liner flanges. |
Multiple-Piece Cylinder Heads
Multiple-piece cylinder heads use a separate cylinder head for one, or a group of two and three, cylinders in a multiple-cylinder engine Figure 16. These cylinder heads are ideal for modular engine designs where part castings shared between different engine models can be used to construct various engine configurations. This allows the same head to be used on a variety of cylinder arrangements. For example, a single cylinder head can be used to build single-, 2-, 4-, 6-, or 8-cylinder engines Figure 17. Servicing is simplified if only one or a few cylinders of an engine are being repaired. However, multiple-cylinder heads tend to move around more, wearing the top of liners or block decks. The result is more frequent head gasket leaks and damage to the block deck that may require engine removal to resurface the block at a machining facility. They cannot provide much structural rigidity to the engine either.
FIGURE 09-16: One cylinder head per cylinder is used on this V-12 locomotive diesel using a modular engine design. Multiple-piece cylinder heads may cover one, two, or even three separate cylinders. |
FIGURE 09-17: This in-line six-cylinder engine uses a single head casting for two cylinders. Note the dowel pins to position the cylinder head on the block deck. |
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1.9. Classification by Valve Train Arrangement
LO 9-02: Describe the functions, construction, composition, types, and applications of diesel engine cylinder head assemblies. |
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Classification by Valve Train Arrangement Cylinder heads are further classified by the arrangement of the valve train mechanism. For example, the camshafts in diesel engines are commonly one of two types—either an overhead camshaft or in-block camshaft Figure 18. Overhead camshaft engines have one or two camshafts located in the cylinder head. The most important reason for the use of overhead camshafts in early engines, built in the late 1980s, was to eliminate injector pushrods. Pushrods can bend during high- pressure injection events. These overhead camshaft engines use unit injectors, which are often actuated directly by a camshaft that enables the highest injection pressures. Without the natural bending and flexing of pushrods, overhead camshafts increase the maximum pressure of the injection event. Valve timing is also more accurate and easier to vary in an overhead camshaft configuration Figure 19. In-block camshafts have only the valves, rocker levers, and bridges located in the cylinder heads above the piston, and the camshaft is located in the engine block Figure 20. This camshaft arrangement is also simply referred to as a pushrod engine because the camshaft actuates the valves through a pushrod and rocker lever Figure 21. |
FIGURE 09-18: A low-mounted camshaft in this block is supported by camshaft bushings. |
FIGURE 09-19: This engine uses a single overhead camshaft and unit injectors. |
FIGURE 09-20: This small two-valve/cylinder industrial diesel engine uses pushrods and an in-block camshaft. |
FIGURE 09-21: Compare A. an overhead camshaft design with B. the in-block camshaft valve configuration. |
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1.10. Video: Valve Train
LO 9-2 Video: Valve Train |
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