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The cylinder head sits at the top of the engine. Its purpose is to seal the top of the cylinder to create the combustion chamber. The head also forms the housing for the valve gear and spark plugs. Inside the cylinder head are complex passageways for coolant and oil. The head, along with the components it houses, is known as the top end of the engine.
In many ways, the head is the mechanical control center of the engine. It is the meeting point for the intake, exhaust, ignition and fuel systems.
Cylinder heads are made from either cast iron or cast aluminium. Cast aluminium is more expensive to produce, but offers considerable weight-saving and dissipates heat better than cast iron. Virtually all production gasoline engines use an aluminium head, while cast iron is still commonly used for diesels due to its higher strength.
The interior of a cylinder head is a complex network of oil galleries and coolant passages, even more complex than the engine block. To form these interior cavities, heads are often cast using the lost-foam casting technique.
This method of casting uses a polystyrene model of the cylinder head, made by gluing together layers. The model is then packed with casting sand and all the voids filled. This creates a mold around the polystyrene, which is then vaporized and replaced by molten aluminium. Once the metal has solidified, the sand is removed and washed before the casting is machined.
The combustion chamber is the space where the fuel and air mixture is burned. It is formed by the cylinder head at the top, the cylinder walls, and with the piston as its floor. The shape of the combustion chamber is formed by a recess in the bottom of the cylinder head. The shape of this recess determines how the fuel mixes with the air, and how that mixture burns.
The objective is to achieve complete combustion of the fuel with an even spread of the flame across the chamber. The fuel needs to be mixed and this is achieved by creating turbulence - very rapid swirling - of the mixture as the piston moves upwards on the compression stroke.
The shape of the combustion chamber has developed over the history of the engine.
- In a wedge shaped chamber, the compression stroke forces the mixture from the narrow end of the wedge into the wider end, creating turbulence and concentrating the mixture around the spark plug.
- A hemi-chamber is a dome-shaped recess which is generally used on single valve crossflow heads. With the spark plug centered in the chamber there is a short flame path to the piston head. The spherical nature of a hemi chamber makes it difficult to accommodate multiple valves which are used on the majority of engines today.
- A pent-roof combustion chamber is similar to hemi chamber but with flattened surfaces that make it possible to accommodate four valves per cylinder. Pentroof chambers are the most commonly used on engines made in the past thirty years.
The ports of a cylinder head are passageways that allow air and exhaust gases to enter and leave the combustion chamber. They are cast into the head and their size and shape has a great bearing on the flow through the engine.
One side of the block is typically connected to the air intake, the other to the exhaust manifold. This is known as a crossflow head, and is by far the most common to be found. Reverse-flow is the term for a cylinder head where valves are found on the same side but this is rarely used as it is less efficient than a cross-flow design.
On engines with multiple intake and exhaust valves per cylinder, there may be one port per cylinder, or a single port for all intake and exhaust valves. A great deal of computation and research goes into the shaping of ports to ensure maximum volume and velocity of air traveling through them.
Cylinder head layouts
In an over-head cam (OHC) engine, the predominant type of engine in use today, the camshaft is found in the head, whereas for an over-head valve (OHV) layout the camshaft is in the block.
Inline engines have just one cylinder head, while flat, V and W engines will have multiple cylinder heads, one for each bank of cylinders.
Modifications & Upgrades
Cylinder head decking
A cylinder head can warp over its lifetime, and the lower face that mates with the block can wear. In order to restore a perfectly flat surface, the face of the cylinder head is machined in a process known as decking, resurfacing or head skimming.
Porting is a process where the ports of the engine are modified - usually opened up and the walls polished perfectly smooth. This will improve the gas flow through the head. Polishing is a time consuming process but can be done at home with minimal tools and so is popular among DIY engine rebuilders.
Modern CFD (Computational Fluid Dynamics) computing now tends to lean towards the theory that even after porting, the walls of the ports are more efficient if they are left slightly rough and not polished.
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