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Fgr__1 A typical late-model cylinder head. Fgr__2 An OHC aluminum cylinder head. by Chrysler LLC. CYLINDER HEAD The cylinder head (Fgr__1) is made of cast iron or aluminum. On overhead valve engines, the cylinder head contains the valves, valve seats, valve guides, valve springs, rocker arm supports, and a recessed area that makes up the top portion of the combustion chamber. On overhead cam engines, the cylinder head contains these items, plus the supports for the camshaft and camshaft bearings (Fgr__2). All cylinder heads contain passages that match passages in the cylinder block. These passages allow coolant to circulate in the head and allow oil to drain back into the oil pan. Oil also moves through some of the passages to lube the camshaft and valve train. The cylinder head also contains tapped holes in the combustion chamber to accept the spark plugs. The sealing surface of the head must be flat and smooth. To aid in the sealing, a gasket is placed between the head and block. This gasket, called the head gasket, is made of special material that can withstand high temperatures, high pressures, and the expansion of the metals around it. The head also serves as the mounting point for the intake and exhaust manifolds and contains the intake and exhaust ports. Cylinder head design is one of the most influential factors that affects the overall performance of an engine. The size and shape of the intake and exhaust ports affect the velocity and volume of the mixture entering and leaving the cylinders. Openings in the cylinder head allow coolant to pass through the head. Coolant must circulate throughout the cylinder head to remove excess heat. The coolant flows from passages in the cylinder block through the head gasket and into the cylinder head. The coolant then passes back to other parts of the cooling system. Ports: Intake and exhaust ports are cast into the cylinder head. One port is normally used for each valve. How ever, on engines with more than two valves per cylinder, the ports for the intake or exhaust valves may be combined. These ports are called Siamese ports (Fgr__3). With Siamese ports, individual ports around each valve mesh together to form a larger single port that is connected to a manifold. Cross-flow ports are used on some engines and have intake and exhaust ports on opposite sides of the combustion chamber. Heads of this design are called cross-flow heads. Aluminum Heads: Attempts to lower overall engine weight have resulted in many lightweight cylinder head designs. An aluminum head weighs roughly half as much as a cast-iron head. Eliminating anywhere from 20 to 40 pounds of weight is a plus for fuel economy, but these heads bring new concerns for technicians. Aluminum expands and contracts almost twice as much as cast iron in response to temperature changes. When an aluminum head is mated to an iron block, the difference in thermal expansion between head and block creates a scrubbing stress on the head gasket. Unless the gasket is engineered to take this, leak age and premature gasket failure can result. The difference in thermal expansion also creates a lot of stress throughout the head. The head wants to expand in all directions as it heats up, but the head bolts keep it from going sideways or lengthwise. The only way it can expand is up, so the head tends to bow up in the middle. Engines with overhead cams and aluminum heads often have the cam journals machined in the head itself. Aluminum makes a fairly good bearing material. It’s soft and provides good embedability to foreign particles. But it lacks the support and rigidity of a conventional steel-backed bearing in an iron saddle. Because of this, overhead cam bores experience more flex than their cast-iron counterparts. The result is usually accelerated wear and egg-shaped bores. If the head overheats and warps, alignment through the cam bores is destroyed. In some instances, the mis alignment can be so bad that the cam won’t turn once the head is unbolted from the engine. Fgr__3 Siamese ports. Fgr__4 A typical wedge combustion chamber. Intake port; Exhaust port COMBUSTION CHAMBER The performance of an engine, its fuel efficiency, and its exhaust emissions all depend to a large extent on the shape of the combustion chamber. An efficient combustion chamber must be compact to minimize the surface area through which heat is lost to the engine's cooling system. The point of ignition (the nose of the spark plug) should be at the center of the combustion chamber to minimize the flame path, or the distance from the spark to the further most point in the chamber. The shorter the flame path, the more evenly the air-fuel mixture will burn. Manufacturers have designed several shapes of combustion chambers. Before looking at the popular combustion chamber designs, two terms should be defined. 1. Turbulence is a very rapid movement of gases. Turbulence causes better combustion because the air and fuel are mixed better. 2. Quenching is the mixing of gases by pressing them into a thin area. This area is called the quench area. Wedge Chamber: In the wedge-type combustion chamber, the spark plug is located at the wide part of the wedge (Fgr__4). The spark travels from the large area in the chamber to a smaller one. This allows for rapid and even combustion. At the end of the chamber's wedge is a very narrow area. The quench area, also called the squish area, causes the air and fuel to be thoroughly mixed before combustion. The mixture in the quench area is squeezed out at high speed as the piston moves up. This causes turbulence in the chamber. The turbulence breaks down the fuel into small particles and helps mix the air and fuel. Both of these contribute to efficient combustion and reduce the chances of detonation. The turbulence also helps the flame front move evenly through out the chamber. The wedge-shaped combustion chamber is also called a turbulence-type combustion chamber. Hemispherical Chamber: The hemispherical combustion chamber gets its name from its basic shape. Hemi is defined as half, and spherical means circle. The combustion chamber is shaped like a half circle. This type of cylinder head is also called the hemi-head. The piston top forms the base of the hemisphere, and the valves are inclined at an angle of 60 to 90 degrees to each other, with the spark plug positioned between them (Fgr__5). This design has several advantages. The flame path from the spark plug to the piston head is short, which gives efficient burning. The cross-flow arrangement of the inlet and exhaust valves allows for a relatively free flow of gases in and out of the chamber. The result is that the engine can breathe deeply, meaning that it can draw in a large volume of mixture for the space available and give a high power output. The hemispherical combustion chamber is considered a nonturbulence-type combustion chamber. The mixture is compressed evenly on the compression stroke. Combustion radiates evenly from the spark plug, completely burning the air-fuel mixture. One of the more important advantages of the hemispherical combustion chamber is that air and fuel can enter the chamber very easily. The wedge combustion chamber restricts the flow of air and fuel to a certain extent. This is called shrouding. Fgr__6 shows the valve very close to the side of the combustion chamber, which causes the flow of air and fuel to be restricted. Volumetric efficiency is reduced. Hemispherical combustion chambers don’t have this restriction. Pentroof Chamber: Many of today's engines have a pentroof combustion chamber. This design is a modified hemispherical chamber. It’s mostly found in engines with four valves per cylinder. The spark plug is located in the center of the chamber and the intake and exhaust valves are on opposite sides of the chamber (Fgr__7). Pentroof chambers have a squish area around the entire cylinder. === Fgr__5 A typical hemispherical combustion chamber. Fgr__6 Shrouding is a restriction in the flow of gases caused by the location of the valves in the combustion chamber. Fgr__7 A cylinder head with pentroof combustion chambers. === Next: part 2 Prev.: Lower End Theory and Service Home Article Index top of page |
