Cylinder head

For vehicles, special attention is most often focused on the knocking potential encountered at high motor speeds in excess of 4000 rpm for which the consequences from the mechanical point of view are considerable and lead very often to mechanical failure such as broken valves or pistons, and rupture of the cylinder head gasket. Between RON and MON, it is the latter which better reflects the tendency to knock at high speeds. Conversely, RON gives the best prediction of the tendency to knock at low engine speeds of 1500 to 2500 rpm. 

Fig. 35. Precompressed cylinder head with radial inlet and outiet valves (174).

Historically, strontium metal was produced only in very small quantities. Rapid growth of metal production occurred during the late 1980s, however, owing to use as a eutectic modifier in aluminum—silicon casting alloys. The addition of strontium changes the microstmcture of the alloy so that the siUcon is present as a fibrous stmcture, rather than as hard acicular particles. This results in improved ductility and strength in cast aluminum automotive parts such as wheels, intake manifolds, and cylinder heads. 

Shortening the projection of the cylinder heads into the cylinder... 

Installing spacer rings between cylinder head and body or under the valves... 

More flexibility can be obtained with a variable volume clearance pocket such as that shown in Figure 11-20. This is a plug built into the outer cylinder head. When moved, the clearance volume of the outer end of the cylinder changes. 

Coolant Jacket Cylinder Head Spark Plug Pushrod Cylinder Block Tappet Crankcase Cam Camshaft... 

This arrangement of valves defines a push-rod engine. In the alternative approach of Figure 2, the camshaft is moved to a position above the cylinder head, eliminating the push rod. This configuration defines an overhead-cam engine. The overhead camshaft is driven from the crankshaft by either a belt or a chain. 

Double Acting Gas compression takes place in both ends of the cylinder, head end and crank end, Figures 12-21, 12-2J, and 12-4B. 

This is the total volume remaining in the cylinder at the end of the piston stroke. This consists of the volume between the end of the piston and the cylinder head, in the valve ports and the volume in the suction valve guards and the discharge valve seats. See Figures 12-12,12-17A, and 12-17B. 

Lowering cylinder wall and cylinder head temperatures reduces loss of capacity and horsepower per unit volume due to suction gas preheating during inlet stroke. This results in more gas in the cylinder for compression. 

Reducing cylinder wall and cylinder head temperatures removes more heat from the gas during compression, lowering its final temperature and reducing the power required. 

In a two-cycle machine, all pistons complete a full cycle each time the crankshaft completes one revolution. Figure 44.9 illustrates the normal action of a two-cycle, or single-action, compressor. Inlet and discharge valves are located in the clearance space and connected through ports in the cylinder head to the inlet and discharge connections. 

An alternative valve design uses a conical discharge valve in the centre of the cylinder head, with a ring plate suction valve surrounding it. This construction is used in compressor bores up to 75 mm. 

Valve and cylinder head design is very much influenced by the need to keep the volumetric clearance (q.v.) to a minimum. 

Smaller machines may have a valved bypass across the inlet and outlet ports in the cylinder head, or a variable clearance pocket in... 

Figure 4.8 Concentric cylinder head valves with cone-seated discharge valve...

Cold suction gas provides cooling for the compressor and is sufficient to keep small machines at an acceptable working temperature. Refrigerantshaving high discharge temperatures (mainly ammonia) require the use of water-cooled cylinder heads. Oil coolers are needed under some working conditions which will be specified by the manufacturer. These maybe water cooled or take refrigerant from the system. 

Small compressors will have motor overheat protection adjacent to the hermetic shell or built into the winding (see Section 4.8) and larger motors will have contactor-starters with overcurrent devices. Overheat protection is also fitted on many machines, to guard against high motor winding, cylinder head or oil temperatures. These usually take the form of thermistor detectors, connected to stop the motor. 

Volume changes also can be mechanically determined, as in the combustion cycle of a piston engine. If V=V(i) is an explicit function of time. Equations like (2.32) are then variable-separable and are relatively easy to integrate, either alone or simultaneously with other component balances. Note, however, that reaction rates can become dependent on pressure under extreme conditions. See Problem 5.4. Also, the results will not really apply to car engines since mixing of air and fuel is relatively slow, flame propagation is important, and the spatial distribution of the reaction must be considered. The cylinder head is not perfectly mixed. 

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