Cylinder block and crank mechanism

4.7.1 Cylinder block

The cylinder block in a multi-cylinder engine is the same cast part as in the single-cylinder engine discussed above. One just took, for example, four single-cylinder engines, connected them together, installed the same crankshafts and camshafts for all - that's all. An example of engine block of a 6-cylinder engine is shown in Figure 4.26.

Figure 4.26 Engine block.

In case of casting a block from an aluminum alloy, the cylinders are made in the form of separate cast-iron sleeves installed in the holes of the upper and lower partitions of the block. It is also worth mentioning the liners that are "cast" into the engine block.

4.7.2 Sleeve

Note
A sleeve, which comes into direct contact with the water circulating in the water jacket is called "wet". In this case, the sleeve is securely sealed in the lower bulkhead of the engine block using a copper or rubber ring. Sleeves that are installed in the engine block so that they do not come into direct contact with water are called "dry". During the manufacture of engine blocks with cast sleeves, the sleeve itself is installed in the casting mold even before it is filled with material for the block. After casting the molten metal, diffusion of materials occurs, due to which the sleeve, as it were, is casted into the engine block structure. This is good from an economic point of view, since no money is spent on processing. But it is bad from the point of view of maintainability – in case if any defects of the sleeve are detected, the entire engine block will have to be disposed of.

The upper plane of engine block is carefully processed. A common head, which covers the cylinders from above, is installed on it.

4.7.3 Piston

The piston is a metal cup, which is installed inside a cylinder with a certain clearance. During the working stroke, the piston perceives the gas pressure. Pistons are made out of aluminum alloys.

4.7.3.a Gasoline engine piston

In gasoline engines, the piston top has a flat surface and thick walls with internal ribs that increase its strength and provide good heat dissipation. However, in modern gasoline engines, the piston top has a more complex shape: starting from grooves for valves and ending with a very complex shape, which is necessary for better mixture formation while using direct fuel injection into the cylinder.

Grooves for installing O-rings are located on the side surface of the piston. Two piston rings and one oil scraper ring (it consists of three elements, see figure 4.27) are installed in modern engines.

Note
There are specific names for separate piston surfaces. Thus, the upper surface, which directly perceives all the loads from expanding gases during combustion of the fuel-air mixture, is called the piston top, and the side vertical surface is called piston skirt.

Figure 4.27 Piston with piston rings

4.7.3.b Diesel engine piston

In diesel engines, the pistons have a different shape, since the combustion chamber is located directly inside the piston, as can be seen from Figure 4.28. Combustion chambers in this case can have a very complex shape, which is due to the desire of designers to more thoroughly mixing air with atomized fuel.

Figure 4.28 Piston of diesel engine with combustion chamber inside the piston top.

4.7.4 Piston rings

Compression rings seal the piston in the cylinder and prevent gases outflow through the gap between the piston skirt and the cylinder wall. Oil scraper rings remove excess oil from the cylinder walls, preventing it from entering the combustion chamber. The place of installation of the rings and their appearance are shown in Figure 4.27.

A straight or oblique cutout, which is called a lock, is made on the ring. This cutout allows the ring to spring in order to compensate for its expansion when heated, as well as to install the ring into the recess on the piston.

 4.7.5 Piston pin

Piston pin is a short steel tube. It passes through the upper connecting rod head and the holes in the piston (in the so-called piston bosses), thus connecting them.

The piston pin can be firmly pressed into the upper connecting rod head. In this case, the type of connection is called "fixed pin".

Piston pins of floating type are used as well. They can rotate both inside the piston bosses (visible in Figure 4.29) and inside the upper connecting rod head. In order to eliminate lateral displacement, the pin is attached on the sides with two spring-loaded retaining rings that are installed in the grooves of the piston bosses.

Note
Non-floating pins are rigidly fixed in the upper connecting rod head or in the piston bosses. When installing a floating pin, its entire surface is working, as a result of which less wear is provided and the possibility of galling is reduced.

Figure 4.29 Piston and piston pin without connecting rod.

Figure 4.30 Piston and piston pin in assembly with connecting rod.

4.7.6 Flywheel

The flywheel is a carefully balanced cast iron disc. In addition to ensuring uniform rotation of the crankshaft, the flywheel after unwinding helps overcoming the compression resistance inside the cylinders while starting the engine. The flywheel, due to the stored energy obtained during rotation, also provides the engine with the ability to overcome short-term overloads, for example, when starting a car, etc. Schematically, the flywheel is clearly visible in Figure 4.1 of this chapter.

The flywheel is bolted to the crankshaft flange. A gear ring is fixed for starting the engine from the starter on the rim of the flywheel. Reference marks are applied for determining the TDC of the piston of the first cylinder and for installing the ignition.