The core around which the engine is built is called either the crankcase, or the engine block. It is usually made from aluminium alloy and built in two halves to allow the insertion of the crankshaft. If the engine is ‘liquid cooled’ a large one-piece jacket houses cylinder liners to provide a cooling liquid container around them.
Depending on the manufacturer of the engine, cylinders are bored into the engine block, or fitted onto the crankcase.
The cylinders of an ‘air cooled’ engine are
‘finned’ to increase the cooling area.
Piston rings are made from cast iron, which contains
a large amount of graphite, a substance which assists self lubrication when the engine is first started.
The cylinders are made of alloy steel. The cylinders resist the pressure of combustion and provide a working surface for the piston rings. If the engine is ‘air cooled’ the cylinders are ‘finned’ to increase the cooling area. Pistons, (see Figure 3.7) made from light alloy, are fitted within the cylinders. Around each piston are fitted two, or, in some cases, three piston rings.
The piston rings are made from cast iron, which contains a large amount of graphite: a substance which allows self lubrication between the rings and the cylinder walls when the engine is first started. The rings ensure that no gases leak from above the piston into the crankcase, which, in a wet sump engine, constitutes a reservoir for the engine oil. For more information on wet and dry sump engines refer to Chapter 5, Lubrication.
The connecting-rod (see Figure 3.8) transmits the forces of combustion to the crankshaft.
Figure 3.7 The Piston, complete with Gudgeon Pin and Piston Rings.
Figure 3.5 The Two Halves of the Crankcase Joined Together.
The crankshaft converts the linear motion of the piston into rotary motion.
The connecting-rod is made from ‘H’-section high tensile steel, which combines lightness with the strength necessary to withstand the compressive and tensile loads imposed as the piston changes direction.
The connecting-rod is joined to the piston by a gudgeon pin (see Figure 3.7) which fits through the ‘small end’ of the rod. The connecting-rod is joined to the crankshaft, at the crank pins by a large circular bearing called the ‘big end’.
The crankshaft converts the linear motion of the piston into rotary motion. It transmits torque, the engine’s turning moment, to the propeller and provides the drive for
accessories.
The journals, which are the main part of the crankshaft, are supported in the ‘main’ bearings within the crankcase. The crank-pins are offset from the journals by a distance termed the ‘crank throw’. The crank throw determines the piston stroke, and there are two ‘throws’ to one stroke.
Figure 3.10 shows a plan view of the pistons fitted to a crankshaft in a
horizontally opposed engine, via the connecting rods. The view shows the two front pistons, those on the left of the picture, both at bottom dead centre, while the two rear pistons, those on the right of the picture, are both at top dead centre.
The cylinder head (see Figure 3.11) is generally made of aluminium alloy and is finned to improve heat dissipation. It seals one end of the cylinder to provide a combustion chamber for the mixture. The cylinder head accommodates the valves and the sparking plugs and supports the valve rocker arms. The valve rocker arms are operated indirectly by a camshaft (or shafts) (see Figure 3.12) which is a shaft with eccentric lobes machined on it. The camshaft is driven by the crankshaft at half the crankshaft speed.
The camshaft is driven at half crankshaft speed because each valve is required to open and close only once per working cycle, that is, once every two revolutions of the crankshaft.
Figure 3.9 The Crankshaft.
Figure 3.10 Crank Assembly.
Each valve is only required to open and close once per working cycle.
Two springs are fitted to each valve, one inside
the other. This provides both a safety factor and eliminates ‘valve bounce’.
The valves are kept concentric to the valve seats by valve guides. The valve seat is ground to form a gas tight seal with the face of the valve. The valves themselves, both inlet and exhaust, open and close the passages for the induction and scavenging of the gases.
The face of each valve is accurately machined to the same angle as the valve seat. The valve and the seat are then ‘lapped’ or ground together with an abrasive paste until a full contact is obtained.
The valve springs are manufactured from special spring steel, and they ensure that the valves remain closed except when they are being operated by the rocker assembly.
The springs are of the helical coil type, the usual practice being for two springs to be fitted to each valve, one inside the other.
This provides a safety factor, and also eliminates ‘valve bounce’, a condition created by the fact that each valve spring will have a resonant frequency (with the engine RPM) where it will be ineffective at closing the valve on its own.
Figure 3.11 The Cylinder Head.
The ‘accessory housing’, an example of which is shown in Figure 3.13, is a casing mounted at the rear of the block.
It encloses the drive gear trains for the camshafts, the fuel, oil, pneumatic and vacuum pumps, electric generator, magnetos and tachometer.