The following are the key parts of an engine that need to be studied well before you apply your hands on it.
(a) Cylinders and Cylinder Blocks. There are many types of cylinder design covering the varying number of cylinders and the required lay out of the engine. Cylinder block, cylinder head and crankcase – these three parts form the foundation and main stationary body of the automobile engine. They serve as support and enclosure for moving parts. The method used for cylinder cooling is by air or water. The cylinder block is usually made from grey cast iron and sometimes with addition of nickel and chromium. Some blocks are cast from aluminium, in which cast iron or steel sleeve are used. For most engines, cast iron has been found to be a satisfactory cylinder wall material, as it has better wearing qualities. The block has cylindrical hole in which piston moves up and down. Water jackets surround the cylinders to help carry away engine heat.
(b) Air Cool Cylinder Blocks. Air cooled cylinders are usually made separately from the crankcase in one casting. This mono block casting made of cast iron gives rigidity and compactness. The cylinder bores may be machined to every accurate limit in casting itself. This is the old method of making cylinder blocks. In modern method, cylinder liners are put in the cylinder blocks.
(c) Cylinder Liners. There may be wet or dry type liners. In wet liner, there is water circulating around the outer periphery of the liner. Dry type is press fitted into the block and thus outer side is not wetted by circulating water. Leakage wet liner is prevented by means of sealing ring. Arrangement of liners is in such a way that, while installing liners leaves a little protruding portion at the top, which provides a nip during cylinder head tightening. The use of liner makes engine life longer because they can be made of centrifugally cast or of especially hardened steel material and they are also easily replaceable.
(d) Cylinder head. The cylinder head is a single casting suitably water jacketed or provided with cooling fins, depending on the method of cooling in use. The cylinder head is usually made of grey iron or aluminium alloy. Aluminium has the advantages of lightness in weight and high heat conductivity. It is cast separately from the cylinder block, so that it may be removed for cleaning carbon and grinding valves. To retain compression in the cylinder, a flat piece of gasket is placed between the cylinder head and cylinder block. The cylinder head is tightened to the block by studs and nuts or set screws. Engine with side valve systems have holes for screwing spark plugs. Water passages and combustion chambers are formed in the cylinder head. Engines having overhead valve system have the opening on the head for the valves, valves sheets, spark plugs, guides, rocker gear and sometimes bearings for overhead camshaft. There are four types of cylinder head, known as ‘L’ head, ‘I’ head, ‘T’ head and inverted ‘L’ head (F type) with inlet over exhaust.
(e) Crank Case. Crankcase is attached to the bottom face of the cylinder block. It acts as the base of the engine. It supports the crankshaft and camshaft in suitable bearings and provides the arms for supporting the engine on the frame. The oil pan and the lower part of the cylinder block together are called the crankcase. The upper part of the crankcase is often cast integral with the cylinder block otherwise it may consist of a single. Casting or it may be built up from more sections depending on the type of engine. The parts are often light alloy casting but ferrous metal are also used, particularly, when a separate sump, usually a steel pressing is fitted. Separate part of the crankcase and sump, if fitted, are secured by nut or bolts and set screws. Oil tight joints are made with gaskets, some of which require the use of joining compound.
(f) Piston. A piston in conjunction with the piston rings forms a sliding gas tight plug to the charge in the cylinder. It also transmits the force of the burning charge to the connecting rod, and because it acts as guide and bearing to small end of connecting rod, it takes the side thrust due to the inclination of the rod. The piston can be divided into two main sections, the ring section and skirt section. The top of the piston is called head. Ring grooves are cut on the circumference of the upper portion of the piston. The part below the ring grooves is called skirt.
(1) The concave crown is less common. The shape of the crown is one of the factors governing the shape of the combustion chamber. The heat generated at the top of the piston causes the piston to expand. Hence, they are fitted with clearance, which allows the free movement of the piston.
(2) Formally cast iron was widely used for manufacturing of pistons, but the aluminium alloy piston is used in modern vehicles. Aluminium piston can be either cast or forged. Aluminium alloys allow better heat conduction is light in weight than the cast iron. The heat conductivity of aluminium is three times greater than that of cast iron.
(g) Piston Rings. The number and type of rings are decided by the manufacturer to meet the performance of an engine. Piston rings are made of a high grade of cast iron, centrifugally cast and precision ground which gives an elastic property and minimize ring vibration, Chromium plated rings are used with ordinary cast iron cylinder to prevent, scuffing and bore wear. There are two types of piston rings, compression and oil control ring. The compression rings are accommodated in the first two grooves in the piston head. These rings effectively seal the gas pressure and transfer excess heat to the cylinder. Oil control rings meter and distribute out the oil evenly on to the cylinder wall. The oil scraper (control) ring clean off any excess oil from the cylinder wall. Piston rings have gap so that they may be installed into the piston grooves and removed when worn out by expanding them.
(h) Gudgeon Pin and their fitments. Gudgeon pin or piston pin or wrist pin connects the piston and the small end of the connecting rod. Piston pin is generally hollow and made from case hardened steel, heat treated to produce a hard, wear resisting surface. There are three methods of connecting piston and connecting rod by the piston pin.
(1) Stationary or Anchored. It is drilled and located by set screws in the bosses of the pistons, the pins being free in the small end bush of the connecting rod.
(2) Semi-Floating. It is locked to the connecting rod with split small end and the pin is free to turn in the bushes of the piston bosses.
(3) Fully- Floating. In this type the piston pin is free to turn in bushes of the small end and the piston bosses. This method usually employs circlips to limit the movement, but in some applications the pin is fitted with soft end pads, such as phosphor bronze or aluminium to prevent scoring of the bore.
(i) Connecting Rod. The purpose of the connecting rod is to transfer the up and down movements of piston to crank pin of crankshaft, and crankshaft converts this reciprocating motion into rotary motion. The connecting rod carries the power thrust from piston to the crankpin and hence. The connecting rod has two ends. The upper end is called small end for fitting with a phosphor bronze bush, or the eye is split and pinched by a setscrew to a nut and bolt when it is designed to clamp, the gudgeon pin. The lower end is called the big end, for connecting the crankshaft. The big end is either split or in one piece.
(j) Crank Shaft. This is the main shaft of the engine to convert the reciprocating motion of the piston into rotary motion. The crankshaft is made of Nickel Chrome Steel Drop Forged. The crankshaft is carried in the main bearings, which are housed in the crankcase. The crankshaft is classified according to the number of cranks. A shaft having one crankpin is known as a’ Single Throw’, four cranks, a four throw shaft and so on. The part of the crankshaft, which runs in the main bearing, is called ‘Journal’. The shaft carries the crankpins to which the big end of the connecting rods are clamped. The journals and crankpins are connected by crank webs or counter weights. Majority of the single cylinder engine have built up crankshafts i.e. the crankshaft has each of its parts made separately and then assembled strongly together by key and keyways. On four cylinders engine the crankshaft is made with 3 or 4 journals, whereas six cylinders engine may have 4, 5 and 7 journals. The cranks are arranged at 180°, 120° or 90°.
(k) Fly Wheel. A Fly Wheel is a heavy steel wheel attached to the rear end of the crankshaft. The size of fly wheel depend up on the number of cylinders and the general construction of the engine. The purpose of fly wheel is to even up the intermittent power impulses given to the crankshaft by the piston. The power output from engine cylinder is not smooth, during the power strokes, the crankshaft speeds up and during the non-power strokes it speeds down, the flywheel helps the crankshaft to run at a constant speed. It absorbs energy during the power strokes and delivers it back during non-power strokes. A ring Gear is mounted on the periphery of the flywheel. The starter motor pinion meshes with this gear to crank the engine. The ring of teeth may be machined on the flywheel itself, but it is more usual for the ring to be made separately and shrunk in to place on the flywheel. It is quite common practice for dead centre position, valve timing, and ignition timing point to be marked on the flywheel.
(l) Vibration Dampers. Vibration dampers are fixed to the front end of the crankshaft to reduce torsional stresses and vibration, reduce the weight of the flywheel, increase the life of crankshaft, increase the life of Journal s and crankshaft bearings.
(m) Camshaft. This is a one piece shaft which has got cams integral with it for opening and keeping a valve open for the correct number of crankshaft degrees. The camshaft is made of steel suitably hardened to resist wear on the cams. The shaft is supported on journals, which revolve in the bearings in the crankcase. Camshaft is driven by the crankshaft through gears or by a chain and sprockets, at the half of engine speed. To enable the camshaft to be timed correctly to the crankshaft, some method of marking is used. This may take the form of marking certain rear teeth, which must be meshed on assembly, or scribed lines on the sprockets may have to be aligned. Camshaft of some overhead valve engines is mounted on the top of the cylinder head.
(n) Timing Gears. These are reduction gears, which transmit a positive drive from the crankshaft to the camshaft through pinion in meshy sprockets, chain and in some cases worm bevel or screw gears are used. Cam shaft runs at half of crankshaft speed.
(1) The master timing pinion is fixed to the crankshaft by a key or bolts.
(2) The camshaft-timing pinion is fixed to the camshaft similarly to the master timing pinion, and having twice the number of teeth as the master timing pinion, will be driven at half Its speed.
(3) The gears are enclosed in Casing, which is secured to the engine block by bolts, the joint being sealed by a gasket. An oil seal is fitted to all shafts that pass through the timing case.
1. For the continuous satisfactory operation of an engine it must have:
(a) Air and fuel must be supplied in correct proportions to the cylinder at correct time in relation of the piston.
(b) Combustion must occur at the correct time, so that maximum advantage can be taken of the expansion of gases.
(c) The hot gases must be expelled from the cylinder after the combustion is ended.
2. In a four-stroke engine, INLET and the EXHAUST valves are so arranged to admit the combustible mixture and expel the burned gases from the cylinder.
3. In a four-cylinder four-stroke engine, for one revolution of the crankshaft there will be two power strokes.
4. In an IC engine, the ignition system employed is a high tension (high voltage) magnet or Coil Ignition System.
5. In two stroke cycle engine, one power stroke is obtained for every revolution of crankshaft.
6. In two stroke engines, INLET PORT, TRANSFER PORT and EXHAUST PORTS are employed, and their opening and closings are controlled by the piston movement..
7. The cylinder block forms the main body of the engine. A water jacket or fins are surrounded the cylinder to help to carry away the heat.
8. There are two types of cylinder-liner used, they are wet or dry type.
9. The compression ring sets as a gas seal and scrapper ring scrap excess oil from the cylinder wall on downward stroke of piston.
10. The gudgeon pin, which is also known as wrist pin, is used to attach piston to the connecting rod.
11. Crank shaft along with connecting rod changes the reciprocating movement of piston to rotary motion of flywheel.
12. Timing gears are the means of transmitting positive drive from crankshaft to camshaft.