Principal Engine Parts of a car
The car's engine is not a singular unit but built up from several differently sized components that perform their intended functions. The present day automobile that derive their power from internal combustion engines operating on fossil fuels have the following principal parts:
The parts of an engine can be categorized into “Mechanical” and “Electrical” parts
• Mechanical components
The engine cylinder is the part or space where fuel is admitted and reciprocating motion of the piston is obtained by burning it. The engine cylinder is characterized by its ‘bore’ and ‘stroke’. Bore represents its inner diameter and Stroke is the effective length along which the piston reciprocates i.e. the distance traveled by the piston. Two terms related to stroke are ‘Top Dead Center’ (TDC) and ‘Bottom Dead Center’ (BDC). Top Dead Center is the uppermost point of the stroke while the Bottom Dead Center is the lowermost point of the stroke. The velocities of the piston at TDC and BDC are zero.
The part of the engine where the cylinder is located is called the engine block or cylinder block. Cylinders are generally lined with liners or sleeves of some other harder material or coated with some wear resistant material like Nikasil. Liners can be easily replaced when worn out. Cylinder blocks are also provided with hollow spaces around and in between the individual cylinders that are known as jackets in case of liquid cooled engines. The coolant is circulated in these jackets which enables effective heat dissipation.
Engine displacement is determined by the cylinder/cylinders. The volume swept by the piston in one stroke i.e. the cross sectional area times the stroke, is called the Swept Volume and it is measured in cubic centimeters (cm3). This is the engine displacement in case of a single cylinder engine. In case of multi cylinder engines, the engine displacement is the swept volume multiplied by the number of cylinders. The volume enclosed by the cylinder head and the piston at TDC is called the Clearance Volume. The sum of swept volume and the clearance volume is equal to the total volume of the cylinder.
The engines are classified according to the dimensions of the cylinders and their orientation. The bore by stroke ratio classifies the engines as undersquare, oversquare or square accordingly as the ratio is less than one, greater than one or equal to one.
It is made of steel alloy/cast iron or aluminium by casting process. The reason for using aluminium is its light weight and good heat dissipation capacity.
The piston is the cylindrical part which moves up and down in the cylinder and enables compression and expansion of the charge during the combustion cycle. The diameter of the piston is slightly less than the bore of the cylinder to avoid direct wear of the cylindrical piston surface. Three rings known as piston rings are fitted in the circular recesses machined on the piston surface. These rings are in direct contact with the cylinder liner thus preventing piston wear. The top two rings are known as compression rings. Compression rings are chamfered on the outer periphery. They prevent the fresh charge or waste gases inside the combustion chamber from going into the crankcase, a process known as ‘blowby’. The lowermost or third ring is called the oil ring. Its purpose is to ensure proper oil distribution along the cylinder walls and also prevent the leakage of oil into the combustion chamber.
The top two rings are made of cast iron or aluminum alloy which have high wear resistance. The oil ring is made from aluminium.
Crankshaft is a part of the engine which has projections bent and offset from the shaft axis. These projections are called crank throws or crankpins. This design converts the sliding motion obtained from the piston into rotary motion via a connecting rod. Crankshaft is placed below the cylinder block in a casing called the crankcase. In multi cylinder engines one crankpin per cylinder is provided to attach the piston by the connecting rod. The crankpin journal bearing is called the big end and has plain or sliding bearings. Crankshafts have some counterbalance weights which are either bolted to the crank body or form an integral part and are called crank balance. Crank balance is provided to counter the torsional vibrations experienced by the crankshaft due to the reciprocating unbalance of the piston which arises due to the jerks from the combustion process. Crankshafts may be manufactured in parts or as a single piece. The single piece design is preferred as it gives superior strength, better fiber flow and good stress bearing capabilities.
Crankshafts are made from steel by roll forging process or from ductile steel through casting. The single piece crankshafts are made from carbon steels followed by heat treatment. Vanadium micro-alloyed steels are also used frequently because they give high strengths without heat treatment and the low alloy content makes them cheaper than the high alloy steels.
4. CONNECTING ROD
The connecting rod is the link connecting the piston to the crankshaft. It converts the linear motion of the piston into rotary motion of the crank. One end of the connecting rod is attached to the piston through a piston pin/gudgeon pin/wrist pin and is called the small end while the other end is attached to the crankpin journal through bolts holding the upper and lower bearing caps and is called the big end. The bearing is in the form of two half shells which is held in place around the crank journal by the big end of the connecting rod. Both the ends of the connecting rod are not rigidly fixed but are hinged so that they can rotate through an angle. Thus both its ends are in continuous motion and under tremendous stress from the pressure from the piston. The connecting rods are the most sensitive parts and are most prone to failure and hence they are manufactured with high degree of precision.
They are generally made from forged steel but are also made from aluminium alloy for light weight and high impact absorbing ability.
5. CYLINDER HEAD
The cylinder head is the part which sits on top of the cylinder block and houses the valves, rocker arms and the ignition elements. It is bolted to the cylinder block with the head gasket in between. In overhead camshaft engines, the camshaft is present in the head and there is no pushrod arrangement for valve mechanism. The inlet and exhaust ports are also machined within the head to which the inlet and exhaust manifold are then attached. Cylinder heads also form part of the combustion chamber that is machined into it on the underside. Holes and channels are made for bolting and for flow of coolant. Inline engines have a single cylinder head for all the cylinders while V-type and horizontally opposed have a separate head for each bank of cylinders.
They are made from cast iron or aluminium alloy. Aluminium is light weight and conducts the heat away more quickly than cast iron and hence is the preferred choice.
Camshaft is a shaft to which cams are fitted or are machined. The function of the camshaft is to operate the valves directly by sitting over them, or indirectly through a mechanism (rocker arm, pushrod). Camshaft rotation decides the valve timing and is of critical importance. The opening and closing of valves is governed by camshaft which is coupled to the crankshaft either directly through a reduction gear or indirectly through a pulley and a timing belt. Engines in which the camshaft is coupled to the crank by a gear require a pushrod and tappet mechanism along with rocker arms. The gear on the crank has half the teeth than on the camshaft gear. This causes the camshaft to run at half the RPM of the crank. In engines where the timing belt and pulley are used, the camshaft is placed inside the head and there is no need of a pushrod. Instead latches are used which rotate the rocker arm and operate the valves. Such a design is called Overhead Camshaft (OHC) design. Some engines use a single camshaft to operate both the inlet and exhaust valves and are called Single Overhead Camshaft (SOHC) while others utilize a separate camshaft for operating the two types of valves which are arranged in two separate rows and are called Double or Dual Overhead Camshaft (DOHC) engines. OHC design reduces manufacturing cost and is less prone to failure. The lobes of the cam are tapered slightly so that the valves lifters rotate slightly with each depression and wear uniformly. There is considerable sliding friction between cam and follower surface and so they are surface hardened.
Chilled iron castings are the most common material used for fabricating camshaft as chilling of iron castings gives them greater wear resistance and surface hardness. Billet steel is also used for making high quality camshafts.
Valves used in the IC engines are called poppet valves. They have a long thin circular rod known as the valve stem at the end of which is a flat circular disk called the valve head. The valve head has a tapered section connecting to the rod which forms the valve seat. The valve slides in a valve guide and sit in the valve seat when closed which is machined in the head. This sliding motion is enabled by the camshaft and associated linkages and the valves are kept closed or returned to their seats when not in use by valve springs. The valves are responsible for intake of fresh charge and exhaust of waste gases. The valve which admits fresh charge is called intake/inlet valve and the one which allows exhaust gases to go out is called exhaust/outlet valve.
The valves are made from steel alloys and they may be filled with sodium to increase the heat transfer capacity
8. ROCKER ARM
The rocker arm transmits the rotary motion of the cam or camshaft through a latch/tappet and converts it into a linear motion of the valve stem which depresses the valve head. Rocker arm rocks or oscillates about a fixed pivot rod (rocker shaft) in the cylinder head.
The rocker arms are made from steel stampings for light and medium duty engines whereas most of the heavy duty diesel engines use cast iron and forged carbon steel rocker arms for greater strength and stiffness.
The metallic case or housing in which the crankshaft is placed is called the crankcase. The crankcase is located below the cylinder block. The crankcase also has the main bearing in which the crank rotates. The main bearing is a plain or sliding bearing with proper oil supply in it. The four cylinder inline petrol engines have three main bearings, one on each end and one in the middle while the diesel counterparts have five main bearings one on each end and one between each cylinder. The crankcase encloses the crankshaft and connecting rod assembly and protects it from dust, dirt and other foreign materials. The crankcase is filled with air and oil and is sealed off from the fuel air mixture and exhaust gases in the combustion chamber by the piston rings.
The crankcase is made from the same materials as the cylinder block i.e. aluminium or cast iron.
10. OIL PUMP AND SUMP
The oil pump pumps the oil to various parts of the engine for effective lubrication, cooling and cleaning. The oil pump in the engine is a gear type pump which is driven by the crankshaft gear. The oil is pressurized to the passages machined in various components, which then lubricates and cools them. They pressure relief valves to maintain the required pressure in the passages and mainly in the crankshaft journal bearing.
The oil is stored in an oil chamber known as the oil sump. The oil is lifted by the oil pump from the sump through a wire mesh strainer which retains debris and dirt. The oil then passes through an oil filter and an oil cooler before being distributed to the engine parts. The oil after doing its job returns through controlled leaks from pistons, rings, valves, camshaft and finally to the crankcase from where it is drained back to the oil sump.
11. FUEL PUMP
The fuel (diesel) in a CI engine is not mixed with air unlike SI engines, but is sprayed into the combustion chamber through a nozzle for ignition. The fuel pump pressurizes the fuel to such a pressure that it atomizes and when it is sprayed in the cylinder, it gets ignited when it comes in contact with the air. The fuel pump consists of a spring loaded piston valve in a cylinder. When fuel from the fuel filter is introduced in the fuel pump, the spring loaded piston applies pressure on it. The pressure is transmitted through the pressure lines and finally to the injector which has a nozzle opening in the cylinder. Thus the fuel gets atomized and ignited.
• Electrical Components
The alternator is an AC generator which charges the car’s battery and powers its electrical system when the engine is running. The alternator is an electromechanical device which converts the mechanical energy into electrical energy. It consists of a rotating magnet known as the rotor turning within a stationary set of conductors which have windings on them and called the stator. The rotation of the rotor changes the magnetic flux around it and induces an e.m.f. in the stator windings in the form of AC voltage. The construction of the alternator is robust enough to drive it by a pulley smaller than the crankshaft pulley via a belt so as to turn it faster than the engine. The alternator uses a set of rectifiers or diodes to convert AC to DC.
2. STARTER MOTOR
The starter motor is a small high torque motor for cranking the engine. For the engine to be started, the piston must move downwards with the inlet valve open to suck in fresh air/fuel. To accomplish this, the starter motor is connected to a key operated switch (a relay) and a starting battery. When the key is turned, the switch turns on, the circuit is completed and it pushes out the pinion on the motor shaft which goes into mesh with the flywheel gear. This turns the crankshaft and moves the piston down via the connecting rod. As the piston moves down sucking in fresh charge, the ignition element or the injection element burn the fuel and the engine gets started. At this point, the starter motor pinion goes out of mesh of the flywheel.
3. SPARK PLUG
The spark plug is an electrical device which ignites the fuel in the combustion chamber at the end of the compression stroke. The fuel gets ignited and as a result expands and pushes the piston down so as to obtain the power stroke. The spark plug is fitted into the cylinder head on the underside of the combustion chamber through threads on the plug body. The spark plug has two electrodes: one central electrode which is connected to the ignition coil or magneto through a highly insulated high tension wire; the other electrode is at the base of the plug and is grounded. There is a small gap between the two electrodes generally between 0.9-1.8 mm. When high voltage current from the ignition
coil/magneto is supplied the air between the gap gets ionized and a spark is generated which is sufficient to ignite the fuel. The electrode gap is of critical importance for proper sparking at all speeds. The spark plug requires a voltage in the range of 12,000 - 25,000 V to fire properly. The spark plug has a terminal, an insulator and its tip, metal jacket and the seals in the body structure. The terminal is connected to the ignition coil, the insulator (made of porcelain) provides insulation and mechanical support, the metal jacket conducts heat away from the plug body and to the cylinder head, and the seals properly seal the recess in the cylinder head where the spark plug is fitted.
4. ELECTRONIC FUEL INJECTOR
The injector does the job of mixing the fuel and air at such a high pressure that it gets ignited. The electronic fuel injection electronically controls the injection process so that always the required amount of fuel is injected into the cylinder before firing. This is controlled by a hardware which is programmed to meter the fuel accurately and optimize the air/fuel ratio at all speeds of the engine. It is assisted by sensors on the crankshaft or camshaft to monitor the engine rotational speed, mass flow sensors, oxygen sensors and throttle position sensors to give the hardware timely feedback. It forms a closed-loop feedback system which helps in proper fuel distribution in all the cylinders and leads to dependable starting, improved engine performance and lesser maintenance.
5. IGNITION COIL
The ignition coil generates the voltage to be supplied to the spark plug. It has two coils, one primary and another secondary. The primary is connected to the battery and the secondary to the capacitor and a distributor and is grounded. The turns in the primary coil are thick and few in number while those in the secondary coil are thin and large in number. The current produced in the primary coil induces a current in the secondary current by mutual induction which is then stored in the capacitor and is supplied to the distributor which distributes the current among the spark plugs.
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