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The Engine’s System
The engine is an important part of the automobile; it acts as the heart of a person, which provides the power that the cars need to run on the road. All the automotive engines today are the Internal Combustion Engines ( ICEs ) because the fuel is burnt inside their cylinders and the energy is provided.
ICEs are those heat engines that burn their fuel inside the engine cylinder. In ICEs, the chemical energy stored in their fuel converted into heat energy during the burning part of their operation. The heat energy is converted into mechanical energy by the expansion of gases against the piston attached to crankshaft that can rotate. The engines that burn petrol are known as petrol engine. Other types of ICEs burn heavier oils, of these types the diesel engine has come into the widest use.
Diesel and petrol engines have the same mechanical parts, except that diesel components are generally stronger and heavier. Both engines are internal combustion engines, but they have different fuel system and use different fuels. With a diesel, only air enters the cylinder during the intake stroke. A petrol engine takes in an air-fuel mixture. Following are some general comparisons between diesel and petrol engines:
In a diesel, the fuel is injected into the cylinder as a fine spray near the top of the compression stroke. With a petrol engine, the fuel is injected into the exhaust ports at the start of the induction stroke.
Ignition in a diesel is by the high temperature from the highly compressed air. A petrol engine needs a spark for ignition.
Diesel engines generally operate at lower engine rpm than petrol engines.
Diesel engines use distillate for fuel, which is less volatile than petrol.
The design of diesel engines makes them noisier than petrol engines and they have a unique diesel knock.
Small diesel engines, as well as petrol engines, are used in passenger cars and light commercial vehicles. Larger diesel engines are used in all heavy commercial vehicles, earthmoving equipment, and farm machinery.
１. Engine Configurations
The term engine configuration refers to the way that the cylinders of an engine are arranged. The cylinders can be in-line, or at an angle ( V-type ). Within these three basic arrangements, there are a number of variations.
In-line engine
With in-line engines, the cylinders are arranged in a straight line, one behind the other. Most in-line engines have their cylinders vertical, but some are slanted. That is, the engine is tilted at an angle to reduce the overall height. These engines are sometimes referred to as slanted engines.
Some in-line engines have their cylinders horizontal, so that the engine is more or less on its side. This reduces the overall height of the engine. This arrangement is
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used mainly in larger commercial vehicles with the engine mounted under part of the cab. The mechanical arrangement of a four-cylinder in-line for a passenger car is shown in Fig.1-9. The cut-away view in Fig.1-10 enables the various parts to be identified.
Horizontally opposed engine
This arrangement has its cylinders arranged in two flat blanks with the crankshaft between them. The engine shown has a short rigid crankshaft with five bearings. A horizontally opposed engine has even firing impulses and good balance. Movement of a piston in direction is opposed by movement of a piston in the opposite direction.
Horizontally opposed engines, with their flat design, give the engine a low height and also help to keep the center of gravity of the vehicle low. A low center of gravity gives the vehicle stability.
V-type engines
With V-type engines, the cylinders are arranged in two blanks at an angle. This reduces the length of the engine and makes it more compact. This also reduces the length of the crankshaft, which can be designed to be more rigid than a long shaft.
２. The Engine Assembly and Components
The complete engine assembly consists of the mechanical components that make up the engine itself and also a number of associated systems. These are the systems that are needed to start the engine and also to control it and keep it running. The mechanical parts of the engine assembly can be broken down into a number of sub-assemblies, or groups of associated components, although these are usually referred to merely as assemblies, for example, cylinder-head assembly and piston assembly. Some parts of an engine are internal, other parts are external, this illustration identifies a number of external parts.
The various components of an engine are assembled together during manufactured and most of these are secured by means of bolts, nuts and other types of fasteners. Some parts are internal and others are attached to the outside of the engine.
The cylinder block is the largest part of engine. The other parts are either fitted into the block or attached to it. As its name suggests, it is basically a block of cast metal, usually cast iron, but it can be aluminum alloy with cast iron or steel liners. Aluminum is used to reduce the weight.
The cylinder block has accurately bored cylinders to take the pistons. The lower part of the block is known as the crankcase and this has bearings that carry the crankshaft. The water-jackets that surround the cylinders are filled with coolant.
The top of the cylinder block has a machined surface for the cylinder head and the bottom of the cylinder block is machined to provide a mounting for the oil pan, or sump.
The cylinders can be arranged in the block so that they are in line, as shown, or they can be set at angle ( V-type engine ).
The crankshaft is mounted in bearings in the lower part of the cylinder block ( the crankcase ). The connecting rods connect the pistons to the crankshaft, which is rotated by the power strokes of the pistons when the engine is running. The up-and-down, or reciprocating, motion of the pistons is changed to rotary motion by the combination action of the connecting rods and the cranks of the crankshaft.
The term crankshaft comes from the word cranked, which also means bent. It is a shaft with a number of cranks or bends. With in-line engines, there a crank for each cylinder, but with some V-type engines, there is only one crank for each pair of cylinders.
The flywheel is a heavy cast iron wheel attached to the rear of the crankshaft. It reduces engine vibrations by smoothing out the power impulses of the pistons. The flywheel absorbs energy during the power stroke and gives up energy during the other strokes and this helps to keep the engine running smoothly. A ring gear fitted to the rim of the flywheel so that the engine can be rotated by the starter pinion when starting the engine. With automatic transmissions, a drive plate and torque converter take the place of the flywheel and perform the same functions as a flywheel, as far as the running of the engine is concerned.
The piston has grooves that carry the piston rings and these provide a seal between the piston and the cylinder wall. Compression rings are used as a gas seal and the oil ring is used to prevent excess oil from finding its way up past the piston into the combustion chamber.
The connecting rod has a removable cap and a split bearing at its lower end where it is connected to the crankshaft. Its upper has a piston pin that provides a wrist-type of action with the piston. Because of its action, the piston pin is sometimes referred to as a wrist pin.
The cylinder head is made of cast aluminum alloy. It is bolted to the top of the cylinder block so that it encloses the cylinders. It has combustion chambers above the cylinders in which the air-fuel mixture is burnt. Cylinder heads can be made of cast iron, which is more resistant to corrosion, but aluminum alloy is used for petrol engines because it has advantages of better heat transfer and lighter weight. The cylinder head has intake ports and exhaust ports. The intake valves open the intake ports to admit the fuel charge into the cylinder during the intake stroke. The exhaust valves open the exhaust ports to allow the burnt gases to leave the cylinder following the exhaust stroke.
The camshaft and the valve mechanism are used to open and close the valves at the correct time. The camshaft is driven from the crankshaft at half the crankshaft speed.
The valve cover, also called the cylinder-head cover, is fitted to the top of the cylinder head. This encloses the valve mechanism. An additional cover on top of the engine covers the ignition coils.
This cylinder-head arrangement, for a four-cylinder engine, has two camshafts and sixteen valves. Other four-cylinder engines can have two camshafts and twelve valves, or one camshaft and eight valves.
The timing belt and pulleys drive the camshaft at half the crankshaft speed ( engine speed ). A toothed drive belt is often used, but gears and chains are also used-in some instances a belt and chain are both used.
On some engines, the timing chain is used to drive the oil pump. On diesel engines, a timing chain or a timing gear is used to drive the injection pump as well as the camshaft.
The oil pan, or sump, holds the oil for the engine lubricating system. The oil pan is made of steel that has been pressed to shape. Other oil pans are made of aluminum alloy that has been cast to shape.
The oil pan is bolted to the underside of the engine so that it closes off the crankcase. Only the ends of the crankshaft that extend beyond the cylinder block are exposed.
Various forms of gaskets and seals are used between the surfaces where the parts are bolted together. Some of these are used to seal against oil, some against coolant and some against heat and pressure. Some provide a seal on flat surfaces, other seal against rotating shafts.
As well as the larger parts of an engine, there are numerous small parts. These include bolts, washers, retainers, spring and brackets which go to make up the mechanical components of the engine.
３. Engine Systems
The engine systems that enable the engine to start and to continue to operate are as follows:
(1) Starting system;
(2) Fuel system;
(3) Ignition system;
(4) Cooling system;
(5) Lubricating system;
(6) Intake system;
(7) Exhaust system;
(8) Charging system;
(9) Engine’s electronic control system.
These are the systems for petrol engines. Diesel engines have similar systems except for the fuel and ignition systems. Some parts of the systems are built into the engine, some parts are attached to the engine and other parts are located on the body panels in the engine compartment.
Starting system
The starter is used to rotate the engine during starting. It consists of an electric motor and a drive. The drive has a small pinion that meshes with the ring gear on the flywheel during starting. The battery supplies the electrical energy to operate the starter and rotate the engine until it fires and runs on its own.
Fuel system
These are four basic types of fuel systems: carburetor systems for petrol engines, fuel injection systems for petrol engines, gas fuel systems ( LPG or NGV ), and diesel injection systems. All these systems operate in different ways, but they all have somewhere to store fuel ( a fuel tank or a cylinder ) and a way of supplying the engine with the fuel. They also have a way of supplying air and fuel mixed in the correct proportion so that it can be effectively burn in the combustion chambers.
Ignition system
Petrol engines and engines operating on gas require an ignition system. This is needed to provide the sparks that fire the charges in the combustion chambers. For this reason, petrol engines are sometimes referred to as spark-ignition engines. This distinguishes them from them diesel engines that do not need a spark because they use compression ignition.
Combustion in a diesel occurs when the fuel is sprayed into the combustion chamber. The air in the cylinder is at high temperature from being compressed-high enough to ignite the fuel that is sprayed from the injector.
Cooling system
A considerable amount of heat is produced in an engine by the burning air-fuel mixture. Some of the heat is used to do useful work, some is transferred to other parts of the engine and some is carried away with exhaust gases. However, there is still enough heat to cause damage unless it is removed. This is still the function of the cooling system, which removes about one-third of the heat that is produced.
The cooling system does not just remove heat; it maintains the engine at a desirable operating temperature. In a liquid-cooled engine, this is done by circulating coolant through the water-jackets. In an air-cooled engine, cooling is by air over cooling fins.
Lubricating system
The engine-lubricating system consists of an oil pump, a relief valve and a filter; also pipes, passages and drillings in various parts of engine through which the oil can flow. A quantity of oil is held in the oil pan. From this, oil is taken by the oil pump and circulated throughout the engine before returning to the oil pan. The oil lubricates all the moving parts and this is not only reduces friction, but it prevents wear and damage. The oil pump is driven directly by the crankshaft. This system has an oil cooler, located at the filter mounting under the filter.
Intake system
In petrol engine with electronic fuel injection, the intake system includes the air cleaner, the throttle valve assembly and the intake manifold. The intake system provides clean air and carries it into the engine through the intake manifold. Nozzles of the fuel injectors spray fuel into the air passing from the intake manifold into the intake ports.
With carburetor fuel systems, a mixture of air and fuel is carried from the carburetor, through the intake manifold and into the engine through the intake ports.
For engines that operate on gas, a mixture of air and gas is carried into the engine by the intake manifold.
In petrol engines with fuel injection, and in diesel engines, clean air only is provided by the intake system.
Exhaust system
The exhaust system carries the burnt gases away from the engine and also reduces noise. The system consists of the exhaust manifold, exhaust pipes, a catalytic converter and a muffler. Arrangements vary with different engines, there may be more than one muffler and more than one catalytic converter. Engines that run on Lead Replacement Petrol ( LRP ) do not have a catalytic converter, neither do diesel engines.
Charging system
The alternator, which is driven by the engine, converts mechanical energy to electrical energy.
The battery supplies energy for the starter, the ignition system and electric fuel pump during the starting period ( petrol engine ), but once the engine is running, the alternator supplies all the electrical energy. It also recharges the battery to replace the energy used during starting.
Engine’s electronic control system
The engine’s electronic control system consists of sensors, a control unit and actuators. The control unit receives signals from the sensors and then sends signals to various actuators.
There are sensors on the engine, in the intake system and in the exhaust system. The injectors are actuators that adjust the spray of fuel. There are also actuators in the ignition system that advance and retard the spark. These are just two examples of where electronic control is used.
４. Engine Operation
During the downward motion of the piston, the air-fuel mixture is sucked from the carburetor into the cylinder. During the upward motion the mixture is compressed by the piston in the cylinder and ignited by an electric spark. When the mixture is burned in the cylinder, the resulting heat causes the gases to expand which exert pressure on the cylinder walls and on the piston. The piston, being movable, is pushed downward by this pressure to the full length of its stroke.
The pressure exerted on the piston is transmitted through the connecting rod to the crankshaft that is made to revolve. The crankshaft turns through one-half of a revolution as the piston moves downward. A flywheel attached to the crankshaft stores up energy. The momentum of the flywheel carries the piston through the balance of its motion until it receives another power impulse. The process is repeated over and over again, the crankshaft is turning continuously and the engine is running.
The events that are repeated make up the cycle of the engine. The number of stokes of the piston required to complete the cycle varies with the type of engine. In modern vehicles, the cycle is extended through four stroke of the piston or two revolutions of the crankshaft. This is called a four-stroke cycle engine. In two stroke cycle engines, the cycle is completed in two strokes of the piston or one revolution of the crankshaft.
In the four-stroke cycle engine, the four strokes are named suction, compression, power, and exhaust in accordance with the operation of the cycle which occur during each particular stroke.
Suction stroke: during suction stroke, the piston is moved downward by the crankshaft, which is revolved either by the momentum of the flywheel or by the power generated by the electric starting motor. The inlet valve remains open and the exhaust valve is closed during this stroke. The downward movement of the piston sucks air-fuel mixture in the cylinder from the carburetor through the open inlet valve. Here the fuel is petrol mixed with air, broken up into a mist, and partially vaporized in the carburetor.
Compression stroke: during compression stroke, the piston moves upward, thus compressing the charge. Ignition and much of the compression also take place during this stroke. The heat produced by the compression makes more homogeneous mixture of air and petrol inside the cylinder. The heat makes the petrol easier to burn, while the compression forces it into closer combination with the air. The mixture, under compression, is ignited by the spark produced by a spark plug, and the combustion is over half-completed when the piston is at Top Dead-Centre ( TDC ). Both the inlet and exhaust valves remain closed during the compression stroke.
Working, power or expansion stroke: the expansion of the gases due to the heat of combustion exerts a pressure on the cylinder and piston. Under this impulse the piston moves downward thus doing useful work. Both the valves remain closed during this stroke.
Exhaust stroke: during this stroke, the inlet valve remains closed and the exhaust valve opens. The greater part of the burnt gases escapes because of their own expansion. The piston moves upward and pushes the remaining gases out of the open exhaust valve. Only a small quantity of exhaust gases remains in the clearance space which will dilute the fresh incoming charge.
Thus, in this type of engine, four strokes of the piston are required to complete the cycle, and the four strokes make two revolutions of the crankshaft. The operations are repeated over and over again in running the engine.
