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Module 1: Engine components and its Principle parts
Spark Ignition (SI) engine: Spark Ignition (SI) Engine is a type of engine in which the combustion takes place by the spark generated by the spark plug. It uses petrol as fuel and works on Otto cycle. In the spark ignition engine the air fuel mixture is inserted into the cylinder with help of carburetor. The compression of the fuel takes place but it has low compression ratio. The fuel is ignited by the spark generated by the spark plug. SI engine produces less noise and vibration and their starting is very easy. They are light in weight and have less maintenance cost. They are mostly used in light commercial vehicles such as scooters, motorcycles cars etc. The four stroke-cycles refers to its use in petrol engines, gas engines, light, oil engine and heavy oil engines in which the mixture of air fuel are drawn in the engine cylinder. Since ignition in these engines is due to a spark, therefore they are also called spark ignition engines. SUCTION STROKE: In this Stroke the inlet valve opens and proportionate fuel-air mixture is sucked in the engine cylinder. Thus the piston moves from top dead centre (T.D.C.) to bottom dead centre (B.D.C.). The exhaust valve remains closed through out the stroke. COMPRESSION STROKE: In this stroke both the inlet and exhaust valves remain closed during the stroke. The piston moves towards (T.D.C.) and compresses the enclosed fuel-air mixture
drawn. Just before the end of this stroke the operating plug initiates a spark which ignites the mixture and combustion takes place at constant pressure. POWER STROKE OR EXPANSION STROKE: In this stroke both the valves remain closed during the start of this stroke but when the piston just reaches the B.D.C. the exhaust valve opens. When the mixture is ignited by the spark plug the hot gases are produced which drive or throw the piston from T.D.C. to B.D.C. and thus the work is obtained in this stroke. EXHAUST STROKE: This is the last stroke of the cycle. Here the gases from which the work has been collected become useless after the completion of the expansion stroke and are made to escape through exhaust valve to the atmosphere. This removal of gas is accomplished during this stroke. The piston moves from B.D.C. to T.D.C. and the exhaust gases are driven out of the engine cylinder; this is also called SCAVENGING. Compression ignition(CI) engine: Compression Ignition (CI) Engine is an engine in which the combustion of fuel takes place by the heat of the compressed air. It uses diesel as fuel and works on Diesel cycle. In the compressed ignition engine, only air enters into the cylinder during suction stroke. It has high compression ratio because of the high ignition temperature of the diesel fuel. The fuel is ignited by the heat of the compressed air. Due to high compression ratio it produces more power. Due to incomplete combustion of the fuel, it produces more hydrocarbons which lead to air pollution. The noise and vibration problem is there in the CI engines. The maintenance cost of the CI engine is more as compared with the SI engines. They are mostly used in heavy duty vehicles such as buses, trucks, railways, ships etc.
- Thermal efficiency High thermal efficiency. Less thermal efficiency.
- Method of ignition Spark plug is used to produce spark for the ignition. Heat of compressed air is used for the ignition.
- Engine Speed High speed engines. Low speed engines.
- Pressure generated Low pressure is generated after combustion. High pressure is generated after combustion.
- Constant parameter during cycle Constant volume cycle. Constant pressure cycle.
- Intake Air + fuel. Only air. Weight of engine Si engine has less weight. CI engine are heavier.
- Noise production It produces less noise. It produces more noise.
Production of hydrocarbon Less Hydrocarbon is produced. More hydrocarbon is produced.
- Starting Starting of SI engine is easy. Starting of CI engine is difficult.
- Maintenance cost Low High
- Vibration problem Less Very High
- Cost of engine Less cost High cost
Volume to power ratio Less High
- Fuel supply Carburetor Injector
- application It is used in light commercial vehicles like motorcycle, cars etc. It is used in heavy duty vehicles likes bus, trucks, ships etc. Cylinder-Arrangements The engine cylinders can be arranged in the following ways (Fig. 2.12). (a) In a row (in-line) (6) In two rows or banks set at an angle (V-type) (c) In two rows, opposing each other (flat, or pancake) Generally, engines with number of cylinders up to six are of in-line type, in which the cylinders are arranged in a bank, one behind the other. The cylinder bank is inclined to vertical either at 30 or 45 degrees. The cylinder centerlines are parallel, one next to the other (Fig. 2.13). They are usually located within a single block, and the connecting rods are connected to a common crankshaft. The engine with eight cylinders and more are V-type in which cylinders are arranged in two banks forming a “V with the crankshaft as the bottom point The angle between two banks of cylinders is either 60 to 90 degrees. However, four and six cylinder engines are also available in V-type. Several rear engine drive automobiles use four or six cylinder engine with half of the cylinder at 180 degrees to the other half. Cylinders work to the same crankshaft. The cylinder centerlines are usually in a horizontal plane and they are therefore called horizontally
- Heat transfer: The cylinder liner receives combustion heat through the piston and piston rings and transmits the heat to the coolant.
- Compression gas sealing: The cylinder liner prevents the compressed gas and combustion gas from escaping outside. It is necessary that a cylinder liner which is hard to transform by high pressure and high temperature in the cylinder. A cylinder wall in an engine is under high temperature and high pressure, with the piston and piston rings sliding at high speeds. In particular, since longer service life is required of engines for trucks and buses, cast iron cylinders that have excellent wear-resistant properties are only used for cylinder parts. Piston:
A piston is a component of reciprocating engines, reciprocating pumps, gas compressors and pneumatic cylinders, among other similar mechanisms. It is the moving component that is contained by a cylinder and is made gas-tight by piston rings. In an engine, its purpose is to transfer force from expanding gas in the cylinder to the crankshaft via a piston rod and/or connecting rod. In a pump, the function is reversed and force is transferred from the crankshaft to the piston for the purpose of compressing or ejecting the fluid in the cylinder. In some engines, the piston also acts as a valve by covering and uncovering ports in the cylinder. Connecting Rod: A connecting rod is a shaft which connects a piston to a crank or crankshaft in a reciprocating engine. Together with the crank, it forms a simple mechanism that converts reciprocating motion into rotating motion. Connecting rods are commonly made from cast aluminum alloy and are designed to withstand dynamic stresses from combustion and piston movement. The small end of the connecting rod connects to the piston with a piston pin. The piston pin, or wrist pin, provides a pivot point between the piston and connecting rod. Spring clips, or piston pin locks, are used to hold the piston pin in place. Crankshaft: A crankshaft (related to crank) is a mechanical part able to perform a conversion between reciprocating motion and rotational motion. In a reciprocating engine, it translates reciprocating motion of the piston into rotational motion; whereas in a reciprocating compressor, it converts the rotational motion into reciprocating motion.
Valve and port timing diagrams: A valve timing diagram is a representation of the positions of the crank when the various operations as inlet valve opening, closing, exhaust valve opening and closing and also the beginning and end of various strokes. The valves cannot open and close abruptly; it requires a finite period of time for its operation so a time advance is given for proper functioning. The timing of the valves is controlled by cam settings. Valve timing for 4 stroke petrol (Spark Ignition) engine. Now let us see the various position of the crank when the inlet and exhaust valves during the various processes. Inlet valve opening: The valve timing is different for low speed and high speed. The inlet valve opens before the piston reaches the TDC during the exhaust stroke. This is to ensure that the fresh charge enters the cylinder as soon the piston as soon as the piston starts to move down. Actual valve timing diagram for actual valve timing diagram for lowhigh speed 4 stroke SI engine speed 4 stroke SI engine. Inlet Valve closing: The closing of the inlet valve takes place during the start of compression stroke (i.e. when the piston move from BDC to TDC after finishing suction stroke and the starting of compression stroke). I f the inlet valve is allowed to close exactly at BDC then less charge than the capacity enters during the suction stroke so the inlet valve closing is delayed to 20°-25 degrees after the crank
reaches the BDC position during slow speed and 40- 5 0degrees after the crank reaches the BDC position during high speed. Exhaust Valve Opening: The exhaust valve opens at the end of expansion stroke. The exhaust valve opening is done before the piston reaches the BDC so as to provide more time for all the burnt gases to escape. The opening of the exhaust valve is necessary because if there are some burnt up gases left in the cylinder it may affect the cylinder walls and the spark plug. So the exhaust valve is opened 30- 35 degrees before BDC for slow speed and 45-50degrees before BDC for high speed. Exhaust valve closing: The exhaust valve closing is also important to let out all the burnt gases. The time between the exhaust valve opening and the exhaust valve closing determines the amount of burnt gases that escapes. Usually the exhaust valve is closed 8°-10°after the piston reaches the TDC position. An important phenomenon in the valve timing diagram is the angle of overlap. Combustion chamber for SI and CI Engine: The design of combustion chamber involves the shape of the combustion chamber, the location of the sparking plug and the disposition of inlet and exhaust valves. Basic requirements of a good combustion chamber are-
- High power output
- High thermal efficiency and low specific fuel consumption
- Smooth engine operation
- Reduced exhaust pollutants. Combustion chamber for SI Engine-
was first introduced by Ford motor in 1910-30 and was quite popular for some time. This design has an advantage both from manufacturing and maintenance point of view. Advantages Valve mechanism is simple and easy to lubricate. Detachable head easy to remove for cleaning and decarburizing without Valves of larger sizes can be provided. Disadvantages Poor turbulence Extremely prone to detonation due to large flame length and slow combustion More surface-to-volume ratio and therefore more heat loss. Extremely sensitive to ignition timing due to slow combustion process
3. I-head (or overhead valve) combustion chamber: The disappearance of the side valve or L-head design was inevitable at high compression ratio of 8 : 1 because of the lack of space in the combustion chamber to accommodate the valves. Diesel engines, with high compression ratios, invariably used overhead valve design. Since 1950 or so mostly overhead valve combustion chambers are used. Lower pumping losses and higher volumetric efficiency from better breathing of the engine from larger valves or valve lifts and more direct passageways. Less distance for the flame to travel.
Less force on the head bolts and therefore less possibility of leakage (of compression gases or jacket water). Removal of the hot exhaust valve from the block to the head, thus confining heat failures to the head. Absence of exhaust valve from block also results in more uniform cooling of cylinder and piston. Lower surface-volume ratio and, therefore, less heat loss and less air pollution. Easier to cast and hence lower casting cost The main drawback of this design is: Both valves are placed in a single row along the cylinder block. This limits the breathing capacity of engine, unless the overall length is increased. However, modern engine manufactures overcome this problem by using unity ratio for stroke and bore size.
4. F-head combustion chamber: In such a combustion chamber one valve is in head and other in the block This design is a compromise between L-head and I-head combustion chambers Advantages :
- High volumetric efficiency
- Maximum compression ratio for fuel of given octane rating High thermal efficiency
- It can operate on leaner air-fuel ratios without misfiring. Drawback:
- Ease of starting.
- Ability to handle variations in speed.
- Smoothness of operation i.e. avoidance of diesel knock and noise.
- Low exhaust emission.
- Nozzle design.
- High volumetric efficiency.
- High brake Mean effective pressure. TYPES OF COMBUSTION CHAMBERS- CI Engines
- OPEN INJECTION (DI) TYPE: This type of combustion chamber is also called an Open combustion chamber. In this type the entire volume of combustion chamber is located in the main cylinder and the fuel is injected into this volume.
- INDIRECT INJECTION (IDI) TYPE: in this type of combustion chambers, the combustion space is divided into two parts, one part in the main cylinder and the other part in the cylinder head. The fuel – injection is effected usually into the part of chamber located in the cylinder head. Choice of materials for different Engine Components : A. Parts common to both Petrol and Diesel engine:
- Cylinder, 2. Cylinder head, 3. Piston, 4. Piston rings, 5. Gudgeon pin, 6. Connecting rod,
- Crankshaft, 8. Crank, 9. Engine bearing, 10. Crank case. 11. Flywheel, 12. Governor,
- Valves and valve operating mechanism. B. Parts for Petrol engines only:
- Spark plug, 2. Carburetor, 3. Fuel pump. C. Parts for Diesel engine only:
- Fuel pump, 2. Injector
Cylinder: A cylinder is the central working part of a reciprocating engine or pump, the space in which a piston travels Gray cast iron, compact graphite cast iron, cast Al alloy. Piston: its purpose is to transfer force from expanding gas in the cylinder to the crankshaft via a piston rod and/or connecting rod Al-Si-Cu-Mg alloy
Fig: Three bearing crankshaft for a four cylinder engine Fig: Five bearing crankshaft for C I engine Camshaft: It’s an internal component of valve train which controls the valve operation timing with pushrod or rocker arms or self controlling lobes. Chilled cast iron, Cr-Mo steel, iron base sintered metal Valve and Valve spring: The valve which allows mixture into the cylinder is the inlet valve; the one through which the spent gases escape is the exhaust valve. They are designed to open and close at precise moments; to allow the engine to run efficiently at all speeds.
Engine Positioning: Front engine
- The large mass of an engine at the front of the car gives the driver protection in the event of a head on collision.
- Engine cooling is simpler to arrange
- In addition the cornering ability of a vehicle is normally better if the weight is concentrated at the front.
