Title: Types of engines
To understand the structure and various types of engines.
At the end of this unit you should be able to:
1. define and classify internal combustion engine.
2. draw and label four stoke and two stoke cycle.
3. explain four stroke cycle engine.
4. explain two stroke cycle engine.
5. explain the process of four and two stroke compression ignition engine.
6. explain the process of the Wankel engine ( Rotary ),
7. explain the process of opposed-piston engine
8. explain the process of turbine gas engine.
In this unit we are to discuss the spark ignition (SI) and compression ignition (CI). These are internal combustion ‘ heat engine’. Their function is to convert potential heat energy contained in fuel into mechanical work.
An internal combustion engine can be classified in two different ways and they are spark ignition (SI) and compression ignition (CI).
1.1 Types of ignitions for the internal combustion engine.
a. Spark Ignition (SI)
An SI engine starts the combustion process in each cycle using a spark plug. The spark plug gives a high-voltage electrical discharge between two electrodes which ignites the air-fuel mixture in the combustion chamber surrounding the plug, Many forms of torch holes are used to initiate combustion from external flame.
b. Compression Ignition (CI).
The combustion process in a CI engine starts when the air-fuel mixture self-ignites due to high temperature in the combustion chamber caused by high compression.
1.2 Four Stroke Cycle
1.2.1 Induction stroke
The air-plus-fuel charge is induced into the cylinder as the piston moves from TDC to BDC. Due to the movement of the piston the pressure in the cylinder is lower than a value between the atmospheric pressure. Air flows through the induction system because of the pressure difference, and the intake valve is opened. Before air is channelled into the cylinder, the air passes through a carburetor in which the metered amount of petrol is added to the air.
1.2.2 Compression stroke.
With both valves closed, the charge is compressed by the piston. At the TDC position the charge occupies the volume above the piston, which is called the clearance volume, and it fills mainly the volume of the combustion chamber. The spark is timed to occur at a point before TDC. The combustion process occurs at almost constant volume, and there is a large increase in pressure and temperature of the charge during this process.
1.2.3 Power stroke
When the gas pressure expands, the temperature also rises. This pushes the piston down the cylinder. This expansion continues and ends near BDC, but in order to assist in exhausting the gaseous products the exhaust valve opens before BDC.
1.2.4 Exhaust stroke.
The piston travels from BDC to TDC in the exhaust stroke, and it pushes most of remaining exhaust gases out of the cylinder, and the pressure during this stroke is slightly higher than atmospheric pressure. In a normal aspirated engine, the clearance volume cannot be exhausted, and at the commencement of the next cycle this volume is full of exhaust gas at about atmospheric pressure. The valve exhaust closes after TDC.
1.3 Two-Stroke SI Cycle
When the piston ascends on the compression stroke, the next charge or air-fuel mixture is drawn into the crankcase through spring-loaded automatic valve. Ignition occurs before TDC, and at TDC the working stroke begins. As the piston descends through 80% of the working stroke, the exhaust port is uncovered by the piston and exhaust release will begin. The transfer port is uncovered later in the stroke due to the shape of the piston or the position of the transfer port in relation to the exhaust port. The charge in the crankcase which has been compressed by the descending piston, enters the cylinder through the transfer port. The piston can be shaped to deflect the fresh gas across the cylinder to assist the scavenging of the cylinder, this is called cross-flow scavenge. As the piston rises, the transfer port is closed slightly before the exhaust port is closed. When the exhaust port is closed compression of the charge in the cylinder begins.
1.4 Compression Ignition Engine Process
1.4.1 Characteristics of CI Engine
a. CI engine does not have carburetor and electrical ignition system, but this
engine has a fuel injection component.
b. Fresh air is induced into the cylinder as the piston moves from TDC to BDC.
c. Temperature is built up during compression of air in the cylinder.
d. Fuel in the form of mist is injected into the compressed air at a high temperature, and the fuel is burnt.
e. Fuel must be injected using a fuel injector pump at a pressure higher than the air compression.
f. In the CI engine, the compression ratio is 21:1 and the pressure becomes 5000 kPa at the end of compression.
1.5 Four-stroke CI engine cycle.
1. First Stroke: Intake Stroke.
It is the same as the intake stroke in an SI engine, the only major difference is that there is no fuel added to the incoming air.
2. Second Stroke: Compression stroke.
It is similar to as in an SI engine except that only air is compressed, the compression is of higher pressure and temperature. Next in the compression stroke fuel is injected directly into the combustion chamber, where it mixes with very hot air. This causes the fuel to evaporate and self-ignites, hence combustion starts.
Combustion is fully developed by TDC and it continues at constant pressure until fuel injection is completed, at this stage, the piston has moved towards BDC.
4. Third Stroke: Power Stroke.
The power stroke continues as combustion ends and the piston travels towards TDC.
5. Exhaust Blow down, similar to an SI engine.
6. Fourth Stroke: Exhaust Stroke. Similar to as an SI engine
1.6 Two-Stroke CI Engine Cycle
The two-stroke cycle of a CI engine is similar to a SI engine, except for two changes. No fuel is added to the incoming air, so that compression is done on air only. Instead of a spark plug, a fuel injector is located in the cylinder. Near the end of the compression stroke, fuel is injected into the hot compressed air and combustion is initiated by self-ignition. This two-stroke CI engine also has a rotary air pump to compress air. The exhaust valve is located in the cylinder head, which is similar to an SI engine.
1.7 Opposed-piston engine
This engine has parallel cylinder barrels. It may be described as an inflow opposed-piston engine since the scavenge air flows continuously from the inlet ports uncovered by one piston to the exhaust ports uncovered by the other, the two pistons moving towards each other for compression and away from each other for expansion.
However, the description is confined to the construction incorporating a single straight cylinder barrel in which the two pistons move in opposite phase towards and away from each other.
1.8 Wankel engine
This engine has rotor that provides three equal working spaces. An exhaust release will occur each time an apex seal overruns the leading edge of an exhaust port, that is, three times per revolution of the rotor, and this exhaust will continue until the following seal reaches the trailing edge of the port.
Induction will commence in the same space about 60° of rotor movement earlier.
Thus, there are three complete four-stroke cycle per revolution of the rotor in different working spaces, but all fired by the same spark plug as maximum compression is reached.
1.9 The practical gas turbine cycle
The most basic gas turbine unit is one operating on the open cycle in which rotary compressor and a turbine are mounted on a common shaft, as shown diagrammatically in Figure 1.9. Air is drawn into the compressor, C, and after that compression passes to a combustion chamber, CC. Energy is supplied in the combustion chamber by spraying fuel into the airstream, and the resulting hot gases expand through the turbine, T, to the atmosphere. In order to achieve net work output from the unit, the turbine must develop more gross work output than is required to drive the compressor and overcome mechanical losses in the drive. The compressor used is either a centrifugal or an axial flow compressor and the compression process is therefore irreversible but approximately adiabatic.
1.9.1. Power turbine
The power turbine is more convenient to have two separate turbines, one of which drives the compressor while the other provides the power output. The first, or high-pressure (HP) turbine, is then known as the compressor turbine, and second, or low-pressure (LP) turbine.
Turbine, is called the power turbine. The arrangement is shown in Figure.1.9.1 The power turbine is mostly used in a jet engine. The population nozzle takes the place of the LP stage turbine.
The aircraft is powered by the reactive thrust of the jet of gases leaving the nozzle, and this high-velocity jet is obtained at the expense of the enthalpy drop from 4 to 5 at the diagram. The turbine develops just enough work to drive the compressor and overcome mechanical losses.
In a turbo-prop engine the turbine drives the compressor and also the airscrew, or propeller. In practice there is also small jet thrust developed in a turbo-prop aircraft.
1.9.3: Regeneration process in the turbine gas unit.
Exhaust gas flows in a turbine which contains great heat and it can be used to heat up the compressed air provided and for burning in the combustion chamber. Transfer of heat occurs in equipment called regeneration.
1.9.4 Regeneration process
Exhaust gas flows in tubes whereas air flows on the outside of the tubes in the opposite direction, and it is heat up by the heat of the gas. The effectiveness of heat transfer depends on the heated area of tubes.
In using regeneration to heat up air, can reduce fuel consumption and improve the cycle of heat.
CHAPTER 2 (CLICK HERE TO DOWNLOAD : PART 1 & PART 2)
Title: Engine process analysis
To understand the structure and various types of cycle engines.
At the end of this unit you should be able to:
1. define the air standard cycle.
2. define constant pressure (cp) and constant volume (cv).
3. draw p-v diagram of Otto cycle, diesel cycle.
4. explain Otto cycle, diesel cycle.
In this unit we are to discuss the meaning of standard cycle, heat supplied at constant volume and heat supplied at constant pressure. An internal combustion engine can be classified into three different cycles and they are otto cycle, diesel cycle and dual-combustion cycle.
2.1 The Otto Cycle
The Otto cycle is the ideal air standard cycle for the petrol engine, the gas engine, and the high-speed oil engine. The cycle is shown on p-v diagram in
Otto cycle process:
a. 1 to 2 is isentropic compression with compression ratio v1 / v2, or rv .
b. 2 to 3 is reversible constant volume heating, the heat supplied Q1
c. 3 to 4 is isentropic expansion, v4 /v3 is similar to v1 /v2.
d. 4 to 1 is reversible constant volume cooling, the heat rejected Q2.
2.2 The diesel cycle
In diesel cycle, heat is supplied at a constant pressure and rejected at a constant volume. The process of cycle is:
Process 1 to 2 is isentropic compression
Process 2 to 3 is reversible constant pressure heating.
Process 3 to 4 is isentropic expansion.
Process 4 to 1 is reversible constant volume cooling.
2.3 The Dual Combustion Cycle
Modern oil engines known also as diesel engine, use solid injection of the fuel. The ideal cycle which is used as a basis for comparison is called the dual combustion cycle or the mixed cycle, and is shown on a p-v diagram in Figure. 3.1 In this cycle, heat is supplied in two parts; the first part at constant volume and the second in constant pressure. Hence the name ‘dual combustion’.
The dual combustion cycle process.
1. Process 1 to 2 is isentropic compression.
2. Process 2 to 3 is reversible constant volume heating.
3. Process 3 to 4 is reversible constant pressure heating.
4. Process 4 to 5 is isentropic expansion.
5. Process 5 to 1 is reversible constant volume cooling.
CHAPTER 3 (CLICK HERE TO DOWNLOAD)
Title: Combustion and characteristic of fuel
To understand the basic of combustion and analyze the major components needed to make an engine run.
At the end of this unit you should be able to:
1. describe the combustion of fuel injection.
2. draw engine pressure Vs crank angle diagram
3. define the term “Knocking”
5. list the effects knocking during engine process.
6. define how to reduce knocking problem during
7. draw the Ricardo Diagram.
8. define the Catena Number.
This section introduces the subject matter that you are going to learn.
In this unit, we are to discuss several automotive manufacturers who have built diesel engine in certain automobiles. For many years the manufacturers try to upgrade the system especially internal combustion engine and general motor; for example, the V-8 diesel engine in large vehicles pickup trucks. Vehicle service is very important, and mechanics or technicians must be able to service all types of vehicles. Hence it is important for the mechanics or technicians to study the basic system of diesel fuel injection and high pressure fuel injection. This unit introduces the principle of diesel fuel injection systems.
3.1 Combustion of fuel injection.
All diesel engines being manufactured today utilize, the injection systems because the injection of diesel fuel is directed into cylinder at the right timing. In other words, diesel fuel is injected at a precise time into the top of the cylinder, during the compression stroke of diesel engine. In order to do this, the pressure to produce atomized fuel must be very high. Injection systems inject at a range of 4,000 to 10,000 psi.
To understand combustion of fuel injection, we must know the definition of common words in this unit.
What is combustion? Combustion is one process in which air and
fuel are burned after being mixed at a correct ratio of 14.7 parts of air to 1
part of fuel.
In this part , we learn the specific requirements for combustion, including
topics such as air, fuel and ignition requirements, timing, air-fuel ratios , compression ratio and engine efficiency. The internal combustion engine has certain requirements for efficient operation. There must be sufficient air for combustion, correct amounts of fuel mixed with the air, and ignition to start combustion.
22.214.171.124 Air, fuel and ignition requirements
This process is very important to produce power and can be used for pushing the vehicle forward. If any one of these three ingredients is missing, the engine will not run.
Figure 126.96.36.199 : Combustion process: Three things are needed for
Combustion; air, fuel and ignition. If one ingredient is
missing, combustion will not happen.
One process of identifying when air, fuel and ignition occur in relation to the crankshaft rotation. This is a relationship between the position of the piston and crankshaft. For the engine to operate efficiently, the air and fuel mixture must enter the cylinder at the correct time. This mean that the intake valve must be opened and closed at the correct time. The exhaust valve must be opened and closed at the correct time too. The ignition must also be timed. The timing of the ignition can change with speed and load; if the process is correctly timed, maximum power will be obtained in converting chemical energy into mechanical energy.
188.8.131.52 Air-Fuel ratio.
Air –fuel ratio is defined as the ratio of air to fuel mixed by the carburetor or fuel injectors. The term air-fuel ratio is often called the stoichiometric ratio. The air and fuel must be thoroughly mixed. Each molecule of fuel must have enough air surrounding it to be completely burned. If the two are not mixed in the correct ratio, engine efficiency will drop, and exhaust emission level will increase.
184.108.40.206 Compression ratio
During engine operation, the air and fuel mixture must be compressed. This will be covered in the discussion of the four cycle principle in this unit. This compression helps to squeeze and mix the air and fuel molecules for better compression. The more air and fuel are compressed, the better will be the efficiency of the engine. According to an engineering dictionary, the meaning of compressed ratio is a measurement of how much the air and fuel have been compressed and the ratio of the volume in the cylinder above the piston when the piston is at BDC to the volume in the cylinder above the piston when the piston is at TDC.
Compression ratio = Volume above the piston at BDC
Volume above the piston at TDC
220.127.116.11 BMEP (Brake Mean Effective Pressure)
This is a theoretical term used to indicate how much pressure is applied to the top of the piston from TDC to BDC. This term is only used when we analyze the results of different fuel in an engine.
18.104.22.168 Engine Efficiency
The term efficiency can be used to indicate the quality of different machines. Efficiency can also be applied to engine. Engine efficiency is a measure of the relationship between the amount of energy put into the engine and the amount of energy available out of the engine. The basic efficiency is defined as;
Efficiency = output energy x 100 %
What is ignition? The ignition system is designed to ignite the air and fuel that have been mixed in the fuel system. It is important to improve this system. Each year, ignition is becoming more and more computerized. Today’s ignition systems are almost totally computer controlled for improved combustion.
3.1.3 Pra- Ignition
This Is one process where the spark is heated up before the ignition begins. It causes rough running and in extreme cases, can do damage to the engine.
3.1.4 Delay period
Delay period is the commencement of injection and it is indicated by the dot on the compression line 15° before dead centre. The period 1 is the delay period during which ignition is being initiated, but without any measurable departure of the pressure from the air compression curve which is continued as a broken line in the diagram as it would be recorded if there were no injection and combustion.
Figure 3.1.4 : Delay period
CHAPTER 5 : Turbo charging and supercharging
CHAPTER 6 : Cylinder block and cylinder head
CHAPTER 7 : Piston and piston ring
CHAPTER 8 : Valve and valve cooling
CHAPTER 9 : Emission control system