ME 2041 Advanced Internal Combustion Engines Syllabus Unit

ME 2041 Advanced Internal Combustion Engines Syllabus: Unit I SPARK IGNITION ENGINES • Air-fuel ratio requirements , • Design of carburettor –fuel jet size and venture size, • Stages of combustion-normal and abnormal combustion, • Factors affecting knock, • Combustion chambers, • Introduction to thermodynamic analysis of SI Engine combustion process. Unit I

ME 2041 Advanced Internal Combustion Engines Syllabus: Unit II COMPRESSION IGNITION ENGINES • • • Stages of combustion-normal and abnormal combustion Factors affecting knock, Direct and Indirect injection systems, Combustion chambers, Turbo charging , Introduction to Thermodynamic Analysis of CI Engine Combustion process. Unit II

ME 2041 Advanced Internal Combustion Engines Syllabus: Unit III ENGINE EXHAUST EMISSION CONTROL • Formation of NOX , HC/CO mechanism , Smoke and Particulate emissions, • Green House Effect , • Methods of controlling emissions , • Three way catalytic converter and Particulate Trap, • Emission (HC, CO, NO and NOX , ) measuring equipments, Smoke and Particulate measurement, • Indian Driving Cycles and emission norms Unit III

ME 2041 Advanced Internal Combustion Engines Syllabus: Unit IV ALTERNATE FUELS • Alcohols , Vegetable oils and bio-diesel, Bio-gas, Natural Gas , Liquefied Petroleum Gas , Hydrogen , • Properties , Suitability, Engine Modifications, Performance , • Combustion and Emission Characteristics of SI and CI Engines using these alternate fuels. Unit IV

ME 2041 Advanced Internal Combustion Engines Syllabus: Unit V RECENT TRENDS • Homogeneous Charge Compression Ignition Engine, Lean Burn Engine, Stratified Charge Engine, Surface Ignition Engine , Four Valve and Overhead cam Engines, • Electronic Engine Management, Common Rail Direct Injection Diesel Engine, Gasoline Direct Injection Engine , • Data Acquisition System –pressure pick up, charge amplifier PC for Combustion and Heat release analysis in Engines. Unit V

ME 2041 Advanced Internal Combustion Engines • Carburetion The process of formation of combustible air-fuel mixture, by mixing correct amount of fuel and air in a device called carburetor, before it enters the engine cylinder. • Factors Affecting Carburetion 1. Carburetor Design has influence on distribution of air-fuel mixture to cylinders. 2. Ambient Air condition Ambient pressure and temperature influence the efficiency of carburetion. Higher ambient temperature increases the vaporization rate of fuel forming a homogeneous mixture. 3. Fuel Characteristics Evaporation characteristics (indicated by distillation curve) is critical for carburetion; presence of volatile HC also is important for quick evaporation Unit I

ME 2041 Advanced Internal Combustion Engines 4. Engine Speed and Load • • At higher engine speed, the carburetion time is less causing strain on carburetor to deliver uniform mixture in a short time; thus provision of venturi has to be such that the carburetion is done efficiently at higher pressure drops Higher loads will demand richer mixture and lower load leaner mixtures. • Types of Air-Fuel Mixtures 1. Chemically Correct Mixture Stoichiometric or balanced chemical mixture in which air is provided to completely burn the fuel; the excess air factor is unity 2. Rich Mixture Fuel is in excess of what is required to burn the fuel completely. The excess air factor is less than unity. 3. Lean Mixture Air is in excess of what is required to burn the fuel completely. The excess air factor is greater than unity. Unit I

ME 2041 Advanced Internal Combustion Engines • Range of Air-Fuel Ratio in SI Engines 9: 1 (rich) to 19: 1(lean) ; The stoichiometric value for gasoline is 14: 1, The SI engine will not run for too rich or too lean mixtures. • Mixture Requirements at Different Engine Conditions The air fuel ratio affects the power output and brake specific fuel consumption of the engine as shown in the Figure 1. Power Output (k. W) BSFC A/F ratio Unit I BSFC (kg/k. Wh)

ME 2041 Advanced Internal Combustion Engines • Mixture Requirements at Different Engine Conditions (Contd. ) • The mixture corresponding to maximum output on the curve is called best power A/F mixture, which is richer than the stoichiometric mixture. • The mixture corresponding to maximum BSFC on the curve is called best economy mixture, which is leaner than the stoichiometric mixture. • The actual A/F ratio requirement for an automative carburetor falls in 3 ranges: § Idling (rich) § Cruising (lean) § High Power (rich) Unit I

ME 2041 Advanced Internal Combustion Engines • Mixture Requirements at Different Engine Conditions (Contd. ) Idling Cruising Power 1 4 A/F Ratio 2 0 3 50% Throttle Opening Figure 2. A/F Ratio Vs Throttle opening Unit I 100%

ME 2041 Advanced Internal Combustion Engines • Mixture Requirements at Different Engine Conditions (Contd. ) Idling Range (1 -2) • • • During idling, engine operates at no load and closed throttle. • • Also, the amount of fresh charge admitted is less due to smaller throttle opening. • Rich mixture improves the contact of fuel and air by providing efficient combustion at idling conditions. • As the throttle is opened further, the exhaust gas dilution reduces and the mixture requirement shifts to the leaner side. The engine requires rich mixture for starting at idling. Rich mixture is required to compensate for the charge dilution due to exhaust gases from the combustion chamber. Exhaust gas dilution prevents efficient combustion by reducing the contact between the fuel and air particles. Unit I

ME 2041 Advanced Internal Combustion Engines • Mixture Requirements at Different Engine Conditions (Contd. ) Cruising Range (2 -3) • • • Focus is on fuel economy. No exhaust gas dilution. Carburetor has to give best economy mixture i. e. . Lean mixture. High Power Range (3 -4) • As high power is required, additional fuel has to be supplied to achieve rich mixture in this range. • Rich mixture also prevents overheating by reducing the flame temperature and cylinder temperature. Unit I

ME 2041 Advanced Internal Combustion Engines • Principle of Operation of Simple Carburettor Unit I

ME 2041 Advanced Internal Combustion Engines • Principle of Operation of Simple Carburettor • The carburettor works on Bernoulli's principle: the faster air moves, the lower its static pressure, and the higher its dynamic pressure. • The throttle (accelerator) linkage does not directly control the flow of liquid fuel. Instead, it actuates carburettor mechanisms which meter the flow of air being pulled into the engine. The speed of this flow, and therefore its pressure, determines the amount of fuel drawn into the airstream. • A simple carburetor consists of a float chamber, fuel discharge nozzle, a metering orifice, a venturi a throttle valve and choke. • • The float and needle valve maintain the fuel level • • During suction stroke air is drawn through the venturi. Fuel strainer is used to trap debris from the fuel and prevent choking of the fuel nozzle. It is removed periodically for cleaning. Venturi accelerates the air causing a pressure drop. Unit I

ME 2041 Advanced Internal Combustion Engines • Principle of Operation of Simple Carburettor • This pressure drop provides vacuum necessary to meter the air-fuel mixture to the engine manifold. • Fuel is fed to the fuel discharge jet, the tip of which is located at the throat of the venturi • • Pressure drop is proportional to the throttle opening or load on the engine. • The choke valve is located between the entrance and venturi throat. It is also of butterfly type. When choke is partly closed, a large pressure drop occurs at the venturi throat, which provides a rich mixture by induction of large amount of fuel as required during idling or high load conditions. Choke valves are spring loaded to prevent excessive choking and are sometimes automatically controlled by thermostat. • For providing rich mixture during idling, an idling adjustment is provided. It has an idling passage and idling port. Throttle valve achieves governing of SI engine by varying the A/F ratio. It is a butterfly valve located after the venturi tube. When the load is less, the throttle is in near closed position and if the load is high throttle is fully opened. Unit I

ME 2041 Advanced Internal Combustion Engines • Principle of Operation of Simple Carburettor • The system operates at starting and shuts off when 20% throttle opening is reached. • Normal venturi depression is not sufficient to provide rich mixture due to lower throttle opening. But this low pressure causes fuel rice in idling passage and it is discharged through idling port downstream of the throttle valve. • The idling air bleed sucks some air for mixing with the idling fuel and vaporizes the mixture. The additional fuel-air supply makes the mixture rich for idling. • Simple carburettor has the drawback of providing rich mixture with increasing throttle opening. Unit I

ME 2041 Advanced Internal Combustion Engines • Compensating systems in Carburettors • For part load conditions, the carburettor must supply economic air-fuel ratio mixture. The main metering system will not satisfy this requirement. The following compensating systems are used to achieve this: • • Unit I Air Bleed Jet Compensating Jet Emulsion Tube Back Suction Control Mechanism Auxiliary Air Valve Auxiliary Air Port Altitude Compensating Device

ME 2041 Advanced Internal Combustion Engines • Compensating systems in Carburettors Air Bleed Jet • • It contains an air bleed to the main nozzle. Air flow through the bleed passage is restricted by orifice. When engine is not operating the bleed passage is filled with fuel. When the engine starts the fuel from the bleed passage is displaced by air flow from the orifice. The air and fuel form an emulsion at the tip of the bleed passage. This causes faster delivery of fuel due to low viscosity and fuel discharged rises. Thus uniform mixture ratio is supplied. Unit I

ME 2041 Advanced Internal Combustion Engines • Compensating systems in Carburettors Compensating Jet • • • The purpose of this is to make the mixture leaner as the throttle opens progressively. An additional jet called compensating jet is provided with the main jet. This jet is also connected to the fuel well and the fuel is metered through compensating orifice. As the throttle opening increases the main jet makes the mixture richer by adding more fuel. The compensating jet makes the mixture leaner proportionately. The total mixture will make A/F ratio constant. When the main jet is lean, compensating jet is rich. Unit I

ME 2041 Advanced Internal Combustion Engines • Compensating systems in Carburettors Emulsion Tube • • It is also known as submerged jet device. Here, the main metering jet is kept at a level 25 mm below the fuel level in float chamber. The jet is called submerged jet. The jet is placed in a well that has holes exposed to atmosphere. When the throttle opening increases, the holes in the well are uncovered causing additional fuel and air to enter the air-fuel stream, causing the faster A/F mixture delivery during part load operation. Unit I

ME 2041 Advanced Internal Combustion Engines • Compensating systems in Carburettors Back Suction Control Mechanism • • In this device, the top of the fuel chamber is connected to air entry by means of a large vent line fitted with a control valve. The second line connects the fuel float chamber to venturi throat via a metering orifice. When the control valve is opened, the pressure in float chamber is p 1 and the throat pressure is p 2 which is lower than p 1. This causes the fuel to flow. When the valve is closed, there is no difference in pressure and hence no fuel flow. Thus the control valve achieves the desired air fuel ratio during part load operation. Unit I

ME 2041 Advanced Internal Combustion Engines • Compensating systems in Carburettors Auxiliary Air Valve • • When the engine is not in operation, the pressure p 1 acting on the valve is ambient. The pressure p 2 acting at the venturi is negative (vacuum). This pressure differential lifts the auxiliary valve against the spring tensile force. Additional air is thus infused in the air-fuel mixture preventing rich mixture during part load operation. Unit I

ME 2041 Advanced Internal Combustion Engines • Compensating systems in Carburettors Auxiliary Air Port • If the butterfly valve is opened, additional air passes through this port, reducing air flow through venturi. Thus pressure differential is comparatively smaller. Thus fuel drawn is reduced to compensate for loss in density of air at high altitudes. Unit I

ME 2041 Advanced Internal Combustion Engines • Compensating systems in Carburettors Altitude Compensation Device • • • This was used in high altitude car driving and for aircrafts. At high altitudes, air density decreases and hence engine power output is affected. A/F ratio is affected at high altitudes as carburettors are designed to operate on sea level. To compensate for the change in air density, fuel flow has to be reduced from the calibrated value at sea level. A mixture control system comprising a needle valve, which restricts fuel flow in proportion to altitude change acts as an altitude compensating device. Unit I

ME 2041 Advanced Internal Combustion Engines • Calculation of A/F ratio for a Simple Carburettor • Let be the difference in height between the tip of the nozzle and fuel level in the float chamber • - Air velocities at inlet and exit - Pressures at inlet and exit - Air density Applying Bernoulli’s Equation across the venturi, As Unit I ,

• Calculation of A/F ratio for a Simple Carburettor Mass flow rate of air through the venturi, Similarly Mass flow rate of fuel, A/F ratio is, Where , A- area of venturi, Af – Area of fuel jet, f – density of fuel Unit I Due to the difference in level between tip of jet and fuel level in chamber

ME 2041 Advanced Internal Combustion Engines • Calculation of A/F ratio for a Simple Carburettor Mass flow rate of air through the venturi, Similarly Mass flow rate of fuel, A/F ratio is, Where , A- area of venturi, Af – Area of fuel jet, f – density of fuel Unit I Due to the difference in level between tip of jet and fuel level in chamber

ME 2041 Advanced Internal Combustion Engines • Combustion in SI Engines • Combustion is the process of oxidation of fuel resulting into the release of energy equivalent to calorific value of fuel. Energy released in combustion is in the form of heat. • Combustion process in spark ignition engine has requirement of the • mixture of fuel and air in right proportion • mechanism for initiation of combustion process and • stabilization and propagation of flame for complete burning • For complete combustion of every fuel there is chemically correct fuel-air ratio also called stoichiometric fuel-air ratio. This fuel air ratio may be rich or lean depending upon the proportion of fuel and air present in mixture. In SI engine this fuel air ratio generally varies between 1 : 7 to 1 : 30 with lean mixture at 1 : 30 and rich mixture at 1 : 7. Stoichiometric fuel-air ratio is around 1 : 14 to 1 : 15 for hydrocarbon fuel. The extreme values of fuel-air ratio permissible in SI engine on both rich and lean ends put limits as ‘lower ignition limit’ and ‘upper ignition limit’. • • Unit I

ME 2041 Advanced Internal Combustion Engines • Combustion in SI Engines • • Varying fuel-air ratio is required in SI engine due to varying loads on engine between no load to full load on engine. The ratio of actual fuel-air ratio to stoichiometic fuel-air ratio is given by ‘equivalence ratio’ or ‘relative fuel-air ratio’. Appropriate fuel-air ratio is maintained in SI engines through ‘carburettor’ (the fuel metering system). Unit I

ME 2041 Advanced Internal Combustion Engines • Stages of Combustion in SI Engines Unit I

ME 2041 Advanced Internal Combustion Engines • Stages of Combustion in SI Engines Combustion in SI engine may be described to be occurring in following significant phase: (i) preparation phase • After compression of fuel-air mixture in cylinder the high temperature spark is delivered by spark plug in the compressed fuel-air mixture. Temperature at the tip of spark plug electrode may go even more than 10, 000ºC at the time of release of spark. • Sparkles released have sufficiently high temperature to initiate the combustion of fuel. For complete combustion of fuel mere initiation of combustion does not serve the purpose instead a sustainable combustion process is required. • After setting up of combustion, a sustainable flame front or flame nuclei is needed so that it proceeds across the combustion space to ensure complete combustion. Thus, this phase in which spark is first released followed by setting up of sustainable flame front is called “preparation phase” and may consume around 10º of crank angle rotation. Unit I

ME 2041 Advanced Internal Combustion Engines • Stages of Combustion in SI Engines • Crank angle rotation consumed in “preparation phase” depends upon the speed of engine, constructional feature of cylinder, piston, location of spark plug, strength of spark, characteristics of fuel, fuel-air ratio etc. • Preparation phase is shown to occur from ‘a’ to ‘b’ with small or negligible pressure rise as initially rate of burning is very small. (ii) Flame Propagation Phase • After sustainable combustion flame is set up, then the flame nuclei get scattered due to excessive turbulence in combustion space causing pressure to rise from ‘b’ to ‘c’. • This phase of combustion depends upon the turbulence inside cylinder, strength of combustion nuclei, fuel-air ratio, strength of spark, cylinder geometry, fuel properties etc. • This phase of combustion is called as “flame propagation phase” and is accompanied by the excessive pressure rise. Flame propagation phase should also be as small as possible. Unit I

ME 2041 Advanced Internal Combustion Engines • Stages of Combustion in SI Engines (iii) After Burning Phase • After the maximum amount of fuel-air mixture is burnt, the residual gets burnt after the piston has moved across the TDC. • This last phase is termed as “after burning phase” and occurs during the expansion stroke. • • • Hence, it can be summarised that the complete combustion in SI engine occurs in three distinct zones i. e. preparation phase, flame propagation phase and after burning phase. In order to have complete combustion in smallest possible time the flame propagation phase and preparation phase should be shortened. Out of total distance travelled in combustion space in first phase i. e. Preparation phase about 10% of combustion space length is covered in about 20– 30% of total time for combustion. Unit I

ME 2041 Advanced Internal Combustion Engines • Stages of Combustion in SI Engines • • Flame propagation phase is spread in about 80% of combustion space length and is covered in 60– 70% of total time of combustion. ‘After burning’ occurs in less than 10% of combustion space in less than 10% of total combustion time. • Abnormal Combustion • Combustion may also sometimes occur abnormally. “Abnormal combustion” is said to occur when combustion begins inside the cylinder on its’ own before the stipulated time for it. This abnormal combustion may be due to pre-ignition (i. e. ignition of fuel even before spark plug ignites it) or auto-ignition (i. e. Ignition of fuel due to hot spots in the combustion space like valve seats, spark plug) and results in uncontrolled pressure rise. Abnormal combustion is also termed as detonation or knocking and can be felt by jerky operation of engine, excessive noise, reduced power output etc • • Unit I

ME 2041 Advanced Internal Combustion Engines • Factors affecting knock • • Fuel A ‘low self ignition temperature’ fuel promotes knock. Induction pressure Increase of pressure decreases SIT and increases induction time; tendency of knock increases. Eg. At full throttle knock tends to occur more. Engine Speed Low engine speed will give low turbulence and low flame velocity and hence knock tendency is more. Ignition Timing Advancing ignition timing increases peak pressure and promotes knock. Compression Ratio High compression ratio increases cylinder pressures and increases the tendency for knock. Combustion Chamber Design Poor design results in long flame path, low turbulence and insufficient cooling all of which increase knock tendency. Cylinder Cooling Poor cylinder cooling increases the temperature and hence the chances of knock temperature’ fuel promotes knock. Unit I

ME 2041 Advanced Internal Combustion Engines • Combustion Chambers Unit I

ME 2041 Advanced Internal Combustion Engines THERMODYNAMIC ANALYSIS OF SI ENGINE COMBUSTION • Burned and Unburned Mixture States Because combustion occurs through a flame propagation process, the changes in state and the motion of the unburned and burned gas are much more complex than the ideal cycle analysis. The gas pressure, temperature and density changes as a result of changes in volume due to piston motion. During combustion, the cylinder pressure increases due to the release of the fuel's chemical energy. As each element of fuel-air mixture burns, its density decreases by about a factor of four. This combustion-produced gas expansion compresses the unburned mixture ahead of the flame and displaces it toward the combustion chamber walls. The combustion-produced gas expansion also compresses those parts of the charge which have already burned, and displaces them back toward the spark plug. Unit I

ME 2041 Advanced Internal Combustion Engines THERMODYNAMIC ANALYSIS OF SI ENGINE COMBUSTION • Burned and Unburned Mixture States During the combustion process, the unburned gas elements move away from the spark plug; following combustion, individual gas elements move back toward the spark plug. Further, elements of the unburned mixture which burn at different times have different pressures and temperatures just prior to combustion, and therefore end up at different states after combustion. The thermodynamic state and composition of the burned gas is, therefore, nonuniform. A first law analysis of the spark-ignition engine combustion process enables us to quantify these gas states. Work transfer occurs between the cylinder gases and the piston (to the gas before TC; to the piston after TC). Unit I

ME 2041 Advanced Internal Combustion Engines THERMODYNAMIC ANALYSIS OF SI ENGINE COMBUSTION • Burned and Unburned Mixture States Heat transfer occurs to the chamber walls, primarily from the burned gases. At the temperatures and pressures typical of spark-ignition engines it is a reasonable approximation to assume that the volume of the reaction zone where combustion is actually occurring is a negligible fraction of the chamber volume even though the thickness of-the turbulent flame may not be negligible compared with the chamber dimensions. With normal engine operation, at any point in time or crank angle, the pressure throughout the cylinder is close to uniform. The conditions in the burned and unburned gas are then determined by conservation of mass : Unit I

ME 2041 Advanced Internal Combustion Engines THERMODYNAMIC ANALYSIS OF SI ENGINE COMBUSTION • Burned and Unburned Mixture States The conservation of energy: where V is the cylinder volume, m is the mass of the cylinder contents, v is the specific volume, xb is the mass fraction burned, Uo is the internal energy of the cylinder contents at some reference point 0, u is the specific internal energy, W is the work done on the piston, and Q is the heat transfer to the walls. The subscripts u and b denote unburned and burned gas properties, respectively. The work and heat transfers are: Where Unit I is the instantaneous heat-transfer rate to the chamber walls.

ME 2041 Advanced Internal Combustion Engines THERMODYNAMIC ANALYSIS OF SI ENGINE COMBUSTION • Burned and Unburned Mixture States Useful results can be obtained by assuming that the burned and unburned gases are different ideal gases, each with constant specific heats. i. e. Combining these eqns. Unit I

ME 2041 Advanced Internal Combustion Engines THERMODYNAMIC ANALYSIS OF SI ENGINE COMBUSTION • Burned and Unburned Mixture States The above equations may be solved to obtain If we now assume the unburned gas is initially uniform and undergoes isentropic compression, then Unit I