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Basics of Internal Combustion Engines by Indranil Mandal
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1. Unit I Part A 1. Describe the major functions of piston, connecting rod, crank shaft, cams and valves. Major Functions are: - (i) Piston: - Pistons are made of aluminum in small engines or cast iron in larger slower-speed engines. The piston both seals the cylinder and transmits the combustion-generated gas pressure to the crank pin via the connecting rod. n 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. (ii) 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. It may also convert rotating motion into reciprocating motion, its original use. Earlier mechanisms, such as the chain, could only impart pulling motion. Being rigid, a connecting rod may transmit either push or pull, allowing the rod to rotate the crank through both halves of a revolution. In a few two-stroke engines the connecting rod is only required to push. (iii) Crankshaft: - It 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. In order to do the conversion between two motions, the crankshaft has "crank throws" or "crankpins", additional bearing surfaces whose axis is offset from that of the crank, to which the "big ends" of the connecting rods from each cylinder attach. It is typically connected to a flywheel to reduce the pulsation characteristic of the four-stroke cycle, and sometimes a torsional or vibrational damper at the opposite end, to reduce the torsional vibrations often caused along the length of the crankshaft by the cylinders farthest from the output end acting on the torsional elasticity of the metal.
2. (iv)Camshaft: - A camshaft made of cast iron or forged steel with one cam per valve is used to open and close the valves. The cam surfaces are hardened to obtain adequate life. In four stroke cycle engines, camshafts turn at one-half the crankshaft speed. Mechanical or hydraulic lifters or tappets slide in the block and ride on the cam. Depending on valve and camshaft location, additional members are required to transmit the tappet motion to the valve stem; e.g., in in- head valve engines with the camshaft at the side, a push rod and rocker arm are used. A recent trend in automotive engines is to mount the camshaft over the head with the cams acting either directly or through a pivoted follower on the valve. Camshafts are gear, belt, or chain driven from the crankshaft. (v) Valves: - Valves are made from forged alloy steel; the cooling of the exhaust valve which operates at about 700• C may be enhanced by using a hollow stem partially filled with sodium which through evaporation and condensation carries heat from the hot valve head to the cooler stem. Most modern spark ignition engines have overhead valve locations (sometimes called valve-in-head or l-head configurations). This geometry leads to a compact combustion chamber with minimum heat losses and flame travel time, and improves the breathing capacity. Previous geometries such as the L head where valves are to one side of the cylinder are now only used in small engines. 2. List the five important differences between the design and operating characteristics of SI engines and CI engines. The five important differences between the design and operating characteristics of SI engines and CI engines are: -  SI engines draw fuel and air into the cylinder.  Fuel must be injected into the cylinder at the desired time of combustion in CI engines.  Air intake is throttled to the SI engine ‐ ‐ no throttling in CI engines.  Compression ratios must be high enough to cause auto‐ ignition in CI engines (CI: 12 to 24), compressed to pressure about 4 Mpa and temperature about 800 K.  Upper compression ratio in SI engines is limited by the auto ignition temperature (SI: 8 to 12).  Flame front in SI engines smooth and controlled.
3.  CI combustion is rapid and uncontrolled at the beginning.  The valve timing in both CI and SI are similar. 3. Suggest the reasons for less power output of two stroke engine than twice that of 4 stroke engine. The prime advantage of the two stroke cycle spark-ignition engine relative to the four-stroke cycle engine is its higher power per unit displaced volume due to twice the number of power strokes per crank revolution. This is because it fires once every revolution, giving it twice the power of a four stroke, which only fires once every other revolution. Significantly, it also has a higher weight-to-power ratio because it is much lighter. This is offset by the lower fresh charge density achieved by the two-stroke cycle gas- exchange process and the loss of fresh mixture which goes straight through the engine during scavenging.
4. 4. At higher cylinder capacities, multi-cylinder engines are more attractive than single cylinder engine – Suggest the reason? Multi-cylinder engines are invariably used in automotive practice. As rated power increases, the advantages of smaller cylinders in regard to size, weight, and improved engine balance and smoothness point toward increasing the number of cylinders per engine. An upper limit on cylinder size is dictated by dynamic considerations: the inertial forces that are created by accelerating and decelerating the reciprocating masses of the piston and connecting rod would quickly limit the maximum speed of the engine. Thus, the displaced volume is spread out amongst several smaller cylinders. The increased frequency of power strokes with a multi-cylinder engine produces much smoother torque characteristics. Multi-cylinder engines can also achieve a much better state of balance than single-cylinder engines. 5. Explain why brake mean effective pressure of diesel engine is lower that of SI engine? CI engines typically run on a leaner fuel-air mixture even at full load operation, as compared to SI engines. The fuel-air mixture cannot be enriched beyond a limiting value in diesel engine due to the tendency of increased smoke formation. It should also be noted that the air-fuel mixture in a CI engine is not homogenous like that in an SI engine. There will be rich and lean pockets as the liquid fuel is sprayed into the cylinder and there is no much time to mix homogenously before the combustion. In contrast, the fuel and air is mixed prior to the intake in an SI engine. But if you compare the global air-fuel ratio (i.e. total air intake/total fuel intake of the engine), SI engine air-fuel mixtures are richer than their CI engine counterparts. 6. Describe combustion efficiency of an IC Engine? As time available for combustion is very short, a small fraction of fuel does not react and exists with the exhaust flow. Combustion efficiency is defined to account for the fraction of fuel burnt, and has typically values in the range of 95 to 98% when an engine is operating properly. Consider an open system which exchanges heat and work with its surroundings.
5. Environment. Reactants flow into the system; products flow out. Consider a mass m which passes through the control volume surrounding engine the engine; the net chemical energy release due to combustion engine [HR(TA) – HP(TA)] The amount of fuel energy supplied to the control volume around the engine which can be released by combustion is mfQHV. Hence, the combustion efficiency – the fraction of fuel energy supplied which is released in the combustion process ɳc = HR(TA) – Hp(TA)/ mf QHV 7. Describe about combustion stoichiometry? Stoichiometric or Theoretical Combustion is the ideal combustion process where fuel is burned completely. A complete combustion is a process burning all the carbon (C) to (CO2), all the hydrogen (H) to (H2O) and all the Sulphur (S) to (SO2).With unburned components in the exhaust gas such as C, H2, CO, the combustion process is uncompleted and not stoichiometric . The combustion process can be expressed: [C + H (fuel)] + [O2 + N2 (Air)] -> (Combustion Process) -> [CO2 + H2O + N2 (Heat)] ENGINE FUEL AIR CONTROL VOLUME EXHAUST GAS
6. Where, C = Carbon, H = Hydrogen, O = Oxygen&N = Nitrogen. To determine the excess air or excess fuel for a combustion system we starts with the stoichiometric air-fuel ratio. The stoichiometric ratio is the perfect ideal fuel ratio where the chemical mixing proportion is correct. When all fuel and air burned is consumed without any excess left over. Process heating equipment are rarely run that way. "On-ratio" combustion used in boilers and high temperature process furnaces usually incorporates a modest amount of excess air - about 10 to 20% more than what is needed to burn the fuel completely. If an insufficient amount of air is supplied to the burner, unburned fuel, soot, smoke, and carbon monoxide exhausts from the boiler - resulting in heat transfer surface fouling, pollution, lower combustion efficiency, flame instability and a potential for explosion. To avoid inefficient and unsafe conditions boilers normally operate at an excess air level. This excess air level also provides protection from insufficient oxygen conditions caused by variations in fuel composition and "operating slops" in the fuel-air control system. If air content is higher than the stoichiometric ratio - the mixture is said to be fuel-lean. If air content is less than the stoichiometric ratio - the mixture is fuel-rich. 8. Draw the Theoretical and actual pv diagrams of two stroke and four stroke engines. a) Two stroke engine: Here you can observe that the inlet port opens and closes while the exhaust port is still functioning. In a two-stroke engine, the end of the combustion stroke and the beginning of the compression stroke happen simultaneously, with the intake and exhaust (or scavenging) functions occurring at the
7. sametime. b) Four stroke engine: Here the Intake valve opens and closes in the first stroke itself, and exhaust valve actuates while the fourth stroke. PART B 1.Explain the following engine design and operating parameters and its effects of the following terms: - i. Compression Ratio ii. Mean effective pressure iii. A/F ratio iv. Volumetric efficiency v. Engine specific weight and volume. i.Compression ratio: - The static compression ratio of an internal combustion engine or external combustion engine is a value that represents the ratio of the volume of its combustion chamber from its largest capacity to its
8. smallest capacity. It is a fundamental specification for many common combustion engines. In a piston engine, it is the ratio between the volume of the cylinder and combustion chamber when the piston is at the bottom of its stroke, and the volume of the combustion chamber when the piston is at the top of its stroke. A high compression ratio is desirable because it allows an engine to extract more mechanical energy from a given mass of air-fuel mixture due to its higher thermal efficiency. This occurs because internal combustion engines are heat engines, and higher efficiency is created because higher compression ratios permit the same combustion temperature to be reached with less fuel, while giving a longer expansion cycle, creating more mechanical power output and lowering the exhaust temperature. It may be more helpful to think of it as an "expansion ratio", since more expansion reduces the temperature of the exhaust gases, and therefore the energy wasted to the atmosphere. Diesel engines actually have a higher peak combustion temperature than petrol engines, but the greater expansion means they reject less heat in their cooler exhaust. ii. Mean effective pressure: - While torque is a valuable measure of a particular engine's ability to do work, it depends on engine size. A more useful relative engine performance measure is obtained by dividing the work per cycle by the cylinder volume displaced per cycle. The parameter so obtained has units of force per unit area and is called the mean effective pressure (mep). Work per cycle = Png / N Where ng is the number of crank revolutions for each power stroke per cylinder (two for four-stroke cycles; one for two-stroke cycles), then mep = -Png / Vd N The maximum brake mean effective pressure of good engine designs is well established, and is essentially constant over a wide range of engine sizes. Thus, the actual bmep that a particular engine develops can be compared with this norm, and the effectiveness with which the engine designer has used the
9. engine's displaced volume can be assessed. Also, for design calculations, the engine displacement required to provide a given torque or power, at a specified speed, can be estimated by assuming appropriate values for bmep for that particular application. iii. A/F ratio: - Air–fuel ratio (AFR) is the mass ratio of air to a solid, liquid, or gaseous fuel present in a combustion process. The combustion may take place in a controlled manner such as in an internal combustion engine or industrial furnace, or may result in an explosion (e.g., a dust explosion, gas or vapor explosion or in a thermobaric weapon). The air-fuel ratio determines whether a mixture is combustible at all, how much energy is being released, and how much unwanted pollutants are produced in the reaction. Typically a range of fuel to air ratios exists, outside of which ignition will not occur. These are known as the lower and upper explosive limits. In an internal combustion engine or industrial furnace, the air-fuel ratio is an important measure for antipollution and performance-tuning reasons. If exactly enough air is provided to completely burn all of the fuel, the ratio is known as the stoichiometric mixture, often abbreviated to stoich. Ratios lower than stoichiometric are considered "rich". Rich mixtures are less efficient, but may produce more power and burn cooler, which produces less stress on the engine. Ratios higher than stoichiometric are considered "lean." Lean mixtures are more efficient but may cause engine damage or premature wear and produce higher levels of nitrogen oxides. For precise air-fuel ratio calculations, the oxygen content of combustion air should be specified because of different air density due to different altitude or intake air temperature, possible dilution by ambient water vapor, or enrichment by oxygen additions. Air Fuel Ratio = ma/mf Where, ma is the air mass flow rate and mf is the fuel mass flow rate. iv. Volumetric efficiency: - Volumetric efficiency (VE) in internal combustion engine engineering is defined as the ratio of the mass density of the air-fuel mixture drawn into the cylinder at atmospheric pressure (during the intake stroke) to the mass density of the same volume of air in the intake manifold. The term is also used in other engineering contexts, such as hydraulic pumps and electronic components. The intake system-the air filter, carburetor, and throttle; plate (in a spark ignition engine), intake manifold, intake
10. port, intake valve--restricts the amount of air which an engine of given displacement can induct. The parameter used to measure the effectiveness of an engine's induction process is the volumetric efficiency q; volumetric efficiency is only used with four-stroke cycle engines which have a distinct induction process. It is defined as the volume flow rate of air into the intake system divided by the rate at which volume is displaced by the piston. The inlet density may either be taken as atmosphere air density (in which case nv measures the pumping performance of the entire inlet system) or may be taken as the air density in the inlet manifold (in which case nv measures the pumping performance of the inlet port and valve only). Typical maximum values of nv for naturally aspirated engines are in the range 80 to 90 percent. The volumetric efficiency for diesels is somewhat higher than for SI engines. v. Engine specific weight and specific volume: - Engine weight and bulk volume for a given rated power are important in many applications. Two parameters useful for comparing these attributes from one engine to another are: For these parameters to be useful in engine comparisons, a consistent definition of what components and auxiliaries are included in the term "engine must be adhered to. These parameters indicate the effectiveness with which the engine designer has used the engine materials and packaged the engine components. 2. The minimum pressure and temperature in an Otto cycle are 100kPa and 27 ºC. The amount of heat added to the air per cycle is 1500 kJ/kg. i) Determine the pressures and temperatures at all points of the air standard Otto cycle. ii) Also calculate the specific work and thermal efficiencyof the cycle for combustion ratio of 8:1. Take for air: Cv = 0.72 kJ/kg-K & ᵧ = 1.4
11. 3. An engine with 200mcylinder diameter and 300 mm stroke works on theoretical Diesel cycle. The initial pressure and temperature of air used are 1 bar & 27 ºC. The cut off is 8% of the stroke. Determine: i)
12. Pressures & temperatures at all salient points. ii) Theoretical air standard efficiency.
13. 4. With the help of neat diagram describe the following ideal models of engine process. i) Constant volume combustion ii) constant pressure
14. combustion iii) limited pressure or dual combustion iv) Throttled constant volume combustion. (a) Constant volume combustion: The Otto cycle is constructed from: Top and bottom of the loop: a pair of quasi-parallel and isentropic processes (frictionless, adiabatic reversible). Left and right sides of the loop: a pair of parallel isochoric processes (constant volume).The isentropic process of compression or expansion implies that there will be no inefficiency (loss of mechanical energy), and there be no transfer of heat into or out of the system during that process.Hence the cylinder, and piston are assumed impermeable to heat during that time. Work is performed on the system during the lower isentropic compression process. Heat flows into the Otto cycle through the left pressurizing process and some of it flows back out through the right depressurizing process. The summation of the work added to the system plus the heat added minus the heat removed yields the net mechanical work generated by the system. The processes are described by: Process 0–1 a mass of air is drawn into piston/cylinder arrangement at constant pressure.
15. Process 1–2 is an adiabatic (isentropic) compression of the charge as the piston moves from bottom dead centre (BDC) to top dead centre (TDC). Process 2–3 is a constant-volume heat transfer to the working gas from an external source while the piston is at top dead centre. This process is intended to represent the ignition of the fuel-air mixture and the subsequent rapid burning. Process 3–4 is an adiabatic (isentropic) expansion (power stroke). Process 4–1 completes the cycle by a constant-volume process in which heat is rejected from the air while the piston is at bottom dead centre. Process 1–0 the mass of air is released to the atmosphere in a constant pressure process. The Otto cycle consists of isentropic compression, heat addition at constant volume, isentropic expansion, and rejection of heat at constant volume. In the case of a four-stroke Otto cycle, technically there are two additional processes: one for the exhaust of waste heat and combustion products at constant pressure (isobaric), and one for the intake of cool oxygen-rich air also at constant pressure; however, these are often omitted in a simplified analysis. Even though those two processes are critical to the functioning of a real engine, wherein the details of heat transfer and combustion chemistry are relevant, for the simplified analysis of the thermodynamic cycle, it is more convenient to assume that all of the waste-heat is removed during a single volume change. (b) Constant pressure combustion
16. The Diesel cycle is assumed to have constant pressure during the initial part of the combustion phase V2 to V3. This is an idealized mathematical model: real physical diesels do have an increase in pressure during this period, but it is less pronounced than in the Otto cycle. In contrast, the idealized Otto cycle of a gasoline engine approximates a constant volume process during that phase. The image above shows a p-V diagram for the ideal Diesel cycle; where p is pressure and V the volume or v the specific volume if the process is placed on a unit mass basis. The ideal Diesel cycle follows the following four distinct processes: Process 1 to 2 is isentropic compression of the fluid (blue) Process 2 to 3 is reversible constant pressure heating (red) Process 3 to 4 is isentropic expansion (yellow) Process 4 to 1 is reversible constant volume cooling (green) The Diesel engine is a heat engine: it converts heat into work. During the bottom isentropic processes (blue), energy is transferred into the system in the form of work W {in}, but by definition (isentropic) no energy is transferred into or out of the system in the form of heat. During the constant pressure (red, isobaric) process, energy enters the system as heat Q_ {in}. During the top
17. isentropic processes (yellow), energy is transferred out of the system in the form of W_ {out}}, but by definition (isentropic) no energy is transferred into or out of the system in the form of heat. During the constant volume (green, isochoric) process, some of energy flows out of the system as heat through the right depressurizing process Q_ {out}. The work that leaves the system is equal to the work that enters the system plus the difference between the heat added to the system and the heat that leaves the system; in other words, net gain of work is equal to the difference between the heat added to the system and the heat that leaves the system. Work (in) is done by the piston compressing the air (system). Heat (in) is done by the combustion of the fuel. Work (out) is done by the working fluid expanding and pushing a piston (this produces usable work). Heat (out) is done by venting the air Net work produced = Q{in}} - Q{out} The net work produced is also represented by the area enclosed by the cycle on the P-V diagram. The net work is produced per cycle and is also called the useful work, as it can be turned to other useful types of energy and propels a vehicle (kinetic energy) or produce electrical energy. The summation of many such cycles per unit of time is called the developed power. The W_ {out} is also called the gross work, some of which is used in the next cycle of the engine to compress the next charge of air. (C) Limited Pressure Cycle (or Dual Cycle):
18. This cycle is also called as the dual cycle, which is shown in Fig. Here the heat addition occurs partly at constant volume and partly at constant pressure. This cycle is a closer approximation to the behavior of the actual Otto and Diesel engines because in the actual engines, the combustion process does not occur exactly at constant volume or at constant pressure but rather as in the dual cycle. Process 1-2: Reversible adiabatic compression. Process 2-3: Constant volume heat addition. Process 3-4: Constant pressure heat addition. Process 4-5: Reversible adiabatic expansion. Process 5-1: Constant volume heat rejection. 5. Derive an expression for ideal cycle efficiency for an Otto cycle. Basic terms used in derivation of air-standard efficiency of Otto cycle: Total Cylinder Volume: It is the total volume (maximum volume) of the cylinder in which Otto cycle takes place. In Otto cycle, Total cylinder volume = V1 = V4 = Vc + Vs (Refer p-V diagram above) where, Vc → Clearance Volume Vs → Stroke Volume. Clearance Volume (Vc): At the end of the compression stroke, the piston approaches the Top Dead Center (TDC) position. The minimum volume of the space inside the cylinder, at the end of the compression stroke, is called clearance volume (Vc). In Otto cycle, Clearance Volume, Vc = V2 (See p-V diagram above). Stroke Volume (Vs): In Otto cycle, stroke volume is the difference between total cylinder volume and clearance volume. Stroke Volume, Vs = Total Cylinder Volume – Clearance Volume = V1 – V2 = V4 – V3. Compression Ratio: Compression ratio (r) is the ratio of total cylinder volume to the clearance volume.
19. Now that we know the basic terms, let us derive expressions for T2 and T3. These expressions will be useful for us to derive the expression for air-standard efficiency of otto cycle. For finding T2, we take process 1-2 and for finding T3, we take process 3-4. Process 1-2: This process is an isentropic (reversible adiabatic) process. For this process, the relation between T and V is as follows: Process 3-4: This is also an isentropic process. The relation between T and V in this process is similar to the relation between T and V in process 1-2: Here,
20. Air-standard efficiency of Otto cycle: It is defined as the ratio between work done during Otto cycle to the heat supplied during Otto cycle. Air-Standard Efficiency (thermal efficiency) of Otto cycle,
21. 6. Derive an expression for ideal cycle efficiency for a Diesel cycle. Diesel cycle comprises of the following operations: (i) 1-2......Adiabatic compression. (ii) 2-3......Addition of heat at constant pressure. (iii) 3-4......Adiabatic expansion.
22. (iv) 4-1......Rejection of heat at constant volume. Consider 1 kg of air. Heat supplied at constant pressure =Cp(T3−T2) Heat rejected at constant volume =Cv(T4−T1) Work done = Heat supplied – Heat rejected =Cp(T3−T2)−Cv(T4−T1) ηdiesel=workdoneheatsupplied =Cp(T3−T2)−Cv(T4−T1)Cp(T3−T2) =1−(Cv(T4−T1)Cp(T3−T2) =1−(T4−T1)γ(T3−T2) ………(i)[CpCv =γ] Let compression ratio,r=v1v2 , and cut-off ratio,ρ=v3v2 i.e.Volumeatcut−offclearancevolume Now, during adiabatic compression 1-2, T2T1 =v1v2(γ−1)=r(γ−1) or T2=T1r(γ−1) During constant pressure process 2-3, T3T2 =v3v2 =ρ or T3=ρT2=ρT1r(γ−1) During adiabatic expansion 3-4 T3T4 =v4v3(γ−1)=rρ(γ−1) (since v4v3=v1v3=v1v2×v2v3=rρ) T4=T3rρ(γ−1) =ρ∙T1r(γ−1)rρ(γ−1) =T1ργ By inserting values of T2,T3 and T4 in eqn. i, we get ηdiesel=1−(T1ργ−T1)γ(ρ.T1.r(γ−1)−T1∙r(γ−1))=1−(ργ−1)γ∙r(γ−1)(ρ−1)
23. ηdiesel=1−1(γ.r(γ−1)[(ργ−1)(ρ−1)] 7. With neat sketches explain the port timing and valve timing diagrams for both SI and CI engines. SI ENGINE VALVE TIMING DIAGRAM FOR FOUR-STROKE PETROL ENGINE: Diagram shows the valve timing diagram for a four-stroke cycle petrol engine. The inlet valve opens 10-30° before the top dead centre position. The air-fuel mixture is sucked into the engine cylinder till the inlet valve closes. The inlet valve closes 30-40° or even 60° after the bottom dead centre position. The airfuel mixture is compressed till the spark occurs. The spark is produced 20-40° before the t.d.c. position. This gives sufficient time for the fuel to burn. The pressure and temperature increases. The burning gases expand and force the piston to do useful work. The burning gases expand till the exhaust valve opens. The exhaust valve opens 30-60° before the b.d.c. position. The exhaust gases are forced out of the cylinder till the exhaust valve closes. The exhaust valve closes 8-20° after the t.d.c. position. Before it closes, again the inlet valve opens 10-30° before the t.d.c. position. The period between the inlet valve opening and exhaust valve closing is known as valve overlap period. The angle between the inlet valve opening and exhaust valve closing is known as angle of valve overlap.
24. PORT TIMINING DIAGRAM According to the diagram the exhaust port is uncovered at about 43° before BDC. The inlet valve is still closed. This marks the end of expansion stroke. This removes the exhaust while the piston has still not reached BDC. When piston is at position of 35° before BDC the inlet valve also opens and the fresh charge is induced, simultaneously driving away the remaining exhaust. Both valves remain open till 35° after BDC when the inlet port closes and the piston is on its way to the TDC. When piston is 43° after BDC the exhaust port closes marking the start of compression stroke since the cylinder is completely closed now. Ignition occurs in advance at position of 20 before TDC and starts the expansion stroke that continues till position of 43 before BDC and the cycle continues.
25. CI ENGINE VALVE TIMING DIAGRAM FOR FOUR-STROKE DIESEL ENGINE The actual valve timing diagram for four-stroke diesel engine. The inlet valve opens 10-25° before the top dead center position. Fresh air is sucked into the engine cylinder till the inlet valve closes. The inlet valve closes 25-50° after the bottom dead center position. The air is compressed till the fuel is injected. The fuel injection starts 5-10° before the t.d.c. position in the compression stroke. The air fuel mixture burns. The temperature and pressure increases. The burning gases expand till the exhaust valve opens. The exhaust valve opens 30-50° before the b.d.c. position. The exhaust gases are forced out of the engine cylinder till the exhaust valve closes. The exhaust valve closes 10-15° after the t.d.c. position. Before the exhaust valve closes, again the inlet valve opens 1025° before the t.d.c. position. The period between the inlet valves opening the exhaust valve closing is known as valve overlap period. The angle between these two events is known as angle of valve overlap.
26. PORT DIAGRAM The complete principle of two stroke engine is divided into two strokes. The details of which is as follows. First Stroke: Assume piston is at a place as shown in above fig (a). Now suppose the piston starts to move downwards uncovering the inlet port and allowing the air to come in the cylinder. Now piston starts to move upward compressing the air sucked before. Second Stroke: Now the compressed air becomes so hot. It is at this time the fuel is injected into the cylinder from the Fuel Injector. Because of the compression of the air the temperature rises so much that when the fuel is injected, it burns and get converted into gas releasing energy. This energy pushes the piston downwards. When piston comes down after the combustion, the exhaust port opens first allowing the burnt gases to go out. After that, the inlet port gets uncovered allowing the fresh air to come in which completes the cycle. Note: In First Stroke the piston moves from the bottom (BDC) to the Top (TDC) while in Second Stroke the piston moves from BDC to TDC.