Combustion in SI engine chaautomobilepter3.pptx

combustion

1. Three essential conditions for the combustion of fuels are  There must be Fuel to burn. There must be Air to supply oxygen .  There must be Heat (ignition temperature) to start and continue the combustion process. In SI engine a carburettor generally supplies the mixture and spark plug initiates the combustion

2. What is ignition limit Ignition limit refers to the minimum energy required for a combustible-oxidant system to initiate a combustion reaction, resulting in the evolution of heat and emission of light.

3. Combustion phenomenon Normal combustion Normal combustion in a Spark Ignition (SI) engine refers to the typical combustion process that occurs when the fuel-air mixture is ignited by the spark plug. Here's a step-by-step explanation: 1. Intake stroke: Air and fuel are drawn into the cylinder through the intake valves. 2. Compression stroke: The air-fuel mixture is compressed by the piston, creating a small, high-pressure chamber.

4. 3. Ignition: The spark plug ignites the fuel-air mixture at the correct time, causing a small flame kernel to form. 4. Flame propagation: The flame kernel grows and propagates through the combustion chamber, burning the fuel-air mixture. 5. Rapid pressure rise: As the fuel burns, the pressure inside the cylinder increases rapidly, pushing the piston down. 6. Power stroke: The piston is driven downward, rotating the crankshaft and producing torque. 7. Exhaust stroke: The exhaust valves open, and the piston pushes the exhaust gases out of the cylinder.

5. Ricardo, known as father of engine research describes the combustion process can be imagined as if it is developing in two stages: 1. Growth and development of a self-propagating nucleus flame. (Ignition lag) 2. Spread of flame through the combustion chamber

6. According to Ricardo, There are three stages of combustion in SI Engine as shown 1. Ignition lag stage 2. Flame propagation stage 3. After burning stage

7. 1. Ignition lag stage: There is a certain time interval between instant of spark and instant where there is a noticeable rise in pressure due to combustion. This time lag is called IGNITION LAG. Ignition lag is the time interval in the process of chemical reaction during which molecules get heated up to self ignition temperature , get ignited and produce a self propagating nucleus of flame. Ignition lag is very small and lies between 0.00015 to 0.0002 seconds. An ignition lag of 0.002 seconds corresponds to 35 deg crank rotation when the engine is running at 3000 RPM. Angle of advance increase with the speed. This is a chemical process depending upon the nature of fuel, temperature and pressure, proportions of exhaust gas and rate of oxidation or burning.

8. 2. Flame propagation stage: Once the flame is formed, it should be self sustained and must be able to propagate through the mixture. This is possible when the rate of heat generation by burning is greater than heat lost by flame to surrounding

9. . After burning: Combustion will not stop at point but continue after attaining peak pressure and this combustion is known as after burning. This generally happens when the rich mixture is supplied to engine.

10. Factors Affecting the Flame Propagation: A/F ratio: The mixture strength influences the rate of combustion and amount of heat generated. The maximum flame speed for all hydrocarbon fuels occurs at nearly 10% rich mixture. Flame speed is reduced both for lean and as well as for very rich mixture.

11. Compression ratio: The higher compression ratio increases the pressure and temperature of the mixture and also decreases the concentration of residual gases. All these factors reduce the ignition lag and help to speed up the second phase of combustion.

12. Load on Engine: With increase in load, the cycle pressures increase and the flame speed also increases. In S.I. engine, the power developed by an engine is controlled by throttling. At lower load and higher throttle, the initial and final pressure of the mixture after compression decrease and mixture is also diluted by the more residual gases. This reduces the flame propagation and prolongs the ignition lag. This is the reason, the advance mechanism is also provided with change in load on the engine

13. Turbulence : Turbulence plays very important role in combustion of fuel as the flame speed is directly proportional to the turbulence of the mixture. This is because, the turbulence increases the mixing and heat transfer coefficient or heat transfer rate between the burned and unburned mixture. The turbulence of the mixture can be increased at the end of compression by suitable design of the combustion chamber

14. Engine Speed: The turbulence of the mixture increases with an increase in engine speed. For this reason, the flame speed almost increases linearly with engine speed. If the engine speed is doubled, flame to traverse the combustion chamber is halved. Double the original speed and half the original time give the same number of crank degrees for flame propagation.

15. Engine Size: Engines of similar design generally run at the same piston speed. This is achieved by using small engines having larger RPM and larger engines having smaller RPM. Due to same piston speed, the inlet velocity, degree of turbulence and flame speed are nearly same in similar engines regardless of the size. However, in small engines the flame travel is small and in large engines large. Therefore, if the engine size is doubled the time required for propagation of flame through combustion space is also doubled.

16. Factors affecting normal combustion in SI engine Engine speed Increase of the engine speed, reduces the time available for a complete combustion.

17. Mixture properties The burned gas fraction in the unburned mixture, due to the residual gas fraction and any recycled exhaust gases (EGR), slows down both flame development and propagation.

18. Induction pressure Increase in the induction pressure reduces flame propagation speed, but also increases the temperatures at the end of compression process which effects the flame speed, and reduces combustion duration.

19. Compression ratio Increase in CR increases the p and T of the charge at ignition, reduces the mass fraction of the residual gases - more favourable conditions are developed for ignition which reduces the first stage of combustion, and increases flame propagation rate in the main stage. Increasing CR, increases Area/Volume ratio of the cylinder, increasing the cooling effects and the quench layers. Final stage of combustion is increased.

20. Combustion chamber design Intake manifold design and combustion chamber shape effects the gas flow and turbulence intensity. Turbulence strongly effects burning rate of the fuel. Spark plug location effects distance traveled by the flame and flame front surface area. Number of spark plugs. Pressure gradiant should be controlled for optimum conditions in terms of total efficiency.

21. Abnormal combustion Knock originates in the extremely rapid release of much of the energy contained in the end-gas ahead of the propagating turbulent flame, resulting in high local pressures. Non uniform nature of this pressure distribution causes pressure waves or shock waves to propagate across the chamber, which may cause chamber to resonate at its natural frequency

22. Effect of engine variable on ignition lag Spark Plug Characteristics : The design and condition of the spark plug can affect ignition lag. Factors such as gap size, electrode material, and heat range can influence the spark intensity and duration, which in turn affects ignition timing and combustion initiation. Fuel Properties : The properties of the fuel used in the engine, such as octane rating, volatility, and chemical composition, can impact ignition lag. Higher octane fuels typically have better resistance to knocking, which can help reduce ignition lag. Air-Fuel Mixture : The air-fuel mixture composition and quality play a crucial role in ignition lag. A stoichiometric mixture (ideal air-fuel ratio) promotes efficient combustion and can help reduce ignition lag compared to lean or rich mixtures.

23. Engine Speed : Higher engine speeds can reduce ignition lag due to increased turbulence and faster mixing of the air-fuel mixture, leading to quicker combustion initiation. Engine Load : Engine load, or the amount of work the engine is performing, can affect ignition lag. Higher loads may require adjustments to ignition timing and fuel delivery to optimize combustion and minimize ignition lag.

24. Compression Ratio : The compression ratio of the engine influences ignition lag. Higher compression ratios can lead to faster combustion due to increased pressure and temperature in the combustion chamber. Ignition Timing : The timing of the spark plug firing relative to the position of the piston can significantly impact ignition lag. Optimizing ignition timing based on engine speed, load, and other factors is crucial for minimizing ignition lag. Engine Temperature : Engine temperature can affect ignition lag by influencing the vaporization of fuel and air, as well as the overall combustion process. Operating the engine at the optimal temperature range can help reduce ignition lag.

25. Spark advance and factor affecting ignition timing Engine Speed:  The ignition timing needs to be adjusted based on the engine speed. At higher engine speeds, the ignition timing needs to be advanced to ensure that the air-fuel mixture ignites at the right moment for optimal performance. Engine Load:  The engine load, which is a measure of how much work the engine is doing, also affects ignition timing. Higher loads usually require advanced ignition timing to ensure efficient combustion. Fuel Quality:  The quality of the fuel used can affect ignition timing. Lower-quality fuels may require adjustments to the ignition timing to prevent knocking or pre-ignition.

26. Altitude and Ambient Conditions:  Altitude and ambient conditions such as temperature and humidity can affect the density of the air entering the engine, which in turn impacts the combustion process and may require adjustments to the ignition timing. Engine Temperature:  The temperature of the engine can also affect ignition timing. A cold engine may require different ignition timing compared to a warm engine for optimal performance. Engine Knock:  Ignition timing should be set to prevent engine knock, which is an undesirable phenomenon where the air-fuel mixture ignites prematurely in the combustion chamber.

27. Modifications:  Any modifications to the engine, such as changes in compression ratio, camshaft profile, or intake/exhaust systems, may require adjustments to the ignition timing to optimize performance. Engine Design:  The design of the engine, including factors such as the combustion chamber shape, piston design, and valve timing, can also influence the ideal ignition timing for that specific engine.

28. Pre ignition Pre-ignition is the ignition of the homogeneous mixture of charge as it comes in contact with hot surfaces, in the absence of spark . Auto ignition may overheat the spark plug and exhaust valve and it remains so hot that its temperature is sufficient to ignite the charge in next cycle during the compression stroke before spark occurs and this causes the pre-ignition of the charge.

29. Effects of Pre-ignition • It increase the tendency of denotation in the engine • It increases heat transfer to cylinder walls because high temperature gas remains in contact with for a longer time • Pre-ignition in a single cylinder will reduce the speed and power output • Pre-ignition may cause seizer in the multi-cylinder engines, only if one cylinders have pre-ignition

30. detonation Detonation, as the name suggests, is an explosion of the fuel-air mixture inside the cylinder. It occurs after the compression stroke near or after top dead center . During detonation, the fuel/air charge (or pockets within the charge) explodes rather than burning smoothly.

31. The harmful effects of detonation are as follows: 1. Noise and Roughness: Knocking produces a loud pulsating noise and pressure waves. These waves which vibrates back and forth across the cylinder. The presence of vibratory motion causes crankshaft vibrations and the engine runs rough. 2. Mechanical Damage: (a) High pressure waves generated during knocking can increase rate of wear of parts of combustion chamber. Sever erosion of piston crown cylinder head and pitting of inlet and outlet valves may result in complete wreckage of the engine

32. (b) Detonation is very dangerous in engines having high noise level. In small engines the knocking noise is easily detected and the corrective measures can be taken but in aero-engines it is difficult to detect knocking noise and hence corrective measures cannot be taken. Hence severe detonation may persist for a long time which may ultimately result in complete wreckage of the piston. 3. Carbon deposits: Detonation results in increased carbon deposits. 4. Increase in heat transfer: Knocking is accompanied by an increase in the rate of heat transfer to the combustion chamber walls.

33. Performance number 1. Power (P): Measured in horsepower (hp) or kilowatts (kW), it's the rate at which work is done. 2. Torque (τ): Measured in pound-feet (lb-ft) or newton -meters (Nm), it's the rotational force that causes the engine to rotate. 3. Displacement ( Vd ): Measured in liters (L) or cubic centimeters (cc), it's the total volume of air-fuel mixture drawn into the engine's cylinders. 4. Compression Ratio (CR): The ratio of the cylinder volume when the piston is at the bottom to the volume when the piston is at the top.

34. Performance number Performance number is the useful measure of detonation tendency. It has been developed from the knock limited indicated mean effective pressure ( klimep ), when inlet pressure is used as the dependent variable Performance number= klimep of fuel/ klimp of iso -octane.

35. 5. Fuel Efficiency: Measured in miles per gallon (mpg) or kilometers per liter (km/L), it's the distance traveled per unit of fuel consumed. 6. Brake Mean Effective Pressure (BMEP): A measure of the average pressure exerted on the piston during the power stroke. 7. Engine Speed (RPM): Measured in revolutions per minute, it's the speed at which the engine operates. 8. Air-Fuel Ratio (AFR): The ratio of air to fuel in the combustion chamber. 9. Ignition Timing: The timing of the spark plug firing, measured in degrees before top dead center (BTDC). 10. Volumetric Efficiency (VE): A measure of the engine's ability to draw in air and fill the cylinders.

36. Highest useful compression ratio The highest useful compression ratio is the highest compression ratio employed at which a fuel can be used in a specified engine under specified set of operating condition, at which detonation first become audible with both the ignition and mixture strength adjusted to give the highest efficiency.

37. Combustion chamber design RM Combustion Chamber Design in SI Engines: The combustion chamber, also known as the cylinder head, plays a crucial role in the performance, efficiency, and emissions of a Spark Ignition (SI) engine. Here are key design considerations: 1. Chamber Shape: Hemispherical, bathtub, or pent-roof shapes are used to maximize volume, minimize surface area, and promote efficient combustion. 2. Volume: The chamber volume should be minimized to reduce heat loss and increase compression ratio.

38. 3. Surface Area: Reduced surface area minimizes heat transfer and reduces emissions. 4. Spark Plug Location: Central or offset spark plug locations are used to optimize flame propagation. 5. Valve Layout: Valve placement and angle affect airflow, combustion, and engine performance. 6. Quench Area: The quench area (between the piston and cylinder head) should be minimized to reduce heat transfer. 7. Squish Area: The squish area (between the piston and cylinder head) helps to create turbulence, promoting efficient combustion.

39. 8. Swirl and Tumble: Chamber design can create swirl and tumble motions to enhance mixing and combustion. 9. Material: Chamber materials (e.g., aluminum , cast iron) affect heat transfer, durability, and engine performance. 10. Cooling: Cooling channels or jackets help to regulate chamber temperature. Design techniques: 1. Computational Fluid Dynamics (CFD): Simulates airflow, combustion, and heat transfer. 2. Finite Element Analysis (FEA): Analyzes structural integrity and thermal stresses. 3. Rapid Prototyping: Allows for testing and refinement of chamber designs. Optimized combustion chamber design can lead to:- Improved engine efficiency- Increased power output- Reduced emissions- Enhanced durability

40. Some type of combustion chamber Here are some common types of combustion chambers used in Spark Ignition (SI) engines: 1. Hemispherical Combustion Chamber: Half-spherical shape, providing a large volume and minimal surface area. Aircraft,luxury,large displacement, 2. Bath Tub Combustion Chamber: Rectangular shape with a curved bottom, promoting efficient combustion and minimizing heat loss.economy , passenger, motorcycle 3. Pent-Roof Combustion Chamber: Five-sided shape with a flat roof, allowing for large valves and improved airflow. High performance high output

41. 4. Flat-Head Combustion Chamber: Simple, flat design with a small chamber volume, often used in smaller engines. Small displacemebt engine motor cycle 5. Divided Combustion Chamber: Separate chambers for each valve, improving airflow and reducing heat transfer. This type of chamber, usually with about 80 percent of the clearance volume in the main chamber above the piston and about 20 percent of the volume in a secondary chamber. The main chamber is connected to the secondary chamber through a small orifice. Luxury , racing, high output

42. 6. Masking Combustion Chamber: A recessed area in the piston crown, creating a smaller combustion chamber and reducing heat loss.economy , hybrid, high efficiency, small displacement 7. Reverse Taper Combustion Chamber: A chamber with a smaller diameter at the top, reducing heat transfer and improving combustion. Hgh performance, luxury, high output. 8. Heart-Shaped Combustion Chamber: A chamber with a heart-like shape, promoting efficient combustion and minimizing heat loss. high efficiency low emission hybrid 9. Squish Combustion Chamber: A chamber with a narrow, curved shape, creating a "squish" area to enhance combustion. High efficiency hybrid economy 10. Turbulent Combustion Chamber: A chamber designed to create turbulence, improving mixing and combustion efficiency. Racing, luxury, high output.

43. Turbulent combustion chamber

44. Squish combustion chamber

45. Heart shaped combustion chamber

46. Reverse taper combustion chamber

47. Masking combustion chamber

48. Divided combustion chamber

49. Flat head combustion chamber