Stages of Combustion in SI Engines
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Questions and Answers

What is the third stage of combustion in a spark-ignition engine?

  • Exhaust Stroke
  • Compression Stroke
  • Combustion (Power) Stroke
  • Ignition (correct)

    • Which stroke in a spark-ignition engine involves drawing in a mixture of air and fuel into the combustion chamber?

  • Compression Stroke
  • Intake Stroke (correct)
  • Exhaust Stroke
  • Ignition

    • What is the main function of the Compression Stroke in a spark-ignition engine?

  • Compressing the air-fuel mixture (correct)
  • Igniting the air-fuel mixture
  • Creating a vacuum to draw in the air-fuel mixture
  • Producing mechanical work to turn the crankshaft

    • What is the primary purpose of the Ignition stage in a spark-ignition engine?

    <p>Igniting the compressed air-fuel mixture</p> Signup and view all the answers

    Which stroke in a spark-ignition engine involves pushing the exhaust gases out of the combustion chamber?

    <p>Exhaust Stroke</p> Signup and view all the answers

    How many strokes are there in one complete cycle of a four-stroke spark-ignition engine?

    <p>Four strokes</p> Signup and view all the answers

    What is the consequence of the charge reaching its autoignition temperature before preflame reactions?

    <p>Lead to knocking combustion</p> Signup and view all the answers

    What happens if the flame front only moves from BB to CC during preflame reactions?

    <p>Charge ahead of CC autoignites</p> Signup and view all the answers

    What can occur when two flame fronts collide during knocking combustion?

    <p>Generation of a severe pressure pulse</p> Signup and view all the answers

    What is a desirable quality for SI engine fuels to avoid detonation?

    <p>High autoignition temperature and long ignition lag</p> Signup and view all the answers

    How can autoignition impact engine components during knocking combustion?

    <p>Cause engine failure</p> Signup and view all the answers

    What method is commonly used to detect knocking combustion in engines?

    <p>Pressure transducer connected to a cathode ray oscilloscope</p> Signup and view all the answers

    What characterizes detonation in a Spark Ignition (SI) engine?

    <p>Uncontrolled and rapid increase in pressure and temperature</p> Signup and view all the answers

    How does detonation differ from normal combustion in an SI engine?

    <p>Detonation leads to a distinctive knocking or pinging sound</p> Signup and view all the answers

    What is the impact of detonation on engine components over time?

    <p>Damage to the engine components</p> Signup and view all the answers

    What happens if autoignition does not occur in the unburned charge?

    <p>There will be no knocking.</p> Signup and view all the answers

    What effect does increasing turbulence have on the flame speed?

    <p>Increases</p> Signup and view all the answers

    How does engine speed affect knocking tendency?

    <p>Decreases knocking tendency</p> Signup and view all the answers

    Why does a larger engine have a greater tendency for knocking compared to a smaller engine?

    <p>Longer time for flame to cross the combustion chamber</p> Signup and view all the answers

    How does combustion chamber shape influence antiknock characteristics?

    <p>Shortens flame travel and combustion time</p> Signup and view all the answers

    What is the purpose of shaping the combustion chamber to promote turbulence?

    <p>Reduce knocking tendency</p> Signup and view all the answers

    How does the location of the spark plug affect flame travel?

    <p>Decreases flame travel distance</p> Signup and view all the answers

    What is one of the factors responsible for detonation in spark-ignition engines?

    <p>Using a lean air-fuel mixture</p> Signup and view all the answers

    How does high compression ratio contribute to detonation in spark-ignition engines?

    <p>It increases temperature and pressure in the combustion chamber</p> Signup and view all the answers

    Which component can become a hot spot leading to premature ignition in spark-ignition engines?

    <p>Carbon deposits on the spark plug</p> Signup and view all the answers

    How do poorly designed combustion chambers affect the likelihood of detonation?

    <p>They promote turbulence and hot spots</p> Signup and view all the answers

    What effect does advanced ignition timing have on detonation in spark-ignition engines?

    <p>It increases the likelihood of detonation</p> Signup and view all the answers

    What is the impact of using lower octane fuels in spark-ignition engines?

    <p>[Increased] likelihood of detonation</p> Signup and view all the answers

    Why is it important to use the recommended octane rating for an engine?

    <p>To avoid knocking or detonation</p> Signup and view all the answers

    What can carbon deposits in the combustion chamber lead to in spark-ignition engines?

    <p>[Increased] risk of hot spots and premature ignition</p> Signup and view all the answers

    How does overheating of engine components affect detonation in spark-ignition engines?

    <p>Increases chances of detonation</p> Signup and view all the answers

    What role can knock sensors play in preventing detonation in SI engines?

    <p>Control ignition timing to prevent detonation events</p> Signup and view all the answers

    What is the octane rating of regular unleaded gasoline in the United States?

    <p>87 octane</p> Signup and view all the answers

    What does the octane rating of a fuel measure?

    <p>Resistance to detonation</p> Signup and view all the answers

    What is the role of the combustion chamber in a Spark Ignition (SI) engine?

    <p>To facilitate efficient combustion</p> Signup and view all the answers

    How does the compression ratio of an engine influence its performance?

    <p>Optimizes engine performance</p> Signup and view all the answers

    What does the shape and size of the combustion chamber affect?

    <p>Turbulence and swirl of the air-fuel mixture</p> Signup and view all the answers

    What can excessive heat in the combustion chamber lead to?

    <p>Engine damage and reduced efficiency</p> Signup and view all the answers

    What is detonation in the context of a combustion chamber?

    <p>Uncontrolled combustion causing engine damage</p> Signup and view all the answers

    Why is it important to use fuel with the recommended octane rating for your vehicle?

    <p>To optimize performance and longevity of the engine</p> Signup and view all the answers

    What are some key requirements for combustion chambers in SI engines?

    <p>Ignition system compatibility and compression ratio</p> Signup and view all the answers

    What environmental consequences can detonations have?

    <p>Water pollution and air pollution</p> Signup and view all the answers

    What is a common disadvantage of T-head combustion chambers?

    <p>High knocking tendency</p> Signup and view all the answers

    Which type of combustion chamber allows for the easiest lubrication of valve mechanism?

    <p>L-Head Type</p> Signup and view all the answers

    In which type of combustion chamber does the air flow have to take two right angle turns to enter the cylinder?

    <p>L-Head Type</p> Signup and view all the answers

    Which type of combustion chamber aims to obtain fast flame speed and reduced knock?

    <p>F-Head Type</p> Signup and view all the answers

    What characteristic makes an I-Head Type combustion chamber superior at high compression ratios?

    <p>Greater freedom from knock</p> Signup and view all the answers

    What distinguishes an F-Head Type combustion chamber from an I-Head Type?

    <p>Exhaust valve in the head and inlet valve in the cylinder block</p> Signup and view all the answers

    Which type of combustion chamber has both valves located on the cylinder head?

    <p>I-Head Type</p> Signup and view all the answers

    What is a main objective of Ricardo's turbulent head design?

    <p>Obtaining fast flame speed and reduced knock</p> Signup and view all the answers

    What makes an I-Head Type combustion chamber superior for high compression ratios?

    <p>Shorter flame travel length leading to greater freedom from knock</p> Signup and view all the answers

    Study Notes

    Stages of Combustion in SI Engines

    • The combustion process in SI engines occurs in five stages:
      • Intake Stroke: Piston moves downward, creating a vacuum that draws in a mixture of air and fuel into the combustion chamber.
      • Compression Stroke: Piston moves back up, compressing the air-fuel mixture, increasing temperature and pressure.
      • Ignition: Spark plug generates an electric spark, igniting the compressed air-fuel mixture.
      • Combustion (Power) Stroke: Ignited mixture rapidly burns, producing high-pressure and high-temperature gas, forcing the piston down, converting thermal energy into mechanical work.
      • Exhaust Stroke: Piston moves back up, pushing exhaust gases out of the combustion chamber and into the exhaust system.

    Factors Influencing Combustion

    • Turbulence: Depends on combustion chamber design and engine speed; affects flame speed and combustion efficiency.
    • Autoignition: Occurs when the unburned charge reaches its autoignition temperature, leading to knocking combustion.
    • Engine Speed: Increases turbulence, reduces knocking tendency.
    • Flame Travel Distance: Shorter distance reduces knocking tendency; affected by engine size, spark plug position, and combustion chamber shape.

    Detonation in SI Engines

    • Detonation: Abnormal combustion of air-fuel mixture, leading to uncontrolled and rapid increase in pressure and temperature.
    • Caused by: High temperature, pressure, or hot spots in the combustion chamber.
    • Characterized by: Distinctive knocking or pinging sound, engine damage, and increased engine wear.

    Factors Responsible for Detonation

    • High Compression Ratio: Increases temperature and pressure, making spontaneous ignition more likely.
    • Advanced Ignition Timing: Igniting the air-fuel mixture too early in the compression stroke can lead to detonation.
    • Lean Air-Fuel Mixture: Higher combustion temperatures, increasing the likelihood of detonation.
    • High Engine Temperature: Elevated engine temperatures contribute to detonation.
    • Low Octane Fuel: Lower octane fuels have a lower resistance to detonation.
    • Carbon Deposits: Hot spots can initiate premature ignition.
    • Poorly Designed Combustion Chamber: Shape and design can influence the likelihood of detonation.
    • Excessive Carbon Buildup: Accumulation of carbon deposits can lead to hot spots and detonation.
    • Engine Overheating: Overheating of engine components increases the chances of detonation.
    • Incorrect Spark Plug Heat Range: Using spark plugs with an incorrect heat range can lead to overheating and contribute to detonation.

    Effect of Detonation on SI Engines

    • Shock Waves: Can cause significant damage to structures and materials.
    • Heat and Fire: Generates intense heat, leading to ignition of surrounding materials.
    • Blast Pressure: Creates a high-pressure environment, causing structural damage, injury, and fatalities.
    • Fragmentation: Can cause additional injuries and damage over a wider area.
    • Crater Formation: Can lead to the formation of craters.
    • Environmental Impact: Can have environmental consequences, such as air and water pollution.

    Octane Rating of Fuel

    • Measures a fuel's resistance to knocking during combustion.
    • Higher octane rating: Greater resistance to knocking.
    • Regular unleaded gasoline: Typically 87 octane; premium gasoline: 91 or higher.

    Requirements of Combustion Chambers for SI Engines

    • Air-Fuel Mixture Formation: Must facilitate proper mixing of air and fuel.

    • Ignition System Compatibility: Must support the ignition system.

    • Compression Ratio: Must be designed to achieve optimal compression ratio.

    • Cooling: Must be designed to dissipate heat efficiently.

    • Shape and Size: Must influence turbulence and swirl of the air-fuel mixture.

    • Detonation Control: Must minimize the likelihood of detonation.

    • Emissions Control: Must contribute to achieving lower levels of pollutants in the exhaust gases.

    • Optimization for Performance and Efficiency: Must balance power output, fuel efficiency, and emissions.### Combustion Chamber Types

    • T-Head Type: Used in early engine development, has a long distance across the combustion chamber, leading to a high knocking tendency; requires two camshafts, which is a disadvantage.

    L-Head Type

    • Modification of T-Head: Provides two valves on the same side of the cylinder, operated by a single camshaft; easy to lubricate valve mechanism; allows for detachable head without disturbing valve gear.
    • Variations:
      • Fig. 3.9(b): Air flow takes two right-angle turns, causing a loss of velocity head and turbulence, resulting in slow combustion.
      • Ricardo's Turbulent Head, Fig. 3.9(c): Aims to obtain fast flame speed and reduce knock; restricts passage to cylinder, creating turbulence; reduces knocking tendency by shortening flame travel length.

    I-Head or Overhead Valve Type

    • Superior at high compression ratios: Both valves located on the cylinder head; characteristics include:
      • Less surface-to-volume ratio, reducing heat loss
      • Less flame travel length, reducing knock
      • Higher volumetric efficiency from larger valves or valve lifts
      • Confinement of thermal failures to cylinder head

    F-Head Type

    • Compromise between L-Head and I-Head: One valve in the cylinder head, the other in the cylinder block; modern F-head engines have exhaust valve in the head and inlet valve in the cylinder block.
    • Disadvantages: Inlet and exhaust valves are separately actuated by two cams mounted on two camshafts driven by the crankshaft through gears.

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    Description

    Learn about the different stages involved in combustion process in spark-ignition (SI) engines, such as intake stroke, compression stroke, and ignition. Understand how each stage contributes to the overall operation of a gasoline engine.

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