Stages of SI Engine Combustion
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Questions and Answers

What characterizes the ignition lag phase in SI engine combustion?

  • It is primarily responsible for the maximum rise of pressure.
  • It is the phase where the heat release begins.
  • It involves the growth of a fully propagating flame nucleus.
  • It lasts between 0.00015 to 0.0002 seconds. (correct)

    • In the combustion process of an SI engine, what happens during the propagation of flame phase?

  • The maximum rise of pressure occurs. (correct)
  • There is a decrease in flame speed.
  • The crank angle is irrelevant.
  • The mixture ratio begins to stabilize.

    • How does engine load affect flame speed in an SI engine?

  • Increased load results in increased flame speed. (correct)
  • Flame speed remains constant regardless of load.
  • Load has no effect on combustion dynamics.
  • Increased load leads to a decrease in flame speed.

    • What role does turbulence play in the combustion of fuel in an SI engine?

    <p>It increases the flame speed linearly with engine speed.</p> Signup and view all the answers

    Which factor is NOT associated with the effects on ignition lag?

    <p>Ambient temperature of the environment.</p> Signup and view all the answers

    During which phase does the heat release primarily occur due to fuel injection?

    <p>After burning phase.</p> Signup and view all the answers

    What effect does engine size have on the RPM in SI engines?

    <p>Similar designs run at similar piston speeds.</p> Signup and view all the answers

    Which of the following is true regarding the mixture ratio in SI engines?

    <p>Richer mixtures reduce the ignition lag.</p> Signup and view all the answers

    What effect do higher initial temperature and pressure have on ignition lag?

    <p>They reduce ignition lag.</p> Signup and view all the answers

    Which of the following factors can lead to an increase in flame speed?

    <p>Supercharging the engine.</p> Signup and view all the answers

    What is the primary cause of knocking in an engine?

    <p>Auto ignition of unburned charge.</p> Signup and view all the answers

    What effect does lowering the electrode gap have on spark voltage requirements?

    <p>Decreases required voltage.</p> Signup and view all the answers

    Which of the following describes pre-ignition in combustion engines?

    <p>Ignition happens when a mixture contacts hot surfaces.</p> Signup and view all the answers

    How does an increased compression ratio affect flame propagation?

    <p>It increases both pressure and temperature.</p> Signup and view all the answers

    What is auto-ignition in the context of combustion engines?

    <p>Spontaneous reaction of the fuel-air mixture.</p> Signup and view all the answers

    Which effect can combustion noise and roughness indicate?

    <p>Presence of knocking in the engine.</p> Signup and view all the answers

    What is the primary effect of high pressure waves generated during knocking in an engine?

    <p>They can increase the rate of wear on engine parts.</p> Signup and view all the answers

    Why is the location of the exhaust valve critical in relation to the spark plug?

    <p>It prevents potential knocking.</p> Signup and view all the answers

    How does the design of the combustion chamber affect turbulence of the mixture?

    <p>It decreases the flame speed thereby increasing knocking tendency.</p> Signup and view all the answers

    What is a consequence of a detonating engine's increased rate of heat transfer?

    <p>Decrease in power output and efficiency.</p> Signup and view all the answers

    How does raising the compression ratio affect the engine?

    <p>It increases both temperature and pressure of the mixture.</p> Signup and view all the answers

    Which type of fuel molecule is generally associated with a greater tendency to knock?

    <p>Long-chain paraffins.</p> Signup and view all the answers

    What is the effect of fuel-air ratio on knocking tendency?

    <p>It primarily affects ignition delay.</p> Signup and view all the answers

    What happens to carbon deposits as a result of detonation?

    <p>They increase due to inefficient combustion.</p> Signup and view all the answers

    How does an increase in atmospheric humidity affect engine knocking?

    <p>It decreases the tendency to knock by decreasing the reaction time of the fuel.</p> Signup and view all the answers

    What effect does a higher coolant temperature have on the knocking tendency of an engine?

    <p>It increases the knocking tendency by decreasing the delay period.</p> Signup and view all the answers

    What is the key feature of a centrally located spark plug in a combustion chamber?

    <p>It facilitates proper combustion timing.</p> Signup and view all the answers

    Which combustion chamber type is described as being prone to detonation due to its design?

    <p>T-head combustion chamber</p> Signup and view all the answers

    What impact does an advanced spark timing have on combustion?

    <p>It increases the fuel burnt before and after TDC position.</p> Signup and view all the answers

    How does the swirl rate in the combustion chamber affect fuel evaporation?

    <p>It promotes a change of evaporation rate and improves air-fuel mixing.</p> Signup and view all the answers

    What is the result of increased residual gases in the combustion chamber?

    <p>Reduced oxygen concentration, which increases ignition delay.</p> Signup and view all the answers

    What determines the sensitivity of the end gas temperature to ignition timing?

    <p>The design of the combustion chamber.</p> Signup and view all the answers

    What is the primary characteristic of the rapid combustion phase in combustion engines?

    <p>It has high heat release characteristics.</p> Signup and view all the answers

    Which factor does NOT influence the ignition delay in diesel engines?

    <p>Fuel density</p> Signup and view all the answers

    During the controlled or diffusion combustion phase, what primarily controls the burning rate?

    <p>The rate of fuel availability for burning</p> Signup and view all the answers

    What happens during the after burning phase of combustion?

    <p>Energy is primarily released in the form of soot.</p> Signup and view all the answers

    Which engine speed range is classified as medium speed?

    <p>Between 1500 and 3000 rpm</p> Signup and view all the answers

    How does reducing intake air temperature affect ignition delay?

    <p>It increases ignition delay.</p> Signup and view all the answers

    What role does combustion chamber design play in ignition delay?

    <p>It impacts fuel evaporation and manipulation of the spray pattern.</p> Signup and view all the answers

    What is diesel knock primarily caused by?

    <p>Rapid combustion and imperfect fuel-air mixing.</p> Signup and view all the answers

    Study Notes

    Stages of SI Engine Combustion

    • Three stages: ignition lag, flame propagation, and combustion after burning
    • Ignition lag (AB): Preparation phase (growth and development of flame)
      • Very short, typically between 0.00015 and 0.0002 seconds
    • Flame propagation (BC): Period from combustion start to maximum pressure increase.
    • Combustion after burning (CD): Heat release due to fuel injection, no significant pressure rise.

    Effect of Engine Variables on Ignition Lag

    • Fuel: Higher self-ignition temperature results in longer ignition lag.
    • Mixture ratio: Richer than stoichiometric ratios yield shorter ignition lag.
    • Initial temperature and pressure: Higher values reduce ignition lag.
    • Electrode gap: Narrower gaps and higher compression ratios increase ignition lag.
    • Turbulence: Increases with engine speed which is directly proportional to ignition lag; less impact on ignition lag.

    Effect of Engine Variables on Flame Propagation

    • A/F ratio: Mixture strength influences combustion rate and heat generated.
    • Compression ratio: Increased pressure and temperature reduce residual gases. Increased load on engine increases cycle pressures and flame speed.
    • Engine speed: Turbulence increases with speed thus increasing flame speed.
    • Engine size: Similar designs run at similar piston speeds, either small engines with higher RPM or large engines with lower RPM are used. Other factors affecting flame speed include supercharging, spark timing, and residual gas quantities.

    Detonation or Knocking

    • Auto-ignition of unburned portions of charge.
    • Pre-ignition: Ignition of homogeneous charge due to hot surfaces.

    Effect of Detonation

    • Noise and roughness: Loud, pulsating noises and pressure waves cause vibrations.
    • Mechanical damage: Increased wear rate due to high pressure waves.
    • Carbon deposits: Increased carbon deposits.
    • Increased heat transfer: Knocking causes increased heat transfer.
    • Decreased power and efficiency: Reduced power and efficiency.
    • Pre-ignition: Increased rate of heat transfer, potentially impacting engine performance and safety.

    Factors Influencing Knocking in SI engines

    • Temperature factors: Higher compression ratio, supercharging, coolant temperature, cylinder/chamber wall temperature increase knocking tendencies.
    • Density factors: Higher charge density favors knocking. Advanced spark timing affects fuel burn and thus knocking.
    • Time factors: Flame travel distance is correlated with knocking risk. Location of spark plugs, and exhaust valves affects knocking tendencies.
    • Engine size: Large engines are more susceptible.
    • Mixture Characteristics: Mixture turbulence affects flame speed (less turbulence=inc. tendency to knock).

    Stages of Combustion in CI Engines

    • Ignition delay period: Time between fuel injection start and combustion beginning.

    Variables Affecting Diesel Knock

    • Cetane number: Fuel quality and ignition timing.
    • Injection timing: Ignition delay at operating conditions.
    • Injection quantity: Varying engine load affects exhaust gas temperatures.
    • Intake air temperature and pressure: Reduce intake air T & P to increase ignition delay. Increases ignition delay, decreasing intake T&P.
    • Engine speed: Related to "temptime" and "pressuretime" relationship.
    • Combustion chamber design: Influences spray impingement on walls and subsequent combustion.
    • Swirl rate: Change in evaporation rate.
    • Oxygen concentration: Lower oxygen concentration means longer ignition delay.

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    Related Documents

    SI Engine Combustion Stages PDF

    Description

    This quiz covers the three crucial stages of combustion in a Spark Ignition (SI) engine: ignition lag, flame propagation, and combustion after burning. Understand how various engine variables such as fuel type, mixture ratios, initial temperature, and pressure affect the ignition lag and flame propagation processes. Test your knowledge on this essential topic in automotive engineering.

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