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Questions and Answers
What does the octane number represent in fuel testing?
What does the octane number represent in fuel testing?
Which of the following blends results in an octane number of 90?
Which of the following blends results in an octane number of 90?
What is one of the operational parameters that differs between the Research and Motor methods for measuring octane number?
What is one of the operational parameters that differs between the Research and Motor methods for measuring octane number?
Which hydrocarbon has an octane value of zero?
Which hydrocarbon has an octane value of zero?
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In the control of combustion knock, what is the purpose of setting spark advance to 1% below MBT?
In the control of combustion knock, what is the purpose of setting spark advance to 1% below MBT?
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What does the antiknock index represent?
What does the antiknock index represent?
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Why is the motor octane number (MON) typically lower than the research octane number (RON)?
Why is the motor octane number (MON) typically lower than the research octane number (RON)?
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Which fuel is known to have the highest Research Octane Number (RON) among the listed options?
Which fuel is known to have the highest Research Octane Number (RON) among the listed options?
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What is the primary function of tetraethyl lead (TEL) in gasoline?
What is the primary function of tetraethyl lead (TEL) in gasoline?
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Which statement about the formula for octane number in relation to tetraethyl lead is true?
Which statement about the formula for octane number in relation to tetraethyl lead is true?
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What is the role of the spark plug in relation to knock in an engine?
What is the role of the spark plug in relation to knock in an engine?
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Which engine parameter increases the propensity to knock as the compression ratio rises?
Which engine parameter increases the propensity to knock as the compression ratio rises?
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How does engine speed affect the likelihood of knock in an engine?
How does engine speed affect the likelihood of knock in an engine?
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What condition must be met for knock to be avoided during combustion?
What condition must be met for knock to be avoided during combustion?
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What is the primary cause of shock waves in an engine during the knock cycle?
What is the primary cause of shock waves in an engine during the knock cycle?
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Study Notes
Combustion in IC Engines
- Internal combustion engines (IC engines) involve combustion processes that produce mechanical energy.
- Various factors influence combustion, including operating conditions, fuel properties, and engine design.
Flame Propagation in SI Engines
- Fuel-air mixtures are compressed and ignited by a spark plug before the piston reaches top center.
- Turbulent flames spread away from the spark discharge point.
In-cylinder Parameters
- In-cylinder parameters are critical for engine performance.
- Various factors affect the in-cylinder temperature and pressure.
- Variables like crank angle and volume fraction burned are shown within the in-cylinder diagrams.
Flame Development
- Flame development angle (Δθf) is the time interval after spark ignition when the flame kernel develops.
- Rapid burning angle (Δθr) is the time interval required to burn most of the mixture.
- Overall burning angle is the combined time of flame development and rapid burning.
Mixture Burn Time vs Engine Speed
- The time for overall mixture burning (t90%) is inversely proportional to the engine speed (N).
- Faster engine speeds require less time for complete combustion.
- Formula cars have larger bore and short strokes because they need high engine speeds.
Mixture Burn Time
- Mixture burn time (tcomb) is related to the ratio of the bore and the combustion speed in a simple calculation.
Mixture Burn Time vs Engine Speed
- Turbulent burning velocity is proportional to the turbulent intensity (S₁ ~ u).
- Piston speed (u₁) is proportional to engine speed (N).
- Higher engine speeds correlate to higher turbulent flame velocities.
Heat Losses During Burn
- Heat loss to the piston and cylinder head is crucial during combustion.
- Minimizing burn time, by increasing flame velocity and turbulence, reduces heat losses.
- Higher laminar burning velocity is observed with combustion mixtures slightly more rich in fuel.
Optimum F/A Composition
- Maximum engine power occurs at an equivalence ratio (φ) of 1.1, giving highest burning velocity and flame temperature.
- Best fuel economy is observed with an equivalence ratio less than 1.0
Spark Timing
- Spark timing relative to Top Center (TC) affects engine pressure development, power, and efficiency.
- Optimizing spark timing before TC maximizes pressure pulse centering around TC.
- The overall burning angle is typically between 40° to 60°, depending on speed.
Maximum Brake Torque Timing
- Early or late combustion can produce less brake torque.
- Optimal ignition timing (MBT timing), where the opposed tendencies cancel, maximizes brake torque.
- Optimum spark timing is specific to various engine speeds and air-fuel ratios (A/F).
Effect of Engine Speed on Spark Timing
- Overall burn angle (90% burn) increases with engine speed.
- This necessitates higher spark advance for suitable combustion.
- Maintaining optimum spark advance across different engine speeds ensures maximum power.
Effect of Throttle on Spark Timing
- Residual gas fraction increases at part-throttle.
- This dilutes the mixture and reduces laminar burning velocity.
- Consequently, the overall burn angle increases, needing a higher spark advance to compensate.
- Modern engine control units (ECUs) adjust spark advance based on throttle position, intake manifold pressure, and engine speed.
Abnormal Combustion in SI Engines
- Abnormal combustion or "knock" is characterized by a pinging noise.
- Unburnt gases autoignite, producing shock waves, causing these sounds.
- Different intensities of knock correlate with different engine conditions.
Engine Damage From Severe Knock
- Severe knock leads to damage through the combination of high temperatures and high pressures.
- Areas susceptible to damage include pistons, piston crowns, cylinder heads, and gaskets
Combustion in CI Engines
- Fuel is sprayed directly into the cylinder where it vaporizes and spontaneously ignites.
In-Cylinder Measurements
- Fuel injection flow rate, net heat release rate, and cylinder pressure are measured in direct injection CI engines.
- Diagrams are helpful to see the different cycles of these measurements
Combustion in CI Engine
- Combustion stages are characterized including ignition delay and premixed combustion.
- Mixing-controlled combustion and late combustion are also outlined.
Four Stages of Combustion in CI Engines
- Combustion occurs in distinct stages— ignition delay, premixed and mixing-controlled, and late combustion—each with unique features.
CI Engine Types
- CI engines are categorized primarily into direct and indirect injection.
- Direct injection engines have a single combustion chamber.
- Indirect injection engines are divided into a main and pre-chamber.
- Design choices for various applications/engine speeds differ due to factors like mixing length.
Combustion Characteristic
- Combustion span the chamber, depending on equivalence ratios dictated by fuel-air mixing.
- Combustion predominantly occurs in rich conditions, leading to soot generation within the head of the fuel injector.
Ignition Delay
- Ignition delay is the interval between fuel injection initiation and the start of combustion.
- Physical processes include fuel spray atomization, evaporation, and fuel vapor mixing.
- Chemical processes are similar to autoignition including reactions on the liquid fuel surface.
Fuel Ignition Quality
- Ignition characteristics are influenced by fuel types.
- Low cetane fuels produce longer ignition delays, often leading to audible knocking sounds (diesel knock).
- High cetane fuels exhibit shorter delay times and smoother engine operation.
Cetane Number
- Cetane number (CN) is a measure of a fuel's ignition quality, analogous to octane number (ON).
- Fuels are blended to define CN values.
- Higher cetane correlates with faster ignition and smoother operation.
Cetane Number Measurement
- Standardised procedures using a single cylinder engine determine CN under specific operating conditions.
- Compression ratios are varied to determine ignition delay times in the blends of test fuels.
Cetane Number versus Octane Number
- These numbers are inversely related- higher octane number implies a lower cetane number.
- The higher the cetane number, the faster the ignition and the smoother the combustion.
- Gasoline and diesel fuels have inverse qualities.
Factors Affecting Ignition Delay Time
- Injection timing, injection quantity, as well as air temperature and pressure affect ignition delay time in CI engines.
- High loads and wall temperatures often increase residual gases and result in higher temperatures which decrease the ignition delay.
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Description
This quiz explores the combustion processes in internal combustion engines (IC engines), focusing on flame propagation, in-cylinder parameters, and flame development. Understand the influence of various factors on engine performance and combustion efficiency.
