Combustion in Internal Combustion Engines

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.