Stages of Combustion in SI Engines

Stages of Combustion in SI Engines

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

Factors Influencing Combustion Stages

• Turbulence: Depends on the design of the combustion chamber and engine speed; increases flame speed and reduces the time available for the end charge to attain autoignition conditions.
• Engine Speed: Increases turbulence, flame speed, and reduces the time available for preflame reactions, resulting in reduced knocking tendency.
• Flame Travel Distance: Influences knocking tendency, affected by engine size, combustion chamber size, and spark plug position.
• Engine Size: Larger engines have a greater tendency for knocking due to longer flame travel time.
• Combustion Chamber Shape: A more compact shape reduces flame travel and combustion time, resulting in better anti-knock characteristics.
• Location of Spark Plug: Centrally located spark plug reduces flame travel, resulting in minimum knocking tendency.

The Phenomenon of Knock in SI Engines

• Knocking occurs when autoignition occurs, causing a rapid increase in pressure and temperature.
• Autoignition is influenced by the properties of the fuel, and occurs when the unburned mixture reaches its self-ignition temperature.
• Ignition lag period and flame travel time affect the onset of knocking.
• High autoignition temperature and long ignition lag are desirable qualities for SI engine fuels to avoid detonation.
• Knocking can cause engine failure, noise, and vibration, and can be detected by a pressure transducer and audible sound.

Detonation

• Detonation refers to the abnormal combustion of the air-fuel mixture in the combustion chamber.
• Characterized by an uncontrolled and rapid increase in pressure and temperature, causing damage to the engine.
• Factors contributing to detonation include high temperature, pressure, or the presence of hot spots in the combustion chamber.

Combustion Chambers

• T-Head Type: Early design, prone to knocking due to long flame travel distance.
• L-Head Type: Modification of T-Head, providing two valves on the same side of the cylinder, reducing knocking tendency.
• I-Head Type or Overhead Valve: Superior to side valve engines, characterized by less surface to volume ratio, less flame travel length, and higher volumetric efficiency.
• F-Head Type: Compromise between L-Head and I-Head types, with one valve in the cylinder head and the other in the cylinder block.

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 increase 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 initiate premature ignition.
• Poorly Designed Combustion Chamber: Promotes turbulence and hot spots.
• Excessive Carbon Buildup: Leads to hot spots and contributes to detonation.
• Engine Overheating: Increases the chances of detonation.
• Incorrect Spark Plug Heat Range: Leads to overheating and contributes to detonation.

Effect of Detonation on SI Engine

• Shock Waves: Powerful shock waves cause damage to structures and materials.
• Heat and Fire: Intense heat leads to the ignition of surrounding materials.
• Blast Pressure: High-pressure environment causes structural damage, injury, and fatalities.
• Fragmentation: Dispersal of fragments or shrapnel upon detonation.### Detonation
• Can cause additional injuries and damage over a wider area
• Can lead to crater formation, especially in explosions involving earth or underground structures
• Can have environmental consequences, such as air and water pollution, depending on the materials involved

Octane Rating

• Measures a fuel's resistance to knocking during combustion in internal combustion engines
• Represented by two numbers, e.g., 87 octane or 91 octane
• Determined by comparing a fuel's performance to a mixture of iso-octane and heptane
• A higher octane rating indicates a greater resistance to knocking
• Higher-performance or high-compression engines often require fuels with higher octane ratings to prevent knocking and optimize performance
• Regular unleaded gasoline in the US typically has an octane rating of 87, while premium gasoline can have octane ratings of 91 or higher

Combustion Chambers for SI Engines

• Must facilitate proper air-fuel mixture formation, typically through carburetion or fuel injection systems
• Design should support ignition system, including spark plug location and orientation
• Affects compression ratio, which is crucial for engine performance and efficiency
• Should be designed to dissipate heat efficiently to prevent knocking and reduced efficiency
• Shape and size influence turbulence and swirl of air-fuel mixture, affecting combustion efficiency
• Should minimize likelihood of detonation through design factors, such as chamber shape and spark plug location
• Impacts emissions formation, with modern SI engines striving to minimize pollutants
• Crucial for achieving a balance between power output, fuel efficiency, and emissions