Stages of Combustion in SI Engines

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.

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.