What do you understand by turbo charging? Why SI engines are not usually turbo charged.Give the boost pressure range for SI & CI engines

 (i) Turbocharging Explanation 

Turbocharging:
Turbocharging is a method used to increase the power output of an internal combustion engine by forcing more air into the combustion chamber. A turbocharger consists of a turbine and a compressor mounted on the same shaft.

• The turbine is driven by exhaust gases from the engine.

• This drives the compressor, which compresses and forces more air into the engine’s intake manifold.

• More air means more oxygen, allowing more fuel to be burned, resulting in increased power without increasing engine size.

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Why SI (Spark Ignition) Engines Are Not Usually Turbocharged:

1. Knocking (Pre-ignition) Risk:

   • Turbocharging increases the intake pressure and temperature, which raises the compression temperature inside the cylinder.

   • This increases the likelihood of knocking, a major issue in SI engines due to uncontrolled combustion.

2. Lower Compression Ratios Required:

   • To avoid knocking, turbocharged SI engines need lower compression ratios, which reduces their thermal efficiency at low loads.

3. Complex Fuel-Air Mixture Handling:

   • SI engines operate with a pre-mixed air-fuel charge. Compressing this mixture can lead to backfires or hot spots that cause ignition too early.

4. Cost and Complexity:

   • Turbocharging requires intercoolers, stronger pistons, and better engine management systems, making it expensive and complex, especially in smaller gasoline engines.

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Boost Pressure Range:

Engine Type	Typical Boost Pressure Range
SI Engines	0.3 to 0.6 bar (or 4.4 to 8.8 psi)
CI Engines	0.7 to 1.5 bar (or 10.3 to 22 psi)

• CI (Compression Ignition) engines are more robust and operate with excess air, allowing higher boost pressures without knocking.

• Modern turbocharged diesel engines can handle much higher boost due to their stronger components and non-throttle operation.

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Summary:
Turbocharging improves power output and efficiency by increasing intake air pressure. While widely used in CI engines, SI engines are less frequently turbocharged due to knocking, complexity, and thermal limitations.