Describe the various stages of combustion in a SI engines with the(P-θ) diagram

 Alright! Let’s break it down simply:

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Stages of Combustion in a Spark Ignition (SI) Engine
(with help of the Pressure vs Crank Angle (P–θ) diagram):

In an SI engine, combustion after the spark plug fires happens in three main stages:

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1. Ignition Delay (Preparation Phase)

• Spark plug fires but no immediate rise in pressure.

• During this time, small amounts of fuel-air mixture start to ignite.

• Flame kernel forms and grows slowly.

• (On P–θ diagram: pressure slightly increases but still close to compression pressure.)

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2. Rapid Combustion (Flame Propagation Phase)

• The flame spreads rapidly across the combustion chamber.

• Most of the fuel burns here.

• A rapid rise in pressure and temperature happens because combustion releases a lot of energy.

• (On P–θ diagram: sharp peak in pressure.)

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3. After-Burning (Late Combustion Phase)

• Remaining unburned mixture (especially near walls) burns slowly.

• Pressure starts falling because the piston is moving down (expansion stroke).

• Combustion is incomplete compared to the rapid combustion phase.

• (On P–θ diagram: pressure gradually decreases after the peak.)

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🔵 P–θ Diagram Overview:

• X-axis = Crank Angle (θ)

• Y-axis = Cylinder Pressure (P)

• Before spark: Pressure rises due to compression.

• After spark:

  • Little delay → Pressure rises sharply (rapid combustion) →

  • Peaks → Then pressure falls gradually during expansion.

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Simple Sketch of P–θ Diagram (text version):

Pressure
   |
   |                /\
   |               /  \   (Peak Pressure after Rapid Combustion)
   |              /    \
   |             /      \
   |            /        \
   |           /          \
   |__________/            \_________ Crank Angle (θ)
         (Spark)

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Quick Recap:
➔ Ignition Delay → Rapid Combustion → After-Burning

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Would you like me to draw a proper labeled P–θ graph image for you too? 🎨🚗 (It’ll make it even easier to understand!)
Let me know! 📚✨

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