Discuss the method of obtaining in the rate of heat release from engines.

 

🔥 Method of Obtaining the Rate of Heat Release in Engines

The Rate of Heat Release (ROHR) is a critical parameter in analyzing the combustion process in internal combustion engines. It gives insight into how quickly energy is released during combustion and helps assess combustion efficiency, timing, and knocking behavior.

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🔍 Purpose:

• To evaluate combustion characteristics

• Optimize injection timing and ignition

• Study emissions and engine performance

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📐 Method of Obtaining ROHR:

✅ 1. Data Collection (Pressure-Crank Angle Diagram)

• A pressure transducer is installed in the engine cylinder to measure in-cylinder pressure as a function of crank angle.

• A crank angle encoder gives accurate crankshaft position.

Output: Cylinder Pressure vs. Crank Angle (P–θ) Data

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✅ 2. Heat Release Rate Calculation Using First Law of Thermodynamics

The combustion process is analyzed using the First Law of Thermodynamics for an open system:

$$\frac{dQ_{net}}{d\theta} = \frac{\gamma}{\gamma - 1} \cdot P \cdot \frac{dV}{d\theta} + \frac{1}{\gamma - 1} \cdot V \cdot \frac{dP}{d\theta}$$

Where:

Symbol	Description
$\frac{dQ_{net}}{d\theta}$	Net rate of heat release per crank angle degree
$\gamma$	Ratio of specific heats (≈ 1.3–1.4)
$P$	Cylinder pressure
$V$	Cylinder volume
$\theta$	Crank angle
$\frac{dV}{d\theta}$	Volume change rate w.r.t. crank angle
$\frac{dP}{d\theta}$	Pressure change rate w.r.t. crank angle

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🔄 Steps to Calculate ROHR:

1. Measure $P$ vs. $\theta$ using sensors during engine operation.

2. Calculate cylinder volume $V(\theta)$ and its derivative $\frac{dV}{d\theta}$ using engine geometry.

3. Numerically differentiate $P(\theta)$ to get $\frac{dP}{d\theta}$.

4. Substitute values into the heat release equation.

5. Plot $\frac{dQ}{d\theta}$ vs. $\theta$ to visualize the combustion behavior.

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📊 Typical ROHR Curve:

The graph of ROHR vs. crank angle usually shows:

• A sharp rise during premixed combustion

• A plateau or slow rise during diffusion combustion (in CI engines)

• Return to zero or negative as expansion begins

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📌 Applications of Heat Release Analysis:

Use Case	Description
🔍 Combustion Phasing	Identify start and duration of combustion
⛽ Injection/Ignition Optimization	Adjust timing for max efficiency
🌡️ Knock Detection	Sharp ROHR rise indicates knocking
🔧 Engine Calibration	Improve fuel efficiency and emissions

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🧠 Conclusion:

The rate of heat release is a fundamental tool in engine combustion analysis. By using cylinder pressure data and applying the First Law of Thermodynamics, engineers can derive real-time combustion performance data, enabling better design, control, and optimization of internal combustion engines.