Diesel Engine Fundamentals and Comparison to Otto Cycle
📌 The primary difference between Diesel and gasoline (Otto cycle) engines lies in the combustion process.
⛽ Gasoline engines mix air and fuel before compression, ignited by a spark plug (busif).
🔥 Diesel engines compress only air; fuel is injected at high pressure near the top dead center, relying on the high temperature from compression (auto-ignition) for combustion, thus lacking a spark plug.

Combustion Requirements (Fire Triangle)
🔺 Combustion requires three components: air, fuel, and high temperature (not necessarily a flame).
🚫 If any of these three elements are missing, combustion will not occur in the engine cylinder.
⚙️ In the Diesel cycle, the fuel injection occurs while the air is highly compressed (high temperature and pressure), causing ignition upon mixing.

Diesel Cycle Characteristics (PV Diagram Analysis)
📉 In the Diesel cycle, heat addition (combustion) occurs at constant pressure (from state 2 to 3), contrasting with the Otto cycle's constant volume heat addition.
🔄 Expansion (isentropic) and heat rejection (constant volume) processes are identical between the two cycles.
⚙️ Diesel cycles feature a compression ratio ($r$) typically between 15 to 30, which is significantly higher than the Otto cycle (8 to 12), resulting in heavier and more robust engine construction.

Thermodynamic Ratios and Calculations (Example Problem)
📈 The cut-off ratio ($\rho$) is defined as the ratio of the volume after combustion to the volume at the end of compression ($\rho = v_3 / v_2$).
🌡️ Assuming cold air standard properties (constant specific heats at $T = 25^\circ \text{C}$), specific heat values are taken as $c_p = 1.005 \text{ kJ/kg}\cdot\text{K}$ and $c_v = 0.708 \text{ kJ/kg}\cdot\text{K}$, with the specific heat ratio $k = 1.4$.
🔢 For an example problem with $r=18$ and $\rho=2$, the temperature after compression ($T_2$) reaches $953 \text{ K}$ ($680^\circ \text{C}$) from an initial $300 \text{ K}$ ($27^\circ \text{C}$).

Key Points & Insights
➡️ Diesel engine combustion is initiated by high temperature achieved through high compression, eliminating the need for a spark plug.
➡️ The defining difference between the Diesel cycle and Otto cycle is that heat addition in the Diesel cycle occurs at constant pressure, while in the Otto cycle it occurs at constant volume.
➡️ Due to higher required compression ratios ($r \approx 15 \text{ to } 30$), Diesel engines are physically heavier and sturdier than their Otto cycle counterparts.
➡️ In the example calculation, the work output ($W_{\text{net}}$) was $1.34 \text{ kJ}$ with an input heat ($Q_{\text{in}}$) of $2.107 \text{ kJ}$, yielding a thermal efficiency ($\eta_{\text{th}}$) of $63.5\%$.

📸 Video summarized with SummaryTube.com on Nov 18, 2025, 13:41 UTC