Otto and Diesel cycles are crucial thermodynamic processes in internal combustion engines. These four-stroke cycles power most vehicles, from cars to trucks, using different ignition methods and heat addition processes.

Understanding these cycles helps grasp how engines convert fuel into mechanical energy. Otto cycle uses spark ignition and constant volume heat addition, while Diesel cycle relies on compression ignition and constant pressure heat addition, resulting in different efficiencies and applications.

Otto and Diesel Cycles

Components of Otto cycle

• Otto cycle is a four-stroke thermodynamic cycle used in spark-ignition internal combustion engines involves intake, compression, power, and exhaust strokes
• Key components of an Otto cycle engine include:
  • Piston moves up and down within the cylinder to compress and expand the working fluid (air-fuel mixture)
  • Cylinder contains the working fluid and provides a space for combustion to occur
  • Spark plug initiates combustion by creating an electric spark to ignite the compressed air-fuel mixture
  • Valves control the flow of air and exhaust gases into and out of the cylinder (intake and exhaust valves)
  • Connecting rod connects the piston to the crankshaft, converting the linear motion of the piston into rotational motion
  • Crankshaft transforms the reciprocating motion of the piston into rotational motion to drive the engine's output shaft
• Otto cycle engines are widely used in gasoline-powered vehicles (cars, motorcycles) and small engines (lawnmowers, chainsaws, generators)

Processes in Diesel cycle

1. Isentropic compression ($1 \to 2$): Air is compressed adiabatically (no heat transfer) and reversibly (no entropy change) as the piston moves upward, increasing pressure and temperature
2. Constant pressure heat addition ($2 \to 3$): Fuel is injected into the compressed hot air, and combustion occurs at nearly constant pressure, adding heat to the system
3. Isentropic expansion ($3 \to 4$): The high-pressure, high-temperature gases expand adiabatically and reversibly, pushing the piston downward and performing work
4. Constant volume heat rejection ($4 \to 1$): The remaining heat is rejected from the system at constant volume as the exhaust valve opens, and the piston moves upward to expel the exhaust gases

Otto vs Diesel cycle principles

• Both Otto and Diesel cycles are four-stroke thermodynamic cycles involving isentropic compression and expansion processes, as well as constant volume heat rejection
• Key differences between Otto and Diesel cycles:
  • Ignition method: Otto cycle uses spark ignition, while Diesel cycle relies on compression ignition (high compression ratios cause auto-ignition of fuel)
  • Heat addition process: Otto cycle adds heat at constant volume, while Diesel cycle adds heat at constant pressure
  • Compression ratio: Diesel cycle engines have higher compression ratios (14:1 to 25:1) compared to Otto cycle engines (8:1 to 12:1), resulting in higher thermal efficiency
• Diesel cycle engines generally achieve higher thermal efficiency due to higher compression ratios, leaner fuel-air mixtures, and constant pressure heat addition process

Efficiency calculations for thermodynamic cycles

• Work in an Otto or Diesel cycle can be calculated using:
  • $W = \int P dV$ for work done by the system
  • $W = -\int P dV$ for work done on the system
• Heat transfer in an Otto or Diesel cycle can be calculated using:
  • $Q = mc_v \Delta T$ for constant volume processes (Otto cycle heat addition and rejection)
  • $Q = mc_p \Delta T$ for constant pressure processes (Diesel cycle heat addition)
  • $Q = \int T dS$ as a general heat transfer equation
• Thermal efficiency ($\eta_{th}$) of an Otto or Diesel cycle can be calculated using:
  • $\eta_{th} = \frac{W_{net}}{Q_{in}} = 1 - \frac{Q_{out}}{Q_{in}}$
  • For an ideal Otto cycle: $\eta_{th} = 1 - \frac{1}{r^{\gamma - 1}}$, where $r$ is the compression ratio and $\gamma$ is the specific heat ratio (typically 1.4 for air)
  • For an ideal Diesel cycle: $\eta_{th} = 1 - \frac{1}{r^{\gamma - 1}} \left(\frac{r_c^\gamma - 1}{\gamma (r_c - 1)}\right)$, where $r_c$ is the cutoff ratio (ratio of volumes at the end and start of the heat addition process)