The Diesel cycle powers compression-ignition engines, offering higher efficiency than spark-ignition engines. It uses higher compression ratios and constant pressure heat addition, resulting in better fuel economy and torque characteristics.

Understanding the Diesel cycle is crucial for grasping the principles of gas power cycles. It showcases how different thermodynamic processes can be combined to create efficient power-generating systems, highlighting the importance of cycle analysis in engineering.

Diesel cycle principles

Key components and stages

• The Diesel cycle is a thermodynamic cycle that describes the operation of a compression-ignition engine, commonly known as a diesel engine
• The main components of a Diesel engine include a cylinder, piston, connecting rod, crankshaft, fuel injector, and intake and exhaust valves
• The Diesel cycle consists of four main stages:
  • Isentropic compression
  • Constant pressure heat addition
  • Isentropic expansion
  • Constant volume heat rejection

Compression ignition and ratios

• Diesel engines rely on the compression of air to high pressures and temperatures to achieve autoignition of the fuel, unlike spark-ignition engines that use a spark plug to initiate combustion
• Diesel engines typically operate at higher compression ratios compared to gasoline engines, often ranging from 14:1 to 25:1, which contributes to their higher thermal efficiency
• The higher compression ratios in Diesel engines require stronger engine components to withstand the increased pressures and temperatures

Diesel vs Otto cycles

Similarities and differences

• Both the Diesel and Otto cycles are four-stroke thermodynamic cycles used in internal combustion engines, but they differ in their combustion processes and engine designs
• The Otto cycle, used in spark-ignition engines, features constant volume heat addition, while the Diesel cycle has constant pressure heat addition
• Diesel engines have higher compression ratios than Otto engines, leading to higher thermal efficiencies but also requiring stronger engine components to withstand the higher pressures

Fuel delivery and engine characteristics

• In the Otto cycle, fuel and air are mixed before compression, while in the Diesel cycle, air is compressed first, and fuel is injected near the end of the compression stroke
• Diesel engines generally have lower power-to-weight ratios compared to Otto engines due to their heavier construction, but they offer better fuel economy and torque characteristics
• The fuel injection system in Diesel engines allows for precise control over the fuel quantity and timing, which can optimize combustion and reduce emissions

Thermodynamic processes in Diesel cycle

Isentropic compression and expansion

• Isentropic compression (1-2): The air is compressed adiabatically from a low pressure and temperature to a high pressure and temperature, following the isentropic process equation ($PV^γ = constant$)
• Isentropic expansion (3-4): The high-pressure, high-temperature gases expand adiabatically, pushing the piston down and performing work. This process also follows the isentropic process equation ($PV^γ = constant$)
• During isentropic processes, there is no heat transfer between the system and the surroundings, and the entropy remains constant

Constant pressure heat addition and constant volume heat rejection

• Constant pressure heat addition (2-3): Fuel is injected into the compressed air, and combustion occurs at nearly constant pressure. The heat addition process follows the ideal gas law ($PV = nRT$)
• Constant volume heat rejection (4-1): The remaining heat is rejected from the system at constant volume as the exhaust valve opens, and the cycle returns to its initial state. This process is approximated as an isochoric process ($ΔU = Q - W$, with $W = 0$)
• The constant pressure heat addition in Diesel engines allows for a more gradual combustion process compared to the constant volume heat addition in Otto engines

Efficiency and work output of Diesel engines

Thermal efficiency calculation

• The thermal efficiency of a Diesel cycle can be calculated using the compression ratio ($r$) and the cutoff ratio ($r_c$), which is the ratio of the cylinder volumes at the end and start of the constant pressure heat addition process
  • Thermal efficiency = $1 - (1 / r^(γ-1)) * [(r_c^γ - 1) / (γ * (r_c - 1))]$
• Factors that affect the thermal efficiency of a Diesel engine include the compression ratio, cutoff ratio, fuel properties, and engine design parameters such as bore, stroke, and valve timing
• Diesel engines typically have higher thermal efficiencies compared to Otto engines due to their higher compression ratios and the constant pressure heat addition process

Work output and power

• The net work output of a Diesel cycle is the difference between the work done during the expansion process and the work done during the compression process
  • $W_net = W_expansion - W_compression$
  • $W_net = m * (h_3 - h_4) - m * (h_2 - h_1)$, where $m$ is the mass of the working fluid and $h$ is the specific enthalpy at each state point
• The power output of a Diesel engine can be determined by multiplying the net work output per cycle by the number of cycles per second (engine speed) and the number of cylinders
• Diesel engines are known for their high torque output, particularly at lower engine speeds, making them suitable for heavy-duty applications such as trucks, buses, and industrial machinery