Diesel Cycle

Definition

The diesel cycle is a thermodynamic cycle that describes the operation of a diesel engine. It is a type of internal combustion engine that uses the heat of compression to ignite the fuel, rather than using an electric spark as in a gasoline engine.

5 Must Know Facts For Your Next Test

1. The diesel cycle consists of four main processes: isentropic compression, constant pressure heat addition, isentropic expansion, and constant volume heat rejection.
2. The high compression ratio in a diesel engine (typically 14:1 to 25:1) allows for the auto-ignition of the fuel without the need for a spark plug.
3. Diesel engines generally have higher thermal efficiency compared to gasoline engines due to the higher compression ratio and lean air-fuel mixture.
4. The constant pressure heat addition in the diesel cycle is achieved by injecting fuel into the compressed air, which then burns and expands, driving the piston down.
5. The constant volume heat rejection in the diesel cycle occurs when the exhaust valve opens, and the hot gases are expelled from the cylinder.

Review Questions

• Explain how the high compression ratio in a diesel engine contributes to its efficiency.
  • The high compression ratio in a diesel engine, typically ranging from 14:1 to 25:1, allows for the auto-ignition of the fuel without the need for a spark plug. This high compression ratio increases the temperature and pressure of the air in the cylinder, which in turn increases the efficiency of the engine. The higher compression ratio results in a greater amount of work being done during the expansion stroke, leading to a higher thermal efficiency compared to a gasoline engine.
• Describe the four main processes of the diesel cycle and how they contribute to the engine's operation.
  • The four main processes of the diesel cycle are: isentropic compression, constant pressure heat addition, isentropic expansion, and constant volume heat rejection. During the isentropic compression, the air in the cylinder is compressed, increasing its temperature and pressure. The constant pressure heat addition occurs when fuel is injected into the hot, compressed air, and it autoignites, causing the gases to expand and drive the piston down. The isentropic expansion process converts the thermal energy into mechanical work, and the constant volume heat rejection occurs when the exhaust valve opens, and the hot gases are expelled from the cylinder. The combination of these four processes is what allows a diesel engine to efficiently convert the chemical energy of the fuel into mechanical work.
• Analyze how the diesel cycle relates to the concepts of the First and Second Laws of Thermodynamics, and how this contributes to the engine's efficiency.
  • The diesel cycle is closely related to the First and Second Laws of Thermodynamics. The First Law states that energy can be converted from one form to another, but it cannot be created or destroyed. In the diesel cycle, the chemical energy of the fuel is converted into thermal energy during the constant pressure heat addition, and then this thermal energy is converted into mechanical work during the isentropic expansion. The Second Law of Thermodynamics states that heat cannot spontaneously flow from a colder to a hotter object. The constant volume heat rejection in the diesel cycle, where the hot exhaust gases are expelled from the cylinder, is an example of this principle. The high thermal efficiency of the diesel engine, compared to a gasoline engine, is a result of the diesel cycle's adherence to these thermodynamic principles, which allow for a more efficient conversion of the fuel's energy into useful work.

Related terms

Compression Ratio: The ratio of the maximum to minimum volume in the cylinder of an internal combustion engine.

Thermal Efficiency:

The ratio of the useful work output of an engine to the energy input in the form of fuel.

Adiabatic Process:

A thermodynamic process in which no heat is transferred to or from the system.