Basics of Diesel Cycle, Components and Procesess

The Diesel cycle is an important thermodynamic process underpinning diesel engine operation, powering various vehicles and machinery.

This article will give you a comprehensive overview of the Diesel cycles and present different equations used for considering diesel cycles.

Unlike the Otto cycle used in gas engines, Diesel cycles rely on compression ignition, where air compression leads to spontaneous fuel combustion. This distinction yields certain advantages, such as improved efficiency and torque.

Engineers and scientists need to understand the Diesel cycle for engine design and optimization, as it reveals how these engines efficiently convert stored chemical energy in diesel fuel into mechanical work.

Definitions:

Here are some definitions to keep in mind with diesel cycles!

Isobaric: the process takes place under constant pressure.

Isentropic: an ideal thermodynamic process that is both adiabatic (no heat transfer) and reversible; there is constant entropy.

Isometric: under equal measurement.

Compression: a reduction in volume.

Addition: i.e. an isobaric addition, where heat may increase and be added under constant pressure.

Rejection: concerning thermodynamics, a rejection usually involves releasing heat to the surroundings during a process.

A diesel cycle has three major components: the piston, the crankshaft and the combustion chamber.

1-2 SC Isentropic Compression

3-4 SE Isentropic Expansion

2-3 PA Isobaric Addition

4-1 VR Isometric Rejection

These are four processes involved, and they are also described as 'strokes'.

• Suction stroke: The Isometric Rejection
• Compression stroke: The isentropic Compression
• Expansion stroke: The Isobaric Addition
• Exhaust stroke: The Isentropic Expansion

Equations Commonly Used For Concerning Diesel Cycles

The following are the variables for the equations : 

• \(V\) = the volume. i.e. V(1) is the volume at state 1.
• \(r(k)\) = Compression Ratio, is the ratio of the volume at the first state to the volume at the second state.

$$r_k=\frac{V_1}{V_2}$$

• \(r(e)\) = Expansion Ratio, the expansion ratio, is the ratio of the volume at the fourth state to the volume at the third state.

$$r_e=\frac{V_4}{V_3}$$

• \(r(c)\) = Cut off Ratio, is the ratio of the volume at the third state to the volume at the second state.

$$r_c=\frac{V_3}{V_2}$$

• Mean Effective Pressure (MEP) = The constant theoretical pressure would produce the same network in one complete cycle if it acted on the piston.

$$\hspace{3.5em}MEP\ =\ \frac{Net\ Work\ For\ One\ Cycle}{Displacement\ Volume}$$

$$MEP=\frac{P_1[D_{eff}-N_{eff}]}{(k-1)(1-\frac{1}{rk})}=\frac{Q_h-Q_c}{(k-1)(1-\frac{1}{r_k})}$$

The mean effective pressure is the average pressure in an internal combustion engine cylinder over the entire engine cycle.

• \(η\) = The efficiency of the cycle
• \(k\) = The Adiabatic Index
• \(P\) = the pressure. i.e. P(1) is the pressure at state 1.
• Where: \(P(1)[D(eff)-N(eff)] = Q(h)-Q(c), Q(h)\) is the amount of heat initially extracted, and Q(c) is the heat expelled.

Note :

$$\eta_{diesel}=1-\frac{r_c^k-1}{k_rk^{k-1}(r_c-1)}$$

The efficiency of a diesel cycle is a function of the compression ratio and the cut-off ratio.

In summary, the diesel cycle is a thermodynamic process used in diesel engines to convert chemical energy stored in diesel fuel into mechanical work. It has four distinct phases: intake, compression, power, and exhaust. Many different equations are used when it comes to diesel cycles, like in calculating different ratios, and engineers and scientists often need to calculate the efficiency of the cycle and the mean effective pressure.

Diesel engines are known for having a higher thermal efficiency and torque, making them well-suited for heavy-duty applications such as trucks, buses, and industrial machinery.

1. Engineering Software. 2023. Diesel Cycle Analysis. [ONLINE] Available at: https://www.engineering-4e.com/diesel-cycle-analysis. [Accessed 26 October 2023].
2. LinkedIn. 2023. Important Thermodynamic Concepts. [ONLINE] Available at: https://www.linkedin.com/feed/update/urn:li:activity:7095576421120548864?utm_source=share&utm_medium=member_desktop. [Accessed 24 October 2023].
3. Physics Stack Exchange. 2023. Apparent contradiction concerning the net work of a Carnot cycle. [ONLINE] Available at: https://physics.stackexchange.com/questions/524480/apparent-contradiction-concerning-the-net-work-of-a-carnot-cycle. [Accessed 26 October 2023].
4. Steemit. 2023. A Challenging Review Problem for Ideal Diesel Cycle. [ONLINE] Available at: https://steemit.com/education/@josephace135/a-challenging-review-problem-for-ideal-diesel-cycle. [Accessed 26 October 2023].
5. Wikipedia. 2023. Diesel cycle. [ONLINE] Available at: https://en.wikipedia.org/wiki/Diesel_cycle. [Accessed 26 October 2023].
6. Wikiversity. 2019. Thermodynamics/The Second Law of Thermodynamics. [ONLINE] Available at: https://en.wikiversity.org/wiki/Thermodynamics/The_Second_Law_of_Thermodynamics. [Accessed 26 October 2023].