Diesel Cycle: Combustion process with p-V and T-s Diagrams

The Diesel cycle is a combustion process of a reciprocating IC engine. It is invented by Rudolph Diesel in the year 1897. It is used in diesel engines. Today in this article we will know about its process in detail with its p-V and T-s diagrams.

Table of contents
p-V and T-s Diagrams
Maximum thermal efficiency
Difference between Diesel and Otto cycles

p-V and T-s Diagrams of Diesel Cycle

The diesel cycle (combustion process) is similar to the Otto cycle.

p-V diagram

To understand and learn the cycle fully we should refer its p-V and T-s diagrams.

T-s diagram

Also read: Thermal conductivity measurement equipment and methods

Diesel cycle has the following four stages:

• Process 1-2 is Isentropic (Reversible adiabatic) Compression
• Process 2-3 is Constant Pressure (Isobaric) Heat Addition
• Process 3-4 is Isentropic Expansion
• Process 4-1 is Constant Volume (Isochoric) Heat Rejection

The maximum thermal efficiency of the Diesel cycle

The maximum thermal efficiency of a Diesel cycle depends on the compression ratio and the cut-off ratio. Air-standard efficiency (or thermal efficiency) of the diesel cycle is given by:

η_Th=1-(1/r^γ-1) ((α^γ-1)/(γα-γ))

where

ηTh is the thermal efficiency
α is the cut off ratio
γ is the ratio of specific heats

Difference between Diesel and Otto cycles

Invented by Nicolas Otto in 1897, the Otto cycle is an idealized thermodynamic cycle for spark ignition engines. We have discussed above the steps included in an ideal diesel cycle. For otto-cycle, the steps are somewhat similar,

• 1–2 is the isentropic compression of air in the cylinder.
• 2–3 is the constant volume heat addition.
• 3–4 is the isentropic expansion of air.
• 4–1 is the constant volume heat rejection.

Please refer to Pressure-Volume and Temperature-Entropy diagram below.

p-V diagram (Otto cycle)

T-s diagram (Otto cycle)

Other differences are discussed below in the table.

Sr. No.	OTTO CYCLE	DIESEL CYCLE
1	Heat addition is isochoric (constant volume)	Heat addition is isobaric (constant pressure)
2	It is used in petrol engines	It is used in diesel engines
3	Compression ratio is 7:1 to 10:1.	Compression ratio 11:1 to 22:1 which is more than Otto.
4	Less efficient	More efficient
5	The complete backward stroke of the piston triggers the adiabatic expansion.	The adiabatic expansion happens during the rest part of stroke after the heat addition is cut-off in the backward stroke.