Diesel Cycle: Combustion process with pV and Ts 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 pV and Ts diagrams.
Table of contents
pV and Ts Diagrams
Maximum thermal efficiency
Difference between Diesel and Otto cycles
Table of contents
pV and Ts Diagrams
Maximum thermal efficiency
Difference between Diesel and Otto cycles
pV and Ts Diagrams of Diesel Cycle
The diesel cycle (combustion process) is similar to Otto cycle.
pV diagram 
To understand and learn the cycle fully we should refer its pV and Ts diagrams.
Ts diagram 
Diesel cycle has the following four stages:
 Process 12 is Isentropic (Reversible adiabatic) Compression
 Process 23 is Constant Pressure (Isobaric) Heat Addition
 Process 34 is Isentropic Expansion
 Process 41 is Constant Volume (Isochoric) Heat Rejection
The maximum thermal efficiency of Diesel cycle
The maximum thermal efficiency of a Diesel cycle depends on the compression ratio and the cutoff ratio. Airstandard efficiency (or thermal efficiency) of diesel cycle is given by:
Î·_{Th}=1(1/r^{Î³1}) ((Î±^{Î³}1)/(Î³Î±Î³))
where
Î·Th is the thermal efficiency
Î± is the cut off ratio
Î³ is the ratio of specific heats
Î·Th is the thermal efficiency
Î± is the cut off ratio
Î³ 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 ottocycle, 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 PressureVolume and TemperatureEntropy diagram below.
pV diagram (Otto cycle) 
Ts 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 cutoff in the backward stroke.

Diesel Cycle: Combustion process with pV and Ts Diagrams
Reviewed by Vishwanath Nayak
on
October 17, 2017
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