Diesel Cycle – Process with P-V and T-S Diagram

Diesel cycle was invented by Rudolph Diesel in 1893. He put
forward an idea by which we can attain higher thermal efficiency, with a high
compression ratio. All diesel engine works on this cycle. Diesel is used as
fuel in this cycle as it can be compressed at higher compression ratio. It is
also known as constant pressure cycle because heat is added in it at constant
pressure. It has high thermal efficiency and compression ratio (11:1 to 22:1)as compared with Otto cycle.
The engine that is put forward by Rudolph consists of an enclosed air
in the cylinder. The cylinder walls are perfectly non-conductors of heat, but
the bottom is a perfect conductor of heat. It has a hot body, cold body and an
insulating cap, which are alternately brought in contact with the cylinder.
The ideal Diesel cycle consist of 4 process, two isentropic
processes, one constant pressure and one constant volume process. 

 Processes

The 4 process are as follows
1. Isentropic (reversible adiabatic) Compression
2. Constant pressure heat addition
3. Isentropic Expansion
4. Constant volume heat rejection

Also Read: Difference Between Otto Cycle and Diesel Cycle

The working of these four processes of Diesel cycle can be easily
grasped with the help of P-V and T-S diagram. 
Diesel cycle – Process with P-V and T-S diagram
diesel cycle process -  piston position

Note: for better understanding the process, watch the P-V and T-S
diagram wherever necessary.

Process 1-2: Isentropic Compression 

In this process the piston moves from BDC to TDC and compression
of air takes place isentropically. It means that during compression the entropy
remains constant and there is no flow of heat out of the cylinder walls
(non-conductors) happens. Here the air is compressed so the pressure increases
from P1 to P2, volume decreases from V1 to
V2, Temperature increases from T1 to T2 and
entropy remains constant ( i.e. S1= S2).

Process 2-3: Constant Volume Heat Addition

In this process the, the hot body is kept in contact with the
cylinder and heat addition to the air takes place at constant pressure. During
this process, the piston rest for a moment at TDC. The pressure remains
constant (i.e. P2 = P3), volume increases from V2 to
V3, temperature increases from T2 to T3,
entropy increases from S2 to S3.

Process 3-4: Isentropic Expansion

In this process, after heat addition, the expansion of air takes
place isentropically and work is obtained from the system. The piston moves
downward during this process and reaches to BDC. The pressure falls from P3 to
P4, Volume increases from V3 to V4,
temperature falls from T3 to T4 and entropy
remains constant (i.e. S3=S4).

Process 4-1: Constant Volume Heat Rejection

In this process, the piston rest at BDC for a moment and the cold
body is brought in contact with the cylinder and the heat rejection takes place
at constant volume. The pressure decreases from P4 to P1,
temperature decreases from T4 to T1, entropy
decreases from S4 to S1 and volume remains
constant (i.e.V4 = V1).
This is the four working process of diesel cycle engine that we
have discussed. Now we will discuss how this cycle is implemented in the 4
stroke diesel engine. When this cycle is used in 4 stoke Diesel engine than we
have 2 extra processes. One is suction process and other one is exhaust
process. For heat addition diesel is used as fuel which burns and adds
heat. 
Let’s discuss all the process in detail

0-1: Suction process (Suction stroke)
1-2: Isentropic compression process (compression stroke)
2-3: Constant volume heat addition (ignition and combustion of
fuel)
3-4: Isentropic Expansion (power stroke)
4-1: Constant volume heat rejection (coolant comes in contact with
cylinder walls)
1-0: Exhaust process (Exhaust stroke)

Also Read: Difference Between Petrol and Diesel Engine

Process 0-1: Suction process

In this process, the inlet valve opens and suction of air takes
place at atmospheric pressure. It is called as suction stroke.

Process 1-2: Isentropic Compression

The air sucked is now compressed isentropically. Due to the
compression, the temperature of the air increases to such level at which the
diesel gets ignites. It is called as compression stroke.

Process 2-3: Constant Pressure Heat Addition

At the end of the compression, the piston is at TDC and at this
time, the diesel fuel is injected into the cylinder through fuel injector in
atomized form. As this atomized diesel fuel comes in contact with the hot
compressed air, it gets ignited and combustion process starts. The combustion
of fuel adds heats to the engine.

Process 3-4: Isentropic Expansion

The combustion process adds a large amount of heat and this
creates a large force on the piston head and it moves downward from TDC to BDC.
It is the power stroke.

Process: 4-1 Constant Volume Heat Rejection

The piston is at BDC and the coolant comes in contact with the
cylinder walls and takes away heat from the engine and makes it cool. After
that piston moves upward.

Process: 1-0: Exhaust Process

In this process, the piston moves from BDC to TDC and exhaust
valve opens. All the burnt gases left in the cylinder escapes out through
exhaust valve. It is the exhaust stroke.
This is how a four stroke compression engine works.

Summary


Summary of Diesel cycle with operation, position of piston and
changes in different parameters are given below:
S.no
Process
Operation
Position of piston
Change in parameter
1.
1-2: Isentropic compression
Compression of air.
BDC to TDC
V: Decreases from V1 To
V2
T: Increases from T1 to
T2
P: Increases from P1 to
P2
S: Entropy remains constant (S1=S2)
2.
2-3: Constant Pressure heat
addition
Heat is added from an external
source (hot body)
At TDC for a moment and moves slightly downward.
V: Increases from V2 To
V3
T: Increases from T2 to
T3
P: Remains constant  (P2 =
P3)
S: Increases from S2 to
S3
3.
3-4: Isentropic expansion
Expansion of air takes place due
to heat addition.
TDC to BDC
V: Increases from V3 to
V4
T: Decreases from T3 to
T4
P: Decreases from P3 to
P4
S: Entropy remains constant (S3=S4)
4.
4-1: Constant volume heat
rejection
 Heat is rejected to a sink.
At BDC for a moment
V: Volume remains constant(V4 =
V1 )
T: Decreases form T4 to
T1
P: Decreases from P4 to
P1
S: Decreases from S4 to
S1

Application

Diesel cycle is used where more power is needed with less amount of fuel. Mainly it is used in two stroke and 4 stroke diesel engines.

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