In 1897, a German inventor and mechanical engineer Rudolf Christian Karl Diesel popularly known as Rudolf Diesel invented the Diesel Cycle. He introduced an idea by which we can get higher thermal efficiency with a high compression ratio from 14:1 to 22:1. I am today, going to explain Diesel Cycle with PV and TS diagram.
The ideal diesel cycle consists of four processes that’s include two Isentropic processes, a constant pressure process and a constant volume process. The Diesel Cycle can be easily understood with the PV and TS diagram as explained below.
Processes 1-2: Isentropic Compression Process
In this process the piston moves from BDC (Bottom Dead Center) to TDC (Top Dead Center) 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 happens. Here, the air is compressed so the pressure increases from P1 to P2 and volume decreases from V1 to V2. On the other hand, Temperature increases from T1 to T2 and the entropy remains constant, S1=S2.
Process 2-3: Constant Pressure Heat Addition Process
After the compression of intake air , the heat is added at constant pressure. During this process, the piston rest for a moment at TDC. Volume increases from V2 to V3 and pressure remains constant, P2=P3, temperature increases from T2 to T3 and entropy increases from S2 to S3.
Process 3-4: Isentropic Expansion Process
After heat addition, in this process the expansion of air takes place isentropically and work is obtained from the system. The piston moves from TDC to BDC during this process. The pressure falls from P3 to P4, volume increases from V3 to V4, temperature decreases from T3 to T4 and the entropy remains constant, S3=S4.
Process 4-1: Constant Volume Heat Rejection Process
In this process, the piston rest at BDC for a moment and the heat rejection takes place at constant volume. During this process, the pressure falls from P4 to P1, temperature falls from T4 to T1, entropy decreases from S4 to S1 and the volume remains constant.
Different Terms Used in Diesel Cycle
1. Cut-off Ratio:
In the diesel cycle, the cut-off ratio is the ratio of volume after heat is added to the volume before heat addition.
2. Compression Ratio:
The compression ratio is the ratio of volume before the compression to the volume of charge after compression. Or it is defined as the ratio of the total volume of a cylinder to the clearance volume.
rc =V1/V2= (Vstroke + Vclearance)/Vclearance
3. Expansion Ratio:
It is the ration of volume after the expansion to the volume before expansion process. From PV diagram, it is stated as,
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