Air standard diesel cycle – used for diesel engine
Roberto Magalhães
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Which cycle is used in diesel engine? Diesel engine cycle – PV and TS diagrams
The diesel internal combustion engine differs from the gasoline-powered Otto cycle in that it uses a higher compression of the fuel to ignite the fuel rather than using a spark plug (“compression ignition” rather than “spark ignition”).
The Diesel cycle is a combustion process in an alternative internal combustion engine. In it, the fuel is ignited by the heat generated during the compression of air in the combustion chamber, into which the fuel is injected. This contrasts with igniting the air-fuel mixture with a spark plug, as in the Otto cycle (four-stroke/gasoline) engine. Diesel engines are used in aircraft, automobiles, power generation, diesel-electric locomotives, and in surface ships and submarines.
In the diesel engine, air is compressed adiabatically with a compression ratio typically between 15 and 20. This compression increases the temperature to the ignition temperature of the fuel mixture that is formed by fuel injection when the air is compressed.
The ideal air standard cycle is modeled as a reversible adiabatic compression followed by a constant pressure combustion process, then an adiabatic expansion as a power stroke and an isovolumetric exhaust. A new charge of air is drawn in at the end of the exhaust, as indicated by processes a and e in the diagram.
PV TS Diagram for Diesel Engines
The Diesel cycle is assumed to have constant pressure during the initial part of the combustion phase. This is an idealized mathematical model: real physical diesel engines show a pressure increase during this period, but it is less pronounced than in the Otto cycle. In contrast, the idealized Otto cycle of a gasoline engine approximates a constant volume process during this phase.
Processes in the Diesel Cycle:
The diesel cycle has four processes. They are:
Process 1-2: Isentropic (reversible adiabatic) compression Process 2-3: Heat Addition with Constant Pressure (Isobaric) Process 3-4: Isentropic Expansion Process 4-1: Constant Volume (Isochoric) Heat Rejection
Process 1-2: Isentropic Compression
In this process, the piston moves from the Bottom Dead Center (BDC) position to the Top Dead Center (TDC) position. The air is isentropically compressed inside the cylinder. Air pressure increases from p1 to p2, temperature increases from T1 to T2, and volume decreases from V1 to V2. Entropy remains constant (i.e. s1 = s2). Work is performed on the system in this process (denoted by Win in the diagrams above).
Process 2-3: Adding Heat with Constant Pressure
In this process, heat is added at constant pressure from an external heat source. Volume increases from V2 to V3, temperature increases from T2 to T3, and entropy increases from s2 to s3.
The heat added in process 2-3 is given by
Qin = mCp(T3 − T2) kJ
where,
m → Air mass in kg
Cp → Specific heat at constant pressure in kJ/kgK
T2 → Temperature at point 2 in K
T3 → Temperature at point 3 in K
Process 3-4: Isentropic Expansion
Here the compressed and heated air is expanded isentropically within the cylinder. The piston is forced from TDC to BDC in the cylinder. Air pressure decreases from p3 to p4, temperature decreases from T3 to T4, and volume increases from V3 to V4. Entropy remains constant (i.e. s3 = s4). Work is done by the system in this process (denoted by Wout in the pV and Ts diagrams above).
Process 4-1: Constant Volume Heat Rejection
In this process, heat is rejected at a constant volume (V4 = V1). Pressure decreases from P4 to P1, temperature decreases from T4 to T1, and entropy decreases from s4 to s1.
The heat rejected in process 4-1 is given by
MCV QOUT = (T4 – T1) kJ
where,
m → Air mass in kg
Cv → Specific heat at constant volume in kJ/kgK
T2 → Temperature at point 2 in K
T3 → Temperature at point 3 in K
This cycle can operate with a higher compression ratio than the Otto cycle because only the air is compressed and there is no risk of the fuel self-igniting. Although for a given compression ratio the Otto cycle has greater efficiency, because the Diesel engine can be operated at higher compression ratios, the engine can actually have greater efficiency than an Otto cycle when both are operated at compression ratios that can be achieved in practice.
PV TS Diagram for Diesel Engines
