Analysis of thermodynamic cycles: Carnot (Derivation of efficiency and efficiency
problems) Otto, Diesel cycles description only.
AIR - STANDARD CYCLES
The working fluid in an I.C Engine doesn’t operate on a cycle. In the accurate study and
analysis of Internal combustion engine, processes are very complicated. To simplify the
theoretical study "Standard Air Cycles" are introduced, these cycles are similar to the open
cycles, but some simplifying assumptions are made.
An air standard cycle is an idealized cycle in which air is taken as the working fluid.
The actual combustion process is replaced by a heat transfer process and exhaust
process is replaced by a heat rejection process. Air standard cycle refers to thermodynamic
cycle being studied with certain assumptions, so as to use the principles of thermodynamics
conveniently. It is the most simplified form of thermodynamic cycle under consideration.
General assumptions made for a cycle to be air-standard cycle are as follows:
i. Air is the working fluid and behaves as a perfect gas.
ii. Working fluid does not get changed in its mass and composition.
iii. Thermodynamic processes constituting cycle are reversible.
iv. There is no heat loss from system to surrounding and vice-versa.
v. During heat addition process, heat is assumed to be supplied from a high
temperature source.
vi. During heat rejection process, heat is assumed to be rejected to a low
temperature sink.
vii. Specific heats of working fluid do not change throughout the cycle.
In air standard cycle, a part of the heat transferred to air is converted into useful work
and the remainder is rejected. Therefore the work done by the air is equal to the
difference between the heat supplied and heat rejected, if there is no mechanical loss.
Therefore, Work done during a cycle = Heat supplied – Heat rejected.
, Air standard efficiency
Thermal efficiency
Thermal efficiency of a process is defined as the ratio of the work done to the heat supplied
during the cycle.
Thermal efficiency = Work done / Heat supplied
Air standard efficiency
Air standard efficiency is the thermal efficiency obtained with air as the working
fluid. i.e. It is the ratio of work done to the heat supplied during a cycle with air is used as
the working fluid.
Air standard efficiency = Work done / Heat supplied
Work Done WD = Heat Supplied (H.S) – Heat Rejected (H.R)
Air standard efficiency = (H.S – H.R)/H.S
= 1 – (H.R / H.S)
Probably the best known reversible cycle is the Carnot cycle, which was first proposed in
1824 by French engineer Sadi Carnot. The theoretical heat engine that operates on the
Carnot cycle is called the Carnot heat engine. Carnot cycle is a reversible thermodynamic
cycle comprising of four reversible processes (two isothermal processes and two adiabatic
processes). (refer figure 1.1)
Thermodynamic processes constituting Carnot cycle are;
Process 1-2, Isothermal expansion process
Process 2-3, Isentropic expansion process
Process 3-4, Isothermal compression process
problems) Otto, Diesel cycles description only.
AIR - STANDARD CYCLES
The working fluid in an I.C Engine doesn’t operate on a cycle. In the accurate study and
analysis of Internal combustion engine, processes are very complicated. To simplify the
theoretical study "Standard Air Cycles" are introduced, these cycles are similar to the open
cycles, but some simplifying assumptions are made.
An air standard cycle is an idealized cycle in which air is taken as the working fluid.
The actual combustion process is replaced by a heat transfer process and exhaust
process is replaced by a heat rejection process. Air standard cycle refers to thermodynamic
cycle being studied with certain assumptions, so as to use the principles of thermodynamics
conveniently. It is the most simplified form of thermodynamic cycle under consideration.
General assumptions made for a cycle to be air-standard cycle are as follows:
i. Air is the working fluid and behaves as a perfect gas.
ii. Working fluid does not get changed in its mass and composition.
iii. Thermodynamic processes constituting cycle are reversible.
iv. There is no heat loss from system to surrounding and vice-versa.
v. During heat addition process, heat is assumed to be supplied from a high
temperature source.
vi. During heat rejection process, heat is assumed to be rejected to a low
temperature sink.
vii. Specific heats of working fluid do not change throughout the cycle.
In air standard cycle, a part of the heat transferred to air is converted into useful work
and the remainder is rejected. Therefore the work done by the air is equal to the
difference between the heat supplied and heat rejected, if there is no mechanical loss.
Therefore, Work done during a cycle = Heat supplied – Heat rejected.
, Air standard efficiency
Thermal efficiency
Thermal efficiency of a process is defined as the ratio of the work done to the heat supplied
during the cycle.
Thermal efficiency = Work done / Heat supplied
Air standard efficiency
Air standard efficiency is the thermal efficiency obtained with air as the working
fluid. i.e. It is the ratio of work done to the heat supplied during a cycle with air is used as
the working fluid.
Air standard efficiency = Work done / Heat supplied
Work Done WD = Heat Supplied (H.S) – Heat Rejected (H.R)
Air standard efficiency = (H.S – H.R)/H.S
= 1 – (H.R / H.S)
Probably the best known reversible cycle is the Carnot cycle, which was first proposed in
1824 by French engineer Sadi Carnot. The theoretical heat engine that operates on the
Carnot cycle is called the Carnot heat engine. Carnot cycle is a reversible thermodynamic
cycle comprising of four reversible processes (two isothermal processes and two adiabatic
processes). (refer figure 1.1)
Thermodynamic processes constituting Carnot cycle are;
Process 1-2, Isothermal expansion process
Process 2-3, Isentropic expansion process
Process 3-4, Isothermal compression process
