Refrigeration is the withdrawl of heat from a substance or space so that temperature lower than that of the natural surroundings is achieved.
Refrigeration may be produced by
- thermoelectric means
- vapor compression systems
- expansion of compressed gases
- throttling or unrestrained expansion of gases.
Vapor compression systems are employed in most refrigeration systems. Here, cooling is accomplished by evaporation of a liquid refrigerant under reduced pressure and temperature. The fluid enters the compressors at state 1 where the temperature is elevated by mechanical compression (state 2). The vapor condenses at this pressure, and the resultant heat is dissipated to the surrounding. The high pressure liquid (state 3) then passes through an expansion valve through which the fluid pressure is lowered. The low-pressure fluid enters the evaporator at state 4 where it evaporates by absorbing heat from the refrigerated space, and reenters the compressor. The whole cycle is repeated.
Internal Combustion (IC) engines have completely revolutionized transportation, power generation and have perhaps altered the way the society operates forever. Typical IC engines are classified as Spark and Compression ignition engines.
The simplest model for IC engines is the air-standard model, which assumes that:
The simplest model for IC engines is the air-standard model, which assumes that:
- The system is closed.
- Air is the working fluid and is modeled as an ideal gas throughout the cycle.
- Compression and expansion processes are isentropic.
- A reversible heat transfer process characterizes the combustion of fuel and air.
- Heat rejection takes place reversibly and at constant volume.
The Otto cycle is used to model a basic Spark Ignition engine, while the Diesel cycle is the basic model for the Compression Ignition engine.
Spark Ignition Engines (Otto Cycle)
The spark-ignition engines are the most common type used in cars. Larger engines operate using a four-stroke cycle, while smaller engines operate on a two-stroke cycle. In a simple four-stroke cycle, a combustible mixture of air and fuel is drawn into a cylinder during the intake stroke, and the temperature and pressure of the mixture is raised during the compression stroke. At near the maximum compression, a spark initiates combustion of the mixture, raising its temperature and forcing expansion. The expanding gases do work on the piston during the power stroke and then the burnt gases are purged during the exhaust stroke. Typically 3000 or more such cycles are repeated in a minute.
The Otto cycle is an air-standard model of the actual cycle. In addition to the air-standard assumptions listed above, the combustion process is modelled as a reversible constant volume heat addition process. The four steps of the air-standard Otto cycle are outlined below:
Spark Ignition Engines (Otto Cycle)
The spark-ignition engines are the most common type used in cars. Larger engines operate using a four-stroke cycle, while smaller engines operate on a two-stroke cycle. In a simple four-stroke cycle, a combustible mixture of air and fuel is drawn into a cylinder during the intake stroke, and the temperature and pressure of the mixture is raised during the compression stroke. At near the maximum compression, a spark initiates combustion of the mixture, raising its temperature and forcing expansion. The expanding gases do work on the piston during the power stroke and then the burnt gases are purged during the exhaust stroke. Typically 3000 or more such cycles are repeated in a minute.
The Otto cycle is an air-standard model of the actual cycle. In addition to the air-standard assumptions listed above, the combustion process is modelled as a reversible constant volume heat addition process. The four steps of the air-standard Otto cycle are outlined below:
- (1-2) Isentropic compression (Compression Stroke)
- (2-3) Constant-volume, reversible heat addition (Ignition)
- (3-4) Isentropic expansion (Power Stroke)
- (4-1) Reversible, constant-volume heat rejection (Exhaust)
Typical pv and Ts diagrams for an Otto cycle are shown below where steps (1-2) and (3-4) are isentropic, and (2-3) and (4-1) are isochoric.
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