Heat Engines.
Heat engine is defined as a device that converts heat energy
into mechanical energy or more exactly a system which operates continuously
and only heat and work may pass across its boundaries.
The operation of a heat engine can best be represented by a thermodynamic
cycle. Some examples are: Otto, Diesel, Brayton, Stirling and Rankine
cycles.
Carnot cycle
The Carnot cycle describes an ideal, reversible cycle between
an inexhaustible hot energy reservoir and an inexhaustible cold reservoir.
This cycle is important because it sets the theoretical limit on the amount
of work that can be provided by a given amount of energy for the hot reservoir.
Carnot imagined a gaseous substance being put through several cyclic stages.
He believed that the particular gas or liquid used in the cycle didn't
matter; however, some substances will work more efficiently than others.
We assume that the gas being used is an ideal gas; that is, it follows
the law that pressure times volume is proportional to temperature.
The Carnot cycle consists of four stages:
1 to 2: An isothermal (constant-temperature) expansion of the gas. The temperature is maintained at the level of the hot reservoir.
2 to 3: An adiabatic (no heat flow Q - insulated from surroundings) expansion that lowers the temperature
from Thot to Tcold. This is where the work of the cycle is done; as the gas expands, it pushes a piston or turns a turbine.
3 to 4: An isothermal compression maintained at the temperature of the cold reservoir. Normally, this compression would add heat, but instead the heat is siphoned away into the cold reservoir.
4 to 1: An adiabatic compression that brings the temperature of the gas back up to the level of the hot reservoir.
The work done each cycle is the area inside of the lines.
Rankine cycle
The most popular thermodinamic cycle today is the Rankine cycle.
The Rankine cycle is the ideal prototype of steam engines in use today. The cycle is 1) water heated at constant pressure 2) evaporated to saturated vapor 3 superheated to gas 4) expanded 5) exhausted to condenser 6) condensed.(the point of the beginning) In the steam automobile a step 4b where the steam is throttled down to decrease power. They're two Rankine cycles. The above describes the complete Rankine cycle. There is also an engine Rankine cycle used to evaluate the performance of a steam engine separate of the boiler. I have developed a program to analyze the automobile Rankine cycle. The automobile Rankine cycle is the complete cycle including the boiler, throttle, engine, condenser and compression. The low heat reservoir is assumed that of the saturated fluid at condenser pressure.
The Renkine cycle is more economic and has less practical difficulties but not as high as Carnot cycle.