A stirling engine is a heat engine, that converts thermal energy to mechanical work. It is an external combustion engine, which means the combustion takes place outside the cylinder. The second type of engine would be an internal combustion engine, where combustion takes place inside a cylinder, like in a car engine.
Stirling engines are divided into three types: alpha, beta, and gamma configuration engines. Although they differ slightly in appearance, they all have the same working principle. While a beta-type stirling engine has a displacement piston and power piston in the same cylinder, then a gamma type engine has them in different cylinders. Let's take a closer look at the construction and working principle of a gamma-type stirling engine. Here we use the Sunnytech hot air sterling engine as an example.
Our stirling engine has two cylinders connected to each other. The displacer piston is loosely fitted in the cylinder, there is an air gap between the cylinder wall and the piston. The power piston however is sized and polished to obtain a close fit.
These cylinders contain a fixed amount of gas ( which is just air ). The displacer piston does not compress the gas during its operating cycle, since there is an air gap between the piston and the cylinder wall. The displacer piston only pushes a certain volume of gas back and forth in the cylinder.
As the gas heats up, the pressure increases, and expanding gas makes the power piston move. If the gas is cooled, then it contracts and pressure will decrease. This heating and cooling of the gas allow the power piston to move back and forth in the cylinder and make the flywheel rotate.
Lets now look at the stirling process on the basis of four engine positions. Note that the power piston and displacer piston are 90 degrees out of phase.
1) Gas is at the cooler end of the cylinder and cooled down to the maximum.
2) The displacer piston moves down. The cooled gas is pushed into the hot side of the cylinder and warmed - up again.
3) The displacer piston is at the lowest point. The gas in the hot cylinder is heated and it expands. Expanding gas exerts a force on the power piston.
4) The displacer piston moves up. The hot air is displaced to a relatively colder part of the cylinder. The gas starts to cool down and the pressure starts to drop.
This process can also be illustrated by a pressure-volume diagram. Such diagrams are used in thermodynamics to show how the pressure and volume of a gas change in a given process.
The gas pressure, volume, and temperature are related by an equation: PV = nRT, where n is a number of moles, R is the gas constant, P is pressure, V is volume, and T is the temperature of the gas.
1 - 2 and 3 - 4 are isothermal processes. Isothermal means that the temperature of the gas remains constant during this process. It can be seen from the equation, that if T is constant and likewise n and R are constant quantities, then pressure and volume are inversely related to each other. Thus, when the gas is compressed at a constant temperature, and the volume decreases the pressure increases. However, if the gas expands at a constant temperature, the pressure decreases, as the volume increases.
2 - 3 and 4 - 1 are isochoric processes. In these processes, the volume of the gas remains constant. When the gas is heated and the volume remains constant, the pressure increases and if the gas cools and the volume remains constant, the pressure decreases.
This is how our small desk gadget converts heat energy into mechanical energy. Some of this mechanical energy is then converted into electrical energy by the generator, and the colorful LED lights up.