A Stirling engine is a heat engine that operates by cyclic compression and expansion of air or other gas, the working fluid, at different temperatures. More specifically, it is a closed-cycle regenerative heat engine with a permanently gaseous working fluid. The inclusion of a regenerator distinguishes the Stirling engine from other static engines such as the Diesel and Otto cycles.
The idealized Stirling cycle can be thought of as an infinite number of Carnot cycles operating between two temperature limits. In practice, however, real engines are not perfectly efficient due to irreversibilities in both the working fluid and regenerator. The Stirling engine was invented by Robert Stirling in 1816 and was initially employed in steam ships and pumps. It has since been used in a variety of applications including refrigeration, dynamic compressors (such as those on board submarines), cryogenic pumps and vehicles powered by solar energy.
How it works: An overview
The Stirling cycle can be divided into four processes which occur in sequence: 1) isothermal expansion; 2) adiabatic cooling; 3) isothermal compression; 4) adiabatic heating (Figure 1). These four processes form a loop that repeats itself indefinitely if no net work is done on or by the system – this is referred to as a closed cycle.
In process 1), also known as the “power stroke”, expansive work is done on the gas while it expands Isothermally against an external pressure Pext driving the piston away from top dead center (TDC). During this stage, heat QH flows into the system from a high temperature heat reservoir TH through heater H1.
In process 2), also referred to as “adiabatic cooling”, No heat flow occurs across any boundaries during this time period so QH = 0 . The volume increases linearly with time resulting in an adiabatic decrease in pressure until valve V2 closes at point B when PV2=K_B T_B = constant . K_B is known as Boyle’s Law constant for an ideal gas .
In process 3 ), also called “isothermal compression”, work Wc is done on behalf of an external force upon the gas while maintaining its temperature at Tc , thus being referred to as Isothermal . Heat QC flows out of system to low temperature reservoir TC through cooler C2 . Valves V1 & V2 open simultaneously at point C when PV1=PV2=K_C T_C =constant letting compressed colder air enter Heater H1 where QC=QH(from equation above). Now we have completed one full thermodynamic Cycle !
There are two main types of stirling engines : Alpha type & Beta type . The main difference between them lies in their configuration : either displacing or non-displacing respectively . Within these two types , there are three common arrangements : double Acting , opposed Piston & free Piston … all with unique characteristics ! Double Acting cylinders are more commonly seen because they’re typically more efficient than their single acting counterparts due to increased thermal efficiency & power output per unit cylinder volume while weight & size stay relatively unchanged . This design provides linear motion instead of reciprocating like most internal combustion engines which use pistons connected to crankshafts via connecting rods … linear motors have many benefits but that’s beyond scope here ! Opposed piston designs have one power piston balanced by another counteracting weightless piston located directly opposite it inside same cylinder chamber separated by dividing wall known as “divider” ; both connected to same flywheel assembly providing rotary force output similar IC engines just without need for crankshaft ! Free Piston stirling engines don’t even use pistons attached to anything !! Instead , they rely on magnets attached around outside perimeter of long thin cylindrical chambers forming what’s called magnetically levitated free pistons which move up/down inside those chambers according their mass relative to opposing forces created my magnetic fields around them ; think maglev trains !! Now that we know general idea behind how these machines function along with some key terminology , let’s get into actual details regarding each step of thermodynamic Cycle starting with ….