Sterling Engine

In last post we discussed on concept of 8 stroke engine. In today’s post we will discuss on sterling engines.

So first idea of this kind of cycle was given by Robert sterling in 1816 and it was first used in 1818. At that time it was an initial attempt to make engine in which working fluid was as air in closed cycle. It was first used to pump water. Though it is not used in daily life but study of this engine is still important.

Ideal Sterling engine cycle consists of two isothermal and two isochoric (constant volume) processes. We can also say that the two adiabatic processes in Carnot cycle was replaced by two isochoric processes.  Difficulties for making this kind of engine which work on ideal sterling’s cycle that the complete isothermal reaction a process have to pass through infinite equilibrium stages which means to complete isothermal process it will take infinitely long time. Which is not possible in practical case. Still some organization are trying to create this engine by modifying the concept.

There two variant of sterling engine.

(1)    Alpha-type sterling engine

(2)    Beta-type sterling engine

Alpha type 

Beta type

Beta type sterling engine has only one cylinder. It is more complex to explain.

We will discuss on Alpha sterling engine.

So Alpha sterling engine consists of hot cylinder, cold cylinder, piston we already know the function of each the hot cylinder is heated with some external heat source and cold cylinder is cooled with fins. Hot cylinder is shown in red color while cold cylinder is shown in blue shaded color.

Now these are four steps in this engine.

(1)    The gas in hot cylinder is heated which results in heating of gas present in overall system. Due to heating of gas it expands and pressure increases. The piston of hot cylinder is at its Top dead Centre (TDC) and cold cylinder is at Bottom Dead Centre (BDC).

(2)    Now the hot cylinder is in contact with cold cylinder which reduces pressure and due to momentum of flywheel hot cylinder piston moves to BDC and cold cylinder piston moves to TDC

(3)    Now all gases gathers at cold cylinder and their cooling is done which will lead to contraction or say decrease in pressure. The cold cylinder piston will move BDC to TDC and hot cylinder piston will move TDC to BDC.

(4)    Now as cold cylinder is continuously reduces the pressure piston will start to move TDC and inversely hot cylinder piston will move to TDC from BDC.

So these were the processes related to alpha type sterling engine.

Sterling engines have useful application where temperature difference is available. But it cannot be started instantly which is disadvantage of this engine.

η_carnot   >   η_sterling

I hope you enjoyed this post. Please comment below for any doubt or query related to this post.