How Stirling engines work

How Stirling engines work

How Stirling engines work

Introduction

Nowadays, engines are used in many products. Since the industrial revolution (from the late 1700s to the mid-1800s) steam engines ECE (External Combustion Engine) was the only widely used engine. Stem engine was the first engine invented back in 1698 by an Englishman named Thomas Savery.  Later, Robert Stirling developed and patented the Stirling engine in Edinburgh, Scotland. Unfortunately, this type of engine always took a back seat as the steam engine was dominating at that time especially after the first commercially ICE (Internal Combustion Engine) was successfully created around 1859 by Étienne Lenoir. In this article, the focus is on the Stirling Engine as it is the least known and uncommon type of engine.

How does the Stirling Engine work?

typical Stirling engine

A typical Stirling engine

Like any other internal combustion engine, the Stirling engine does have a piston cylinder and a crankshaft and it does rely on gas pressure to create motion; however, it doesn’t require any explosion, intake valves or exhaust. In fact, the involved gas which is air is sealed inside the machine, and the displacer which is in a loose-fit within its cylinder plays the role in transferring heat to the energy cycle creating mechanical motion. The energy being cycled here is the thermal energy.

Exploiting the thermal energy requires a temperature difference that is the hot and cold air in the displacer cylinder. In this engine, the bottom metal plate is hot while the upper metal plate is cool. It should be noted that the displacer itself is made of plastic which provides some thermal insulation between the plates. The small cylinder with the tight-fitting power piston is obviously the power cylinder, it is the mechanism producing the force to create motion. 

The large air volume under the displacer is exposed to heat provided by any heat sources placed under the bottom plate. This gas is picking up energy and expanding which results in an increase in a pressure that exerts force throughout the closed machine including on the power cylinder. This force pushes the power piston up turning the crank and rotating the flywheel, and since the displacer is also connected to the flywheel through a crank it automatically lowers the displacer exposing the air to the upper cold plate. Therefore, the air loses energy, its pressure drops and the momentum of the rotating flywheel moves the power piston back to the bottom and moves the displacer up once again exposing the air to the hot plate. The air expands and the increasing pressure starts the power piston on another power stroke.

The flywheel plays an important role since its momentum maintains the motion when the gas is contracting the machine into the next power stroke. This cycle repeats as long as there is a temperature difference between the upper and lower plates. Eventually, the small temperature difference between the hand and the ambient air is enough to start this cycle.

Stirling engine design

  • Alpha Stirling engine

An alpha Stirling engine has two Power Pistons, for that reason they have a higher power to weight ratio than the Beta Stirling engine

Alpha Stirling engine

In terms of its functionality, the principle is applied here where the red cylinder is the hot cylinder and the blue cylinder is the cold cylinder, the main difference is that both pistons are on the same point on the crankshaft; therefore, the hot piston is 90 degrees ahead of the cold piston while rotating. 

An Alpha Stirling engine has four strokes:

  • Expansion: Most of the air within the engine is in the hot cylinder. The gas heats and expands and drive both pistons inwards to the crankshaft.
  • Transfer: The momentum of the flywheel carries the engine through the next 90 degrees, this causes most of the air to be transferred over to the cold cylinder
  • Contraction: The majority of the expanded gas has shifted over the cold cylinder, it cools and contracts which sucks both pistons outwards, away from the crankshaft.
  • Transfer: The momentum of the flywheel carries the engine through the next 90 degrees, this causes most of the air to be transferred over to the hot cylinder, completing the cycle.

 

  • Beta Stirling engine

The beta Stirling engine has only one cylinder

Beta Stirling engine

Beta Stirling engine

Within that cylinder is the power piston and the displacer, same loose-fit for the displacer to be able to shuffle air back and forth within the cylinder. As the displacer moves towards the flywheel, it forces the air within the cylinder to go to the hot end of the engine. This causes the air to heat up, expand and the power piston is forced outwards. Now as the displacer moves in the opposite direction, it causes the air to go to the cold side of the cylinder. The air cools, contracts and the power piston is sucked inwards. Timing is really important in this type of Stirling engines and it is set by the crankshaft. The displacer must be 90 degrees ahead of the power piston, and the displacer’s connecting rod goes through the piston.

Advantages and Disadvantages of using Stirling Engine

  • Advantages

Using a Stirling engine can bring a lot of benefits to the table. Here are some pros that one can benefit from by using a Stirling engine:

  • Stirling engines have a low maintenance cost; therefore, they could be used for longer times without having to worry about their maintenance.
  • Stirling engines could use various different fuels such as Helium, Hydrogen, alcohol…
  • Stirling engines could be used for various different purposes, such as energy generation, electricity producing…
  • Stirling engines could be adapted for industrial purposes as well as domestic purposes; in other words, Stirling engines could be used in factories, laboratories, as well as houses.

 

  • Disadvantages

  • Stirling engines have a low-efficiency rate, between approximately 25 percent and 12 percent. In some cases, it can even go as low as 10 percent.
  • Stirling engines may have a low maintenance cost but their fuel is expensive (helium and hydrogen)
  • In order to generate a good amount of electricity and energy, the engine must be great in size which could be a problem

Conclusion

In conclusion, Stirling engines are a good invention which could be optimized and developed further in order to minimize the drawbacks and cons of the system.

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