Introduction and basic technical description of Stirling Engines

The New Stirling engine technology is a substantial advancement of the Stirling engines referred to here as "classic". First, the thermodynamics on which all Stirling engines are based and the functioning of the classic engines are described here.

A Stirling engine is a periodically operating heat engine that converts thermal energy into mechanical energy ("Heat-To-Power"). In the Stirling engine, an enclosed volume of gas is heated in a space (cylinder) that is kept continuously hot, causing it to expand.

The expansion of the gas drives the machine. In another room (cylinder) the gas is cooled and compressed. It oscillates back and forth between these two spaces. Stirling engines are mostly designed as piston engines, but there are also other designs.

The machine can deliver work because the work required for compression at cold temperature is less than that released for expansion at hot temperature.

The heat is supplied from the outside to the enclosed gas mass, so the machine can be operated with any external heat source. Since the gas is not exchanged, a particularly suitable gas such as helium or hydrogen can be used.

Conventional Stirling engines ("classic machine") store the heat contained in the working gas in a reservoir (regenerator) on the way from the hot to the cold room in order to improve efficiency. The regenerator releases the heat when the gas flows back from the cold to the hot room.

"Yet the general knowledge and understanding of Stirling engines is still at such low level that, even among experts a wide divergence of opinion can be found, not only as to their basic applications or desirable constructional features, but even as to the analytical approach appropriate for their design and optimisation."
T. Finkelstein (Foreword for Allan J.Organ: Thermodynamics and Gas Dynamics of the Stirling cycle Machine)

Types of Stirling Engines

There are different types of Stirling engines ("Alfa", "Beta" and "Gamma"), which are distinguished from one another by the way in which the working and displacement pistons are arranged. The New Stirling Engine which we are talking about here is an Alfa-type machine as built by Alexander Kirk Rider, therefore also called "Rider Motor".

With the Alfa type, two pistons are housed in separate cylinders and act on a common crankshaft offset by approx. 90°. The offset of the cooled cylinder ensures that the gas can be expanded or compressed by one piston, while the other piston moves little near top or bottom dead center. Since both cylinders are connected to each other by a pipe and a regenerator, the work cycle (expanding and also compressing) continues in the following cycle on the other top of the piston.

Alfa-Stirling engine

Example of a modern "classic" Stirling Engine

The Cycle of an Alfa-type Stirling Engine

The Stirling process consists of 4 steps. The following figures show these cycles using an Alfa type machine.
Alfa-Stirling motor

1: isothermal expansion
Alfa-Stirling motor

2: isochoric cooling
Alfa-Stirling motor

3: isothermal compression
Alfa-Stirling motor

4: isochoric heating

Thermodynamic Process

Stirling process

1-2: isothermal expansion
Stirling process

2-3: isochoric cooling
Stirling process
3-4:isothermal compression
Stirling process

4-1: isochoric heating

The thermodynamic cycles in detail

Isothermal Expansion inside the hot cylinder

T=const. during gas expansion.
The heat Q supplied to maintain the temperature corresponds to the work W done.

Isochoric Cooling inside the Regenerator

At constant volume, the temperature and pressure of the gas decrease.

Isothermal Compression inside the cold cylinder

T=const. during gas compression.
The volume change work W corresponds to the amount of heat Q to be removed .

Isochoric Heating inside the Regenerator

At constant volume, the temperature and pressure of the gas increase.
Formula Stirling Cycle 1-2
Formula Stirling Cycle 2-3
Formula Stirling Cycle 3-4
Formula Stirling Cycle 4-1

How to build the perfect Stirling Heat Installation

The best engineers have struggled to improve the Stirling Machine for decades. Everyone who deals with the Stirling Engines immediately recognizes that there is still more to be gained. However, they could not untie the Gordian knot. That's how it's done:
  • It is necessary to maintain a high temperature difference between the working and compression cylinders.
  • Large amount of heat recovered by the regenerator. Every joule saved through internal recovery does not have to be supplied by burning fuel.
  • The regenerator must not lose efficiency at high flow rates.
  • Avoid temperature losses, which can be 200 K at the heater and 50 K at the cooler.
  • Small losses due to friction and gaps.
  • Few losses due to heat dissipation through the material from warm to cold areas of the system.
  • The mechanics and the flow paths must be designed in such a way that the thermodynamic cycle comes as close as possible to the theoretical four cycles without overlapping.
  • High compression ratio of the pistons without escaping of working gas into dead spaces (piping, regenerator).
  • The conflict of objectives between maximizing heat exchanger surface area and minimizing dead volume must be resolved.

The Future

The New Stirling Engine Technology presented here meets all of the above requirements.

Stirling motors designed and manufactured according to the "Classical technology" only deliver small amounts of power, have a low degree of efficiency and therefore only a limited area of ​​application.

They only have the basic principle in common with the New Stirling Engine Technology presented on the following pages. After carefully reading them, it becomes clear that in reality the Stirling engine is more predestined to deliver high performance, to replace stationary or ship-based power generation systems and to use previously unused waste energy.

It is the best option for a "Heat To Power" application.

On the following pages the [] New Stirling Engine Technology is explained in detail.

"Problems can never be solved with the same mindset that created them" (A. Einstein)

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  • Dipl. Ing. Thomas Seidenschnur

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