Ericsson cycle

The Ericsson Cycle is named after inventor John Ericsson, who designed and built many unique heat engines based on various thermodynamic cycles. He is credited with inventing two unique heat engine cycles and developing practical engines based on these cycles. His "first" cycle is very similar to what we now call the "Brayton Cycle" except that it was external combustion. His second cycle we now call the "Ericsson Cycle".

Ideal Ericsson Cycle

The following is a list of the four processes that occur between the four stages of the ideal Ericsson cycle:
*Process 1 -> 2: Isothermal Compression. The compression space is assumed to be intercooled, so the gas undergoes isothermal compression. The compressed air flows into a storage tank at constant pressure. In the ideal cycle, there is no heat transfer across the tank walls.
*Process 2 -> 3: Isobaric Heat-addition. From the tank, the compressed air flows through the regenerator and picks-up heat at a high constant-pressure on the way to the heated power-cylinder.
*Process 3 -> 4: Isothermal Expansion. The power-cylinder expansion-space is heated externally, and the gas undergoes isothermal expansion.
*Process 4 -> 1: Isobaric Heat removal. Before the air is released as exhaust, it is passed back through the regenerator, thus cooling the gas at a low constant pressure, and heating the regenerator for the next cycle.

Comparison with Stirling, Carnot and Brayton cycles

The Ericsson Cycle is often compared to the Stirling cycle, since the engine designs based on these respective cycles are both external combustion engines with regenerators. The Ericsson is perhaps most similar to the so called "double-acting" type of Stirling engine, in which the displacer piston also acts as the power piston. Theoretically, both of these cycles have so called "ideal" efficiency, which is the highest allowed by the Second law of thermodynamics. The most well known ideal cycle is the Carnot cycle, although ironically, a real "Carnot Engine" is not known to have been invented.

Comparison with the Brayton Cycle

The first cycle Ericsson developed, is now called the "Brayton Cycle", commonly applied to the rotary jet engines for airplanes.

The second Ericsson cycle is the cycle most commonly referred to as simply the "Ericsson cycle". The (second) Ericsson cycle is also the limit of ideal gas-turbine Brayton cycle, operating with multistage intercooled compression, and multistage expansion with reheat and regeneration. Compared to the Brayton cycle which uses adiabatic compression and expansion, the second Ericsson cycle uses isothermal compression and expansion, thus producing more net work per stroke. Also the use of regeneration in the Ericsson cycle increases efficiency by reducing the required heat input. For further comparisons of thermodynamic cycles, see Heat engine.

Ericsson Engine

The Ericsson engine, (see figure), is based on the Ericsson cycle, and is known as an "external combustion engine", because it is externally heated. To improve efficiency, the engine has a regenerator or recuperator between the compressor and the expander. The engine can be run open-cycle or closed-cycle. Expansion occurs simultaneously with compression, on opposite sides of the piston.

The Regenerator

Ericsson coined the term "regenerator" for his independent invention of the mixed-flow counter-current heat-exchanger. However, Rev. Robert Stirling had invented the same device, prior to Ericsson, so the invention is credited to Stirling. Stirling called it an "economiser" or "economizer", because it increased the fuel economy of various types of heat processes. The invention was found to be useful, in many other devices and systems, where it became more widely used, since other types of engines became favored over the Stirling engine. Interestingly, the term "regenerator" is now the name given to the component in the Stirling Engine!

The term "recuperator" refers to a separated-flow, counter-current heat exchanger. As if this weren't confusing enough, a mixed-flow regenerator is sometimes used as a quasi-separated-flow recuperator. This can be done through the use of moving valves, or by a rotating regenerator with fixed baffles, or by the use of other moving parts. When heat is recovered from exhaust gases and used to preheat combustion air, typically the term recuperator is used, because the two flows are separate.

History

In 1791, before Ericsson, Barber proposed a similar engine. The Barber engine used a bellows compressor and a turbine expander, but it lacked a regenerator/recuperator. There are no records of a working Barber engine. Ericsson invented and patented his first engine using an external version of the Brayton Cycle in 1833 (number 6409/1833 British). This was 18 years before Joule and 43 years before Brayton. Brayton engines were all piston engines and for the most part, internal combustion versions of the un-recuperated Ericsson engine. The "Brayton Cycle" is now known as the gas turbine cycle, which differs from the original "Brayton Cycle" in the use of a turbine compressor and expander. The gas turbine cycle is used for all modern gas turbine and turbojet engines, however simple cycle turbines are often recuperated to improve efficiency and these recuperated turbines more closely resemble Ericsson's work.

Ericsson eventually abandoned the open cycle in favor of the traditional closed Stirling cycle.

The Ericsson Cycle Engine (The second of the two discussed here) was used to power a 2000 ton ship, The Caloric Ship Ericsson and the engine ran flawlessly for 73 hours. The combination engine produced about 300 horsepower. It had a combination of 4 dual-piston engines; the larger expansion piston/cylinder, at 4.267 meters or 14 feet in diameter, was perhaps the largest piston ever built. Rumor has it that tables were placed on top of those pistons and dinner was served and eaten, while the engine was running at full power. At 6.5 RPM the pressure was limited to 8 psi. The one sea trial proved that even though the engine ran well it was underpowered. Sometime after the trials the Ericsson sank. When it was raised the Ericsson cycle engine was removed and a steam engine took its place.

Ericsson designed and built a very great number of engines running on various cycles including steam, Stirling, Brayton, externally heated diesel air fluid cycle. He ran his engines on a great variety of fuels including coal and solar heat.

Ericsson also was the inventor of the screw propeller for ship propulsion, in the USS Princeton.

References

* Ericsson's patents. 1833 British and 1851 USA
* The evolution of the heat engine, by: Ivo Kolin Published Moriya Press, 1972 by Longman
* Hot Air Caloric and Stirling Engines, by: Robert Sier. Published 1999, by L A Mair.

Links

* 1979 RAND report on a new "Ericsson Cycle Gas Turbine Powerplant" design [http://www.rand.org/pubs/reports/R2327/]