An Ericsson Cycle Engine is also known as a "hot-air engine," and is named after John Ericsson. It is often compared to a Stirling engine.

The Ericsson cycle can be most easily described by comparing it to the Brayton cycle.

The Brayton cycle consists of four steps: isentropic compression of cool gas, heating at constant high pressure, isentropic expansion of hot gas, and cooling at constant low pressure. The amount of work done compressing or expanding a gas is equal to the pressure times the change in volume. Both the compression and expansion steps have the same pressure difference. But hot gas takes up more volume than cool gas so the expansion step produces more work than the compression step consumes and the net work output of the engine is positive.

The Ericsson cycle differs from the Brayton cycle in two ways. Isothermal compression and expansion instead of adiabatic, and regeneration instead of heating and cooling.

In adiabatic compression the temperature increases. Pressure increases both from the decrease in volume and increase in temperature. In isothermal compression, the gas is cooled as it is compressed to keep it at a constant temperature. So the pressure increases only from the decrease in volume. Isothermal compression takes less work than adiabatic compression because the pressure climbs slower as the gas is compressed. Likewise, isothermal expansion produces more work than adiabatic because the pressure stays higher if the gas is heated as it expands.

The gas coming from the compressor is cool and high pressure, and we want it to be hot and high pressure to send it to the expander. The gas coming from the expander is hot and low pressure, and we want it to be cool and low pressure to send it to the compressor. The solution to this is to bring the two into thermal contact and heat will flow form the hot gas to the cool gas. This is called regeneration. It reduces the heat input required to bring the high pressure gas up to high temperature.

So Ericsson has two efficiency wins over Brayton: isothermal compression and expansion produces more net work than adiabatic, and regeneration reduces heat input.

Thermodynamically, Ericsson is more similar to the Stirling and Carnot cycles. All use isothermal compression and expansion at the hot and cold temperatures. To transition the gas between the hot and cold temperatures Carnot uses adiabatic compression and expansion, Stirling uses constant volume regeneration, and Ericsson uses constant pressure regeneration. All are theoretically capable of Carnot efficiency if there are no losses. But I find the comparison to Brayton easier to understand. Both Brayton and Ericsson can be realized by the same simple machine, a turbine compander. And a Brayton cycle with regeneration and an infinite number of stages of intercooling and reheat is equivalent to Ericsson.