The engine shown in Figure 1 is an external combustion type engine which has two air ports, working in a way similar to a two stroke internal combustion engine.

The hot air goes to the burner and is used for improving its efficiency. A three way control valve directs the air flow to the burner or to an exhaust gas pipe. A pulsation dampener keeps the air flow stable.

The compression rate can be changed by displacing the piston head regarding the sliding block. This is done through a worm gear set that rotates the piston stem. The piston head has a elliptical shape that compensates for the change in the position of the piston ending points. Smaller rate values can be set during the engine start-up and higher ones when engine is warm.

A potential market for this engine are the farms in the remote areas, where raw combustible materials are available. Thus the farmers would be able to generate their own electrical energy and also to be suppliers and earn additional money.

Figure 2 shows an engine that works under the same principle of the previous one but now the crank shaft was replaced by an hydraulic pump and an hydraulic actuator.

This engine converts thermal energy to hydraulic power, which can be used in a hydraulic motor coupled to a generator. An accumulator stores an amount of oil under pressure as to keep the hydraulic motor smoothly running. It also provides the initial energy for the engine start-up.

It requires an electronic control system in order to command the hydraulic directional control valve and the solenoid valve.

The hydraulic cylinder moves the piston for the air compression phase. It also assists in the final air exhaust, as it can not be well defined by the air expansion alone.

It is possible to control not only the compression rate but also all the speeds and times during the entire engine working cycle. It can be controlled the time the piston rests in the top position and therefore the heat input. This way an optimal efficiency cycle can be achieved.

Environmental friendly power generation in small to medium sized plants could be the use of such a type of engine.