How does it work?

When a confined body of gas (air, helium, whatever) is heated, its pressure rises. This increased pressure can push on a piston and do work. The body of gas is then cooled, pressure drops, and the piston can return. The same cycle repeats over and over, using the same body of gas. That is all there is to it. No ignition, no carburetion, no valve train, no explosions. Many people have a hard time understanding the Stirling because it is so much simpler than conventional internal combustion engines.

The Stirling cycle is described using the pressure-volume (P-v) and temperature-entropy (T-s) diagrams shown in Figure 1. The P-v and T-s diagrams show the state of a "working fluid" at any point during the idealized cycle. The working fluid is normally a the Stirling engines being produced to us, the working fluid is air.

In the idealized Stirling cycle heat (i.e., energy) is transferred to the working fluid during the segment 2-3-4. Conversely, heat (energy) is extracted from the working fluid during the segment 4-1-2. During segment 2-3 heat is transferred to the fluid internally via regeneration of the energy transferred from the fluid during segment 4-1. The means that (ideally) heat is added from an external source only during segment 3-4, and that heat is rejected to the surrounding environment only during segment 1-2. Note that this is the idealized cycle; it is not clear how well we approach this cycle in engine being built in IITK.

 [P-V,T-S diagram]
Figure 1: P-v and T-s diagrams used to define the idealized Stirling cycle

A sketch of the Stirling engine is shown in Figure 2.* The engine involves two pistons: the "displacement" and "power" pistons. Both pistons move up and down in their respective cylinders as the crankshaft rotates. Since the power crank connected to the power cylinder is oriented perpindicular to the crankarm connected to the displacement cylinder, the position of the power piston is always 90 deg "ahead" of the displacement piston.

 [sketch of Stirling engine]

Figure 2: Sketch of the Stirling Engine, Showing the Power and Displacement Pistons and Associated Linkages

Note that the displacement piston does not cause any compression of air during operation. That is, the displacement piston has an OD of 38 mm., whereas the displacement cylinder has an ID of 46 mm. Hence there is a 4 mm. gap around the periphery of the displacement piston; air can easily "leak" around the displacement piston. As the displacement piston moves up and down during engine operation, it simply pushes a constant volume of air back and forth from the bottom (hot end) to the top (cold end) of the displacement cylinder.

Conversely, the power piston does cause compression of air. The ID of the power cylinder is honed to produce a very smooth surface, and the OD of the power piston is sized and polished to obtain a close fit (ideally friction-free and airtight). As the power piston moves up and down within the cylinder the total volume of air within the engine is therefore increased and decreased accordingly.The variation in total air volume is entirely due to motion of the power piston; the minimum volume of air (about 242 cm-3) occurs when the power piston is at bottom-dead-center, whereas the maximum volume of air (about 273 cm-3) occurs when the power piston is at top-dead-center.

The variation of volume of air contained within the "hot end" of the displacement cylinder and the "cold end" of the displacement cylinder are determined solely by the position of the displacement piston and do not correspond to cyclic compression/expansion of the air. Heat is transferred to the air when it fills the hot end of the displacement cylinder, whereas heat is transferred from the air when it fills the cold end of the displacement cylinder.

In order to separate these high temperature portion and low temperature portion, this engine has the displacer.The optimum size of displacer diameter is 98% of its cylinder diameter. In short, there is a optimum clearance between the displacer and its cylinder.

Sketches of the engine are shown in Figure 3. These figures and show the relative positions of the power and displacement pistons at four points during the engine cycle. In Figure 3a the power piston is shown at the top-dead-center position (roughly corresponding to point 1 in Figure 1). As the power piston moves towards the bottom-dead-center position (Figure 3b) a reduction in total volume occurs. Also, heat (denoted Q) is transferred from the air contained within the cold end of the displacement cylinder to the surrounding atmosphere (the cold end of the cylinder is a finned aluminum structure to accelerate heat transfer away from the working fluid) . Ideally, heat transfer is complete once the power piston reaches bottom-dead-center (Fig 3b); this roughly corresponds to point 2 in Fig 1. Heat is then transferred to the air forced into the hot end (Figure 3c), which roughly corresponding to point 3 in Fig 1. Heat is applied to the hot end via some external heat source (e.g., an alcohol lamp), and the hot end is not finned to minimize heat loss to the surrounding atmosphere. Finally, the power piston moves back towards the top-dead center position (Fig 3d), which roughly corresponding to point 4 in Fig 1, and completing the cycle.

Figure 3: Relative Piston Positions at Four Points During the Stirling Engine Cycle

 [Power piston at the top,moving down]

(a) Power piston at top-dead-center (power piston stationary, displacement piston moving down)

 [Power piston at the bottom,moving down]

(b) Power piston at bottom-dead-center (power piston stationary, displacement piston moving up)

 [Power piston at the bottom,moving up]

(c) Power piston at bottom-dead-center (power piston stationary, displacement piston moving down)

 [Power piston at the top,moving up]

(b) Power piston at top-dead-center (power pistonstationary, displacement piston moving up)


Although everything is blended together with respect to the main "steps" in the Stirling cycle, there is clearly some regenerative action. As air is forced from the cool end to the hot end by the displacement piston, the rush of air through the 4 mm. gap formed by the cylinder wall and displacement piston surface will cool the wall and heat the gas. Conversely, as air is forced from the hot end towards the cool end the cylinder wall will be heated and the air will be cooled. This regenerative action improves the overall efficiency of the engine by reducing the external heat required.