2122 Stirling engine

The Stirling engine, invented by Robert Stirling, first built in 1816 and subsequently produced on a small scale, is not an internal combustion power unit. Its working gas is cycled in a closed circuit, passing through a heat exchanger on the way round. Gases such as hydrogen, helium and freon have been used in the closed circuit. Originally, it was a viable alternative to the steam engine, for example in marine propulsion, but it has yet to be proved competitive with the internal combustion engine for road vehicle applications. However, it could become attractive owing to its virtually zero oil consumption and long intervals between oil changes, long service life, relative silence, a thermal efficiency potentially of about 40 to 45% at part load, acceptance of a wide variety of fuels in a continuously burning heater, and a very clean exhaust. Its disadvantages are complexity, bulk and weight. The specific weight of a 10 kW engine is about 10 kW/kW, but becomes lower as the power output increases.

It operates on a four-stage working cycle. One type of Stirling engine is shown diagrammatically in Fig. 21.22, which shows what is termed the displacer type. This has the disadvantages that each cylinder contains two pistons, and it can be built up into a multi-cylinder unit only by, in effect, bunching together a group of single-cylinder engines: it cannot be simplified by integration. There is, however, a less bulky double-acting, or piston-displacer, type, which will be explained later.

In the first type, the upper piston is the displacer and the lower one the working piston. The four stages in the working cycle, Fig. 21.22, are:

(1) Both pistons at the outer extremes of their travel, and the working gas in the cold space between them.

(2) The working piston has compressed the gas at a low temperature, while the displacer piston has remained stationary.

(3) The displacer has now descended and pushed the gas through a cooler,

Fig. 21.22 Showing the four stages in the Stirling engine cycle, in a piston displacer-type of unit

regenerator and heater into the hot space above it, while the working piston has remained stationary.

(4) The hot gas has expanded and pushed both the displacer and working pistons down to their lowest positions. Therefore, the working piston remains stationary while the displacer moves up again, pushing the gas back through the heater, regenerator and cooler into the cold space once more, until stage 1 is repeated and the cycle begins again.

The mechanism by means of which the pistons are moved in this manner is called a rhombic drive, Fig. 21.23, in which the two crankshafts are geared together to rotate in opposite directions. Although the motion is complex, the diagram is self-explanatory. With such a mechanism, all the inertia forces can be balanced. The function of the cooler is to reduce the volume of the gas entering the cold chamber which, because of the intrusion of the piston rod, is inherently of smaller volume than the hot chamber. The regenerator takes heat energy from the exhaust and transfers it to the gas on its way to the heater which, of course, supplies additional heat energy to the gas, to increase the total available for conversion into work in the expansion chamber.

Stirling Engine Diagram
Fig. 21.23 Diagram of the double acting, or piston-displacer, type of engine: H is the heater, R the regenerator and C the cooler

There is also a two-cylinder layout, or multiples of two cylinders with single-acting pistons, in the closed circuit of which the gas passes between a cold chamber over one piston to the hot chamber over the piston in the other cylinder, and back again. This of course has a conventional crankshaft, as well as a similar cooler, regenerator and heater arrangement.

However, the double-acting, or piston-displacer, Stirling engine, again with only one piston per cylinder, can be made simpler and more compact than any of the other layouts. With two or more cylinders and double-acting pistons, the cold chamber can be below and the hot chamber above the piston in each cylinder. Again, these chambers are interconnected in a closed circuit, through a cooler, regenerator and heater, in an arrangement represented diagrammatically in Fig. 21.24. The cycle of operations is of course similar to that already described.

Stirling Pipe Heater

Fig. 21.24 The rhombic drive of a piston-displacer unit having an experimental head operating on the heat-pipe principle. Heat applied to the finned top evaporates liquid sodium from its porous lining, which then condenses on the tubes H containing the working gas. The regenerator is R and the cooler C

Fig. 21.24 The rhombic drive of a piston-displacer unit having an experimental head operating on the heat-pipe principle. Heat applied to the finned top evaporates liquid sodium from its porous lining, which then condenses on the tubes H containing the working gas. The regenerator is R and the cooler C

Since the bulkiest and most complex component of a Stirling engine is its regenerator, burner and heater, there is much to be gained by placing these in one sub-assembly serving all the cylinders. This can be done by bringing the cylinder heads close together by adopting either a narrow angle V-layout or a swash-plate-type engine having its cylinders arranged around and parallel to a central shaft, Fig. 21.25. As the swash-plate rotates, it moves the pistons alternately up and down.

In general, the sealing of piston rings in Stirling engines is not difficult. This is because there is no combustion in the cylinders, the maximum temperatures are relatively low and, for any given mean pressure, the peak pressure is much lower than in an internal combustion engine. Furthermore, there is no side loading, since this is taken by the crosshead. Sealing of piston rods, however, does pose problems, various seals from a complex sliding seal assembly to a rolling sock type having been used.

Fig. 21.25 A swash-plate-type Stirling engine with heat-tube type head

A combustor, rotary recuperator and heater unit, developed by Ford for a Stirling engine installed experimentally in its Torino car, is illustrated in Fig. 21.26. The use of ceramics or other materials resistant to very high temperatures in this area could lead to an increase in thermal efficiency from its current maximum of about 35% to approximately 40%.

Since a Stirling engine rejects about twice as much heat to coolant as an Otto cycle unit, its cooler has to be correspondingly larger. In all other respects, it is in effect a conventional radiator. Even so, the sheer bulk, weight and cost of such a large radiator is a major disadvantage.

The combustor is not unlike that for a gas turbine engine, comprising a thimble-like casing within which is a cylindrical combustion chamber having

Air inlet Heater tubes Cylinder heads Regenerator seals Exhaust

Rotary regenerator Combustion zone

H Fuel atomiser J Igniter K Combustion can L Main housing M Motor N Baffle

Air inlet Heater tubes Cylinder heads Regenerator seals Exhaust

Rotary regenerator Combustion zone

H Fuel atomiser J Igniter K Combustion can L Main housing M Motor N Baffle

Fig. 21.26 Combustor and regenerator assembly for the Ford Torino engine

large perforations in its peripheral walls. Mounted in its closed end are an electric igniter and a fuel atomiser jet. Air enters the annular space between the casing and the combustion chamber, and passes radially inwards through the perforations, to mix with the atomised fuel. The burning gases expand axially along the combustion chamber and pass from its open end into the centre of a cylindrical heat exchanger. This comprises a set of finned parallel tubes arranged in a ring coaxial with that of the burner assembly, but mounted directly on top of the cylinder block. The hot gases from the combustion chamber pass radially outwards between the tubes, inside which the working gas is flowing.

Now let us look at the path of the air from the time that it enters and ultimately, becoming the burned exhaust gas, leaves the unit. The burner assembly is contained coaxially in an even larger cylindrical housing, on opposite sides of which are the air intake and exhaust ports. As can be seen from Fig. 21.26, the in coming air first passes through a rotary heat exchanger, which is actually the regenerator, the principle of which is described in Section 21.8, in connection with gas turbines. The function of the regenerator is to take from the exhaust gases heat that would otherwise go to waste, and to use it to preheat the air immediately before it enters the combustion chamber.

After passing through the upper segment of the rotating heat exchanger disc, the air enters the open end of the housing of the burner, through which it proceeds, as previously described, through the burner to the heat exchanger through which the working gas is flowing. From the periphery of this heat exchanger, the gas passes back through the lower segment of the rotary heat exchanger to which it gives up some of its heat. It is then released through the exhaust port.

There are two control systems. One is a closed-loop control over the fuel supply. This is regulated by a cylinder head temperature sensor, and maintains the heater tubes at a constant temperature. The other is for control over power output. This can be done by pumping working gas into and out of the closed circuit. Alternatively, what is termed dead volume control can be employed to open the system to one or more fixed volume chambers, Fig.

21.27. For automotive applications a more suitable method, giving more rapid response and better part throttle efficiency, would appear to be to vary the angle of the swash-plate.

A typical indicator diagram for a Stirling engine is illustrated in Fig.

21.28. Note its large area. The performance of such an engine is similar to that of a petrol unit, but its thermal efficiency is higher under all conditions of load and speed, and the torque curve is significantly flatter. Consequently, there might be potential for economy by simplifying the transmission. Inevitably, however, the cost would be of the order of at least 8% above that of an ic engine.

In 1958 General Motors became a licensee of Philips of Einhoven, who had been developing the engine since 1938. Then, in 1968, both United Stirling, of Sweden, and MAN/MWM, of Germany, also became licensees. By 1971, Philips had installed a rhombic drive Stirling engine in a DAF bus, after which Ford, working in conjunction with Philips, installed a double-acting swash-plate type in their Torino car. For information on further developments such as the use of a heat accumulator, a heat pipe and a free

E Dead space valve

F Equalisation valve

G Cooler

H Heater J Hot space K Cold space

E Dead space valve

F Equalisation valve

G Cooler

H Heater J Hot space K Cold space

Fig. 21.27 The dead volume control system a «

Volume, V

Fig. 21.28 Typical Stirling indicator diagram, with phases from Fig. 21.22

piston version, the reader is advised to refer to the CME, May 1979, and the Philips Technical Review, Vol. 31, No. 5/6, 1970.

Solar Stirling Engine Basics Explained

Solar Stirling Engine Basics Explained

The solar Stirling engine is progressively becoming a viable alternative to solar panels for its higher efficiency. Stirling engines might be the best way to harvest the power provided by the sun. This is an easy-to-understand explanation of how Stirling engines work, the different types, and why they are more efficient than steam engines.

Get My Free Ebook


Post a comment