121

The engine of the Aerojet unit operated as a thermocompressor inhaling helium at a low pressure l.2MN/nr (1801b per sq in) and exhaling it at a higher pressure (1.5 MN/rn"; 215 1b per sqin) through inlet and outlet valves. The llow was therefore controlled by valves and the engine qualified for classification as an Ericsson rather than a Stirling engine using the definitions given earlier. It was, in fact, an Ericsson engine of the Bush variety.

The Aerojet engine was a single-cylinder machine containing a reciprocating displacer with a heater located at the upper end of the cylinder and a cooler at the lower end. The regenerator was the long narrow annulus around the circumference of the displacer. The displacer was guided in its motion by a linear bearing on a centre-post along the axis of the cylinder. The centre-post also provides the reversing cavity or. in the jargon of other free-piston Stirling engines, the bounce space or gas spring.

Operation of the engine can be understood by reference to Fig. 17.9. The sequence of operations is divided into four phases shown as (a), (b), (c), and (d).

At state (a) both inlet and outlet valves are closed, the pressure in the cylinder and in the reversing cavity is low, and the displacer is at the top matrix

ItHUlatcd

Regenerator matrix

Sterling Engine Sequence

Cooler Owlet check Reversing matrix

ItHUlatcd

Regenerator matrix

j-CompTmion-j— Delivery —{—Expansion-»f*- Induction—^

Time (arbitrary units)

j-CompTmion-j— Delivery —{—Expansion-»f*- Induction—^

Time (arbitrary units)

l-lti. 17.9. Operating sequence ol Aerojet thermocompressor engine.

dead-point and beginning the downstroke. As the displacer moves, lluid flows from the cold end through the regenerator to the hot end. Although the volume enclosed in the engine cylinder remains constant the pressure-is raised because the mean temperature level increases as more of the gas is heated. As the displacer descends, fluid in the reversing cavity is compressed as the volume of the reversing cavity decreases and consequently the pressure increases. The rale of increase of pressure in the reversing cavity is less than the rate of increase in the engine cylinder. The different rale of increase in pressure causes a pressure differential to be established across the transverse faces of the displacer over the area of the centre-post. The gas force acting on the displacer is, in fact, the pressure difference between the cylinder and reversing cavity pressures times the cross-section area of the reversing cavity centre-post.

The gas force on the displacer then acts to drive the displacer towards the bottom of the cylinder, thereby increasing the gas flow to the hot space which in turn further increases the pressure in the cylinder and consequently the downward force acting on the displacer. This compression process continues until the pressure in the cylinder reaches the pre-set delivery pressure, at which point the outlet valve opens and pressurized helium gas leaves the engine cylinder (Fig. 17.9(b)).

The delivery process continues until the displacer reaches the end of stroke at the bottom dead-point. The pressure of gas in the reversing cavity is highest at that time and on Fig. 17.9 is shown to be identical to the delivery pressure on the cylinder. This is not a necessary condition but a similarity of pressure levels does ease the sealing requirements between the cylinder and reversing cavity.

The pressure energy stored in the gas spring of the reversing cavity is sufficient to 'jump' the displacer upwards from the bottom position. In practice this is assisted by the provision of a mechanical 'reversing' spring not shown in Fig. 17.9 but which can be seen in Fig. 17.8.

The upward motion of the displacer (I;ig. 17.9(c)) causes fluid to flow back from the hot space to the cold space resulting in reduction in the mean temperature of the fluid and hence a reduction in the pressure so the delivery valve closes. The pressure in the cylinder decreases, and as before, at a faster rate than the pressure in the reversing cavity. Consequently, a pressure differential force is established which causes the displacer to continue moving up the cylinder. This in turn increases the flow to the cold space and further decreases the cylinder pressure and consequently increases the accelerating force on the displacer.

The expansion process continues until the cylinder pressure falls to the pre-set value at which the inlet check valve opens and a fresh charge ol low pressure (1.2 MN/nr; 1801b per sq in) helium is inhaled. The induction process continues until the displacer reaches the top dead-point and the cycle is complete. Fluid pressure in the reversing cavity pressure is lowest at this point.

A schematic diagram of the complete Aerojet heart assist system is shown in Fig. 17.10. The pressurized helium gas flowed from the engine to a pneumatic actuator driving the blood pump. The actual pump work to the blood was accomplished through a novel magnetic coupling allowing the use of a rigid hermetic seal enclosure to contain the helium at the actuator.

Use of the helium working gas for the pump actuation was clearly an attractive feature for it eliminated the pneumatic/hydraulic conversion process of the McDonnel-Douglas machine. However, the advantage was partially offset by the need, in the Aerojet machine, to provide a liquid (physiologic saline) cooling circuit to carry heat from the engine to the vicinity of the blood pump for eventual dissipation in the blood.

Critical features of the Aerojet engine as regards the target 10-year design life were the helium check valves, the centre-post bearing and the displacer reversing spring. Considerable endurance testing has been carried out on these components and on complete systems. Moise and l aeser (1977) claimed a total test time of 67 000 hours and appeared sufficiently confident to project that the 10-year operating life target would be met. The centre-post bearing was perhaps the most critical

Heat source

Insulated displacci piston

Inict p chock / valve

Cheek valves

Mullifoil insulation

0 High pressure (215 psO

lleutci main*

Regenerator Q Low pressure ( ISO psi) matrix

1- Reversing tappe: îjvj Reversing cavity pressure .{. Rcverciog spring1" "j- Shaft bearing

Cooler matrix

I ligh pressure accumulator

Low pressure Actuator bearing^,-,,1-:—n—rn accumulator Reversing Follower magnetK J^n,rol,crll£i cavity .Siiline compliance fe;

Compliance J. I Xii/i fluid bag j

1 Icrmetic seal . _ _ enclosure

Coolant pumping chamber

Shuttle valve

Pusher plate j) blood pump

Fig. 17.10. Schematic diagram of Aerojet heart-assist system (marl: VL1 engine).

component. This was a dry lubricated rubbing unit of an alumina sleeve riding on an alumina shaft. It is difficult to share the sanguine view of Moise. With data from only 3943 hours of actual engine running time, to project the wear rate for 10 years (63 000 hours) he allowed that Ihe total quantity of debris generated from all sources in ten years of operation would be 0.05 cm1 (0.003 in3) . A speck or two of this in the wrong place on that most critical bearing surface would wreck the chances of achieving the 10-year life. Wear debris on the valve seats or faces would also prevent satisfactory operation of the engine.

A most interesting feature of the Aerojet engine is the use of long hollow fine bore glass 'straws' as the regenerative matrix arranged in annular form around the inside of the cylinder. A cross section of square straw element is shown in Fig. 17.11. The working fluid flowed around and through the straws in passing from the hot space to the cold space. A variety of different cross-section forms for the straws had been evaluated but no technical details of the optimum forms were released. Straws of improved 'arc matrix' form shown in Fig. 17.12 were investigated theoretically by Hoffman (1976) using the Aerojet engine simulation program. He obtained the results shown in Fig. 17.13 for net pneumatic efficiency as a function of the engine cyclic rate with different section straws and radial gap dimensions. Details of the Aerojet simulation work and the degree of correlation between predicted and observed performance have not been published.

Artificial Heart Cross Section

Fio. 17.11. Cross-section of square straw regencrutive clement (after Holtman 1976).

Typical dimensions lor A and C arc, respectively, 0.055 and 0.015 in,

Fio. 17.11. Cross-section of square straw regencrutive clement (after Holtman 1976).

Typical dimensions lor A and C arc, respectively, 0.055 and 0.015 in,

The Aerojet artificial heart development involved considerable animal experiments with components and part systems to establish and evaluate various physiological and anatomical compatability criteria (sec-Fig. 17.14). Animal test-work for the Aerojet program was carried out in cooperation with medical and veterinary personnel of the University of California. Davis.

A complete discussion of all aspects of the Aerojet program may be found in the series of annual reports, for example see Andrus (1976).

Stirling Engine Explanation
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.

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