Gm Research Engines

Component development

Percival (1974) has indicated that in the first five years of research and development work on Stirling engines cITort was concentrated on component development, specifically:

(a) seals for the piston and piston rods in rhombic drive engines to prevent both leakage of gas and ingress of oil to the working fluid,

(b) reduction of gear noise,

(c) improvement in combustion and burner nozzle designs,

(d) durability of the preheater.

(e) engine speed governing.

(f) reduction of regenerator cost.

(g) endurance testing,

(h) refinement of the cycle analysis.

(j) numerous studies and demonstrations of thermal energy storage systems in combination with Stirling engines.

Effort in the last five years was concentrated in other areas including:

(a) cooler and heater heal transfer.

(b) rolling seal quality control,

(c) low-cost preheaters.

(d) swash-plate drive bearing studies,

(e) reduction in engine volume.

(f) stress analysis of heater cylinders,

(g) vehicle applications.

(h) exhaust emissions.

(j) governor refinements, (k) controls reliability, (I) reduction of friction. Percival's report contains a wealth of detail about all these matters.

In twelve years between the start of the program in 1958 and its abrupt termination in 19711 over 30 000 hours operating experience on Stirling engines had been gained. The accelerating pace of the project was such that over 50 per cent of the operating experience was gained over the final three years and 75 per cent in the last five. In addition many thousands of hours of operation were accumulated on seal, bearing, combustion, regenerator, and heat transfer rigs.

Ground power unit The principal visible achievement of the G.M. Research program was the Ground Power Unit Stirling engine generator. These units sustained a decade of development. The final model, a GPU3, is shown in I-ig. 13.1. The engine was a single-cylinder machine with rhombic-drive having a bore of 6.98 cm (2.75 in) and a stroke of 3.05 cm (1.2 in). Figs. 13.2 and 13.3 show the operating characteristics of the engine (reproduced from Percival 1974). In the course of development the engine experienced many vicissitudes, principally in the governor control and hydrogen compressor systems (sec Chapter 101. However, by 1967 the engine was capable of stable operation to within ±5 revolutions per minute and satisfactory response to sudden increases or decreases in load. Reliability was progressively increased so that in 1969 the system passed a rigorous 500 hour military qualification test.

One of these engines was used in an experimental vehicle installation, the Stir-Lee 1, the hybrid Stirling-engine electric-drive car shown in Fig. 13.4. Details were reported by Agarwal el ul. (1969). The GPU3 engine

Bus Philips Stirling Engine
Pic.. 13.J. General Motors Stirling engine generator GPU3 (after Hcffner 1966).
Fig. 13.2. Performance data for GPU3 engine {alter Percivnl 1974).

was installed in the rear compartment of a standard Opel Kadett. The engine was not connected directly to the drive train of the car but drove, at constant speed, a three-phase alternator. Alternator output was rectified to charge electric storage batteries. The battery power was modulated and inverted to a three-phase induction motor driving the car.

This exercise was largely a demonstration of the feasibility of a low-emission engine/electric-drive hybrid vehicle. Agarwal el al. (1969) conclude that \.. the extra cost and complexity of the hybrid system would make it difficult to compete with simpler propulsion approaches for private passenger vehicles ..."

liarlier vehicle work was undertaken in 1964 with the installation of a 22 kW (30 hp) Stirling engine in a Corvair. This vehicle, christened the

Carnot Rpm Efficiency
Engine speed (rpm) FlO. 13.3 Performance data lor GPU3 engine (after Pcreival 1974).

'Calvair', was energized from a tank of heated alumina and formed part of the studies in progress for thermal energy storage systems.

Underwater power systems The double-acting Stirling engine was invented at Philips in the late 1940s in both swash-plate and crank configurations, but was abandoned because of problems with the piston seal. In 1965, the concept was revived at G.M. Research specifically for advanced torpedo motors. An important paper about this work was published (Mattavi et al. 1969) which reviewed various studies of compact double-acting engines and contained performance data projected for a number of underwater pow-erplant engines up to UOkW (150 hp). The paper also contained some discussion of energy-storage materials and metal combustion systems.

Battery' charging control Hydrogen reservoir

DC to AC modulating Lead acid batteries

Starter motor

k mvener controls Induction motor (3-phase)

air blower

Fig. 13.4. Stir-Lee hybrid Stirling engine electric-drive car (after Agarwal el al. 1969).

Battery' charging control Hydrogen reservoir

Stirling engine

Radiator and fan

DC to AC modulating Lead acid batteries

Starter motor k mvener controls Induction motor (3-phase)

air blower

Fig. 13.4. Stir-Lee hybrid Stirling engine electric-drive car (after Agarwal el al. 1969).

Metal combustion involves the rapid oxidation of a liquid metal using the resultant heat of reaction as the primary energy source. For underwater power systems it is attractive primarily because the reaction products can be retained 'on board' thereby avoiding detection and, in deep submergence systems, avoiding the need for compression equipment to discharge the effluent.

Vehicle engines

These studies for compact underwater engines coincided with the development of public interest in the emission characteristics of vehicle engines. The promising efficiencies and specific outputs achieved with Stirling engines coupled with their low noise and low exhaust emission characteristics generated intense interest at General Motors in potential vehicular applications. In 1967 the Pontiac and Oldsmobile Divisions cooperated with G.M. Research in a study of a 1S5 kW (25(1 hp) swash-plate Stirling engine for a Torino car. At the same time the Truck and Coach Division was interested in a Stirling engine of 110 kW (150 hp) for city buses as a replacement for the dicsel engine. It was said that the advantages of reduced noise, smoke, odour, hydrocarbon, and NO, emission with essentially no oil consumption would be worth up to S15 000 more per bus.

Another possibility carefully considered in the late 1960s was the production of small cars for shopping and short-distance city travel. Part of the work involved study of a 18 k\V (25 hp) swash-plate Stirling engine complote with accessories, transmission, and power train. The compact engine studies carried out embraced by 1968 the five fundamental arrangements shown in Fig. 13.5. Comparative sizes of these five arrangements showed the double-acting engines to be about half the size of rhombic in-line or rhombic opposed-piston configurations. Thereafter, detail designs were undertaken for compact double-acting engines of 90 kW (120 hp) output from four cylinders. The results, shown in Fig. 13.6, indicate that swash-plate design is not necessarily the most compact form of double-acting engine. Subsequently (in 1968) the design of a four-cylinder engine of 110 kW (150 hp) was undertaken for vehicular applications as part of the Truck and Coach Division demonstration program. The engine, designated Type 4L23 was a double-acting, four-cylindcr, in-line, single-crank configuration with crossheads and piston displacement of 377 cm1 (23 in*). The engine was designed for operation at 2000 revolutions per minute with a mean hydrogen pressure

Fig. 13.5. Compact engine arrangements (after Percival 1974): (a) Single-acting, single-cylinder rhombic-drive oi in-line; (b) Single-acting, opposed-cylinder rhombic-drive; (c) Double-acting, multiple-cylinder in-line; (d) Double-acting, multiple-cylinder, swash-plate drive; (e) Double-acting multiple-cylinder, opposed-piston, in-line.

Fig. 13.5. Compact engine arrangements (after Percival 1974): (a) Single-acting, single-cylinder rhombic-drive oi in-line; (b) Single-acting, opposed-cylinder rhombic-drive; (c) Double-acting, multiple-cylinder in-line; (d) Double-acting, multiple-cylinder, swash-plate drive; (e) Double-acting multiple-cylinder, opposed-piston, in-line.

Stirling Engine

Fto. 13.6. Relative volumes of double-acting Stirling engines of 120 hp with different drive mechanisms iaftcr Percival 1974«: in!1 Swash-plate; (bj Crank and connecting rod: (c) Scotch yoke.

Fto. 13.6. Relative volumes of double-acting Stirling engines of 120 hp with different drive mechanisms iaftcr Percival 1974«: in!1 Swash-plate; (bj Crank and connecting rod: (c) Scotch yoke.

of 10.3 MN/nr (1500 lb per sq in). To provide a margin for future development of the hot parts the drive mechanism was designed for a mean pressure of 20.6 MN/m2 (3000 ib persq in).

Calculated performance data for the model 4L23 engine are shown in Fig. 13.7 and a comparison of the engine with equivalent General Motors diesel engines is given in Table 13.1. Percival (1974) notes laconically:

'Approximately 95 per cent of the basic engine parts wore on hand early in 1970 and the engine was being motored on the dynamometer .is a part of the balancing procedure when the programme was halted on February 27, 1970, Target date for start of the couch installation had been May I. 1970.'

Publications

In the course of their duodecade of effort on the Stirling engine, G.M. Research contributed several papers of permanent interest and value to the open literature in addition to those referenced above. The first by Flynn ci al. (1960) was a lengthy paper primarily surveying the Philips rhombic-drive engine but included an interesting historical section. Another paper by Flynn ef al. (1962) surveyed the possibilities for heat

Engine speed (rpm)

Fio. 13.7. Performance data lor (he Type 4L23 Stirling engine: a four-cylinder in-line double-acting encine of optimized design for use in buses {after Percival 1974).

engines in combination with thermal energy storage systems with particular reference to the Stirling engine. Subsequently Heffner (1966) reviewed the progress of the G.M. Research program with emphasis on the Army GPU. The paper was also of interest because of the photographs and passing references to the other larger engines, to some aspects of the analytical procedures and to work on the reciprocating seals.

Percival (1967) discussed naval applications of Stirling engines and covered some of the same ground, but less substantially than the subsequent paper by Mattavi et al. (1969).

A report on the favourable environmental characteristics of Stirling engines in terms of smoke, odour, noise, and exhaust emissions was presented by Lienesh and Wade (1968) and contained measured data on the GPU 7.3 kW (11) hp) engines. Other measurements made by Philips

Tabic 13.1. Calculated performance data lor the General Motors Stirling engine Type 4L23 double-acting vehicle engine and a comparison with General Motors diesel engines of comparable power and size.

Tabic 13.1. Calculated performance data lor the General Motors Stirling engine Type 4L23 double-acting vehicle engine and a comparison with General Motors diesel engines of comparable power and size.

Parameter

Units

gmr Stirling

DDAD Diesel

DDAD Diesel

model

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