Technological Advances

Over the course of the next 30 years, several other designs appeared offering innovations such as the open cycle engine (Ericsson, 1860) which replenished the hot air in each cycle with fresh cold air, doing away with cooling difficulties but causing great heat losses and an efficiency of just over 2% [9]. Ways to increase heat transfer area such as using a toothed displacer arrangement (Young and Kirk, 1865) were experimented with, and even the idea of discontinuous displacer motion (Lauberau, 1869) was attempted but failed due to mechanical complexity [9]. In 1870 Swedish inventor John Ericsson built the first solar powered Stirling Engine, shown in Figure 9. Its performance is unknown though Ericsson is recorded as saying that “A solar engine of one horse-power demands the concentration of solar heat from an area of 10 square feet.”

Technological Advances

Figure 9: First solar powered Stirling Engine, built by John Ericsson in 1870 [9]

The first recorded a-type Stirling Engine was the Rider-engine, pictured in Figure 10. It was built in 1875 by the Rider Ericsson Engine Company, and was described in a journal excerpt as having “been in successful operation for some twelve years, many hundreds being in use in [England] alone. .The engine, as introduced into [England] from the States, was in most parts perfect so that for years no improvements were considered necessary.” [14]

Technological Advances

Figure 10: Rider-engine, a-type engine built in 1875 [9]

The next significant improvement came about in 1880 from A. E.H. Robinson. His engine featured an air pre-heater, a device that channelled incoming combustion air around the hot exhaust in order to pre-heat it, resulting in an improvement in overall efficiency by as much three times [9]. Unfortunately, by the time this engine was made, the rise of the internal combustion engine had already begun thanks largely to Nikolaus Otto and Karl Benz. During the late 1800’s and early 1900’s steam engines still dominated industry, shipping and railroads [11] and this was rapidly shifting towards the internal combustion engine. The only commercial success being experienced by the Stirling engine at this time was small engines of a “few or fractional horsepower used for pumping water, powering small domestic or farm machines, laboratory stirrers and toys” [11]. These engines found success due to their safe, reliable and versatile operation. The ability to run on such fuels as household rubbish if necessary was useful in a time when mains electricity was not widely available [11].

By the 1930’s the Stirling Engine had been all but forgotten, that is until 1937 when the Philips Corporation took an interest in them as a potential method of powering their radio sets. Philips had built up a sizeable market in Europe selling their radios, however wanted to expand to more remote areas of Africa and Asia where mains supply was not yet available in most places [11]. This meant the radios would have to be powered by batteries, which were expensive at the time and hence a barrier to potential customers. In addition to this, the radios used vacuum tubes which had a rather high rate of power consumption. This meant a generator was the best option, and Philips decided that a Stirling Engine would make the ideal power source for the generator, based on the fact that it could be small, silent, low-maintenance, safe and economical, as well as running on paraffin oil which was widely available.

Over the next three years in their laboratory the Nat. Lab in Eindhoven, The Netherlands, Philips developed a number of engine designs ranging in power output from 6 W to 1 HP [11]. Some earlier engines included experiments on pressurizing the working fluid and using hydrogen instead of air, both of which significantly increased output power but reduced the engine life span of the experimental engines. In 1941 the Type 10 engine was designed with the novel feature of transferring heat to the cylinder head via a liquid metal (K-Na eutectic) pumped with an electromagnetic pump. This technology was found to be unsuitable for the Stirling Engine application but is now used in the cooling of fast-breeder reactors in nuclear power stations [11]. Using high temperature sodium heat pipes was also tried, however these proved unstable and after an explosion in the lab they ceased experiments in this area [11].

Experiments were performed on an already built Type 10 engine in 1941 to run it as a heat pump by driving the shaft of the engine with an electric motor. The initial motivation was to find a possible alternative to the Freon compression-evaporation cooling systems found in refrigerators and air conditioners, however it was quickly found that the Stirling cycle cooler was much more suited to a cryocooler application, as temperatures as low as -100°C were reached even with minimal insulation around the cylinder head [11]. By 1945 this had reached -200°C with only minor changes to the engine.’

When World War II broke out in 1940, The Netherlands was invaded by the Germans who proceeded to take control of the Nat. Lab, slowing research progress though not halting it completely. During this time a patent was granted for the double-acting Stirling Engine, shown in Figure 11, dubbed Type 19 by Philips. This engine, as its name suggests, was Philips’ 19th prototype engine and held great promise with its compact size and high power output. Following from this design in 1943 the Type 20 engine was designed, another 4- cylinder double-acting engine with a swept volume of 2.9 litres and a target power output of 50 HP. This engine was fast and smooth-running, had a good torque capability and had a respectable efficiency of 15% [11].

Technological Advances

By 1948 Philips had produced a number of V-engines with 2 cylinders, the Type 24 which also included the subsequent variations Type 24b, 24c, and 24d. The 24d was the most successful, making an indicated 20 HP though it still had problems with sealing and lubrication. It was intended for use in a motor car and was even demonstrated to Henry Ford Jr., though it never found its way under a bonnet [11].

In 1949 an engine known as the SMF-Kroon engine was built as a result of a joint venture between Philips and the S. M.F group, itself a joint venture of four separate engine builders. This engine was rather successful, making 45 HP from its 4 litre displacement with an efficiency of around 20%. However in October 1949 the engine exploded unexpectedly, tragically killing one person. The cause of the explosion was determined to be oil vapour that had been forced into the hot chamber and under the 5 MPa engine air pressure formed an explosive mixture that was set off by the hot piston head. Since this incident a number of safety measures were adopted, the most significant being the use of inert gases such as nitrogen and helium, and even hydrogen which was found to be safer than air [11].

In 1953 Philips invented the rhombic-drive mechanism, demonstrated in a small beta-type engine. It solved the problem of balancing a small displacer-type engine, as well meaning better piston sealing through lack of side-loading forces. Another major benefit was being able to pressurize the engine working gas without pressurizing the crankcase, meaning a significant weight and cost reduction. This was possible because of the perfect seals around the piston rod that result from no lateral thrust acting upon them [11]. Other benefits realised were higher possible working pressures, fewer vibrations and a compact design. The rhombic drive mechanism is shown in Figure 12 in a beta-type engine where the parts are labelled.

Technological Advances

Figure 12: Rhombic drive mechanism used in a Philips beta-type Stirling Engine [11]

1 = Power piston; 2 = Hollow power-piston rod; 3 = Yoke; 4,4′ = Connecting rods pivoted at the ends of yoke 3; 5,5′ = Cranks on the two oppositely rotating shafts coupled by gears 10,10′; 6 = Displacer piston, 7 = Displacer piston rod; 8 = Yoke; 9,9′ = Displacer con-rods pivoted at the ends of this yoke; 11 and 12 = Gas tight stuffing boxes; 13 = Buffer space containing the working gas at the filling pressure of the engine.

Several smaller engine-generator sets were devised and built over the next few years, and although they worked well they could not be made cheaply enough to compete commercially and subsequently Philips stopped all engine production in 1953 in favour of continuing work on the Stirling cycle refrigerator which later continued to the only commercial success experienced by the entire Philips Stirling engine research effort.

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