Peak-pressure type

These are included largely because they are easily confused with the true autographic indicators. Most of them do, however, provide a record in the form of a pressure-reading taken from engraved scales.

The conventional autographic indicator was, by the standards of its day, a highly sophisticated tool. It was also expensive. Among its greatest advantages was an ability to record the changes of pressure within a pressure-vessel (e.g., an engine cylinder) throughout an entire stroke. However, indicators were not particularly easy to install and operate; and the problems increased when the numbers of cylinders began to multiply.

Most steam engines with three or more cylinders were large and costly, and comparatively little additional expense was incurred in the provision not merely of three springs, but three separate indicators. The situation changed with the rise in enthusiasm for internal-combustion engines in the decade prior to the First World War, when comparatively small engines with four, six or eight cylinders appeared. Among the most important goals of the design of internal-combustion engines was consistent performance, particularly when several cylinders were operating continuously and near-simultaneously.

Analysing the performance of these with a conventional autographic indicator, preferable though it may have been under laboratory conditions, was often extremely difficult under normal circumstances. An easier way of checking performance was to ensure that each cylinder was achieving the same level of compression and the same amount of power when the compressed charge was ignited. The maximum-pressure indicator was developed specifically to ease the task of ensuring consistent ignition in multiple-cylinder engines.

A few of the earliest continuous-recording steam engine indicators recorded maximum pressures only, but were out of step with the comprehensive pressure/time traces provided by the Watt, McNaught and Richards instruments. An alternative approach was shown by the simple and sturdy instruments that could show the maximum pressures generated in the cylinder during the firing process. It is assumed that they were inspired by the work of the Belgian engineer Rodolphe Mathot, but the earliest patents that could be traced were filed in Switzerland in September 1905 and in France in October 1905 by Albert Peloux '('résidant in Suisse'). French Patent 358422 shows a variety of methods of showing the pressure, including rapid-pitch screws raising pointers against scales and graduated rods protruding through collars. There is no evidence that the Peloux peak-pressure indicator was made in quantity, and the Okill type, described below, probably takes this particular laurel. However, it was cited by many subsequent patentees and it is likely that the influential Swiss Züblin design of the late 1940s was a direct descendant.

The first to offer a true peak-pressure indicator was an Englishman, John Okill (1875–1947), who received an appropriate British patent in 1907. The drawings accompanying the written specification showed several ways of achieving a balance between pressure in the cylinder, acting upward through a valve against a constrained piston, and that of a sturdy spiral spring. A finger-wheel allowed the pressure of the spring to be altered until a balance point was found, when the piston stopped vibrating. This could be seen either through a sight hole cut in the instrument body or by a vibrating pointer attached to the body side. The pressure could be read off scales engraved on the outside of the body. Not surprisingly, the pointer was much more convenient to use; if any of the 'sight hole' indicators were made, none is known to survive.

Whether many Okill indicators pre-dated the First World War is open to question. In June 1920, John Okill obtained a British Patent to protect an improved form of the vibrating-pointer indicator with a counter, driven by a gear train, which showed the pressure setting numerically (a comparable grant in the U.S.A. was delayed until September 1923). This was a considerable step forward, and surviving examples of what was advertised as the 'New Type Standard' invariably embody the counter system.

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The Okill maximum-pressure indicators of 1907 (left) and 1920 (centre), and a page of a 1920s leaflet extolling the virtues of the 'Standard' and 'Research' patterns. Note the separate bulb-box that accompanied the latter.

Okill indicators were all made by George Taylor (Brass Founders) Ltd of All Saints' Street Works, Bolton, Lancashire, England. The range originally consisted of the ND1, ND2 and ND4 patterns — identified by their serial-number prefixes and respectively calibrated for maximum pressures of 1000, 2000 and 4000 lb/sq.in. — but had soon been extended to include the NP, NKD1 and NKD2 types. The NP was a low-pressure indicator, rated to only 400lb/sq.in; the others were metric-system instruments were calibrated for 70- and 140kg/sq.cm.

Another patent was granted in 1929, to John Okill and John Tate, protecting a variation of the basic Okill indicator developed for laboratory use. This relied on a variation of the 1920 system to activate a small bulb, contained in a separate box with its associated switchgear, each time the pointer moved. When the bulb ceased to illuminate, the pointer was static and the pressure could be read from a combination of circumferential rings engraved on the body and inclined rings on the spring cap. The production version of the 'New Type Research' instrument differed from the patent drawings, as the ball-tipped lubricating plunger was co-axial with the spring instead of protruding angularly from the lower body, and the recording graduations reverted to one of the forms that had been patented in 1907: circumferential rings engraved on the body and eight arrowhead flanges projecting downward from the spring cap.

Okill and Tate then developed adapted the 1920-type indicator to handle extremely high pressures without the use of an excessively stiff spring. This instrument was horizontal instead of vertical. A right-angle connection between the engine cylinder and a chamber in what the designers called the 'cylinder block' allowed pressure to be registered. Two rods entering the chamber in diametrical opposition were anchored to two crossbars which, linked with bars along each side of the cylinder block, were allowed to slide longitudinally. If their diameters were equal, the opposed rods would be 'in balance': no matter now high the pressure rose in the chamber, no movement would be detected. But by making the rod connected with the spring assembly larger than its counterpart, the pressure balance was disturbed and the larger rod would move back — but only proportionately to the true pressure, the precise relationship being controlled by the rod-diameter differential. A vibrating pointer lay on top of the instrument, anchored in a fork-like collar connecting the spring chamber and the cylinder block. High-pressure indicators of this type were offered commercially as the 'SP' ('Super-Pressure'), readily distinguishable by their unique construction. They were intended to be used with 'airless' injection engines, where the pressures could not only rise as high as 10,000lb/sq.in but also give what was effectively a hammer blow to the indicator piston.

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Left: a page from a leaflet promoting the Okill SP or 'Super Pressure' indicator, made in accordance with a patent granted in Britain in 1931. Right: a typical 1936-type 'ND2' indicator, capable of handling pressures as high as 2000lb/sq.in. This particular example was probably made after the end of the Second World War. Museum of Making collection.

The 1936-type Okill & Tate peak-pressure indicator was an improvement of the 1920 patent, with a free-floating piston and a pressure block which could move axially without twisting the spring. Alternative methods of registering pressure were proposed, but the production version, which usually acknowledged the U.S. Patent granted in September 1938 instead of the earlier British equivalent, retained the gear-driven counter. This instrument replaced the 1920 type in production; it was certainly made in ND1, ND2, NKD1 and NKD2 versions, though ND4 seems to have been superseded by the SP.

Others soon followed where Okill had led, though rarely with commercial success. One of the first was John Pearson, whose British Patent of August 1922 protected a simple indicator consisting of a spring-loaded plunger skidding vertically within its casing. The pressure was measured by the maximum height of the plunger-tip above the casing (where a sliding finger was held by a spring) or by a line engraved helically on the spring-cap.

The 'Acrometre', patented in France by Mazellier & Carpentier, relied on a thin disc-like valve to allow combustion gases to pass until the pressures on each side of the disk were equal. The level at which this occurred could then be read on a conventional Bourdon-type gauge. This particular system was unsuccessful; the disc valve was prone to damage and, owing to the dimensions of its seat, was also improperly balanced. An alternative approach was taken by the German inventor Robert Bosch, who set an indicator into the fuel line of the injection system. This could be adjusted against a pre-calibrated spring (by way of a micrometer thimble) until the valve began to leak.

None of these, however, were as simple, sturdy or reliable as the Okills, which remained supreme in Britain until a peak-pressure indicator was developed shortly after the end of the Second World War by Gebr. Sulzer AG of Winterthur, Switzerland. Application for the relevant Swiss patent was made in October 1948, though the grant was delayed until the summer of 1950. U.S. Patent 2673464 (sought in August 1949 but not granted until March 1954) names the inventor as Marcel Wilhelm Züblin of 'Dumbarton, Scotland, Assignor to Sulzer Frères S.A., WInterthur'.

The specifications described and illustrated several ways that screws and springs could be used to rotate a cap against a graduated scale in response to rises in pressure. The commercially-successful version embodied expansible bellows attached to a push rod, which was in turn anchored in a conical nut. Operation was simple: when pressure was applied to the bellows, through the valve communicating with the pressure-generating vessel (usually an engine cylinder), the bellows expanded to force the push rod up against the counter-pressure of a calibrated co-axial coil spring. A conical nut was raised from its seat, but a clock-spring anchored in the cap instantly rotated the nut back again. The amount of rotation was directly proportional to the pressure being generated in the bellows, and an appropriate reading could be taken directly from a graduated thimble or cap.

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Left: a vertical-section drawing of the Dobbie McInnes version of the Sulzer peak-pressure indicator, showing the bellows and the conical nut. Right: the front of the Dobbie McInnes leaflet, probably dating from the early 1960s, clearly shows the construction of the standard indicator. Note the vulcanite casing and the graduated thimble.

Production of the Sulzer indicator was licensed to Haenni Präzisions-Maschinenfabrik AG (now Baumer Bourdon-Haenni AG) of Jegensdorf, Switzerland, and Dobbie McInnes Ltd of Glasgow. It was the first of its type to successfully challenge the supremacy of the Okill design, largely by offering automatic operation. All that was required of the operator was to re-set the instrument prior to each measurement and note the final reading. The British-made instruments were offered as the 'Air Cooled Cylinder Pressure Type' used with a standard indicator cock (usually calibrated for 200–1400lb/sq.in or 15–100kg/sq.cm), with ventilation holes in its fluted vulcanite body; the steel-bodied 'Water Cooled Cylinder Pressure Type', attached directly to the engine cylinder; and an 'Oil Fuel Pressure Type', designed to be inserted in fuel lines, which could handle pressures as high as 10,000lb/sq.in.

Many attempts were made in the U.S.A. in 1910-25 to develop indicators suited to internal-combustion engines, including, for example, the 'Gas-Engine Recorder' patented in November 1915 by Horace Morrow of Willston, Ohio. The subject of U.S. Patent 1161875, this relied on a trace drawn on a circular tablet controlled by a spring and ratchet. The mechanism advanced one click for every cycle.

The introduction of Okill instruments into North America persuaded Robert Wasson of Cranford, New Jersey, to develop a peak-pressure indicator that he claimed to be simpler, more accurate and more easily understood than its English antecedent. Protected by U.S. Patent 1950532 of 13th March 1934 (sought as early as November 1925), the Wasson design used a graduated sleeve and an electric lamp to show the point at which pressure of the spring balanced that in the cylinder. Whether the Wasson indicator was ever exploited commercially is currently unknown, but it undoubtedly influenced the patent granted in July 1936 to Rudolf Ulrich of Pittsburgh, Pennsylvania (U.S. no. 2046801). This protected another modification of the Okill principle, but incorporating a lifter attached to the piston to activate a tension spring controlled by an indexing sleeve.

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Left: drawings taken from the U.S. Patent granted to Robert Wasson in 1934. Note the light-bulb inside the top casing. Right: Bacharach-made Premax indicator no. 2650, shown with its flash-lamp unit to the left. Courtesy of the U.S. Government Patent Office, Washington DC, and Bruce Babcock, Amanda, Ohio.

A combination of the Wasson and Ulrich patents was subsequently made by the Bacharach Industrial Instrument Company as the 'Premax'. Its operation was described in Use of the Indicator for Diesel Engine Maintenance, published by Bacharach, which stated that it was '…composed essentially of a piston exposed to the engine pressure, the helical tension spring against which the piston force acts, index sleeve, which is used to adjust the tension of the spring, and the contact of the neon circuit which gives a visual method of checking piston motion… By inspection [of a diagram] it is seen that a force upon the piston is transmitted to the spring through a pusher tube. The opposite end of the spring is connected to a micrometer sleeve so that in rotating this sleeve downward, the spring will be deflected, eventually giving a force which balances the upward thrust of the piston. A direct pressure reading can then be taken from the micrometer scales… A visual means is supplied for determining the equilibrium point and thus the compression or firing pressure. As the piston moves up due to the gas pressure, the switch closes and the neon light flashes. The circuit is broken when the cylinder pressure drops during expansion. A continuous flashing of the neon light occurs until the sleeve is rotated to stop the piston motion. When the exact point of balance between the two forces is reached, the switch will remain open [the flashing ceases] and the cylinder pressure is then read from the micrometer.'

Bacharach was also the assignee of a U.S. Patent, no. 2610508, sought in October 1946 and granted in September 1952 to Joseph Stein and John Wagner. This combined the tension-spring and indexing-collar system with a digital counter.