Although Paxman's earliest compression-ignition engines were not introduced until 1927, by late 1930 the Company was preparing to launch what might be described as its next generation of diesels. This was Paxman's first monobloc type, known as the Heavy Duty Diesel Engine. One of the earliest ordered, under Order No 17703 dated 9th December 1930, was a six cylinder VX for the British Industries Fair, Birmingham where it was to be exhibited the following February.
As explained on the page Heavy-Fuel-Oil Engines, Paxman's first generation of compression-ignition engines were constructed with separate 'A' frames, each supporting a single cylinder. Like its predecessor, the new monobloc engine was a four stroke design with a bedplate carrying the main bearings and crankshaft, surmounted by separate 'A' frames which formed the crank chamber. The key distinguishing feature of the new engine was its single cast-iron cylinder block (monobloc) which housed all the cylinder liners and was carried on top of the 'A' frames. This block was tied to the lower part of the bedplate by long high-tensile bolts which passed through the block and the 'A' frames to the bedplate. The bolts were tightened up to a degree greater than the tension which could be imposed on them by the firing loads of the engine so that they absorbed the whole working load. In its publicity literature Paxman explained that with this method of construction the cast-iron framing was always in compression and not in tension. The resultant greater rigidity of the framework made it possible to keep the crank chamber inspection doors very large without detracting from the strength of the structure.
Cut-away illustration of a twin camshaft type of Paxman Heavy Duty Diesel Engine. One of the long through bolts is visible just to the right of the opening in the crankcase.
The earlier Heavy-Fuel-Oil engines were originally 'open' types with relatively basic forms of lubrication even for their main bearings. In 1929 the Company had introduced, as an option across its standard Heavy-Fuel-Oil engine range, enclosed types with forced flow lubrication. The new monobloc engines were designed from the start as fully enclosed and pressure lubricated throughout. All moving parts except the flywheel were enclosed, thus preventing the ingress of dust or moisture which could cause wear. More importantly, total enclosure allowed the use of full pressure lubrication not only for the main and connecting-rod bearings but also for camshaft bearings and valve gear. Lubrication of the camshaft and valve gear was effected by a secondary pressure lubrication system delivering clean oil which was drawn from and returned to the camshaft trays. These innovations made it possible to run the new engines for long periods unattended, and at higher speeds. They also assisted in reducing wear on moving parts to a minimum and maintaining a cleaner operating environment.
Valve Gear: For ease of maintenance inlet and exhaust valves were carried in separate cages incorporating removable valve seats. The cages were bolted to cylinder heads and by undoing a few bolts could quickly be removed without disturbing any other components. Spare cages, with valves ground in and ready for use, could be installed in a few minutes, leaving regrinding of the removed valves to be done later when convenient. The valves themselves were operated by rockers carried in the cages, one end of each rocker having a roller cam follower bearing on the camshaft.
On VZ and VX types the valves were arranged horizontally, one in each side of the cylinder head. The VY and VYL types had their valves positioned vertically and fitted in the top of the head.
Camshafts: Types VZ and VX were fitted with twin camshafts, placed on top of the cylinders, one on each side of the engine, operating the inlet and exhaust valves by single rockers. Types VY and VYL had a single camshaft which operated the valves by means of rocking levers.
The camshaft drive was arranged at the flywheel end of the engine, using a roller chain driven from a sprocket on the crankshaft. On types VZ and VX the drive was to a lay shaft and thence through spur gears to the two camshafts. On VY and VYL types the chain drive was taken direct to a sprocket carried on the camshaft.
Accessibility: Throughout its history, going back to early steam days, Paxman has always paid careful attention to the issue of accessibility for maintenance and repair. Although the Heavy Duty Diesel was fully enclosed, ease of access was a feature kept clearly in mind at the design stage. The photograph here shows a six cylinder engine with the covers partly removed. Large easily removable crankcase doors gave good access to main bearings and big end bearings. They were also of a sufficient size to allow pistons and connecting rods to be removed through them, without disturbing cylinder heads. The sliding covers on the upper part of the engine gave access to camshafts and valve gear for inspection and servicing whilst preventing the ingress of dirt and escape of lubricating oil.
Fuel Injection: It is clear from contemporary literature that when the VX engine was introduced it was fitted with the spring injection fuel system like its predecessor (1). Also, in a paper he delivered in late 1931 on diesel rail traction, Edward Paxman referred to the fact that many manufacturers had adopted Bosch type pumps but specifically mentioned that his Company had used spring injection for the engine recently fitted in an experimental locomotive (a reference to the LMS Railway's No 1831).
Spring injection worked very successfully with the slower Heavy-Fuel-Oil engines but did not perform ideally at the higher speeds of the new engines. Not long after the Heavy Duty Diesel Engine was introduced its spring injection was replaced with the fuel injection system brought out by Bosch in 1927. After this change VZ and VX types were fitted with a Bosch-type monobloc fuel pump driven from an extension of one of the main camshafts. VY and VYL types were fitted with separate fuel pumps mounted adjacent to each cylinder and driven direct from the main camshaft.
The introduction of Paxman's new monobloc engine in 1931 was not the only major landmark in the development of the Company's diesel engines that year. As an alternative to the cast-iron through bolt construction of the new engine in its standard form, a fabricated steel frame version was also brought out. Paxman was the first firm in Britain to construct diesel engines with entirely welded steel framework and showed what was probably the first of this type at the Shipping, Engineering and Machinery Exhibition at Olympia in September 1931. On display was a six cylinder VXS, developing 300 bhp at 600 rpm, direct coupled to a 200 kW DC generator. The engine was run at the exhibition to demonstrate the smoothness of operation and freedom from vibration. However, the outstanding feature, as reported in technical journals at the time, was its welded steel frame (2).
The design of the frame was the subject of patents (3) taken out by Mr C H Stevens of the Steel Barrel Co Ltd, Uxbridge which built the frame for the engine exhibited at Olympia. Paxman subsequently used this method of construction under licence from Mr Stevens. The frame was designed so that all the firing stresses were taken direct from the cylinder head right down to the seating of the crankshaft bearings in continuous steel plates or slings, without any load being imposed on the welding. As shown in the photograph here, the vertical plates or slings were held together by two suitably slotted horizontal plates which were worked into position and welded to them. The top plate acted as a seating for the cylinder heads while the lower plate served as a housing for the cylinder liners. The simplicity of the construction allowed access to all parts of the framework, making it possible to achieve good quality welds throughout the whole structure.
The advantage of welded steel frame construction was a major reduction in engine weight combined with great strength. The weight of the framework itself was less than half that of the equivalent cast-iron through bolt construction used in Paxman's standard Heavy Duty Diesel Engine. The lighter weight and robustness of the steel framed version were important considerations for portable and semi-portable applications. Substantially reducing the engine's weight while retaining the same power output, enhanced its suitability and attractiveness for rail traction and certain marine applications. Already we are seeing signs of Paxman's constant quest to increase the power to weight ratio of its engines. Records suggest that only six of these steel framed Heavy Duty Diesels were built, two for installation in the LMS Railway's first diesel locomotives and four for auxiliaries in naval ships. Nevertheless it may be argued that these contracts were significant in shaping the future direction of the Company. The high power to weight ratio of its engines led to rail traction and naval applications becoming two of Paxman's most valuable markets throughout the second half of the 20th century. It is interesting to note that the Ventura engine, launched in 1960, had a fabricated steel crankcase and that up to the late 1990s some Valentas built for the Royal Navy also had fabricated crankcases because of their lighter weight and greater shock resistance.
The letter 'S' was added to the engine mark of the steel framed versions of the Heavy Duty Diesel Engine (e.g. VXS and VZS) to differentiate them from the cast-iron ones.
The earliest catalogue we have seen for these Heavy Duty Diesel Engines is No 1022. It bears no date but the introduction reproduces a press cutting of 21st June 1933 so is no earlier than this date. It was possibly prepared to be available in time for the Shipping Exhibition in September that year where the Company exhibited a 4MZ (marine version of the VZ) engine. The types, power outputs and speeds of engines offered in catalogue 1022 are as shown in the table below. Cylinder dimensions are not given in the catalogue; those shown below are taken from a data sheet found in Paxman's Service Department.
| Type | Number of Cylinders | BHP per cylinder | Speed (RPM) cont duty | Bore & Stroke | |
| Normal Load | Max Overload for 1 hr | ||||
| VW | 2, 3, 4, 5, 6, 7, 8 | 17.5 | 19.25 | 1,000 | |
| VZ | 2, 3, 4, 5, 6, 7, 8 | 25 | 27.5 | 750 | 6½" x 10" |
| VX | 2, 3, 4, 5, 6, 7, 8 | 50 | 55 | 600 | 9" or 9½" x 12" |
| VY | 3, 4, 5, 6, 7, 8, 9, 10 | 100 | 110 | 500 | 13" x 16" |
| VYL | 3, 4, 5, 6, 7, 8, 9, 10 | 100 | 110 | 330 | 13" x ??" |
Speeds of the S (steel frame) types could be increased by 25% of the above.
The VYL had a lengthened piston stroke to maintain the same power output as the VY but at the lower speed where the customer required a slower running engine.
A later catalogue, No 1042, offers the same types and sizes of engines as those shown in the table above except that there is no reference to the Type VW and the smallest VZ offered is now a 3 cylinder version. It is fair to assume the Type VW had been withdrawn and there is no evidence to suggest that any were ever built. Catalogue No 1042 refers to the diesel-electric paddle boats built for Queensferry so cannot be earlier than 1934.
The VY and VYL Series engines were still be advertised as late as January 1938, then being offered in 5 to 8 cylinder forms, developing 600/1,000 bhp at 333/500 rpm (4). It is believed that very few VY or VYL engines were built.
Types MZ, MZS, MX and MY were marine versions of the VZ, VZS, VX and VY respectively.
Like the earlier Heavy-Fuel-Oil Engines, one of the most important applications of the Heavy Duty Diesel Engine range was electrical power generation. Engines were sold to a variety of industrial and commercial concerns for this purpose but scarcely a handful to local authorities.
Right - An 8 cylinder VX similar to those supplied for the Bank of England.
Within a few months of the new engine being launched, major orders were received for two prestigious London projects. These were placed by a noted firm of electrical contractors, Drake & Gorham Ltd, on behalf of their clients. The first batch of orders, placed on 22nd June 1931, was for four 8 cylinder 450 bhp VX engines for the Bank of England. The engines, Nos 23830 to 23833 inclusive, were delivered between 19th December 1931 and 16th January 1932.
The second batch of orders was placed by Drake & Gorham just a few weeks later, in September, for another well-known financial institution. This was for five 6 cylinder 300 bhp VX engines for installation at the Prudential Assurance Company's Head Office at Holborn Bars in which more than 5,000 people were employed. The engines, Nos 23881 to 23885 inclusive, were delivered during the summer of 1932.
Left - This photograph showing the five 6VX engines installed at the Prudential Assurance Co's Head Office appeared in Paxman's Heavy Duty Diesel Engine catalogues.
Paxman did not have the success it appears to have hoped for in selling its Heavy-Fuel-Oil Engines into the marine auxiliary market. It fared considerably better in selling its Heavy Duty Diesels for on-board power generation; a market which from then on grew to be a major one for the business.
Two Portuguese naval vessels, "Gonçalo Velho" and "Gonçalves Zarco", built in 1932/3 were each fitted with a 3MZ auxiliary. Four Royal Navy Halcyon Class minesweepers built in 1934, "HMS Halcyon", "Skipjack", "Harrier" and "Hussar", were each equipped with a steel-framed 3MZS auxiliary.
Right - 3 cylinder Heavy Duty Diesel Engine direct coupled to a generator for marine auxiliary purposes.
Auxiliaries were also supplied for passenger ferries and coastal traders. For four of the latter owned by Coast Lines Ltd, "British Coast" (1933), "Atlantic Coast" (1934), "Pacific Coast" (1935) and "Ocean Coast" (c.1936), no less than ten MZ engines were ordered. These comprised eight 4MZ and two 3MZ engines, driving Campbell & Isherwood 220 volt DC generators at 750 rpm, with outputs of 60kW and 45kW respectively. It is interesting to note that Coast Lines Ltd had a large shareholding in David MacBrayne so perhaps it was Paxman's earlier dealings with MacBrayne over "DEV Lochfyne" which provided the entrée to the Coast Lines business.
This application is essentially a specialised form of electrical power generation. Two of Paxman's earlier Heavy-Fuel-Oil Engines had been supplied as the prime movers for what is believed to have been the first diesel-electric vessel to operate in British coastal waters, MacBrayne's "DEV Lochfyne". Building on that pioneering step Paxman Heavy Duty Diesels were supplied as prime movers for at least five diesel-electric vessels constructed during the 1930s.
Right: David MacBrayne's "MV Lochnevis".
One of these was another MacBrayne ship, "MV Lochnevis", built by William Denny & Brothers Ltd of Dumbarton in 1934 and powered by two 6 cylinder MY engines, rated at 650 bhp each. To minimise vibration being transmitted to the hull, the engines and generators were mounted on specially designed bedplates which incorporated springs. This innovative feature was further developed on a later Paxman-engined MacBrayne ship, "Lochiel".
Also built by William Denny in 1933/34 were two paddle wheel driven ferries, "Queen Margaret" and "Robert the Bruce", for the Firth of Forth crossing at Queensferry, north of Edinburgh. The first diesel-electric paddle ferries to be constructed for service in the British Isles, each was fitted with two 8 cylinder MZ engines with individual ratings of 175/192½ bhp at 750 rpm. Paxman records suggest that their four engines were the only 8 cylinder MZs ever made.
Left: "Queen Margaret" by the Forth Railway Bridge.
The first diesel-electric tug to operate on the River Thames, "Sir Montagu", was built c.1936 with an 8 cylinder VX engine, rated 400/440 bhp at 600 rpm, for main propulsion. Even more unusual than the paddle wheel ferries was "Vehicular Ferry No 4" built by Ferguson Brothers of Port Glasgow in 1938. Operating on Glasgow's Govan-Partick crossing, this vessel had an elevating vehicle platform to handle different states of the tide, twin screws at each end, no rudders, and two 6MX engines for propulsion.
Fuller details of all the above installations can be found on the page Diesel-Electric Marine Propulsion.
Reference has previously been made to the application of welded steel frame versions of the Heavy Duty Diesel for rail traction. A 6 cylinder VXS engine, with a rating of 412 bhp at 750 rpm, was fitted in an experimental locomotive, LMS 1831. Believed to have been the first diesel powered locomotive built for a British railway company, No 1831 was built by the LMS Railway and entered service in 1932. A second LMS diesel locomotive, No 7054, was built by the Hunslet Engine Company of Leeds in 1934. This was fitted with a smaller version of the VXS, a 6 cylinder VZS with an output of 198 bhp at 900 rpm. The locomotives had hydraulic and mechanical transmissions respectively.
Right: Locomotive 7054 built for the LMS Railway in 1934 by the Hunslet Engine Company.
In hindsight, Paxman's early involvement in diesel rail traction was an excellent move. In time it led on to the Company becoming a major player in a large new and profitable market which developed post-War. Other pre-War Paxman-engined locomotives were mainly small industrial shunters. However, in the late 1940s and early 1950s Paxman was to become one of the best known suppliers of engines for locomotives during the dieselisation of the British rail network and of many overseas railways. The rail traction market remained an important source of Paxman business right up to the end of the 20th century. That story is told on the pages Diesel Rail Traction and Paxman Diesels for the World's Railways.
Paxman sold a good number of its earlier Heavy-Fuel-Oil engines to municipal water undertakings and pumping stations. This market also yielded a useful number of sales for the Heavy Duty Diesel Engine range.
Details of Order Numbers, engine types and customers for many of Paxman's Heavy Duty Diesel Engines are to be found on the Oil Engine Listing page.
I am not aware of any surviving example of the Paxman Heavy Duty Diesel Engine but please contact me if you have any knowledge of one.
The Heavy Duty Diesel Engine had only been on the market for a year or so when another watershed was reached in the story of Paxman's diesel engine development. 1932 was the year of AGE's financial collapse which brought down Paxman in its wake. No doubt the Company faced a great deal of uncertainty and disruption while new financial backing was sought to allow the business to continue and a new Board of Directors appointed. Edward Paxman now joined the Board as Engineering Director. Despite all the upheaval caused by AGE's fall, Paxman had the vision, motivation and energy to start developing its third generation of compression-ignition engines. The design of Paxman's first high speed diesels was undertaken in collaboration with Sir Harry Ricardo. The story of these engines and their successors is told on the page Paxman Engines since 1934.
References
1. Paxman catalogue 987 (c.1931), Heavy-Fuel-Oil Engines, p.47.
Motor Ship, July 1931, p.137.
Motor Ship, September 1931. p.247.
2. Engineering, July 17, 1931.
Motor Ship, September 1931. p.247.
3. British Patents, numbers 342610 and 362759, see:
Paxman catalogues Nos 1022 and 1042, Heavy Duty Diesel Engines, p.13.
4. Motor Ship, January 1938. p.88.
© Richard Carr 2006
Page updated: 17 NOV 2007