Thursday, 1 September 2016

Jowett flat-four engines: Early development

During early prototype testing of the Javelin it became apparent that the engine was mechanically noisy in a harsh resonating manner. This was deemed to be a major problem given that the Javelin was to be a luxury small family car and as such a remedy was required. Two types of crankcase had been manufactured, one in cast-iron, the other in aluminium. Experimental testing on these early designs made it clear that there was considerable whirl of the crankshaft and a certain amount of flexing on the crankcase was evident from edge marking on the bearings.

The next stage of development was to manufacture a cast-iron crankcase and a three bearing crankshaft, the bearing-cap joint faces now being horizontal, so the crankshaft could be dropped out of the bottom of the engine. The first engine tested of this type was a 1200cc unit, but when experiments were carried out with larger diameter liners, increasing the capacity to 1500cc, considerable crankcase ‘thump’ was experienced. To overcome this for testing purposes a boiler plate was bolted across the bottom of the bearing-caps, however this only highlighted the inherent weakness of this crankcase design. Subsequent development led to the adoption of light-alloy crankcases split vertically, which permitted the use of tie-bolts, making the whole assembly much stiffer.
Alloy crankcase of the production Jowett flat-four

Jowett had noted that cast-iron crankcases resulted in a quieter engine, but it was decided that the alloy crankcase should be proceeded with as it had been designed for die-casting. The cast-iron version was approximately 10% quieter than the alloy one, but was naturally heavier. In view of this and the difficulty at that time of obtaining iron castings, the spilt alloy crankcase was decided upon. It was also at this stage that the 1200cc project was dropped, as there was a large performance difference between it and the 1500cc engine.

When the revised engine was power tested, a considerable drop in performance was seen above 4250rpm. This was attributed to inadequate breathing and poor turbulence in the stepped head. The valve lift was increased from 0.275in to 0.315in and the ports ‘cleaned up’. Weslake was called in to inspect the combustion chamber and he evolved a semi-pancake head with 14mm plugs easier to produce and increasing top-end power by 15%, while providing smoother running. The exhaust system was changed from streamlined exhaust ports brought out to the bottom face of the head, to a manifold bolted to the underside of the head, the off-side manifold feeding into a pipe running round the front of the engine to enter the near-side manifold and take benefit thereby of extractor action. The main exhaust pipe led from the back of the near-side manifold. It had 1 3/8in inside diameter and the power drop with silencers was only 3 BHP compared with an open pipe. This new exhaust arrangement gave a power increase of 1.5% and resulted in the cylinder heads no longer being handed, a production and servicing advantage.

Jowett engine sectioned

Snatchy running below 20mph lead to an increase of flywheel diameter to the limits of the bell-housing. Another alteration required following testing, was to change the main bearing clearance due to rapid crankcase expansion. A steel housing giving 0.0003in to 0.0018in clearance at assembly temperature was adopted.

The Javelin emerged as the first really new British post-war car. It was a comfortable, brisk 5/6 seater saloon giving 75/80 mph and 28/32 mpg with advanced aspects such as the flat four engine, torsion-bar suspension and wind-defeating body form

The prototype engines, developing 40-45 BHP had been satisfactory in respect of bearings, but long-distance driving on the Continent with the early production versions showed up a tendency to run big-end and main bearings.

With the previously mentioned improved breathing 50-52 BHP was developed at 4500rpm, and it was decided that white-metal bearings must be replaced by copper-lead bearings, if possible in conjunction with the existing EN 12 steel crankshaft. The flat-four layout led to higher oil temperatures than are experienced in in-line designs, which contributed to the bearing failures.

Initially sintered copper-lead bearings with a white metal flash of 0.00125in were utilised with the un-hardened steel crankshaft. These bearings showed no signs of fatigue, but were extremely sensitive to dirt and scuffing on the crankshaft. A hardened crankshaft was therefore adopted with special care being paid to assembly and running-in. It was also found that the stepped location on the big-end led to distortion on tightening, so a new con-rod was devised, the big-end having an offset serrated face and clamp bolts increased to 0.375in and 400lb/in tightening torque. A dirt trap hole of 1/16in diameter had a negligible effect on oil pressure and consumption. The crankshaft was induction hardened on the journals and pins to a hardness of 512-530 Brinell, and the bearing surfaces were lapped to a finish of 8-12 micro-inches against the former 12-24 micro-inches. A softer bearing material of 30% lead, 1.2% tin and 68.8% copper with a 0.00025 in plated white-metal layer for running-in was used with the new rods and crankshaft and the bearings now stood up to 50 BHP and 4750 rpm in spite of the higher oil temperatures and compact bearings of the flat-four layout.

The lubrication system was thoroughly tested in the initial stages of development, an engine being rigged for measurement of oil spillage from bearings, relief valve, ancillary services etc. As a result, the feed to the main bearings was increased and the size of the oilways increased to 7/16in diameter to obviate a possible danger of bearing starvation under cold-start conditions with the full-flow filter system adopted to ensure clean oil for the hydraulic tappets. The pressure relief valve exhausted beneath the sump oil level to avoid aeration and later the discharge was by-passed to the pump suction side, within the cover.

Initially the three-bearing crankshaft engine suffered with oil swirl due to air transferring from one side of the crankcase to the other. To prevent this, a surface baffle was fitted which allowed the free passage of air only. Originally the oil pump had been mounted on a bearing cap, but the vertically split cases obviated this location, so it was moved to the timing-case wall and driven by spiral gears from the crankshaft. The distributor was also repositioned to allow it to be mounted vertically and use a common drive-shaft as the oil pump from the spiral bevel gear on the crankshaft. The oil pump capacity was also increased which meant oil pressure rose from 50lb/ to 65lb/ Following use on Jowett’s competition cars, an oil-cooler built to Jowett’s specification was incorporated on production engines in 1952. Initially this was placed rewards for accessibility, but later was moved to a location between the fan and radiator. With the oil-cooler in circuit, pressure pulsations occurred at audible frequencies until the previously mentioned dirt-trap holes in the big-end caps were deleted.

In vehicle testing it was shown that louvers in the bonnet became ineffective in terms of extracting air from behind the radiator above 50mph. Pressure areas were checked and it was found possible to take air from behind the radiator via apertures in the front wheel arches.

An issue with early production engines was noisy valve gear, even with the zero lash hydraulic tappets. Improved manufacturing tolerances were introduced, but the noise was still deemed unacceptable. Jowett then went on to investigate the effect of valve opening/closing ramp sizes and also velocity profiles. After much experimentation Jowett settled on cams with a 0.008in opening ramp and 0.020in closing ramp (from the fierce initial 0.002in and 0.006in ramps on the inlet and exhaust respectively). Unfortunately hydraulic tappets became unobtainable during 1950 and the noise level rose somewhat with the enforced use of conventional tappets.

Experiments were made with the material for camshafts and tappets. Excellent results were found with a high duty 1% chromium cast iron camshaft with a tip hardness of 40-45 Rockwell C and chilled iron tappets of a similar hardness and a finish of 7-10 micro inches. A phosphate process on cam and tappet faces to retain oil during running-in was found to be beneficial, but not necessary.

Five different types of liner/piston combinations were used in the course of development. Vacrit high-duty manganese chromium iron liners with a 280-270 Brinell surface hardness were originally used, in conjunction with split-skirt pistons in LO-EX or LM13 alloys withtwo D/26 radial thickness pressure rings and a slotted oil-control ring.

A taper-faced Vacrome chromium-plated top piston ring was adopted to cut oil consumption, without complete success. Liner distortion was suspected and investigation showed that whilst 0.008-0.010in gasket nip at 38/40 lb/ft cylinder head tightening torque was satisfactory to retain gas and water seals, this was highly critical; any degree of higher torque loading or excessive nip caused local liner collapse and consequently distortion. To counteract this, the liner section was stiffened and an internally-stepped second ring fitted to facilitate quick bedding-in of the chromium-plated piston ring. Following this a Javelin ran 80,000 miles in the course of testing by Avon India Rubber Co. Ltd., gave an average of 3,700 mpg of oil at 37.39 mph average speed, and maximum bore wear averaged 0.002in, equal to 40,000 miles per thou.

Due to the Javelin’s unusual firing order of 1-3-2-4 carburation was paid special attention. Cylinder 1 and 3 are fed from one carburettor via siamesed ports, and 2 and 4 from the other carburettor. To prevent a weak mixture in the front two cylinders of each bank caused by inlet tract surge, a 0.55in diameter balance pipe was added between the two carburettors. Intake noise was a problem on the Javelin and Jowett went on to test many different types of air filter and silencer, but no satisfactory solution was found. Instead Jowett evolved their own baffle box which was located in the alligator-bonnet, tuned to length to suit the induction system, and connected to a resonance chamber which was coupled to the air intakes by vertical pipes having squash rubber connections which broke as the bonnet was lifted. A non-spill oil-bath air filter was incorporated.

In addition to the early bearing failures and excessive oil consumption, gasket blowing was an issue on some Javelins. It was subsequently found that this was due to a too small an asbestos content at the fold of the gasket, however it was only with the increased output for competition purposes that this cause was identified.

Sunday, 7 August 2016

A product from Yorkshire: The Jowett Javelin and Jupiter flat-four engines

Yorkshire may not be renowned for automotive engineering, but in its West Riding on the outskirts of Bradford there was a hive of such activity. This article focuses on the Jowett car company who ended up with their factory at Idle just north of Bradford.

The Jowett name goes back to 1901 when Benjamin and William Jowett along with Arthur Lamb set up a cycle business that went on to manufacture V-twin engines for driving machinery. I do not intend to describe the whole Jowett history, but instead will focus the developments of their most famous engine; the flat-four fitted to the Javelin and Jupiter cars.

Following the Second World War, many vehicle manufacturers were conservative design-wise as they re-started production for civilian products. This was not the case with Jowett. Their Javelin saloon introduced in 1945 was inspired to some degree by the popularity in England of the Lancia Aprilia. Gerald Palmer had joined Jowett as Chief Engineer during the War at the mere age of 30. The Javelin was his first vehicle. This advanced car utilised a flat-four cylinder engine which offered many benefits such as:

  • Compact engine allowed spacious seating (up to 6) within a given wheelbase
  • Notably vibration free
  •  Low centre of gravity
  • Excellent visibility due to the low engine height

Jowett flat-four

The new engine had overhead valves and was initially available in two forms, a 1200cc unit of 69.5 by 78mm bore and stroke, and a 1500cc version of 72.5 by 90mm bore and stroke. The smaller unit was intended for the home market, whilst the bigger engine was destined for export. Both engines had a compression ratio of 7.25:1. Subsequently there was a change in Jowett policy which caused Palmer to concentrate on the larger 1500cc engine.

In its early form the engine had a two-bearing crankshaft running in white-metal bushes carried on the crankcase, a circular spigotted cover at one end eliminated the need for a split crankcase. The crankcase was cast in aluminium alloy to D.T.D. 424 specification. And wet cylinder liners sat on a joint washer at the base and were clamped down by the detachable cylinder heads.

Cast iron cylinder heads were utilised which incorporated a vertical inlet port leading to siamesed valve ports which allowed the use of one carburettor per pair of cylinders. The exhaust ports were at the bottom of the cylinder head, with the exhaust gas being taken away by an integral manifold under each head. A metal duct directed cooling water to each valve seat. The combustion chamber was a stepped type; the inlet platform providing a squish area to promote charge turbulence over the exhaust valves, from which side the mixture was ignited.

The valves were push-rod actuated with hydraulic tappets also being incorporated into the mechanism to account for thermal expansion of the aluminium crankcases. These tappets were lubricated from one of the main oil galleries, with lubricant filtered by the full flow method.

The new engines, in both sizes, were extensively tested both on the bench and roads. The prototypes had 10mm spark plugs, but subsequently 14mm plugs were used. The 1496cc version produced 40 BHP, but this was soon increased to 50 BHP due to improved engine breathing.

To be continued.

Wednesday, 20 July 2016

Paul Dunstall: Genuine Domiracer Equipment

In 1958 Paul Dunstall went somewhat against the grain and started to race a Norton Dominator motorcycle. This was quite unusual at the time, as generally they were not considered to be a machine fit for this purpose and Norton already produced a very able racing motorcycle in shape of the Manx. Through his racing exploits Paul developed the Norton Twin with considerable success and following his retirement in 1960, was able to offer a range of special and tuning equipment for the various models. In 1961 he became an official Norton dealer.
In 1963 Dunstall purchased the works Norton Domiracer that had been ridden by Tom Phillis to 3rd place in the 1961 Senior TT. He also purchased all remaining spares and stock relating to Doug Hele’s experimental  lowboy project.

Tom Phillis 1961 Senior TT

This is a short blog on his catalogue from the early 1960’s listing the parts available for these models.

Dunstall provided race bikes for many riders over the years, the following are listed in his catalogue:-
Chris Conn
Dave Degens
Dave Downer
Joe Dunphy
Dereck Minter
Sid Mizen
Fred Neville
Tom Phillips
Colin Seeley

He continued to develop the Norton Twin throughout the 1960’s and in 1969 started the season with a radically different spine-framed machine designed by Eddie Robinson. This frame was manufactured for Dunstall by Jim Lee at his Birstall premises in Yorkshire and was considerably lower and lighter than the previous lowboy design. Jim went on to produce some very successful spine-framed machines of his own, the most successful being the TR2B Yamaha raced by Mick Grant.

Dunstall Drainpipe

And so to the catalogue.

By 1966 Dunstall was building complete machines, based on the Norton, BSA and Triumph twin cylinder models available at the time. As well as offering various light-alloy and tuning modifications, options included seats, tanks and fairings, all of which gave the machine a distinct Dunstall look. In 1970 he became a Honda dealer and parts were included in his later catalogues for these machines. He also put his name to a 3 cylinder Kawasaki racing motorcycle designed and built by Alan Baker. Again the frame and tanks of this machine were built by Jim Lee in Yorkshire and it used a double disc front brake designed by Eddie Robinson which had previously been used on the ‘drainpipe’ Norton bike.

Dunstall continued to sell customising parts for Honda, Kawasaki, Suzuki and BMW machines until he sold the company name in 1982.

Tuesday, 5 July 2016

The NSU Prinz engine

During the 1950’s there was a dramatic shift away from motorcycles to car ownership. This resulted in decreasing markets for motorcycle manufactures, which in the case of some companies meant that diversification into the four wheeled market was an obvious step.

One such manufacturer was NSU, who in 1955 received the directive to produce a small four-seater car. The specification of the car was for it to have a mass of approximately 500 kg and a speed of above 100 km/h. Another notable design constraint for NSU was that it would have to utilise its existing equipment and machinery that was developed for the production of motorcycles.
NSU Prinz powertrain

The engine for this new vehicle was to be a twin-cylinder four stroke. During early scheming a 400cc engine capacity was considered, however NSU settled on a 600cc unit. The engine would be air-cooled as NSU had extensive experience of this through its motorcycle manufacturing and also due to production cost reasons.

The engine, gears and differential for the new car would be housed in a single transversally positioned block in order to ensure a compact, sturdy unit with a favourable centre-of-gravity and low production cost. The clutch was mounted on the left of the crankshaft as was the blower wheel, which was attached to the clutch driving wheel. The gear drive was obtained by helical spur gears between the clutch and main bearing. On the other side of the crankshaft ‘Dyna’ starting equipment was mounted, which controls lighting whilst the starter also acted as a flywheel mass. Cooling air from the blower wheel was guided into a sheet metal sleeve which forced it around the cylinder block and cylinder head.

The twin cylinder unit utilised a common ignition source, so for this reason the crankshaft had only one throw and consisted of three drop-forged parts, i.e. the two crankpins and two bearing pins to the right and left and of the compact centre piece, which was developed as a counter-weight and flywheel mass. Following the initial machining and grinding of the crankpins, the crankshaft was assembled with a press-fit and the insertion of an intermediate ring, which allowed their separation (by pressure) in the event of repairs. The final grinding of the main pins was completed after the press-fit of the components.

Lubrication for the connecting rod bearings was carried through channels leading from the main bearings into the hollow crankpins whose ends were sealed by blanking plugs.  Besides the two main bearings, there was one step bearing on the side of the clutch and one on the side of the lighting attachment. The clutch and pinion were held on the left pin by a pair of needle roller bearings. This method of supporting the crankshaft proved very successful as it resulted in lengthened intervals between overhauls. The big-end bearings in the connecting rod were not of the split-type.  The bearings for the mains and big-ends were conventional tripartite components (supporting plate of steel, lead bronze and a third layer for the purpose of reinforcement). The rods with their built-in bearings are fitted before pressing together the crankshaft assembly. Oil flung from the big-end bearings was sufficient to lubricate the cylinder bores and gudgeon pins.

NSU Prinz engine layout

As mentioned previously, the complete housing of crankshaft, gears and differential was constructed of an aluminium alloy pressure die-casting with high degree of rigidity. Extensive ribbing of the block surfaces was used to evenly distribute stresses at locations such as the thrust blocks and where heavily loaded screws were inserted. The lower part of the housing also acts as an oil sump, with a capacity of approximately 2 litres.

The two cylinders and the channel for the valve drive gear was cast in a single piece. The cylinder bores were induction hardened and, in order to reduce wear of the cylinder bores, an alloy of grey-cast iron and chromium was used as the material for the cylinder blocks. Rid-shaped openings for the inflow of cool air were placed between the two barrels and between the housing for the camshaft drive. The cylinder had passages for both the pressurised oil feed and oil draining from the upper valve gear. The cylinder bore measured 76mm and with a 66mm stroke resulted in a cubic capacity of 598cc.

The cylinder head for the two cylinders was once again cast in a single piece of chill-cast aluminium. Pressure die-casting was not possible due to the complex shape of the ports. The combustion chamber was semi-circular, which allowed valves with large diameters to be fitted. The valves were fitted at an angle of 32° to the cylinder centre. Both the valve seats made from grey cast iron and bronze valve guides were shrunk into the head. The cylinder head had comprehensive finning on all surfaces including the centre section between the two combustion chambers. The inlet valve seats had hardened surfaces, whilst the exhaust valves were armoured with a special heat resisting steel alloy. Two valve springs were fitted to each valve.
NSU Prinz engine layout showing valve gear

Two cam ladders were fitted onto the cylinder head for the camshaft which was die-forged and case hardened. It was also hollow in order to provide lubrication to the camshaft bearings and cam tracks. The rockers were also case-hardened and fitted with a hardened chromium rubbing layer.
NSU cam drive layout

Probably the most technically interesting aspect of the engine was the drive methodology for the camshaft. On the side of the lighting attachment, the crankshaft was fitted with a control pinion which operates a control wheel on a short shaft. On the same shaft are two narrow eccentrics. Light connecting rods were utilised to transmit this rotary motion to similar eccentrics on a short shaft in the cylinder head, substantially in line with the camshaft. The weight of these ‘side rods’ was compensated by counterweights. In order to prevent distortions through heat expansion of the cylinder or cylinder head, the upper eccentric shaft runs on roller bearings in a separate housing. This housing is pivoted to the cylinder head to one side. A third thin connecting rod, non-rotating serves to maintain the true distance between the eccentric shafts in the crankcase and the cylinder head. The non-rotating intermediate rod was placed close to the rotating connecting rods and as a consequence is equally heated, while still keeping the necessary distance for the control gear, quite independent of the heat expansion of cylinder and cylinder head. Transmission from the upper eccentric shaft to the camshaft which, although placed in line are not necessarily coaxial, is obtained by a pressed-in pin on each of them and a trailing link connecting these pins. All supporting points of the valve gear were generously lubricated, which also acted to remove heat. This system of cam drive was first used by NSU on their Max motorcycle. It was found to require practically no maintenance whilst also ensuring high levels of rigidity between the crankshaft and camshaft. With this valve drive system NSU’s Sport-Max proved to be a highly successful 250cc racing machine which was even able to win the World Championship.
NSU Prinz ventilation system layout

The car was heated by fitting heat exchanges around the exhaust manifold. Intake air flowed through an oil-bath/ air filter to a Solex carburettor. An NSU developed diaphragm pump fed fuel into the carburettor.

The drive from the engine was transmitted via spur gears to the synchronized gears. For this reason the gearshafts are parallel to the crankshaft and rear axle-shafts. The output was transmitted from from the gears to the helical differential drive-wheel. Universal joints for the swinging rear axle shafts are fitted to the two outlets of the differential housing.

The engine unit was suspended in the vehicle using one rubber mounting to the front and two to the rear.
Complete NSU Prinz drive assembly

With this powertrain, the Prinz-4 weighed 565 kg and was capable of reaching 120 km/h. Fuel consumption was approximately 5.7 litres/100 km. In the Sports-Prinz, the vehicle weight was 555 kg, whilst a top speed of 133 km/h was achievable as a result of a more aerodynamic body shape.
NSU Prinz performance