Scott two-stroke car engines

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[Although it is still popular for motor cycles, in the car world the 2-stroke engine is often heartily disliked, except for lightweight runabouts. But it still has its enthusiastic adherents and Saab in Sweden, who won the recent R.A.C. Rally, and the Daimler-Benz owned Auto Union/D.K.W. small cars use 3-cylinder power units of this kind. In the following exclusive article George Stevens, Editor of Yowl, the Scott O.C. Magazine, recalls some ambitious pre-war Scott car-type multi-cylinder 2-cycle engines that are of lasting interest to engineers and all 2-stroke advocates.—Ed.]

For over half a century the name Scott has been synonymous with high-performance 2-stroke machines, usually motorcycles whose speed and smooth acceleration—coupled with an exciting exhaust cadence—became bywords in the vintage era.

Scott power units however, were by no means reserved for 2-wheeled thoroughbreds. As long ago as 1915 Alfred Scott cut adrift from the motorcycle factory he had founded six years earlier and devoted his talents to the development of 3-wheel guncars. After the war a civilian version appeared—the Scott Sociable. The ” Crab “—as it became nicknamed—incorporated a very interesting 2-stroke engine with rotary inlet valves. The capacity was 578 c.c., and lubrication was by a 1 : 32 petroil mixture. Commercially, Scott’s cyclecar was a failure—mainly due to production and management troubles. The mOtorcycle Company, too, had its share of problems, culminating in yet another windingup in the slump of 1930. The Chief Financier sank some more capital and the Company was re-formed as Scott Motors (Saltaire) Ltd. .

As most of the ,old staff had left, he engaged some newcomers. The new Chief Designer was a clever young engineer named William Cull, and he was responsible for most of the interesting power units made at Shipley in the ‘thirties.

Motorcycle soles in 1931 were too low to keep the little Shipley works fully occupied and part of the factory was let to a tyre firm. Scotts accepted all sorts of machining subcontracts to pay their way, and Cull was encouraged to find new outlets for the skill and facilities at his disposal: he was given a free hand in experiment and research.

His attention turned to engines which were clearly designed with light sports vehicles in mind; his first brainchild was a super charged yee-four 2-stroke which utilised two of the T.T.-Replica motorcycle cylinder blocks on a common undivided crankcase and shaft. A rotary blower fed mixture direct to the cylinder inlet ports via a rotary valve—a system which avoided crankcase com pression and thus made possible a one-piece four-throw crankshaft, splined to a car-type clutch and flywheel. Mainand big end bearing lubrication was simplified by the separate induction! compression unit, as oil could swill around the common crankcase quite freely without enriching the mixture. Lubrication of the multi-feed rotary distribution valve, however, was a very different problem, and persistent troubles led to abandonment of this interesting unit, which never got beyond the prototype stage.

Cull went back to the well-tried ” Day-cycle ” and conceived a completely new engine—an in-line 3-cylinder with normal piston controlled induction and transfer. An experimental 750 c.c. unit was built and installed in a motorcycle frame for road-tests during 5933. The results were sufficiently encouraging to justify limited production of a 73 x 78 nun. 980 c.c. version in a heavyweight motorcycle. This Model 35 with 5.8 to 1-c.r. gave 47 b.h.p. at 6,000 r.p.m. and over 60 lb./sq. in. b.m.e.p. Up to 3,000 r.p.m. output was higher than most equivalent o.h.v. units in spite of the low c.r.

Several of these big 2-strokes machines were sold, and one at least has survived. Another went into a Morgan 4/4 chassis, and was subjected to lengthy tests, during which lubrication and cooling problems were satisfactorily solved. [I tested this car for Motor Sport just after the war. — Ed]

By far the most interesting engine made at the West Riding works, however, was the 2-litre, 6-cylinder unit of 1936. This engine has recently been put on display at the Birmingham Museum of Science & Industry.

The 22 1/2 in. long crankcase (a single alloy casting) surmounted a 2 1/2-gallon sump which was held by no fewer than 26 peripheral studs. Sides and bottom of the sump compartment were finned, and the internal crankchambers higher up were separated by five crankcase webs. These—and the rear and front walls—housed the eight double-row roller bearings which, in turn, were carried in alloy drums that exactly fitted the crankcase webs.

The 6-throw crankshaft was of the built-up type, each pair comprising full discs appropriately machined and splined. The front three throws were as in a conventional 6-cylinder engine, but the rear three were at 120 to each other—number four being at 180 to number one, giving a power stroke, at every 60° of crank-shaft rotation.

Big-ends were triple-row roller races, and the ribbed connecting rods were similar to the well-known motor-cycle pattern, with partially cut away little-ends. Crankcase compression for each chamber was maintained by the oil film in the mains roller races, which were of a special close-tolerance type, each with a positive oil-reed from one of two metering pumps situated fore and aft of a chain-driven auxiliary shaft. There were actually four oilpumps working in the same system: the main primary pump drew lubri:cam from the sump and fed it under pressure to the intakesof the two metering pumps. A pressure relief valve discharged oil to the auxiliary shaft bearings and chains, and a scavenge pump withdrew surplus oil from each erankchamber and returned it to the sump. The ” metering pumps ” were operated by swashplates coupled to the throttle linkage: as the opening increased, so did the stroke of the six plungers in each pump body. (One to each main bearing, and one to each cylinder bore.)

Cylinder construction was novel and provisionally patented : the lower section of each bore was formed by a sleeve in which ports were accurately machined, and which was pressed into position up a -counter-bore in the cylinder proper—a chromidium iron casting with normal exhaust and transfer ports. The protruding port-sleeves spigoted into the crankcase, while the upper junction of each liner and cylinder was adapted as an oil-groove communicating with a surrounding gallery fed from one of the swashplate pump plungers. Evenly spaced notches admitted the oil to conventional Scott heat-treated Y-alloy deflector pistons, and small cuts aroundthe skirts distributed it evenly around the bore.

The light-alloy cylinder head was secured by 33 studs and the usual copper-asbestos gasket interposed between upper and lower machined surfaces. A large water take-off was bolted to the bead casting at the front, and sparking plugs were centrally disposed. The lower hose connectiOn was situated directly in line with the six exhaust ports, and specially contoured passages ensured that in coming coolant thoroughly circulated around the hottest point of the engine. Effective water cooling was largely due to the highcapacity Nang pump chain-driven from the front of the crankshaft; it had blades at 90° to the rotor, and a drain-tap at the lowest point.

As on nearly all Scott engines, transfer-port covers were detachable. Actually, a pair of 3-cylinder side-plates was used, end-to-end; these had a specially shaped inner aspect which formed a smooth venturi in each passage. Piston and ring inspection was not quite so simple as on the normal Scott min-engine—these transfer covets were held in place by 42 screws! Immediately beneath these plates was a cast-alloy inspection cover bearing the inimitable ” Scott ” scroll.

The dynamo was belt-driven in conventional car style—by a ” Texrope ” belt made in the factory next door.. (FrankWigglesworth Ltd., a firm long established as makers of all-British transmission drives, grew up alongside the old ScOtt Engineering Co. Ltd.) A Delco-Remy was used and electrical equipment was completed by a starter-motor with the usual Bendix pinion, .erigaging the toothed flywheel which housed a Borg -& Beck clutch. (The gearbox illustrated was not a Shipley product, but a remotechange job of famous origin).

Rod linkage actuated two R.A.G. carburetters, fed from a mechanical Amal fuel pump—and thence the swashplate tilt mechanism or the old pumps. (The overriding regulators, with small eccentric dials, may he seen at the front and rear of the nearside view or the crankcase, level with the float chambers). 

When put on test, on the Shipley dynamometer this 2-litre engine (73mm. x 78 mm.) developed 86 b.h.p. in standard, notrun-in condition—not fabulous, but an output which brought fourth praise from both Laurence Pomeroy and Cecil Clutton. It had a safe speed of 6,600 r.p.m., limited only by the flywheel safety factor; but of the specific fuel consumption no record (mercifully?) has survived. Total weight was 350 lb.

 

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