A FOUR-CYLINDER TWO-STROKE RACING ENGINE
Full details of the Jameson power-unit, and of the two cars to which it has so far been fitted.
THh; even torque and high theoretical efficiency of the two-stroke engine has engaged the attention of numerous inventors, hut the practical results so far . attained have been a little disappointing. With the three-port engine
using crank-case compression, an assymetrical piston with a deflector head has to be used, scavenging is poor, and trouble is experienced with oil being sucked in with the mixture. A supercharged engine is the obvious solution, and the improvements which have lately been effected in these components makes them suitable for everyday use.
The Jameson engine employs an almost spherical combustion chamber, and the top of this is closed by a piston-type inlet valve. This is operated by a crank-shaft which runs along the top of the engine, and the valve is extended, so that it passes round the shaft and carries a gudgeon pin at the top. A short connecting rod communicates the motion of the crankpin to the valve. This arrangement gives a long dwell at the closed and open positions, and a quick opening and closing, which is the ideal timing for an inlet valve. The inlet ports are conical in shape, and the energy lost in compressing the mixture is regained when it escapes into the cylinder. Two 14 mm. sparking plugs are used, and the domed top of the piston, which carries three rings, projects into the combustion chamber. The exhaust ports are disposed on each side of the cylinder, and are uncovered by the piston just before the bottom of the stroke. The centrally placed inlet valve gives good scavenging, and the design of piston and combustion space induces a high degree of turbulence, so that straight petrol may be used with a compression ratio of 7 to 1. A 500 c.c. engine built by the Jameson Engine Company gave the remarkable output, of 52 horse power at 5,000 r.p.m., and unlike most high
speed two-strokes ran steadily down to 300 r.p.m. The Jameson Company are now building 4 cylinder racing engines with a capacity of 996 c.c., this size being chosen in case the should regard the 100 c.c. displaced by the piston valves as part of the swept volume. Running on petrol-benzol the power output fulfils the high promise of the single cylinder engine,
with readings of 72 h.p. at 3,500 r.p.m., and 110 at 4,800 and is expected to give 140 at 6,000 r.p.m. These figures are well in advance of anything which has been obtained with a four-stroke engine of similar capacity. The four-cylinder engine employs the
same lay-out of inlet valve and cylinder as that used on the single-cylinder, but two vertically disposed superchargers are used, their casings being embodied in the cylinder block, one at each end. The block and crank-case form a single casting, and the crank-shaft is carried in five roller bearings.
The housings are split in the same way as is the practise with white metal bearings, and a solid crankshaft with disc webs is used. The throws are arranged in pairs at 180 degrees, the forward and the back pairs being arranged at 90 degrees to each other. The firing order is 1,4, 2, 3 and the torque is equivalent to that of an eight-cylinder four-stroke.
Rollers are used for the big-ends, which are split like the main bearings. The little-ends run on needle-bearings.
The rear supercharger is bevel-driven from the crank-shaft, and a further bevel drive at its top end operates the overhead crank-shaft for the inlet valves. A crossshaft drives two magnetos. A further bevel-drive at the front end drives the forward supercharger, which is not directly connected with the crank-shaft. Besides its normal duty of pumping the mixture. this blower steadies the overhead mechanism.
The engine has an inlet and an exhaust pipe on each side, and it has been found, as was the case with the old racing Scott motor-cycles, that efficient silencers add to the power-output. A finned aluminium sump is fitted, but this merely acts as an oil-collecter, as
dry-sump lubrication will be used. There is a large oil-pump driven off the rear crankshaft bevel, and a water pump is fitted at the front end of the engine. A dynamotor is coupled direct to the front end of the crankshaft. The engine with a cast-iron block and
crank-case weighs 330 lbs., while with an aluminium casting and liners the weight will be reduced to about 220 lbs. The castings are very substantial, and should withstand the high power-output without difficulty. The first of these engines has been secured by Dudley Froy, who would have run it in the 500 Miles Race if it had been ready in time. The chassis, which was constructed by Mr. W. Fensom, is of normal design, upswept front and rear, and . is fitted with a racing Delage front axle. An E.N.V. self-changing gear-box is used, and since the torque is high, 132 lbs. at 5,000-6,000 r.p.m., the box is nearly as big as that fitted to Straight’s Maserati. Another engine has been fitted to a special light underslung chassis, and the owner, Mr. Sebag-Montefiore, plans to nbe it in
strenuous touring events such as the Alpine Trial and the Monte Carlo Rally. It will not he ready for the 1934 Rally, however. Sir Malcolm Campbell and Whitney Straight are interested in the new engine, and may be using it in the forthcoming season.
The engines are built by the Jameson Engine Co. (U.K.), Ltd., 729, Salisbury House, London, E.C.2.