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Aviation History
1947
1947 - 0122.PDF
FLIGHT JANUARY 2 3RD, 1947 T "C H O E L D W C R RE EN ST" maintenance and operation. It is a normally aspirated flat-four two-stroke in which the opposing cylinders fire together (i.e., it has the same firing interval as an in-line two-stroke or a 4-cylinder four-stroke engine). The mix- ture is supplied by a Zenith down - draught carburettor direct to the plain centre- bearing of the crankshaft, and distribution ports in the crankshaft admit it alternately to the two crankcase com- partments where it is compressed by the inward movement of the pairs of pistons (normal crankcase compression). Uniform Heat Distribution Transfer to the combustion chambers is via a port belt in which the transfer jets alternate with the exhaust ports, thus ensuring a uniform heat distribution around the cylin- der walls and preventing distortion of the cylinder liners. Cylinder heads are hemispherical, and flat-topped pistons are used, thus eliminating the hot-spots from which the old deflector type of two-stroke piston is so prone to suffer under continuous load. Being flat topped, the pistons can also be machined all over. The engine is, in fact, symmetrical, except for its off-set gudgeon pins which pro- vide, that degree of desaxe which has a desirable effect on the port timing, and, pev se, the engine's flexibility. Lubrication is provided by a constant supply of oil fed from a separate tank to the centre bearing by a meter- ing oil pump interconnected to the throttle, but for the first 50 hours of the engine's life this is augmented by a 5 per cent addition of oil to the petrol for upper-cylinder lubrication. There would probably be no harm in continu- ing the petroil addition indefinitely, but it is apt to be rather messy and is not really necessary after the running- in period. The makers are emphatic, however, that there must be no economizing with the metered oil; in fact they seal the adjustment and all guarantees become void if it is tampered with. It must be noted that by interconnect- ing the oil pump with the throttle, the supply of lubricant (it is, of course a total loss system) is varied according to the load—which, incidentally, is also a good feature of the petroil system. Some idea of the unit's sturdiness may be gained from the fact that its massive four-throw crankshaft accounts for o^lb of the engine's total weight of 851b. The ignition unit is coupled to the rear end of the crankshaft, and the TINY BUT TRIl^l : The pleasing lines of the new Wren single-seater will appeal to theowner-pilot. The nicely proportioned control surfaces should result in gocd manoeuvrability. left-handed tractor airscrew is driven via a 0.64 reduction If gear. The crankshaft is hardened and ground to take theJ" split, three-track roller-bearing big-ends and, as the unit is intended to run at a maximum of 4,000 r.p.m. for take- off, all reciprocating parts are "mirror" balanced. Motor cycling two-stroke enthusiasts will be particularly attracted by the makers' claim that this engine will not four-stroke unless the mixture is deliberately fouled, and will idle evenly down to 500 r.p.m. driving a 6£lb air- screw ! They also say that the throttle can be flicked wide open, when the pick-up is instantaneous ; there is no hesi- tancy or choking. Reference to the data panel will show that the bore (2.75m) is greater than the.stroke (2.5m) which makes for a comparatively slow piston speed despite the high revolu- tions—a good feature when reliability and long life are-:•* • to be combined with reasonable power output. The con-" sumption figures given by the makers indicate the low operating cost and should be con- sidered in conjunction with the compression ratio of 8 to 1, and the fact that 73 octane- petrol is the intended fuel. Ordinary maintenance, should certainly be ; well within the scope of the average owner- . pilot, for the few accessories are very '' get- at-able " and there are, of course, no valves, valve-gear, or other highly stressed details call- ing for expert attention. Thus a top overhaul could surely be tackled by anyone capable of holding a spanner, leaving only a complete overhaul (which should be at very infrequent intervals) to call in the services of a licensed aircraft engineer or the makers. Even top overhauls should be agreeably far apart, for the hemispherical heads are reported to give a high degree of turbulence with very rapid -and complete combustion. So much so that, says Mr. Carr, " no flame is visible at the ports under any conditions, and, moreover, the exhaust gas is relatively cool. (We understand that it is STURDY SIMPLICITY : Designed for reliability and low upkeep costs,the flat-four 2-stroke Wren engine develops 35 h.p. for take-off and is claimed to be immune from four-stroking. The sketches merely showthe general layout of the power unit. y possible to insert the fingers in the short exhaust stubs for several seconds at full throttle without discomfort.) Inci- dentally, the exhaust pipes have been designed to down the harmonics which normally exist in the pipe ai cylinder, and interfere with the slow-running and idling of a two-stroke engine ; so here lies part of the engine's immunity from four-stroking. Makers' performance figures give a top speed of 95 m.p.h., a cruising speed of 75 m.p.h. and a landing speed, of 42 m.p.h. Wing loading is 6.2 lb/sq. ft. Type Bore Stroke Volume Compression Gear ratio ... Weight of engine Flat four, 2-stroke 2.75in 2.5in 973 c.c. 8to I 0.64 85 1b. ENGINE DATA Take-off 35 h.p. at 4,000 r.p.m. Max. normal 30 h.p. at 3,500 r.p.m. Cruising ... 25 h.p. at 2,800 r.p.m. Fuel 0.62 lb/b.h.p./hr at 3,500 r.p.m. 0.54 Ib/b.h.p./hr at 2,800 r.p.m.
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