THE A.E.C.
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OIL ENGINE
A Description of the New Compressionignition Engine, Which is Now in Production, W,ith an Iopression of Its Performance on the Road
WE have from time to time made reference to the six-cylindered oil engine which is manufactured by the Associated Equipment Co., Ltd., Southall, Middlesex, and now record its manufacture on a produc. tion basis. The combustion principle employed is that of the A.cro system, which makes use of solid injection by a Bosch pump, mounted on the near side of the engine.
This power unit is not an experimental machine ; it has been the subject of continuous research for some years. Although many Londoners have not realized it, a sixwheeled 104-seater bus has been Operating in the Metropolitan area for a long time using an engine of this class. Tests have been carried out with a 5-ton lorry, obtdining a fuel consumption of about 10 m.p.g. In the 104-seater bus it also gave very satisfactory operation.
The new engine is not an adaptation or conversion of the standard A.E.C. petrol unit. Primary research work was carried out with the singlecylindered engine. Particular care was taken in constructing the new engine to see that it is interchangeable with the corresponding A.E.C. petrol power unit.
Finally, the design standardized has been a six-cylindered 110 mm. bore and 142 mm. stroke engine, having a swept volume of nearly e24 630 cubic ins. Monobloe construction of the cylinder-barrel casting has been employed, and the mainbearing bolts are brought up through the crankcase to fix the cylinders. The crankcase is thus relieved of gas-pressure loads. The heads are arranged in two groups, each covering three cylinders and carrying the air-inlet and exhaust valves, also the Acro eornbustion chambers.
The valves are operated by push rods, clearance adjustments being provided in the rocker levers. Seven bearings are provided for the crankshaft, which has main journals of approximately' 3 ins., whilst the crankpins are 211,6 ins, in diameter. The pistons are of Y alloy and the gudgeon pins are 40 Min, in diameter. Nickel-chrome steel is employed for the connecting rods, which carry the bearing metal directly in the big-ends ; the gudgeon pin of
each is clamped in the upper end by a pinch bolt.
Pressure lubrication is employed throughout ; t h e camshaft, pistons and gudgeon pins, however, are lubricated by splash. There is an individual feed to the overhead valve mechanism, a hollow rocker shaft being employed. The distribution drive is by Triplex roller chain and all the auxiliaries are grouped on the near side to suit forward control vehicles.
A governor limits the minimum speed, but
not the maihnum ; this is necessary to correct the difference in operating conditions between a hot and a cold engine, also to take care of a peculiarity of fuel pumps in that, for a given setting, the quantity of fuel discharged increases slightly as the speed rises. It sometimes happens that, after accelerating, the engine does not slow down. sufficiently rapidly to allow eaSy gear-changing, so the governor is provided to overcome this difficulty. When overrunning the engine the fuel is cut off above 400 r.p.m., so that a -very effective engine brake is provided.
A Bosch pump, driven at halfengine speed, supplies fuel to the inclined injector nozzles, which discharge the spray into the throats of the air cells ; the camshaft gives a rate of injection specially suited to the high speed attained by the engine. Fuel is delivered from the tank to the Bosch pump by an A.C. pump at a 3-lb. pressure ; an exhauster pump provides the vacuum for servo braking. A vacuum tank with a non-return valve
is installed. This exhauster is of the rotaryvane type.
The new engine has a striking weight-topower ratio of 14 lb. per b.h.p.; the maximum is over 100 b.h.p. and the speed range is 300 r.p.m. to 3,000 r.p.m. — considerably above the normal for an oil engine.
This new compression-ignition engine is now available for six of the eight A.E.C. chassis, the prices of which, equipped with it, are as follow :—Regal, £1,400; Ranger, £1,425; Regent, £1,500; Renown, so
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£1,700; Majestic, £1,360; Mammoth, £1,460; Mammoth (.6-ton tipper), £1,437; Mammoth (7-8-ton tipper), £1,467. As compared with the petrol engine of the corresponding type, the oil engine is about 4 ins, greater in length.
A* demonstration was given of the new engine installed in a complete bus of the ST type, operated by the LondonGeneral Omnibus Co., Ltd. At the time the body carried about 30 passengers. When idling, the amount of motion perceptible at the radiator cap was materially less than we have seen with other oil engines, and, from 'the point of view of a passenger in the bus, would probably not be noticed except by a technical observer. The mechanical noise at the idling speed of 300 r.p.m. consisted of the well-known " kettele-drum " of the compression-ignition power unit, The course covered during the demonstration included main roads and lanes. The odour of the exhaust, when ticking over, distinguished the engine as one of the oil-using type, but the smell was not noticeable when in the bus. The exhaust, as regards smoking, was exceptionally clear throughout the range of speed from 300-3,000 r.p.m. It may be said with accuracy that the exhaust of the new A.E.C.-Acro oil engine 2500 is as clear as that of its petrol counterpart.
On top gear from 8 m.p.h. to 15 m.p.h. the familiar " kettle-drum " is present, but the volume of sound would not be distinguishable by the layman from the pinking due to overadvanced ignition in the petrol engine. Above 15 m.p.h. the noise disappeared, and from that speed up to 40 m.p.h. the general performance was indistinguishable from that of the petrol engine. The acceleration was not quite so rapid as that afforded by the normal power unit in the bus in question.
The rear-axle ratio of the particular bus we tried was stated to be 61. to 1, which is, of course, that employed in many petrol-engined chassis of this type. The exhauster system of servo operation gave Instant application of the four-wheel brakes.
Despite the high rate of revolution —considerably higher than any other production engine working on the compression-ignition system— there was no vibration period. It is a creditable feat to have produced an engine, to be interchangeable with a petrol unit, which runs so well and at the same time, has such a low weight. The extra cost of any A.E.C. chassis with the new unit is about £300.