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G.M.C. FOUR-STROKE DIRECT-INJECTION DIESELS

10th January 1964
Page 50
Page 55
Page 50, 10th January 1964 — G.M.C. FOUR-STROKE DIRECT-INJECTION DIESELS
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AS recorded in our issue of Decern13, 1963, the G.M.C. Truck and Coach Division of the General Motors Corporation, U.S.A., has been experimenting with a four-stroke V-6 diesel engine of up to 170 gross b.h.p. output, and on Monday details of this new design were officially released in America. As is well known, until now all G.M.C. automotive diesels have been of the two-stroke type, so this new development marks an important change in the Corporation's power-unit policy.

Only one size of engine has been developed so far, this having a cubic capacity of 7-83 litres (477-7 cu. in.), but it is available with two ratings. The D478

unit has gross and net outputs of 150 and 135 b.h.p. respectively at 3,200 r.p.m. and 275 and 266 lb. ft. torque at 2,000 r.p.m. The higher rated DH478 engine has gross and net ratings of 170 and 155 b.h.p. respectively, and 310 and 298 lb. ft. torque respectively, the speeds at which these outputs are delivered being the same as for the D478.

The less-powerful engine is to be standard in G.M.C. commercials of up to 10-7 tons solo or 15-6 tons train gross weights, whilst the other engine, which is optional at extra cost in these lighter chassis, will be standard in solo chassis of up to 17.4 tons or prime movers of up to 20 tons gross ratings, and the units will be fitted in all types of chassis, including normal-control fourand sixwheelers, short-cab forward-control chassis and all-steel tilt-cab models. Besides having a low initial cost, G.M.C. claim that the new engines have a lower fuel consumption than any other diesel engine in the same class, and this is put down to the use of toroidal piston-crown combustion chambers which also, incidentally, results in G.M.C. having adopted the name of Toro-Flow for the new engines.

The design has cylinders at an angle of 60° and the cylinder blocks and crankcases are a single fine-grained chromiumnickel-alloy iron casting, the skirt of which extends to 3 in. below the crankshaft centreline. The banks of cylinders are staggered and the bores are unlined. The forged-steel crankshaft is carried in four main bearings, and to give maximum rigidity the main-bearing caps are secured not only by vertical bolts but by horizontal bolts passing through the crankcase walls. The connecting rods are steel drop forgings and each crankpin carries two rods. A balance shaft with two counterweights is gear-driven from the crankshaft at twice engine speed, and this is carried in the block in four steelbacked bronze bearings.

Both cylinder heads have a similar material specification to the main block casting, and incorporate specially shaped inlet ports which gives swirl characteristics to the air. An unusual feature of the manifolding is that the inlet and exhaust manifolds are on the same sides of each head—on the outside of the engine— whilst each head is secured to the block

by 14 bolts so that, in effect, each bore is surrounded by six bolts. The undersides of the heads are completely flat, and the single inlet and exhaust valves to each cylinder arc well spaced in order to give maximum head life. Each valve is positively rotated and operated from a central camshaft through a conventional system of push rods and rocker arms. The inlet valve.s are of high-chromium alloy steel, whilst the exhaust-valve seat inserts are of Steelite F. a nickel chromium-tungsten-cobalt alloy.

An American Bosch distributor-type fuel-injection pump is employed in conjunction with four-hole injectors. The pump incorporates automatic advance mechanism and is gear-driven at engine speed from the crankshaft.

Other engine features include the use of a ball-bearing-mounted water pump capable of delivering coolant at a rate of 88 Imp. eat, per min. at 3,200 r.p.m., an oil cooler as standard on the higherrated engine and optional on the other version, positive ventilation from the heads and crankcase to the air cleaner. and a.c. generating equipment. The new engines have 130 min. (5-125 in.) bores, the stroke dimension being 98 mm. (3-86 in.). The compression ratio is 17-5 to I, and the quoted dry weight, less accessories and clutch, is 950 lb. Production is to start later this month_


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