The Case for the V Engine
Page 68
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SAFE PROVISION FOR HIGHER R.P.M., AND THUS OUTPUT, CAN GIVE "SOMETHING FOR NOTHING"
The Cummins Y8 12.8-litre diesel engine, which has a gross output of 265 b.h.p. at 2,600 r.p.m.
ACCORDING to the Cummins Engine Co., an increase in •power output obtained by raising the r.p.m, is an example of getting something for nothing, provided good combustion efficiency can be maintained and safe piston speeds are not exceeded. In their opinion, limiting piston speed rather than r.p.m. is the most important single factor ensuring reliability under arduous operating .conditions. The larger the bore of a power unit of a given capacity, the lower the piston speed at a given r.p.m., and the engine r.p.m. of a large-bore engine can therefore be raised to provide a highe'r power output: Crankshaft length is, however, a function of bore diameter and, above a critical length, . the necessary increase in the size of the journals and webs (in the cause of rigidity) and the need to use heavier crankcase scantlings, raises the cost per horsepower and the
weight-power ratio. .
Whilst, therefore, the six-cylinder in-line engine may continue indefinitely to meet the needs . of operators who prefer to employ relatively slow-speed engines, its development in naturally aspirated form is limited by the nonacceptability of higher piston speeds or the greater length of a large-bore engine structure. In contrast, the V engine enables the ideal to be more nearly approached by building the engine with box" dimensions which offer the optimum stiffness-dimension ratio. A larger bore and higher r.p.m. can, therefore, he employed 'without increasing the dimensions or cost of the crankcase assembly unduly. • In the typical diesel engine, piston speed is limited to around 2,500 ft. per min. compared with 3,500 ft. per min. allowable in the case of a mien petrol engine, A restriction on rubbing velocity is imposed by the difficulty, of lubricating the piston—in particular the ring pack— and although advances in lubrication technology since the war have enabled higher piston speeds to be used, it is improbable . that further improvements will offer more than a marginal return. The lower allowable piston speed of diesel engines is a function of the heavier
ring pack required to ensure good compression (combined with adequate lubrication of the compression rings) and minimum oil consumption.
It is pertinent to compare certain technical features of the Cummins six-cylinder in-line engines with corresponding features of the 90° V6 and VS, units. A comparison of dimensions will show the installation advantages of the V engine for many types of application.
Taking the case of the Cummins V6200 (200-b.h,p. V6 unit) and.cOrnparing it with the Cummins NHE 195 (in-line six-Cylitider engine developing 195 b.h.p.) the V6 has a bore and stroke Of 5+ in.
and in, respectively, and a capacity of 588 cu. in., whilst the bore, stroke and displacement of the in-line engine arc respectively 5+ in., 6 in: and 743 cu. in. The net weight of the former is 1,588 1h. and that of the in-line engine 2,435 lb., the corresponding lb. per b.h.p. being 7'95 and 12.87. At the governed speed of 2,600 r.p.m. the piston speed of the V engine is about 1,800 ft. per min., whilst the larger-bore in-line engine operates at a piston speed of 1,930 ft. per min. at the rated r.p.m. of 1,950. Outputs per Cu. in, are respectively 0-34 b.h.p. and 0-26 b.h. p.
• V6 Advantages
Proportionate gains offered by the V6 engine comprise respective reductions of 38 per cent and 7-2 per cent in lb. per b.h.p. and piston speed, and an increase of 30 per cent in output per cu. in., and these gains typify the advantages of the naturally aspirated V6 and VS engines in the standard range in comparison with in-line engines of Similar output. The smallest V6 engine, the V6140, develops 140 b.h.p. at 3,300 r.p.m. and the output of a larger V8 unit, the V8265, is
265 b.h.p. at 2,600 r.p.m. Ratings of automotive in-line engines vary between 175 b.h.p. at 2,500 r.p.m. (this unit has a bore of 4T„ in. and stroke of 5 in.) and 380 b.h.p. at 2,300 r.p.m. This higher b.h.p. rating is obtained from the turbocharged version of the 855-cu.-in. engine with a bore of 54 in. and a stroke of 6 in.
Dimensionally the V6200 engine shows to advantage with regard to length over
its in-line counterpart by some 23 in., the overall lengths of the two units being respectively 37 in. and 60 in. The width of the V engine is 32 in., which is about 4 in, greater than that of the in-line unit. Heights are respectively 4021 in. • and
48+ in. ecause of the space available between the two cylinder blocks, V engines are particularly suitable for the application of turbocharging.
When reviewing the claims for a Largebore, high-speed engine (with any configuration of cylinders) it is necessary to consider the effect on combustion of raising the r.p.m. and " flattening " the combustion chamber. Engine speed may, in certain cases, be limited by a reduction in combustion efficiency above a critical r.p.m. and a relatively shallow chamber is not approved by all makers.
Technicians of Cummins Engine Co. state that they have "plenty in hand" with regard to the efficiency of combustion at the higher speeds of the Vengines, and claim that the four-valve cylinder head and centrally placed injector of the Cummins PT (pressure time) injection system nrovide scone for raising the r.p.m. without introducing cornbustion problems. Of related interesf, injectors of the same basic design are fitted to all Cummins engines in the automotive and industrial .ranges, covering outputs of 90 b.h.p. to 700 b.h.p.
The torque of a high-speed engine is inevitably reduced for a given b.h.p. output, and the provision of a well-matched gearbox is, therefore, of importance. Because of the increased speed and "low torque, the size, weight and cost of the gearbox can, however, be reduced.
Criticisms have been made of V6" engines on account of the out-of-balance forces produced, but it is pointed out by Cummins designers that these forces do not impair the mechanical wearing properties of the unit and that the resultant vibration can be accommodated by suitable mountings. • Integral construction, which is considered essential to rigidity, is employed for the cylinder blocks and crankcase of the V6 and V8 engines. Well-proven techniques can be employed in the production of the engine and it is possible to provide ample water passages. A single water pump feeds the two cylinder banks.
P. A. C. BROCKINCTON, A.M.I.MECH.E.