Vee versus in-line
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Will the turbocharged in-line six compete successfully with the V-8 diesel? With improvements in turbochargers the in-line engine possibly has the greater potential.
WHILE IT IS generally assumed in Britain that the naturally-aspirated V-8 diesel will take over from the naturallyaspirated in-line six for applications in which
an output of more than say 200 bhp is required, protagonists of the in-line engine believe that the turbocharged in-line six has a greater potential for powers that will be required in the foreseeable future.
Weighing up the relative merits of the two types of unit could be an important current exercise in the deliberations of a number of engine makers. And the favourable reaction of UK operators to the performance of the Volvo FB88 42-ton-gross outfit ("Futuristic Trunk to Glasgow," COMMERCIAL MOTOR, November 10) has stimulated interest in the in-line engine particularly with regard to its low fuel consumption and noise level.
For given combustion and mechanical efficiencies the output of an engine is a function of combustion pressure and engine rpm. The latter is limited in practice by the requirement to restrict piston speed to a maximum determined in part by piston weight. Improving the output of a naturallyaspirated engine by raising the rpm necessitates reducing the stroke and increasing the bore. But an increase in bore size can only be provided by a corresponding increase in the length of the crankshaft. If the length of the shaft exceeded a critical limit, the dimensions required to give sufficient robustness and torsional rigidity to withstand the stresses of a highly-rated engine would be unacceptable in that they would result in an abnormally heavy and long power unit.
In contrast, the large-bore V8 engine has a relatively short crankshaft and a much higher rpm can be employed without increasing the weight of the structure or creating torsional vibration problems. In many applications its configuration also facilitates installation, and having eight cylinders instead of six its breathing capacity is more favourable at a given speed. The volumetric efficiency of any unit tends to fall off at higher speeds.
Turbocharging remains the only means of raising the output of an in-line unit to give a bhp/litre or bhp/lb. comparable to that provided by a modern naturally-aspirated V8. But turbocharging can be applied to V engines without undue complications (although it is preferable to fit a turbocharger to each bank), so if outputs well in excess of 300 bhp are eventually required the turbocharged V would certainly have an unchallengeable claim to prior consideration as the most suitable type of unit.
An output of 300/320 bhp may well, however, be the maximum horse power that heavy vehicles will require for an indefinite period and whether the turbocharged in-line , six or V8 gains the ascendency will depend on first cost, fuel consumption, weight, combinability with available transmissions, installation advantages and established production facilities.
To what extent does combustion efficiency depend upon stroke/bore ratio? By all accounts the most economical diesel in the world, the Gardner 6LXB, has a ratio of about 1 to 1.26 and produces its maximum output at 1,850 rpm. The ratio of the smaller Perkins 6.354 (which develops its peak horse power at 2,800 rpm) is 1 to 1.29, which is regarded as the near-ideal by the Perkins company. The ratio of the Volvo and Scania Vabis is around 1 to 1.14.
In this country, the Gardner is still regarded by many leading operators as the "boss's engine" and demand for the unit exceeds supply. Gardner units have successfully been turbocharged by operators and it is known that Gardner technicians no longer discredit turbocharging as a future possibility.
The majority of engine makers accept the modern turbocharger as a "suitable tool" with regard to performance, reliability and long life and are interested in the potential of the exhaust-gate turbocharger (the use of which should enable torque back-up to be increased) and of series turbocharging. Acceleration lag has been reduced to minimal proportions by the use of lightweight components and it is probable that the problem will be completely eliminated in the near future.
Saving in fuel
Turbocharging can give a saving in fuel consumption of 10/15 per cent, it facilitates exhaust silencing and apart from a tendency to produce excessive smoke momentarily when the engine is accelerated (which is currently the subject of intensive research) its use reduces smoke density. Moreover, a turbocharged lower-speed engine operates at a lower level of radiated mechanical noise than a higher-speed naturallyaspirated engine as evidenced by Volvo and Scania Vabis units.
Upwards of 75 per cent of heavy commercial vehicles sold in Sweden are equipped with turbocharged engines; it would appear that operators accept a vehicle on its performance merits regardless of whether it is turbocharged or naturally aspirated.
A long stroke is favourable to fuel consumption because it enables a more compact combustion chamber to be used with a lower surface/volume ratio, Other things being equal, less heat is dissipated to the cooling system as a waste product and more heat is converted into useful energy. This is a fundamental parameter; there are many others, including injector spray pattern, volumetric efficiency and swirl rate. Many engine designs represent a compromise in that combustion efficiency is to some extent sacrificed to speed and therefore output. Obviously a big gain would justify a small sacrifice; for example increasing the rpm of an engine from 1,850 to 2,200 with a corresponding improvement in output might incur a minor consumption penalty that would be more than offset by turbocharging.
The makers of V8 units may compromise in the other direction, in that they may use a longer stroke than is compatible with maximum output to improve consumption. Increasing the stroke may be limited by the acceptable spread of the cylinders with regard to installation facility.
In practice, the future of the turbocharged in-line six may be determined by piston design, for example by the size of the gudgeon pin that can be employed and by the loadcarrying capability of the piston structurally. Because the height of an in-line engine is relatively unimportant in terms of installation facility compared with the spread of the cylinders of a V power unit, the designer is given more latitude in the design of the piston and in providing a connecting rod/crank ratio that give an ample bearing surface combined with a relatively low side thrust.
And stricter noise regulations favour the lower-speed turbocharged engine because radiated mechanical noise increases with speed and is difficult to control or to damp. Although structurally compact and robust, the V8 does not provide the journal bearing area that is available with an in-line engine. The crankcase assembly of the majority of in-line engines has an excess stress potential that is more than adequate for relatively high pressure turbocharging. And the increase in peak pressure produced by turbocharging is small compared with the increase in output because it improves combustion.
Moreover, if the starting difficulty with reduced compression ratios were overcome, engine makers would typically reduce compressions by at least two ratios and this would enable outputs to be substantially increased without raising the peak pressure.
Finally, if a substantial number of specialist vehicle builders require a "flat" engine—and chassis/body design developments may foster the use of horizontal power units—the V power unit would not fit in. And this could represent a valuable market.