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HE CORNER .
Possibilities of All-hydraulic Transmission, Controlled-slip Differentials, Supercharg!Fig, Fully Automatic Gearboxes and Two-strokes with Petrol injection
By P. A. C. Brockington, A.M.I.Mech.E. THERE are signs of radical changes in design within the next few years. Some of them will be made to fit into the pattern of the increased use of automatic machines in production. Others will be forced by operating considerations.
Although some developments in the car factories _might touch only the fringe of the commercial-vehicle industry, many new ideas may be .put to the test by experiments with commercial vehicles before being adopted for cars.
Other new ideas may be used for lorries and buses and rejected for cars, because of the commercial vehicle's relatively long mileage life and the better return given for a greater monetary Outlay. Taking an example from established practice, a notable case is that of the oil engine, which, contrary to popular opinion, has failed to find favour among motorists mainly because the annual mileage of the majority of cars is too small to offset the higher first cost.
What I, consider to be a negative to catch a positive was the rumour, since dissipated, of an all-hydraulic transmission for the new Ferguson project. A transmission system based on an engine-driven variable-delivery pump feeding wheel motors would be welcomed, particularly by commercial-vehicle manufacturers, if it were efficient and could be produced at a reasonable cost.
Apart from difficulties of housing the wheel motors, it offers scope for saving valuable space and reducing weight. In addition to the probable reduction in the mass of the transmission components, the freedom given to chassis and body designers would enable weight-saving structural B10 modifications to be introduced and the platform or floor height to be lowered. If combined with reverseaction hydraulic braking and with hydraulic suspension of the Citroen type, it would herald a new design era.
Could such a transmission be effi
cient? Judging by basic principles the answer is "no "—fluid friction would absorb too much power—but the only example of the system in this country is said to be performing satisfactorily, and moreover to be doing so with an efficiency of 95 per cent. It is fitted to the experimental tractor developed by the National Institute of Agricultural Engineering.
The tractor incorporates a Vickers variable-output pump, chain driven from the engine and feeding threecylinder piston-type motors mounted on the hubs Which rotate with the wheels. This layout would have to be drastically modified for road vehicles to reduce unsprung weight.
A single lever is the only transmission control required, and the exact speed regulation that it provides is, it is claimed, of great advantage for ploughing. The system would be an even greater boon to commercialvehicle drivers, whether or not it was combined with automatic control.
Despite these promising claims, fluid friction may prevent the allhydraulic system from becoming a
practical proposition. The torque converter was so. promising, so reliable and so popular among drivers, but failed because of a mere 15-per-cent. drop in efficiency. The tractor engine develops some 12 b.h.p. and wheel speed is low. The rate of fluid flow from a 100 b.h.p. engine driving a lorry at 50 m.p.h. would be excessive.
All the Difference The possibility of employing the system to provide four-wheel drive at low cost and only a small sacrifice in weight stimulates thoughts of icy roads and loss of traction on hills. In a conversation with a driver of a. trunk vehicle during January of last year, when many roads were covered with snow, I was told that an extra driving axle at the rear made all the difference between apprehension and confidence when starting out on a long journey.
"Drive on all the wheels would be a big. thing for us fellows in the winter, but I suppose it would cost too much," he said. Much the same observation was made by a civilengineering contractor with regard to tippers on slippery sites.
Although a mechanical all-wheel drive system could never be produced at a price that the great majority of operators would be willing to pay, a much more simple device could be used to increase traction. This is the German ZF controlled-slip differential fitted to many racing cars, which provides a normal action when the difference in wheel speeds is small, but tends to lock the two side shafts together when the difference increases above a certain figure.
The principle is simple; differential action causes plungers to oscillate in their guides and create a frictional resistance to single-wheel slipping, so that free rotation of one wheel at speed is virtually impossible.
Shock Loading Avoided
Tn . practice, total traction is a function of the combined action of the two wheels. It is not limited by the coefficient of friction of the slipping wheel and, apart from improved traction, this obviates the shock loading of a side shaft caused by a wheel gripping suddenly after reaching a high r.p.m. when rotating freely. The wastage of parts that shock loading can cause is illustrated by the report of the civil-engineering contractor that on rough site work in wet weather the average incidence of half-shaft failure was six a day.
A development engineer suggested to me that progress in supercharging would have an influence on the form which future gearboxes would take. In support of his contention he quoted Sir Harry Ricardo's claim that a positive-displacement supercharger could double the low-speed torque of a power unit and that this could be done without modifying the bearings or crankshaft assembly.
Several engine manufacturers are experimenting with the RicardoWel I worth y wobble-plate supercharger fitted to standard engines, and it may soon be possible to compare results, based on extensive operational experience, of the performance of this unit with that of engines equipped with exhaust-driven blowers.
In both cases the value of the supercharger to the engine manufacturer is that it can enable a small number of basic units to provide a wide horse-power range, and therefore simplify production and reduce overall costs, despite the additional cost of the supercharger.
The turboblower has the advantage that its use can reduce specific fuel consumption, and there are indications that maintenance will be simplified. This is a factor of extreme importance and possibly one that will be decisive in the deliberations of the makers with regard to the type that they will adopt.
To the majority of technicians it is a foregone conclusion that superchargers will be the rule rather than the exception in six or seven years' time. In the .words of a designer: "The battle of the blowers is on."
Operators will need a great deal of persuasion that blown engines are reliable and that they will not add materially to maintenance troubres. Nothing short of convincing tests, equivalent to a mileage of more than 150,000, will satisfy them, and the unit will have to be one that can be serviced without special facilities.
The positive-displacement supercharger absorbs power that would otherwise be converted into tractive effort, but it is claimed that the extra consumption when using a Ricardo unit is negligible.
If double the torque is available under operational conditions, this will enable a much smaller unit to be employed in many applications. Alternatively—and this is of possibly greater importance — a simplified transmission system could be used with engines of the same size, providing freedom from gear-changing over a much wider speed range. In practice, this could improve overall efficiency and reduce fuel consumption.
No definite information is yet available regarding the cost of manufacturing the Ricardo supercharger or the smaller types of turlaoblower suitable for vehicle engines. The Ricardo unit is of multi-cylindered type, but the working parts are not Finished to minute limits and the cost of machining should be relatively low.
The turboblower has one main moving component, but its very high rotational speed and the elevated operating temperature of the turbine necessitate the use of high-grade materials and precision finish.
Apart from first cost, the Ricardo unit will probably create fewer maintenance problems and it does not have the disadvantage of acceleration lag, which is a feature of turboblower perforfnance. Although the lag is negligible with some of the latest types of blower, because of the very light weight of the rotating part, t h e necessity of such weight paring might react unfavourably on reliability or maintenance requirements.
Air unusual scheme that has been suggested for supercharger drive is one which would provide an even greater range of speeds without alteration of the gear ratio. If the same epicyclic gear train is employed for both the blower drive and the drive to the rear wheels, an increase in traction load can be made automatically to raise the speed of the blower at a given engine r.p.m.
It is possible that in the reasonably near future, vehicles will have highly supercharged engines with two ratios providing sufficient_torque for normal running conditions.
Intercooling
What about intercooling? To many people the subject is as whimsical as the idea of using an atomic-fired unit in a vehicle, but it deserves brief mention.
Recent developments in the industrial and marine fields have shown that cooling the supercharged air before reaching the cylinder increases the output of the engine by some 30 per cent. without raising the exhaustvalve temperature. Given an ample supply of coolant, the system represents a way of getting something for nothing with a slight improvement in fuel consumption. Before it can be applied to road vehicles, a more efficient form of intercooler will, however, have to be developed.
Nevertheless, the system cannot be relegated to the status of the com pletely impracticable, and if flights of technical fancy can be allowed, it might be described as the means by which the oil engine could be employed to act as a pump to a gas • turbine. Brake mean effective pressures of 200 p.s.i. and upwards are envisaged and the power pack would be far more efficient than that of the burner-energized turbine and might show to advantage in comparison with the " straight " oil engine.
The future of the fully automatic gearbox as a component of a conventional commercial-vehicle transmission system probably depends upon its general development for private cars. Car makers have a greater incentive than commercial vehicle manufacturers to produce such a unit. After its adoption for cars, hauliers and bus operators would benefit from the more extensive knowledge of maintenance that repairers and garages would have obtained.
If it were proved, however, that fuel consumption could be reduced by fitting a fully automatic gearbox, its application on a large . scale to commercial vehicles might well precede its general use for cars. Even the most experienced drivers do not change gear under all conditions to conform with the ideal technique for optimum fuel economy.
A perfected automatic system could be pre-set to provide this advantage and a higher first cost would then be acceptable. Such a transmission would, of necessity, have to provide a positive drive without fluid losses.
Concurrent with the noisy outbreak of the Ferguson rumours it was whispered that a people's car was being developed in this country powered by a two-stroke engine with direct petrol injection.
To the comparatively few commercial-vehicle operators who were vouchsafed the information it had added interest because of the outstanding fuel consumption obtained during a road test by The Commercial Motor of a German Borgward Microbus fitted with such a power unit.
Of less than 700 c.c., the Borgward engine develops about 30 b.h.p., and the fuel-consumption rate'at a steady speed in excess of 30 mph., with the vehicle unladen, is 87 m.p.g.
Direct injection of the petrol practically eliminates wastage of live gas, because pure air is largely employed for scavenging. The erratic .supply of mixture from a carburetter is replaced by exact metering of the fuel and this gives smooth running and improved torque.
Specific consumption compares favourably with that of many fourstroke petrol engines and the unit would have many advantages for vehicles in the light-delivery class on the grounds of performance, economy and savings in weight and space.
Petrol v. Oil
Although the use of the oil engine is progressively extending downwards to the ranks of lightweight vehicles, the popularity, of the petrol engine would be greatly enhanced by an improvement in consumption of the order of 20 per cent. Many users would then prefer it to the heavier and noisier oil engine. The reversion to petrol by some coach operators shows that, even without such an improvement, the frequent replacement of vehicles to satisfy a demand for modernity favours the petrol unit.
The possibility of employing direct petrol injection in conjunction With a modified cylinder head or piston, which would enable a high compression ratio to be maintained at low loads, is not beyond the bounds of possibility. Experiments by the British Internal Combustion Engine Research Association indicate how it could be done, and if it were commercially practicable, a consumption improvement of over 50 per cent. at reduced loads should be reasonably possible.