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TURBO-SUPERCHARGING COMMERCIAL VEHICLES

many users of motor vehicles the thought of super charging conjures up visions of fast sports cars, con suming inordinate quantities of petrol in the search for ever more flashing acceleration and speed. The observant reader of the Technical Press will, however, have noted from time to time in the past few years the advocation of supercharging for quite another reason, also that buses in the Yorkshire town of Halifax have been so equipped, with, we are told, every satisfaction.

The excellent performance of the American Boeing . Flying ForireSs bomber has brought a new development into prominence in the form of the turbo-supercharger, a mechanism in which the energy of the exhaust gases is utilized.

The operation of .this device is of great interest, the turbine being interposed in the exhaust system in such a manner as to make use of the kinetic energy of the gases. The revolving turbine member is connected to a centrifugal blower, which induces air or mixture into the engine under pressure. If fuel-air mixture bp passed through the supercharger a slight advantage results in that, due to the latent heat of evaporation of the fuel, a decrease in temperature occurs, thus, to some extent, offsetting the ill-effects of the increased temperature -due to compression.

Aircraft requirements are, however, considerably different from those of a road' vehicle and it is inecessary that these be understood so that incorrect comparisons will not be drawn.

The higher the altitude the greater the decrease in the density of the atmosphere, and as the power of an engine is largely dependent on the amount of oxygen that can be inspired, the power output must consequently diminish. The remedy, of course, is to fit some form of booster so that the induction air is compressed to something like the atmospheric pressure which obtains at ground level. This was the original procedure, but the difficulty was that, at low altitudes, when little supercharge was required the blower would still deliver its full output. The cure was to close the throttle in stages as altitude was lost, although the pilot was still able to operate the throttle between the closed and limited opening positions.

A Seemingly ideal Principle

Research in France and America, however, brought to light the exhaust turbo-blower, which appeared to be the answer to all problerns, for although the exhaust pressure would not vary with altitude (if induction pressure be retained), the external pressure drops. Consequently, the pressure difference across the turbine increased with altitude as did its rate of rotation. As the turbine is connected to the centrifugal blower, the supercharge pressure increases with altitude. This, on the face of things, seems ideal, but it may be significant that neither in this country nor in Germany is it used.

From the many patents filed for cooling systems for the aircraft turbine, it would appear that, even to-day, complete satisfaction is Still not achieved, and to those with the motor vehicle in mind it should be remembered that probably a 300 or 400 m.p.h. air stream may be available in the case of aircraft.

As turbines are kinetic-energy machines they must run at high speed; in aircraft practice up to 30,000 r.p.m. is employed, and if such a unit be considered for use on a road vehicle this high revolution unit would have to be tolerated if the necessary compact size is to be Achieved. This, at once, suggests servicing difficulties.

Unless complicated and expensive reduction gearing be employed, a positive displacement blower would not be permissible as -this type operates at apprOximately engine speed, and therefore a centrifugal type is necessary. For normal engines this latter form of blower could not be employed for it asserts itself only when a considerable rotational speed has been achieved, but this difficulty is not so marked on the turbo-driven machine for, even at Low engine speeds, it Still does useful work.

This high rotational velocity may well be the reason for' the lack of popularity of the turbo system, because owing to the inertia of the revolving parts and the high speed necessary, some delay must be expected before the blower reaches the appropriate speed of the engine.

In the case of a stationary unit four seconds were required at starting before the blower had attained the required speed. The engine would have heavy flywheels and other heavymoving parts and, consequently, poor acceleration potentialities. How much worse would it be on a road-vehicle engine where flexibility of performance is essential?

Centrifugal blowers of the engine-driven type present difficulties in that the high speed of rotation makes them prevalent to driving-shaft breakages, unlesS reinforced couplings be used, but this is not the case with the turbine drive, for the column of exhaust gas has a definite cushioning effect. , Modern practice is to make use of the energy of the exhaust gases, to extract those of a following cycle by carefully designed manifolding. This is difficult to achieve when a restriction, in the form of a turbine, is interposed, whilst 'back pressure, usually of about 3 lb, per sq. in., is caused in the exhaust manifold. This back pressure reacts unfavourably ,on engine performance and the turbine must, obviously,. cancel out this loss before beginning to show an overall improvement.

Difficulties of Cooling

As already stated,, cooling is a •problem fraught with difficulties and, apart from mechanical considerations, it is important that radiated and conducted heat be not transferred to the in going air, for this would result in a*

rarefied charge and, therefore, reduced efficiency. The bearings will require a copious flow of lubricant for cooling . purposes, and the lubricating system must be, at least, as foolproof as that supplying the engine, because a seisure, due to oil breakdown, may. have disastrous results.

One great' advantage of the exhaust-driven turbine is that the position,of the unit is independent of the need for a drive. A drive to any extra component can cause surprising difficulties, 6/en in the design stage.

As with normal supercharging systems on petrol engines, the charge may be induced in two ways—one by' blowing into the carburetter and, two, by inhaling from the carburetter and blowing into the engine. In the latter case, the cooling effect of the fuel charge is not so marked, and in the former system balance pipes, leading from the blower to the carburette; float chamber and petrol tank, are necessary in order te equalize the pre.,sure.

Lliscussion, so far, has been confined to theoretical aspects of the case, but practical test is, of' course, the final criterion. Unfortungtely, actual figures are not readily obtainable,, although, no doubt, many undisclosed experiments have taken place. We have, however, the word of a representative of one of our best-known commercialvehicle chassis builders, who, at a meeting of the Institution of Automobile Engineers, said that his concern had, without unqualified success, conducted considerable research into the matter: One set of test figures, published by a Continental' company, showed that an engine fitted with a turbo-supercharger gave an•optimurn fuel consumption of .35 lb. per b.h.p. hour, and the turbine speed varied between 5,000 and 1,400 r.p.m. This showed a saving, compared with the .375 lb. per b.h.p. of a mechanically driven supercharged engine. The power increase, due to supercharging, was 50 per cent, and the weight increase 10 per cent. Unfortunately, no figure is ayailable for this engine fitted with a mechanically driven blower. It may be that a performance equal to that of the turbo-driven machine would result, although this is, -of course, mere speculation. In genera!, however, the addition of a mechanically driven blower causes a slight reduction in mechanical efficiency, owing to the power loss in the supercharger .drive.

Excess Air Supply

Some part of the increased therrilal efficiency is due to the turbine system only indirectly, because, in order to maintain the temperature of the mechanism within operating limits, a greater amount of air than is required for combustion is admitted by allowing a greater overlap than is normally used, so that the excess passes into one port and directly out of the other.

Consequently, the exhaust valve and cylinder head are effectively cooled and the boundary layer of stagnant fuel vapour, which tends to ding to the cool walls and thus dilute the lubricating oil, is swept away. This method is, of course, totally impracticable for engines other than those employing fuel injection, because, for obvious reasons, it would be impossible to allow unburnt fuel to pass into the exhaust manifold. We cannot, therefore, anticipate so marked an improvement from a ,petrol engine as item an oil engine.

As a normal petrol engine has a lower thermal efficiency than has an eil engine, it follows that the exhaust temperature must be higher. As the turbine imposes a back pressure, the temperature will be increased still further, with a consequent increased exhaust-valve temperature. As the operator will require some return in the form of increased efficiency, for his outlay on a supercharger, the problem of finance will have to be studied carefully, but I will make au attempt to analyse the position. In this matter present examples are of little use, for they are little better than experimental models.

We will conSider a unit of 140 b.h.p. having the fuelconsumption characteristics of an engine burning _fuel, at 8d. per gallon, of a specific gravity of .8. Fuel saved per hour = 100 x .01 -= 1 lb„ as 1 gallon of fuel weighs 10 x .8 Ib. fib. — ,12 gallon, If the average 10 x .8

speed of a vehicle be 10 m.p.h., then .12 gallon .sill be saved everY 10 miles, and, at 8d. per gallon, this equals id. per 10 miles; therefore, the saving in 100,000 miles will be 10,000d., or L41The figure which has been taken for thermal efficiency will, at best, be very empirical, for this is an optimum result and will vary with speed.

As we are dealing with a small increase in efficiency, it follows that, in order to obtain this, only the finest workmanship is permissible, and, in order to resist the high temperature and rotational speed, high-grade materials must be employed, desiderata which are, obviously, not conducive to low cost. A 'considerable offset would be the reduction in engine size resulting from the increased specific output.

Fully to retain reliability, the crankshaft and big-ends, main bearings, connecting rods, etc., would, in all probability, be irreducible, because if the output torque he unchanged then so would be the loading, so that the engine would require to be designed as a supercharged unit from the beginning.

It may be argued that some existing engines run quite well with the addition of a supercharger, but the answer surely is that, in such cases, the designer has, in the first place, been over-generous with his proportions.

A Price-weight Ratio A rough approximation of the decrease in cost of a supercharged unit may be arrived at by reducing first cost by a Percentage Saving in weight, for motor vehicles follow an approximate law of a price-weight ratio, although this could be considered only if we neglect the cost.of fuel pump and injectors, for they will, obviously, be the same in either case,

It will be appreciated from the foregoing that there is no easy road to obtaining free power from exhaust turbines, and only by careful experiment over a long period can the problems involved be overcome, even if the mechanism be desirable at all.

[In our isae dated July 7, 1939, we dealt with the first Halifax Corporation bus to be fitted with a supercharger. It was an A.E.C. double-decker with a Comet Mark I4

8.8-litre compression-ignition engine. The Centric supercharger, which measured 14 ins, by MI ins, diameter with a bore of 71-k ins., ran at twice engine speed and, driven by four V-belts running on pulleys mounted on the , shaft between the power unit and the Wilson gearbox, gave a pressure of 5 lb. per sq. in. in the engine feed pipe. Some idea of the effect which followed the, fitting of this supercharger may be gathered from the results

obtained. The power output increased from 130 b.h.p. to 170 b.h.p., permitting the raising of the axle ratio from 6.5 to 5.75 to 1 with still a gain in acceleration and hill-clitileing powers.

During the trial which we conducted, the bus, unladen, reached 22 m.p.h. in third gear up a gradient on in 9,

and in second gear the speed was 15 m.p.h. up I in ei with the throttle barely half open. We were informed, at the time, that the double-decker could surmount the latter.gradient in second gear, when fully laden.

Fuel consumption, in terms of m.p.g., rose slightly, but in terms of pts, per b.h.p. the figure was slightly more favourable than without the blower.—En.]