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FUTURE TRENDS IN POWER UNITS

IT is somewhat presumptuous to attempt a paper of this nature at a time when considerable changes in vehicle design are impending, when a number of possible power units of quite widely differing types are in or entering the development stage, and when it is not quite clear how the very severe traffic problems of the future are to be solved. Nevertheless, the main requirements which the vehicle will demand of the power unit are reasonably predictable and the technical characteristics and economics of possible power units are sufficiently well known to allow at least general discussion and prediction.

In the past, operating conditions, road conditions and living conditions have led to quite wide differences in vehicle and engine types and characteristics in different parts of the world. In recent years, however, the automotive industry has become completely international in nature and outlook and this, together with improved roads and improved standards of living, have meant that we have been moving quite rapidly towards common basic requirements in truck and engine design, and this will continue even further.

The main factors influencing forward design requirements of the vehicle power unit in a general sense are improved road systems and higher vehicle weights. Both these are creating a demand for units of higher power output, for which the vehicle designer is prepared to allow less and less space. The natural pressure which already exists for improved efficiency will be emphasized greatly by the requirement of increased performance, as operators will be anxious to offset the cost of the latter in any way possible. The need for greater efficiency is likely to lead to the greater use of specialized units with a performance closely matched to a given type of operation. Some evidence of this is already to be seen in the United States in the great number of options which manufacturers offer in engines, transmissions and equipment, and the care with which individual operators establish detailed specifications for their vehicles.

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It is not within the scope of this paper to examine in detail the changes which are likely in vehicle design over the next 10 years, although there must be some interaction between vehicle and engine design. it is reasonable, however, to assume that the vehicle designer will always be prepared to provide for a suitable power unit in the best possible way and that vehicle design will not inhibit engine development.

Neither is it within the scope of the paper to examine the pattern of development of possible fuels, although this could affect engine design in a major way. Again the assumption is made that fuel development will not inhibit engine design, although equally we cannot assume that engine design will be promoted by fuel developments in the way improved fuels encouraged the development of the gasoline engine in the past.

Until atomic energy can be provided in economic small mobile units, there will be increasing pressure to conserve natural petroleum resources and to minimize the increase in consumption by the maximum efficiency of use and minimum wastage in processing. We cannot be certain yet what the required characteristics of fuels will be in the future, but direct engine fuel economy and the simplification, rather than the elaboration, of fuel specifications must be kept very much to the fore in engine development programmes. At the same time, the petroleum industry must play its part in ensuring that the fuel factors which are critical in the various types of engine are known and made available in the most economic way.

Much has already been written regarding the trends of development of the existing diesel engine over the next few years' 2.3:4* and it is only intended to refer briefly to the immediate future developments. It is more important now to examine the characteristic of possible competitors to the diesel engine to see where it will retain its present dominance, what long-term development is necessary, and

what other power units will displace it in the course of time.

In the immediate future the most important development is the introduction of a number of V-6 and V-8 engines. Among the V-6s, engines with both 900 and 60° V angles have been announced. The 60° V-6 engine is attractive, but the need for individual crankpins for each bank of cylinders and for a separate balancer unit loses much of the advantage in length, weight and cost which the sixcylinder has over the corresponding eight. The 90° V-6 is easier in these respects, but is very difficult to mount satisfactorily in a chassis because of its out-of-balance characteristics, and demands a drive train with a torque capacity substantially higher than the nominal output torque of the engine, because of the bad output torque c.haraeteristic. Below 150 b.h.p. the V-6 engine shows no advantage over the in-line six, except in engine length', and above 150 b.h.p. little real advantage over a V-8. It is felt hat the V-6 engine is not likely to establish a stronghold 311 its market, although the 60' V-6 could well be successful or a period.

The V-8, on the other hand, has many advantages, )articularly in easing the installation problems of higher)ower engines without increasing engine compartment tizes, and is likely to become the standard engine for lowers above 150 b.h.p Below this the existing and well:stablished in-line six-cylinder engines are likely to :ontinue, although increasing use is likely to be made of hem in their horizontal or nearly horizontal forms.

The advantage which the V formation has in respect of :ngine length, together with the disadvantage which it has n width, has led to a move toward oversquare engines of tery short stroke. Although the increased bore gives some )enalty in engine length, this can generally be accommofated comparatively easily in view of the short overall ength; the shorter stroke allows an appreciable reduction n engine width and height, which simplifies installation woblems.

These short-stroke engines permit operation at higher 'otational speeds, which allows greater power to be )btained from a given cubic capacity, and reduces still Urther the specific size of the engine in h.p. per cubic ft.' nthe specific weight in h.p. per lb.

This is not obtained without penalty however, and nevitably fuel consumption and maximum torque suffer ny comparison with the more conventional longer-stroke 'ngine. The combustion engineers responsible for the vork on these new engines have done a remarkable job in .chieving the fuel-consumption figures which have been eported, but these are not yet down to the level achieved .s routine by other engines. Also, the lower maximum orque for a given maximum power output impairs .ppreciably the gradability of the vehicle, and this can ffect the gearbox specification by calling for a greater lumber of ratios. It is important to remember that the liesel has reached the dominant position which it has in ommercial transport because of the operating economy it llows, and it is wrong to sacrifice this economy, both in Drms of direct fuel consumption or in overall costs, for he apparent attraction of other features.

The application of supercharging, particularly by the arbo-supercharger. is on the development programme of nost manufacturers of current diesel engines. After some *jai difficulty a number of highly successful turbocharged iesel engines have appeared on the market. It is noticeble, however, that these have all been of the relatively )w-speed heavy-duty types, and that the manufacturers of le high-speed lightweight engines have been uniformly nsuccessful in producing a good turbocharged unit based

on their existing normally aspirated engines. It is now well realized that the successful low-cost lightweight engines are not built with the factors of safety necessary to carry the additional loads imposed by turbocharging.

This has, however. merely caused a temporary hold-up in the wider application of the turbocharger and has not stopped it altogether. Now that the problems involved are better understood, we can expect to see turbocharged engines appear on the market which have been designed and developed specifically as turbocharged units and which would not be economic to market in their normally aspirated form. The need to obtain increased power from a given engine size is quite strong enough to maintain interest in this.

The work carried out by M.A.N., in Germany, on tuned manifold systems is very interesting and attractive, and deserves more attention. It is not easy to apply these principles to an engine with a very wide speed range and still obtain a satisfactory torque curve, and the engine is more difficult to apply to different installations. However, it is still true that the four-stroke diesel engine has lagged behind its larger two-stroke counterpart and the petrol engine in taking advantage of pulse effects in the manifolds.

The other two aspects of development of the present diesel engine which will require major attention from manufacturers within the next few years are smoke and noise. Legislation already exists in a number of countries covering one or other of these features, and as time goes on we will see standards of increasing stringency being applied. Engine manufacturers can only support these movements, but it is hoped that legislation badly based technically, such as that on smoke in France, will be removed and that they will be given internationally accepted standards to work with. This hope, however, may be too optimistic.

There is no doubt that the effective immediate approach to reducing the average level of exhaust smoke in any country is by improving the standard of maintenance in the held Operators. however, can rightly expect support from the manufacturers in providing engines which are less sensitive to changes in condition of injectors, fuel pumps, valves, and other components which may affect the situation. Some reduction in engine ratings may be necessary, and it may not be advisable to take as much advantage from turbo-blowing or manifold tuning as might be possible.

There seems little prospect of getting a substantial reduction in noise level of an engine from combustion system and component development only, and if the noise levels which are being discussed for future application are to be met much will have to be done to contain the basic engine noise within the engine or within the installation. Both these approaches have been shown possible. Carrying out the silencing completely on the engine itself, however, will necessitate a cost increase on the engine of something over 20 per cent, and it may therefore be more economic to approach the problem partly on the engine and partly on the installation. It should be possible to restrict the sound radiated from an engine, particularly in the forward direction. with only relatively slight increase in cost for special covers or reinforcement, leaving a further reduction to be obtained by better sound insulation of the installation as a whole. Whatever approach is used, however, both vehicle manufacturers and operators must recognize that costs will go up.

The development of the present diesel engine is thus far from static, and there is still room left for considerable advances. THE petrol engine still dominates the smaller sizes of commercial vehicle, .although the diesel enaine is still ipreading its field or application. In the U.K. and Europe the increased availability of small diesel engines in the 40-70 b.h.p. range has led to a diesel penetration of up to 20 per cent in the 15-20-cwt. size of vehicle. In the U.S., the diesel engine now has over 12 per cent of the market in the 20.000-30,000-lb. G VW class, and successful experiments in smaller vehicles indicates that the diesel is likely to take a hold on the I0,000-lb. (VW vehicle class in substantial quantities. There are signs, however, that the petrol engine will make some bid to. hold its position.

' The petrol engine has made unspectacular, but nevertheless substantial, improvements in power output, and particularly in fuel economy, in the past two decades. Part of this improvement has been due to the availability of better fuels allowing higher compression ratios; but it is believed that we are approaching the limit of economic fuel quality and we are unlikely to see conventional petrol engines with compression ratios higher than 10 or 11 I in the commercial field.

A considerable contribution to the improvements, however, has been due to better ,carburation, manifolding and combustion chambers. Petrol injection has appeared on high quality and high performance private cars, and it is a little surprising that it has not yet appeared in the commercial field. While the improvement in fuel economy with injection rarely reaches 10 per cent, except under stop/start driving conditions, quite a reasonable pay-off time can be shown for the additional cost of the equipment. The advent of cheaper fuel injection systems may excite more interest; but there is little doubt that these have suffered from the too' extravagant claims made for their advantages. Petrol injection does not give a great advantage in economy, except when it is used to improve an engine which is suffering from bad distribution—something which can usually be done more cheaply by conventional methods.

Modern combustion systems and manifolding have allowed the present production petrol engine to achieve specific fuel consumptions down to 0.45 lb./b.h.p./hr. at full load, and it is thought that figures of the order of 0-42 lb./ b.h.p./hr. are possible. Considerable advances have also been made in economy at part load where figures for one quarter load of 0.63 to 0.65 lb./b.h.p./hr. have been achieved, and 0:58 lb./b.h.p./hr. may well be reached. These figures are making it increasingly difficult to justify the use of the diesel engine in, for instance, light van operation, except under very intensive stop/start conditions or where extended idling periods are experienced. The diesel engine still does have an advantage, as corresponding figures on an equivalent small high speed diesel engine (even with the necessary indirect combustion system) are 0-405 lb./b.h.p./hr. for full load and 0.5151b./b.h.p./hr. for quarter load, but the improvements in the petrol engine are raising the annual mileage necessary for the diesel to pay-off its cost, it is hoped in certain quarters that further improvements in fuel consumption on petrol engines can be achieved by the development of stratified charge combustion systems. Two different types of these are shown in Fig. 1, the object of both being to provide a local charge of ignitable mixture in the vicinity of the sparking plug instead of the homogeneous mixture throughout the combustion chamber which is necessary on the conventional petrol engine. Economy is thus not limited by the weak limit of combustion.

The problems in maintaining the correct combustion conditions over a wide range of speeds and loads are severe in such engines, and it has been difficult to achieve the

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leoretical advantages. The engines are inevitably more omplicated, either by the necessity for a fuel injection ystern, or for an ante-chamber with additional valve gear. tt the moment, the improvement in fuel consumption which an be obtained does not justify the additional expense and omplication involved; but the theoretical advantages hown in Fig, 2 are such that additional development may ring this engine into the field as a competitor to the smallSt sizes of diesel. Because of the nature of the combuson system, however, there will always be a risk from an ngine of this nature of increased atmospheric pollution. 'his may become a major disadvantage.

The stratified charge principle can also be applied to two kroke engines where an engine of high specific output is .quired, as this will overcome the disadvantage of many f the present two-stroke engines, namely bad fuel conamption. It is, however, not expected that these will be ,rious contenders in the commercial vehicle field.

totary Engines ? OTARY engines have always been a field for the inventor, and since publicity was given to the developlents of Dr. Wankel and N.S.U., a vast variety of rotary ngines have been put forward. Other than the Wankel, nly Renault and Marshall engines (of those which have een disclosed) show any prospect of being practical operatig units. Neither of these two engines, which are essenally the same, are as attractive as the Wankel unit, although ley do avoid certain of the difficulties inherent in the latter. he Renault engine does not offer the cost or size advaniges of the Wankel, and in these respects in fact will differ ttle from present conventional units. The Marshall engine as problems in arranging adequate porting, and is likely be more bulky than the Wankel. All these engines have le problem of maintaining adequate sealing.

The prime advantages of the Wankel engine over the onventional four-stroke petrol engine are compactness nd low weight, smoothness and simplicity. It suffers from le disadvantages of a bad combustion chamber shape, eavy loadings of the crankshaft and bearings, and difficulty maintaining a seal on the rotor over extended periods. A very large effort, however, has been devoted to the 31ving of these problems in a number of countries, and le development has proceeded sufficiently far to allow a umber of companies to make preparations for its producon. The characteristics of a representative N.S.U. Wankel ngine are shown in Fig. 3. The remarkably good fuel 3nsumption figure and relatively low speed for maximum 3rque indicate the very considerable advance which has een made on the sealing efficiency. It is difficult to be stegoric on the question of engine life at this stage before large number of production units have been tested under variety of operating conditions. Nevertheless, it is known tat the life of the unit is already adequate for very many pplications, although it is thought not yet to be ready for se in commercial vehicles. Now that the sealing problem better understood, there seems to be no reason why conderable further improvements in life cannot be obtained, nd there is in any case the compensating factor that the )tor seals which limit the engine life can in fact be changed a very short time indeed.

The comparison of size, weight and cost of the Wankel sgine with the conventional engines is not straightforward ad depends very much on the size of the engine being 3nsidered. At 40-50 b.h.p. the Wankel is about the same ze as a high output two-stroke engine and its advantages tcrease with size. By comparison with the conventional nir-stroke petrol engine, again at •about the 50 h.p. level, present data indicate that the Wankel can be expected to give a reduction in specific volume, weight and cost of the bare engine in the region of 21 per cent, 47 per cent and 46 per cent. The Wankel is, however, penalized more than the conventional engine by the nature of the accessories and gearbox, and when these are included the advantages fall to 4 per cent, 18 per cent and 18 per cent respectively.

At this stage these figures can only be based on broad assessments, particularly in relation to cost. Much work remains to be done on the production development of this engine_ Its basic simplicity is a major advantage, and although the machining problems are different to those of the conventional engine there is little inherently difficult in them. It is an interesting thought that, with the advanced use of die-casting, adhesives and automatic. assembly, we might even achieve the throw-away engine.

The first applications of the petrol Wankel engine will be those which can take advantage of its smoothness and small size. It is not likely to be used in large numbers in the commercial vehicle field for a considerable period, if ever, because of its greater fuel consumption; although there are a number of interesting snecialized applications such as special refrigerator units, if the cost can be brought down to the level expected.

The diesel version of the Wankel engine magnifies the problems of sealing and of high loadings substantially, and is a much more difficult proposition than the petrol version. The bad shape of the combustion chamber may not be a problem for adequate fuel mixing, as advantage may well be taken of the regular air pattern which exists; but it certainly will be a problem in maintaining adequate combustion temperatures over a wide enough operating range. The influence of compression ratio on engine size makes the diesel Wankel engine of a given capacity substantially larger than the petrol unit. Thus, by comparison with the smaller sizes of diesel engine, where the standards of life and fuel consumption are lower than those of larger diesel engines, the diesel Wankel is likely to show little advantage in weight or cost. In the larger sizes where the advantages of size, weight and cost could be quite substantial, the conventional unit holds its market because of its long life and good economy, which the diesel Wankel is unlikely to equal.

From this one must not rule out the diesel Wankel as a possible power unit, as there are many existing diesel applications where its advantages could be made use of; but these are not in the commercial vehicle field, where the diesel Wankel is unlikely to make much headway against the development of present and future power units, Fuel Cells THE fuel cell is to this generation what the gas turbine was to the last, and is likely to be equally disappointing. The attraction however, of producing electricity directly from liquid fuel without an intermediate mechanical stage, and of avoiding the limitations of Carnot is enormous. Military and space research will be devoting increasing attention to the fuel cell, so that comments on its applicability to the commercial vehicle field are necessary.

In this field the main attraction will be maximum overall efficiency of between 50 and 60 per cent, and for certain types of operation increasing efficiency at light load as contrasted with the decreasing efficiency of present power units.

Two types of basic fuel cell are being worked on, with a number of variants of each type. The low temperature fuel cell operates at normal ambient temperatures or only slightly above, using oxygen and hydrogen or other rela lively expensive and pure fuels. The high temperature fuel cell operates at substantially higher temperatures up to 800°C. and uses air together with commercially available hydrocarbon fuels, one of the most• common used so far being methanol. Of the two types, the low temperature cell so far has been the more successful in practical operation.

To be successful in widespread commercial use in a mobile application a fuel cell must use air as an oxidant to avoid having to carry around a separate supply of this. Its fuel must be a naturally occurring hydrocarbon, preferably liquid rather than gaseous, or at least something which can be derived cheaply from naturally occurring hydrocarbons. It must also operate at a temperature level which can be handled without too much special installation and protection, and which can be achieved in a practical start-up time.

It must also be at least reasonably competitive with other forms of power unit in terms of specific weight, specific size and cost. While the elect ro-chemical problems themselves are far from being solved, it is in these other respects that the fuel cell shows up so badly by comparison with other possible power units that one is forced to wonder whether it is likely to have any potential at all.

The electric motors and switchgear necessary for control must be taken into account in assessing these criteria, and by present standards these alone without their power supply do not compare favourably with existing units. However, it must be remembered that we go to consider able pains to dispose of the waste heat from the existing internal combustion engine, and perhaps we should examine the possibility of doing this on the same scale on traction motors and take advantage of the lower heat rejection from the fuel cell itself.

There is little prospect of the fuel cell entering the commercial vehicle field as a serious competitor to other power units within the next 10 years. However, the potential is there and beyond this period we are likely indeed to see the fuel cell developed as a practical mobile power unit, and particularly in smaller sizes. Its progress will undoubtedly be speeded when the programme becomes more one of engineering development than of scientific research.

Gas Turbines THE gas turbine from its start has appeared as an attrac

tive power unit offering small size and low weight with long life and low maintenance costs. It has the major technical attraction of a very advantageous output torque curve, which would allow the simplification of vehicle transmissions.

Its main technical disadvantages is a fuel consumption which is generally poorer than that achieved by the diesel engine, at least at part load if not over the full load range. This characteristic rules out immediately the possible use in commercial vehicles of the simple cycle gas turbine with the separate power turbine. The first requirement of a vehicle gas turbine thus is a heat exchanger to return the exhaust heat which would otherwise be lost back to the intake air, and so increase efficiency (Fig. 4). Two types of these heat exchangers have been used, the first being the recuperative type or static unit with the heat transfer through metal wails in the form of tubes or plates. This is bulky, has limitations in the 'efficiency of heat transfer which can be achieved, particularly over a wide range of operating conditions, and can be sensitive to fouling.

The second type is the regenerator in which a matrix of heat absorbing elements passes continuously from the exhaust to the inlet carrying heat from one to the other. Get? his latter type is now generally preferred to the recuperar, as its advantages in lower bulk and higher efficiency 7ercome the disadvantages of gas losses from the inlet to e exhaust and the need for a slow speed drive for the atrix drum. The major difficulty in its development has :en the provision of efficient sealing over extended periods ;tweet' the cold and hot gas zones; but it is believed that is problem has now been generally overcome.

The Chrysler Corporation have applied the regenerative pe of heat exchanger to the simple cycle gas turbine with parate power turbine with some success. They have a imber of passenger cars in service for customers evaluam, fitted with a 140 h.p. gas turbine, which are achieving reasonable fuel consumption.

Ford and General Motors on the other hand have gone more complicated units with better output characteristics and particularly with better fuel consumptions, Ford are working on a two-stage engine which has reheat between the stages, in addition to recuperators (Fig. 5). General Motors have announced a turbine consisting of a simple cycle with regenerator but with a variable coupling between the separate power turbine and the compressor turbine.

This last approach appears a promising one and some of the many ways of linking the output and compressor shafts may well emerge as desirable in vehicle turbines. One such is shown in Fig. 6 where a differential arrangement is used. This improves the acceleration characteristics of the engine and allows for engine braking, which is absent with the simple power turbine.

-While there are a number of promising lines of development to improve its efficiency, it will inevitably suffer from a limitation in top temperature and therefore in efficiency. This situation can be improved substantially, although not reversed, with the greater use of cooled components at the expense of greater complication.

The problem of stopping the gas turbine-driven vehicle is also one of considerable magnitude, which is frequently overlooked. The unit can drive a vehicle an appreciable distance after the main fuel is turned off, and the situation is made worse by the complete lack of engine braking where a simple free power turbine is used. The approach taken by Chrysler is to fit variable power turbine guide • vanes which can be reversed so that the gases brake the turbine rather in the way that reverse thrust is used in the aircraft turbine. This, or some other similar type of control, will be necessary on all practical automotive units, in which the free power turbine is used.

The outcome of whatever line of development is followed is to increase the complexity and therefore the cost of the practical vehicle gas turbine. The materials used in the engines also are inevitably more costly than those which are satisfactory in the conventional piston engines. The advantage of intermittent combustion in allowing the use of lower grade materials, while not limiting severely the maximum temperatures, is frequently not recognized. Much work is being done and still remains to be done on the development of cheaper materials for high temperature operation. The Chrysler Corporation, who have made a major contribution in the gas turbine field, claim already to have made substantial advances here.

The other factor, so far not completely known, is the effect of building gas turbines in automotive quantities on their cost, and production development will undoubtedly lead to further reductions in first cost. Nevertheless, taking all these into account, it is unlikely that the gas turbine can ever come down to the costs achieved by present conventional engines in sizes below about 300 h.p. and at the same time give operating fuel consumptions which are competitive.

However, we cannot rule out the gas turbine as a contender for a place in the commercial vehicle field. As truck sizes and performance increase, and engines of 400 h.p. and above become necessary, the field will open for gas turbine applications.

Diesel Compound Engine IT IS possible to overcome some of the basic cycle limita tions of the gas turbine by substituting a diesel engine for the combustion chamber and turbine which drives the compressor of the gas turbine (i.e., by using a supercharged diesel engine as a gas producer, and retaining the free power turbine). This gives a considerable improvement in efficiency over the gas turbine, particularly at part load conditions, although at a sacrifice of weight and bulk. The good torque characteristics of the gas turbine is retained; but the efficiency is not as good as that of the diesel engine as it is difficult to maintain good matching of the turbine over the speed range, even using variable turbine nozzles.

It is now believed that it is better to overcome this basic difficulty by gearing the power turbine back to the diesel engine crankshaft, as in Fig. 7. Although this loses the advantage of an improved torque curve, it allows a very great improvement in specific power and weight while retaining an efficiency which can be better even than the straightforward diesel engine.

The two forms of compound engine mentioned above give two particular combinations of characteristics: but a considerable number of forms of compound engines are possible, When used as part of a compound engine, the considerations controlling the choice of the diesel engine are different from those if it is to be used directly, the major point being the possibility of a revival of the twostroke engine (which even two-stroke enthusiasts must admit is disappearing from use in its direct form). The basic limitations of the two-stroke are heat flow and bad air utilization: but the compound cycle makes it possible to use excess air for cooling and scavenging under much less wasteful conditions than in the non-compound engine, as much of the energy can be recovered in the turbine. The two-stroke engine, particularly in the opposed piston form, is likely to be used in compound engines, particularly in those applications where emphasis must be placed on size, weight and fuel consumption, even at the expense of cost and torque characteristic. One such engine proposed by Witzky, in the U.S.A., is shown in Fig. 8.

As with the gas turbine, various forms of coupling of the power turbine to the diesel engine crankshaft are likely to be used in order to give engines of different characteristics. If, for instance, variable drive ratios can be used between the turbine and the diesel crankshafts, the range of efficient operation can be extended considerably without too great a sacrifice of the improved torque characteristics obtainable with the free power turbine. One of the most attractive possibilities here is the compound engine incorporating a differential gear, the three elements of which couple the compressor, turbine and diesel engine, such as have been proposed by Dr. F. J. Wallace (Fig. 9) (6).

One other development which will certainly be finked with the compound engine is the use of charge air cooling, the benefits from which amply repay the additional cost and complexity involved. The extent of the use of this in vehicles will be dependent on the parallel development of inexpensive air-to-air coolers and it is hoped that the initial steps which some companies are taking will be continued.

During the next decade the development of the present conventional diesel engines will undoubtedly move in th direction of the compound engine. The main incentive fc this will be improved fuel consumptions; but the bigge advantages will actually be obtained in major reductions i size and weight. If the market is content with torque outpt characteristics of the present form, then the size and weigt advantages will be great. However, it is felt that th importance of obtaining a torque output characteristic whic: is more suitable for driving a vehicle is such that there wi: be some sacrifice of these possible power and weight advar tages in favour of the better torque characteristics, aLthoug even with this sacrifice they should still be an improvernen over present standards.

As with the gas turbine, it is inevitable that there will bi some cost penalty because of the additional complication of the engines. The reduction in size, however, togethe r,Vith the much lower usage of expensive material, make the production cost of the compound diesel engine substan flatly better than that of the gas turbine. The increase, cost will be compensated for by the better fuel consumptiO but will impose some restriction in the, use of the corn pound engines to the higher h.p. ranges and intensive usage

Differenlial.Diesel Engine THE differential principle which has been mentioned.

connection with both the gas turbine and the dies( compound engine can also be applied directly to presen diesel engines with a mechanical supercharger, as shown i Fig. 10. This arrangement is such that the degree c supercharge of the diesel engine is determined not by th engine or vehicle speed but by the torque requirement c the vehicle; it thus gives a good approximation to th constant h.p. engine for which so many have been searchin

for so long. It has an additional advantage that th differential gear can be arranged to give an overdrive effect thus .allowing a wider range of output shaft speeds tha engine speeds, the overdrive effect increasing as the loa diminishes.

With this arrangement an output torque ratio of we over 2: 1 can be obtained over the operating speed rang If this is allied to a torque converter designed for a hig stall ratio, an output torque range of over 7 : 1 can b obtained, which is sufficient for many vehicle operations. Si far the development of this system has been directed towards increasing the engine power at both maximum and minimum r.p.m. in order to give a better balance of truck performance between good top speed and good gradeability, and the possible maximum torque ratio obtainable from the engine alone is not fully explored. Nevertheless, as shown in Fig. 11, with only two forward speeds such an arrangement can give a torque ratio of 12 :1, which is more than sufficient for all normal commercial vehicle operations.

The differentially supercharged diesel engine, even in its present state of development, gives appreciable advantages in size and weight over conventional normally aspirated or turbocharged diesel engines with conventional transmissions. It has the additional advantage of completely automatic operation without the penalties in efficiency, cost and reliability of automatic transmissions. It is important to recognize that it is not itself an automatic transmission, in that with it the engine speed is reduced when increased torque is required, whereas with the automatic transmission engine speed is increased. This is of great importance in giving greater reliability and life both from the-engine and transmission.

it is difficult to make a general statement regarding the. overall efficiency of the differential diesel in comparison Ivith present conventional units. Its specific fuel consumption is worse than these because of the mechanical supertharger; but this is compensated for in operation on the road by the substantial reduction in average engine speed, and over the bulk of the operating range it gives an appreciable advantage in road fuel consumption. Even so, there are considerable possibilities of further improvement with the development of economy devices to reduce the power absorption of the mechanical supercharger when high boost pressures are not required.

The advantages of the differential diesel in size and weight over the conventional unit are not as great as those which can be expected from the gas turbine or the diesel .:ompound engine. It will be less costly than either of :hese, however,. and its ability to give very nearly an ideal :orque curve for vehicle operation at high efficiency over the operating speed range will always allow it to retain a athstantial place in the commercial vehicle market.

It is poSsible to obtain the same output characteristic ts the differentially supercharged diesel from a turbotharged diesel engine fitted with a waste gate to reduce the amount of supercharge progressively over the higher engine speeds, and so avoid the losses occasioned by the mechanical supercharger. This system itself is not without losses, and the state of turbocharger development is such that an output torque ratio of substantially less than 2: 1 over the speed range would be obtained with such a system. This in turn would entail a penalty on the transmission to be used behind the engine, and would entail at least one additional gear ratio in the gearbox for a given application.

What is more likely in the course of development of this system is that the turbocharger will be allied to a mechanical supercharger driven from a differential gear to allow a reduction in the size and power consumption of the latter, even at higher degrees of supercharge. This will result in further improvements in size, weight and efficiency.

A very important feature, however, of the differential diesel arrangement is that it uses existing components within the existing range of knowledge. The amount of development necessary to bring it to the point of production is thus substantially less than for any other power unit offering comparable advantages, and there is no doubt that this is the next major development.

General comparisons

THE discussions of the various forms of prime mover so far have been necessarily qualitative, and it is in fact exceedingly difficult to make comparisons on a quantitative basis except by studying particular applications. While recognizing the dangers inherent in making generalized comparisons, the author has felt it worth. while to attempt this in the following figures.

These are based on a survey of existing engines, both production and prototype, so far as they are known. Emphasis has been placed on the larger engines in the power range from 150 to 400 h.p. in view of the likely very big increase in usage of such engines in the future. It involves the comparison of engines at sizes which are not necessarily optimum for any given type, and therefore some licence has been taken in weighting some of the results. Despite the shortcomings of such an approach it is believed that the figures give a reasonably realistic comparison between the units covered, at least in general, and representative of likely production engines for commercial vehicle usage up to 10 years ahead. It will always be possible to build particular units to give individual results, better than those shown, in some particular characteristic.

Specific volume

FIG. 12 indicates that the two-stroke version of the diesel compound engine is Hi, r.ly to give the best specific size by a surprisingly large amount, and this figure is based on the assumption that b.m.e.ps. of the order of 300 lb/sq. in. can be achieved successfully on production engines.

It must be noted that in this and the following figures two conditions are shown for the diesel four-stroke turbocharged engine. The worst of these two demonstrates the point that the present production turbocharged engines are based on relatively large and heavy normally aspirated engines which, in terms of specific volume, are still not as good as the more compact and lighter high-speed, normally-aspirated engines, which so far have been difficult to turbocharge satisfactorily. The better figures indicate what is thought possible with engines designed specifically for turbocharging. In a comparison such as this the advantages of the turbocharged engine appear small, but they are still felt to be worth while.

Specific weight THE comparison of specific weights shown in Fig. 13 I broadly reflects the same picture as the specific volumes. The major difference is that the two-stroke compound diesel loses its leading position to the gas turbine, which is understandabler in view of the fact that much of the gas turbine consists of air surrounded by sheet metal. Specific fuel consumption THE comparison of fuel consumptions shown in Fig. 14 is very much the reverse of the comparisons of specific size and weight. The one exception here is the two-stroke compound turbo-diesel engine, which is amongst the leaders in all three charts. This chart is more a measure of the maximum efficiency likely to be achieved than a reflection of the road fuel consumptions which can be obtained from any of the engines. It cannot show the effect of differing part load efficiency characteristics, between units, which can have a major effect on the road performance.

Output torque characteristics

THE question of obtaining output torque curves which

are more suitable for vehicle propulsion than the output characteristics of present engines must always be of extreme importance. Fig. 15 indicates the main possibilities here The minimum torque ratio required for normal truck operation lies between 10 and 14: 1. Certain types of trunk road operation will be satisfied with torque ratios of 7: 1 minimum, particularly where the vehicle has a good power/weight ratio. The fact that present engines can offer a maximum ratio of about 1-2 : 1 illustrates their basic unsuitability for their purpose.

The gas turbine offers a considerable advantage over present diesel engines, in that a torque ratio of at least 3: 1 can be achieved from the unit alone. The differential diesel can offer a range very nearly comparable to that of the gas turbine. Unfortunately, the straightforward compound diesel engine can only offer a curve which is comparable to that of the present normally aspirated diesel, and this must be regarded as a major drawback for this type of unit.

The fact that it does not seem possible at the moment to anticipate the full torque ratio required by the vehicle being achievable on the engine alone means that either gearboxes must be retained in a simplified form, or a torque converter used, or both. While it is disappointing that this is so, it does not minimize the .importance of obtaining the best possible characteristics. In the end it is likely that torque converters, or some form of hydromechanical split path transmission, will be adopted widely rather than conventional gearboxes, or perhaps in combination with these because of the increasing demand for automatic operation. It is unlikely, in the long term, that the straightforward automatic gearbox will be able to satisfy the overriding need for efficiency in commercial vehicle operation.

Operating cost

THE prediction of comparative operating costs is even

more fraught with danger and difficulty than the comparisons above. The assumptions here must be even more arbitrary than those based on technical considerations alone. Nevertheless, some attempt must be made to put forward a picture of these, as ultimately this is the main criterion on which selection of power units must be based.

The effect of engine size on various factors influencing costs is such that it is not possible to generalize without breaking down the power range into different categories. This has been done arbitrarily for small, medium and large vehicles, for which it is assumed that engines of 100, 200 and 400 b.h.p. will be required within the next decade, and tentative figures put forward for the engines which are likely to be considered in each category. Although there is no guarantee that the situation will continue, the assumption has been made that the present fuel cost structure in this country will be maintained.

The estimated situation on power units for small vans d the smallest size of trucks is shown in Fig. 16. This Jicates that there is likely to be little basic change in the ttern which exists at the moment, with the gasoline gine maintaining its hold on that part of the market tere first cost is important and the four-stroke diesel gine probably increasing its proportion of the market here overall operating costs predominate without disicing the gasoline engine completely. The turbocharged :sel and the rotary piston engine may be encroaching rnewhat on the fields of their more conventional counterrts, because of their attraction in lower size and weight, t will be unlikely to have made a major impact, Fig. 17 shows the estimate of the situation in respect -of :dium and large trucks. It should be noted that, in wssing the first cost of the various units, a factor has been :bided to account for what will be relatively large differes in the volume of engines produced. The implication the picture in both categories is that the differential !sel is likely to become a significant factor in the market. The position of the turbo-compound two-stroke diesel 11 not have reached the stage where it will have a major luence except in those applications where its advantage physical size can be realized. More than 10 years will necessary before it becomes a dominant factor, although irnately it is likely to do so.

The gas turbine shows up rather badly. Again this is ied on an assessment of the stage it will have reached, I the gap can well be narrowed in the course of further mlopment, although it is unlikely to overtake the turbonpound diesel, except in the largest sizes.

Finally, it must be remembered that the existing types of sel engine which predominate over the greatest part of commercial vehicle market give a combination of first ,t and operating economy which is of a standard which kes it difficult for more advanced engines to displace in, despite their advantages in other directions. Even 10 years' time the bulk of the engines in use will still recognizable as derivatives from those we have at sent in production. However, the greater variety of engines which will be available to vehicle manufacturers will enable them to offer vehicles more efficiently matched to different types of operation, and a gradual encroachment into the preserve of the present diesel engine will occur.

Implicit in this is the recognition that first cost itself is important only in a small sector of the market and that the commercial vehicle operator looks beyond this to the overall economy in his operations.