223 Introduction

ENGINES FOR PETROL COMMERCIAL VEHICLES. 223 ENGINES FOR PETROL COMMERCIAL VEHICLES. By W. D. WILLIAYSON INTRODUCTION. THE purpose of the first portion of the paper is to put formd for consideration certain factors governing the sizes of engines suitable for use in commercial vehicles, and by promoting discussion upon the points raised to endeavour to direct effort towards greater uniformity and possibly a closer co-operation among commercial vehicle manufacturers and engine builders, to the general advantage of the industry. Secondly, by an examination of well known successful types of engines, an endeavour is made to outline modern practice, and to arrive at simple. formulae covering those parts where commercial vehicle engines may be said to depart from current pleasure car design on account of the different conditions of service. In a paper to be kept within reasonable limits, it is impossible to go deeply into all the points worthy of attention-a volume might be written on the possibilities of the split pin-but if only some of the points touched upon prove of interest, or serve to bring forward helpful discussion, the author’s purpose will be served. In the present paper it is proposed to deal only with enginea for vehicles having a useful capacity of two tons and upwards, EO that artificial restrictions imposed by a tax based on the bore of the cylinders do not apply. The problem presented in the design syld construction of engines for commercial vehicles differ very considerably from those involved in engines for pleasure car work. The pleasure car designer has constantly befom him the question of horse-power output for cylinder capacity, and having done all Downloaded from pau.sagepub.com at Kungl Tekniska Hogskolan / Royal Institute of Technology on June 4, 2016 224 THE INSTITUTION OF AUTOMOHILE ENGINEPHS. he can to obtain the maximum torque from a given oylinder, devotes his energies to the pursuit of “revolutions.” With him, and even more so with the aeronautical engineer, horse-power for weight is a serious consideration. The designer of engines for use solely in commercial vehicles has more scope, in that, unhampered by considerations other than those of strict utility and manufacturing cost, he can set out to design an engine to give the best possible results in the hands o€ the average user, and under the conditions in which it is intended to operate. Within reasonable limits, the question of weight in other than the reciprocating pa&i is not of great moment pro- vided always that additional weight is serving a useful purpose in increasing the wearing qualities of the engine or presenting other advantages in the way of accessibility or fuel efficiency. The user demands an engine capable of propelling his vehicle at a reasonable speed with a high fuel efficiency as expressed in ton-miles per gallon. This engine must be of robust design to give long life and freedom from breakdown under severe conditions, and all parts must be rn accessible as possible for djust- ment or replacement. Here, again, the questions of bore and weight are not involved. Other things being satisfactory, the user is often favourably impressed by the fact that an engine is, say, Bin. larger in bore than a competitor, and few users would object to mrrying quite an appreciable extra weight if it would emure the absence of breakdown, or decrease the cost of running. From this it must not be understood that the ideal engine from the user’s point of view is of huge proportions and terrific weight, but rather that bore and weight are of secondary importance and should not be allowed to have consideration at the expense of other more material requirements. Silence of operation, which; is of the greatest importance for engines for pleasure vehicles, has not been an outstanding feature of commercial vehicles in the past, and some of the most successful types have not been notable for quietness. Comparatively few owners drive their own lorries, consequently this feature has given way to other claims. In the past, too, the engine noise has been drowned by the general uproar, but in these days of live axle drives, enclosed chains and quiet gear boxes, considerably more attention is being paid io silencing the engine. Many heavy vehicles of to-day leave little to be desired in this respect. Motor buses and char-a-bancs set the fashion; fare-paying passengers quite naturally select the Downloaded from pau.sagepub.com at Kungl Tekniska Hogskolan / Royal Institute of Technology on June 4, 2016 ENGINES FOR PETROL COMMERCIAL VEHICLES. 225 vehicle of greatest general comfort, and when once it occurred to the manufacturer of commercial vehicles that it was possible and even desirable for heavy vehicles to run more quietly, it did not take long to effect considerable1 improvement. The silencing of one portion of the chassis threw the noise from another into prominence; this in turn was given attention, and a remedy found, when a third became apparent, and so on. To-day, on reasonably good roads, high class motor lorries are operating with a lack of noise and fussiness undreamt of only a few years ago. From the manufacturer’s standpoint the engine must first satisfy the user in the points covered above; secondly, that engine must be capable of being produced at a competitive price. These two considerations are related to each other mope closely than io apparent at first sight. In spite of the fact that commercial vehicles are strictly utilitarian, there is undoubtedly a fashion in them, or a preference shown in some districts for one make over another. In many instances this may be due to the fact that one make is particularly suitable for a certain district; on the other hand, there is little doubt that where certain vehicles are working in a district and are known to be giving entire satisfaction they are purchased again and again to the exclusion of other and possibly equally good vehicles. A purchaser in one district will insist on having, say, “A” vehicles even if they do cost him a little more than “B.” The manufacturer of “A” vehicles can therefore consolidate his position by putting that extra price or a portion of it into the vehicle in the way of little improvements or extra good workmanship, and so endeavour to make his satisfied customer even more satisfied. Although the policy may appear strange to American manufacturers with large outputs of one model, few British builders of commercial chassis devote themselves exclusively to one size or type of vehicle, but build a range of chassis having useful carrying capacities of perhaps from two to five tons with possibly one or two sizes specially adapted for passenger work. This means the provision of a suitable range of engines, and, from a manufacturing point of view, it is highly desirable to obtain this range or series of engines with the fewest possible changes. The engines may be grouped so that one sine is made suitable for two or three different types of vehicles, and provision can be made in some case5 for larger cylinders and pistons to be used in conjunction with standard crankcases where additional power is required. 15‘1 I, L 1A MSON . P Downloaded from pau.sagepub.com at Kungl Tekniska Hogskolan / Royal Institute of Technology on June 4, 2016 226 THE INS'L'ITUTION OF AUTOMOBILE ENGIREERS. Further, standardisation can be carried out with respect to the method of attaching the engines to the frame, so that it is possible to fit, say, either a 30 h.p. engine or a 40 h.p. engine in the same frame without a lot of alterations. The manufacturer is then in a position to make combinations of engines with the remainder of the chassis, also suitably altered, to meet special requirements, at little extra cost. Where a manufacturer is building vehicles for general purposes and not to suit one particular set of requirements, a little forethought in this direction may me&n both advantage Q himself and increased satisfaction to the user. ENGINESIZE. In fixing the size of the engine, the first and most important consideration is the most satisfactory ratio between engine speed and road speed. For this it is necessary to examine the various factors governing the actual speeds of engine and vehicle, and discover the most economical speed at which to run the engine, and the road speed at which the best results are obtained from the vehicle. The determination of the road speed, as presenting fewer diffi- cultiw, may be taken first. The commercial vehicle user is obvi- ously interested in getting loads from one point to another as quickly as possible. High road speed reduces the charges per mile or per ton-mile, due to proportion of driver's wages, interest on capital and garage expenses, and, further, by affording a greater mileage per vehicle per day, makm fewer vehicles necessary for any given amount of work, and so decreases capital expenditure. On the other hand, tyre costs, petrol consumption and upkeep generally are increased by high speeds. In ordinary delivery work for loads of two to four tom, a very satisfactory maximum speed is 15 miles an hour. On reasonably good roads the chassis should take no great amount of harm at this speed, but if it is exceeded to any great extent, particularly on a bad road and when the vehicle is running light, the effect is not conducive to long life of the mechanism. For loads of four and five tons the maximum should be reduced to 10 or 12 miles an hour.

Load more