Points in Goods-vehicle Chassis Selection
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By A. W. Haigh,
A.M.I.A.E.
In this, the First of Two Articles on the Subject of Buying a New Vehicle, Our Contributor Deals with the Power Unit. Differences Between Mass Production and Quantity Production are Explained, Together with Points in Design
IN the issue of " The Commercial Motor " dated" April 2, 11943, I gave reasons for my belief that the present so-called orthodox design of goods-vehicle chassis can he improved upon only in detail and not in general layout.. In making our choice of . vehicle let us start, then, on the assumption that unorthodox design is ruled •out, and, further, that manufacturers are once more catering solely for public demand and that it is possible to purchase exactly the vehicle one wants.
Every operator of goods-carrying vehicles has his,own loading problems; so that he must, first of all, be sure that the vehicle he contemplates purchasing will be suitable for ha tiling his heaViest loads without exceeding the manufacturers' maximum .loading stipulations. Having decided on the capacity reqUired, the .choice of vehicle rests on two main 1actors7-that of price and operationLI economy; the latter embraces -reliability an.da"cbruhination of the most efficient, chassis units possible. . The initial purchase price is an item needing a great deal of thought. It does not necessarily follow that the cheapest chassis is the worst or that the dearest is the best, as the usual reason for low cost is mass production. Many people argue that, because an article is mass produced it is naturl all3i inferior to what niay be termed the " hand-made ' product.
Individual production is an inducement to avoid fine limits of machining. If a slight mistake has been made in any piece, it can 'Still be fitted by the skill of the man at the bench, but when that piece requires replacernent, after usage, a correctly machined item is sent out by the service department, which,. obviously, cannot be fitted without more work than should be necessary being done on it. • Accuracy Essential in Maas PrOduction . Mass producti on the other hand, ensures that every item on a chassis fits like a jig-saw puzzle. If, by some utifortnnate circum'stance, the slightest mistake be made M the production of any item it is at once discarded, so that. no servicing problem can subsequently arise. Furtherthore, the fact that each part used in the assembly of a mass produced chassis is produced on special machines, which can turn out many times the number of parts per unit of time than can be manufactured by individual production— parts which, Moreover, are held to closer limits because they must fit without resort to the attentions of a skilled craftsman—is a sufficient reason for the lower cost of vehicles produced by this method. .
One argument in favour of individual production is that, because of the comparatively few vehicles turned out, each chassis can be given a separate test, The engine can be thoroughly tested and run-in on the road, whereas the limits to which each part is manufactured in mass production must ensure satisfactory, operation without resort to individual attention, But even this slight advantage of the small-production job is being evened out. Since the advent of super finish in bearing surfaces many experiment's have been carried out, especially in America; on the smoothness to which main and big-end bearings and cylinder bores must be finished to reduce friction to a minimum in the shortest running-in time.
It has been found • that a finish of 60 micrp ins. (60 millionths of an inch), that is the average depth of the .,alleys present on all surfaces,no mattes'how carefully finished, gives the best results. Smoother or rougher surfaces than this'ine:rease running-in time and frictional losses. As super finish is hardly likely to be employed on individually produced engines, it is not difficult to appreciate that
4.5r the difference in actual running-in times, as between engines produced by the two methods, cannot be very great.
However, the choice is entirely a matter of per carol preference,, but no one need be afraid of purchasing a mass-produced vehicle because it is cheap; it is ale reliable.
Itt the heavier classes there can be no preference, for the demand is not sufficient to justify Mass production. Neither may it be small enough for individual production, sr that a vehicle is built which receives most, of the individual care of the ". hand-built " method and a great deal of the attention to 'detail, which is the essence of mass production.
The Value of a Vehicle Record
Operational economy is something which can be decided only by close inquiry into the performance And reliability of each make of .vehicle marketed, or by an examination f the construction of each unit of the chassis. A great deal of assistance in the choosing of a suitable vehicle could be obtained from what Might be termed a vehicle record Tt would consist of a complete list of all goods vehicles manufactured, but would differ from the usual S.M.M. and .1. lists in that, instead of giving the general dimensions and particulars of the chassis, the purchase price, the average cost of running per mile with full load, and the number of failures per annum of each unit would be given.
Such particulars could all be obtained from 'their members by the various associations, and would, without doubt, be of immense help to operators contemplating the pur chase of a. new vehicle. Such information would give a good picture of the reliability, or otherwise, of al 1 chassis.
There is no list of this nature in existence, so that until such time as one is compiled a more tedious method must be used if we are to obtain the best value for our money_ Let us, then, examine each main unit in turn, in an effo-t to decide which design is most suitable.
Manufacturers rarely underpower their vehicles, so that little thought need be given to the power unit other than to ensure that, coupled with the bottom-gear and back-' xle ratios provided, sufficient power is available to enable the vehicle, when fully_ laden, to climb the steepest hill likely
to be encountered. Usually the manufacturer states a gradient which the vehicle will climb, and this is. -alculated by the designers and later checked by*actual test.
The method of calculation is this:— Gross weight of vehicle, 8.tons 10 cwt. 0 qr 0 lb. Max. engine torque, 150.1b./ft = 1,800 lb./ins.. Combined bottom-gear and rear-axle ratios, 50 : 1. Rolling radius of tyres, .16.5 ins.
'Rolling resistance, 30 lb./ton. . Assume gradient to be 1 in" X"
Force doWnhill due to weight of vehicle = 3.5 ions 19,040 lb. Therefore, ti.e vehicle will hold its own on a 1 in 3.6=1 gradient assuming,, the overall efficiency to be..104 per cent At 80 per cegt.,` effociency.; the usually assumed figure, the gradient is 4.63 to 1. If the engine gives its maximum torque at 2,000 r.p,m, the load speed will be: The figure of 150 lb./ft. maximum torque can be taken as a minimum for an engine to be used on a 6-ton. vehicle. A torque of about 18-lb.lft. per ton of gross weight gives good hill-climbing power and, so long as the gear ratios are correct, a point upon which no manufacturer is likely to err, the road speed will be reasonable'.
The revolutions at which an engine develops its maximum torque is of importance to operators, and particularly so when the vehicle has to operate in hilly districts, for it is this feature which determines the speed at which a hill, requiring maximum engine output, can be climbed, and the minimtim road speed for top-gear work. An engine which develop...i-As maximum torque at comparatively high
revs, will give pod speed, whilst low-speed torque ensures power at low road speed in top gear.
Nature ot Work Governs Engine Choice The district' in which deliveries are to he. rpade should, thefefore, be the deciding factor in the choice of engine; if it be so hilly that little low-speed top-gear work can be pempioyed, a, high-speed engine would be preferable, so !that time on hills could be saVed; but, if the country be Comparatively fiat, and opportunities for low-speed topgear ope• ration numerous, such as on local delivery work, a sibw-revving engine.is to be preferred.
Apart from its actual power output, there is a number .. of factors worthy of consideration, before a decision is made' to purchase. For instance, how many cylinclers should there be? The choice lies between four, six'and eight and, considered from the point of view of number of parts to serviced, the four-cylindered unit is obviously. the:best. When, however, balance and smooth running ' are con sidered, the four-cylindered engine comes a bad third. _ Comparing the three types pf engine, a " four can be balanced primarily, but the secondary unb"alanced forces, which reciprocate vertically and act at twice crankthaft speed, cannot be eliminated. Thus, considerable vibration is transmitted to the engine supports, and thence to the frame.
The six-cylindered engine can be perfectly balanced for both primary and secondary forces. Whilst the primary forces of an eight-cylindered V-type engine, with a single ,plane crankshaft, can be balanced, the secondary forces can be only partiallly coped with. The degree of balance obtained depends upon the angle at which the cylinders lie to each other, the forces being both rotary and horizontal reciprocating. With a two-plane crankshaft, however, it is possible entirely to eliminate all unbalanced forces so that sixand eight-cylindered engines are equal in this respect.
Points in Favour of Six Cylinders
With regard to rough running, caused by fluctuation of the turning moment, the " four " is again the worst,
being approximately three times as bad as the "six the " eight " is the best.. Therefore, if balance and general vibration are to be considered, both vital factors in the comfort of the driver, a decision must be made between the sixand eight-cylindered engine. Both are equal in balance, and Uwe is little discernible difference in smooth• ness. Therefore, because of the -fewer parts in the six: cylindered engine this would seem to be the final choiCe.
It should be thentioned, however, that recent advances in engine-mounting methods ensure that any discomfort due to engine vibration has been reduced to a minimum, so that Abe "four !' is not now at such a great elisadvantage in this respect,
With regard to special features, such as valve inserts and cylinder liners, there is little to be said, although there is a choice between wet and dry liners. The former type is pressed into the cylinder-block shell and forms the corn
plate cylinder and one wall ot the water jacket, hence the name " wet " liner. The dry liner is merely a thin tube of .special material, pressed into a normal cylinder block (usually being shrunk in with the aid of liquid air or sometimes scud carbon dioxide) and does not cos e into direet contact with the cooling water. There is little to choose between them except that the use of wet lineis may produce a slightly cheaper engine.
One point with regard to valves mat' be stressed, and that is, there is no reason, so far as 'power output is concerned, for the use of overhead valves, a feature dealt with at some length in an article published in "The Commercial Motor" dated August 20,1943. The fact that servicing may be more simple is offset by the reduced cost and greater simplicity of the side-valve engine.
Fuel Economy of the Oil Engine
.The increasing popularity of the compression-ignition unit demands that it be compared with the petrol engine, so that any operator who is doubtful about the advisability of purchasing an oil-engine a chassis may have a basis,tipon which he can make a final decision: The main advantage of the compression-ignition engine lies in -its low fuel consumption. Cases have been recorded where the .consumption has been only one-half that of the petrol .engine, whilst it rarely exceeds three-quarters, so
that,:a ,great saving in running costs is ensured. Fuel delivery to the engine is by direct injection from the pump. The _quantity :of fuel required under all operating conditions is actueately,metered so that there can be no wastage, whereas, with ?orthodox carbur ition, the mixture fed to the engine 04440. be said to be igiedl at all times, and a certain quantliK of free petrol is bound to be lost.
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lit an oil engine the mixtu.e is evenly distributed to all Cylinders but no matter how carefully the induction system of, a. 'petrol engine be designed, even mixture distribution is imposiible, hence thepower per cylinder differs which, to mention only two faults, causes torsional vibration in ;the' crankshaftand lower efficiency than the possible maxith
Striking a Balance in Engine Types
Assuming that the mixture in both types of engine be equally homogeneous, the rate of flame travel in the compression-ignition engine is far more rapid than in the petrol engine. Each molecule in the former is a potential explosive particle, which ignites at the same time as its fellows when ignition temperature is reached. In the petrol engine the flame must travel from the sparking plug and, consequently, must take a longer time to ignite the charge. Oil-engine torque output is fairly constant, and thus a more flexible unit, over a wider speed range, is prOvided. There is no crankcase dilution, as there is no unburnt fuel. Volumetric and thermal efficiencies are both higher and, because of the latter, less heat is lost to the cooling water, thus making the use of a smaller radiator permissible. There is no carburetter or ignition system to service and an oil engine will start up immediately in the coldest weather, and pull away without having to be warmed up. Although there may be considerable smell from the exhaust, there is but little carbon-monoxide present, To offset these advantages, the oil engine is both heavier , and more bulky than its petrol counterpart, which means that pay-load ig slightly reduced. Because mass-production methods have not yet been employed in its manufacture, the conipression-ignition engine is more expensive and, although an ignition system and carburetter are not required, the fuel pump, a complicated mechanism, must be serviced,
The most detrimental feature, however, is the phenomenon of " diesel knock," Its cause 1fas been put down to the extremely high rate of pressure rise which induces detonation and which, ,similarly to the petrol engine, is responsible for ' the hammering noise. Experiments have been carried out to effect a cure for this, and exCellent results have been obtained. If the rate of pressure use be kept below SO lb: Per sq in. per degree Of crank angl6. ,theta is no knock. It can also be controlled by --the, 'use of turbulent air 'cells in the head, and by the employment of two-stage fuel injection, that is, a small quantity of fuel is provided before the main charge is injected