Commercial-vehicle Petrol Engines.
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We continue below, from page 40 in our last issue, the reproduction of the interesting paper read by Mr. W. D. Welliamson recently before the Inst. of Auto Engineers. The present instalment is not of a particularly mechanical nature, and should make an appeal even to those of our readers who have but little mechanical knowledge,
Hall-Compression and Starting Devices.
We feel that the author has dismissed the question. of the employment of self-starters for commercialvehicle-engines a little too briefly. It is true that no data, properly vouched, are, to our knowledge, available to show the saving that can be effected in petrol by the use of such a device. It is so far doubtful if sufficient economy can be made in this respect to warrant the additional first cost in the employment of yet another auxiliarywith its attendant complications, but that we shall eventually come to regard some starting method other than the manual one to be part and parcel of the equipment of all internalcombustion engines employed for motor vehicles—be they touring cars or commercial vehicles—is our own conviction, and a review of what has been done already in this direction might very well and very usefully have been inserted in the present paper. The subject is dismissed in the following terms :— The author is of the opinion that for the larger engines, say 41 in. bore and upwards, a simple arrangement for releasing compression for starting purposes is a decided advantage in many cases. For vehicles running regular services with only very short stops, or for motorbuses having relatively few engine stops, this fitting is not necessary But for lorries that are parked in the open or for vehicles such as furniture removers' vans, that may have to stand for some hours in the coldest weather, the small complication of, say, a sliding camshaft does appear to be worth while. If it becomes necessary to have female drivers for the heaviest vehicles, anything that helps to ease matters for them will find favour, and it may be that the self-starter proper will find advocates. For the present, the fitting of self-starters to commercial vehicle engines has riot been attended with success. They have been tried with the idea of saving petrol by encouraging the driver to stop his engine when the vehicle is making only a momentary halt for the delivery of perhaps a single package, but the saving effected has not been all that was expected.
Cylinder Design.
We may perhaps leave the rest of the paper without introductory comment.
OyEnder design calls for little comment. cylinders oast in pairs are the rule. The most notable exceptions are the Austin engines, and the Albion engine shown in Fig. 7 [Not reproduced.—ED.]
Austin cylinders are separate castings, and the Albion is a monobloc.
Ample water jackets with carefully considered arrangement round the valve pockets and stems are required, and are found in most modern engines. The author is of the opinion that the water jacket should extend down the cylinders to the level of the top ring or at least the top of the piston when the crank is on the bottom dead centre, although some designs appear to operate successfully with jackets shorter than this.
The plan adopted in the Napier and Pageficld engines of having the cylinders cast with large openings in the jacket at top and sides has much to recomniend it, as it simplifies moulding and ensures the jacket cores being thoroughly cleared. With the ordinary small core plugs it is often almost impossible to clear the jackets about the valve pockets.
Crankcase.
Crankcases differ very little in general from those in pleasure cars, although naturally they are more robust in design. The usual practice is to split the case horizontally on the centre of the crankshaft, carrying the bearings in the upper half, and to
use the lower half principally as an oil retainer and dust exclude'. External ribs on the lower half, although they may be of distinct advantage for oil cooling, are not general. Perhaps the most noteworthy departure from the usual design is that adopted in London General "B "-type engines, where the lower half is little more than an oil tray (Fig. 18). The plan adopted in this engine of scraping the face to form an oil-tight joint between the two halves without the necessity of packing is one that is to be recommended. The oiled paper joint is not an unmixed blessing. It does not always prevent leakage, and usually requires renewing if the engine is dismantled. The fitting of inspection doors as required by the Subsidy scheme to the crankcase is coming into general favour. It is not suggested that it is advisable or even possible in most engines to attempt to carry out repairs through these doors, but for inspection of the internal mechanism, location of trouble, and in some cases the cleaning or removal of the oil filter, they are very useful. Any tendency they may have to weaken the case is usually. overcome without special provision by the extra metal required for the facing and to carry the studs.
Oil Indicator..
All lubrication systems should embody some form of indicator on the dashboard. This indicator is best arranged in series with the oil leads to the bearings. The plunger type indicator,arranged so that it can ba seen in daylight and to be within easy reach of the driver's hand for night driving, is probably the most convenient. There appears to be room for a reliable indicator of somewhat stronger construction than the usual patterns for use on commercial vehicles.
Engine Suspension.
As long as a vehicle is intended for ordinary commercial purposes and is not built with the idea of crossing broken ground, there does not appear to be any necessity to make special provision against frame distortion.
The ordinary underframe method of construction has much to recommend it, and experiments the author has made show that for any distortion that might occur under normal conditions this arrangement is quite sound, and would appear to be more satisfactory than extending the crankcase arms to the main frame.
A Test for the Effect of Frame Twist.
In one experiment made with a four-ton chassis having the engine onn-a sub-frame, the engine was started and throttled down so that it just continued to run. The near side front wheel was then gradually raised. When the wheat was 12 in. clear of the ground, no difference was discernible in the running of the engine. From 12 ins. to 15 ins, the twisting of the fraane did have an 'effect, and at the latter height the engine showed signs of stopping. The engice was switched off, and when tested at the starting handle was found to be stiff With the clutch either in or out. The chassis was gradue ally allowed to fall back again, and at 12 ins. the whole of the stiffness had disappeared.
Three-point Suspension Important for Gearboxes.
Standing at the back of the chassis, the shape and twist taken by the near side long memIel of the frame made it appear that three-point suspension is more important for the gearbox than for the engine. The front of the chassis was tilted on to the off side front wheel, and no doubt helped by the fact that the front spring acted in some measui e to restrain the bending of the side member in front, very little distortion was apparent in front of the dashboard. The greatest change of shape was encountered between the dashboard and the front bracket of the rear sprang, where the frame was both bent and twisted.
Many three-point suspensions are three-point in name only and not in effect, and while they may do no harm, at the same time, unless they make for simplicity, as for iestance in the Daimler, they are not worth while. Where conditions are such that great frame distortion may be expected, the best method would appear to be to have an underfrarne supported at three points arranged in such a way that these point allow flexibility. Any tendency to whip, even the small amount caused by overcoming the necessary friction in the suspension point, is thus prevented from reaching the aluminium casting. In any engines having a tendency to vibrate at certain critical speeds flexible suspension may be a decided disadvantage.
Cooling. Bigger Pipes and Jackets Nowadays.
The problemof cooling a commercial vehicle engiee, which appeared to present considerable difficulty only a few years ago, has now been satisfactorily solved. Extended running under full load at low road speeds when climbing stiff gradients threw excessive work -upon the cooling eystem. Ample water jackets and pipes of larger bore have helped to overcome the original troubles, and a better grasp of the conditions and of the amount of heat to be dissipated has enabled the necessary provision to be made, so that lorries now work under the most severe conditions without showing signs of boiling. Practically all cooling systems for commercial vehicle engines embody a circulating pump, and this pump is invariably of the centrifugal type. Other types of pump have been tried, but the centrifugal, as having the fewest wearing parts, is now firmly established. Cooling fans are of robust design, and in most cases mounted on standard type
bail bearings. A fan with arms and boss cast an aluminium in one piece secured to a malleable iron centre to serve as the fan pulley, appears to be the most satisfactory type.
Driving-belt Weaknesses.
In many engines the fan-driving belts are much too light and weak for the work they have to perform. At maximum engine speed these belts have to transmit quite an appreciable power, and in addition are subject to shocks due to fan inertia and fan momentum when the engine speed is altered quickly. The positive drive for the fan in the Hanford engine (Fig. 28) is specially worthy of attention.
Turning to the radiator proper, the type in most general use, and the one that appears to meet most of the requirements, is the tubular type farmed of vertical water tubes soldered to tube plate, and having top and bottom boxes of cast aluminium.
B40
Honeycomb Radiator Not Suitable.
Honeycomb radiators are, as a rule, too delicate for heavy vehicle work. They are apt to leak through vibration when used in a vehicle having solid tyres, and are not as easy to repair with the limited facilities often at hand, as are those of tubular type.
Opinion seems to be divided as to whether the tubes should or should not have gills, although-the general practice favours gills. The usual tube is of 26-gauge brass or copper 154. in. external diameter with -gills la in. diameter or tr an. square, soldered in position at about e in, pitch. Experiments carried out by a. large maker of radiators showed the most useful ratio of gill diameter to tube diameter to be three to one. In
these experiments where in. tube was used with in. gills for appearance instead of i .in., the efficiency fell approximately 10 per cent.
Where plain copper tubes, are used they are usually in. external diameter by 28 or 30 gauge.
Further experiments went to show that the relative effieienty of gilled to plain tubes is about three to one by reason of the much increased surface of the gills exposed to the air current, In other words, three times as many tubes are required in a tube stack having plain tubes as in a stack having gilled tubes. Trouble with tubular radiators usually occurs where the tubes join the tube plates, and the reduction of the number of joints, and consequently of the possibility of leaks, is a strong recommendation for the gilled tube type. The gills, in addition, help to stiffen the tubes and afford some measure of protection against damage by impact. It might be suggested that in order to further stiffen the tubes and increase the surface area, the tubes might be threaded through a number of plates instead of being fitted with gills. This method would make the isolation and repair or replacement of a single tube much more difficult.
When makers of vehicles cemmeneed to deal with the trouble IA insufficient cooling, the tendency at first was to err on the other side. In some cases very large radiators were fitted, having more cooling surface than is now found necessary. Current practice is very well represented by the following formula for a four-cylinder engine:—
Bore in ins. X stroke in ins. x 8 = ft. of lein. tube with a in. gills. Objection may be taken to this formula. in that it does not take into account variations in cylinder design, disposition of valves, and other factors, but it has the advantage that it is easy to handle and gives a result which can be relied upon for for satisfactory cooling. There are engines an commercial vehicles with cooling surface considerably less than that given by the formula. In one case the cooling surface was carefully cut down, starting from bore x stroke x 7, and at bore x stroke x 6.2 the limit appeared to be reached beyond which it was unsafe to go. It as probable that in engines where other conditions were not as favourable, the cutting down process would have been forced to stop before this.