THE VENT ATION OF FORWARD-C NTROL CABS
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ADETAILED examination of the complete vehicles and chassis equipped with cabs as shown at Olympia last month revealed a surprising neglect of the need for generous ventilation for forward-control cabs. In the case of a normal-control
vehicle, fumes from the engine' are less likely to be troublesome on account of the chances of escape which are provided, but, when the driver is seated in an enclosed compartment which also serves to cover the power unit, the position is very different.
It must be remembered that there are two undesirable elements below the bonnet which it is required in the interests of physical well-being, shall be excluded from the cab. One is unduly heated air ; naturally, the air under the bonnet rises to a temperature much above that of the atmosphere. It is advisable to keep the temperature of the air surrounding the exterior of the bonnet as close as possible to that of the atmosphere outside the vehicle. For especiallY cold weather proper heating can easily be arranged. A stuffy cab is conducive. to drowsiness and reduced efficiency, whilst it also increases the risk of catching cold.
The second undesirable feature consists of fumes, either emanating from the crankcase and exuding through breathers, etc., or under-bonnet air tainted with carbon monoxide escaping from a defective exhaust system. Whilst hot air may be somewhat unpleasant, the fumes can be positively dangerous if breathed in sufficient quantities.
Such a condition obtains in the case or a longdistance vehicle, where a driver is probably ensconced in the cab for continuous periods of four hours or more.
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The physical effects of the breathing of a tainted or unduly heated atmosphere must result in a reduced driving efficiency—hence the need for a more intensive study of the ventilation problem.
In the case of a commercial vehicle, the area of the radiator is large and, in consequence, the induced volume of air is considerable. It must also be remembered that there is an appreciable increase in the volume of the air, in consequence of the heat imparted to it by the radiator and by the engine; the exhaust manifold, of course, is the hottest item and, to a large extent, adds to the temperature. If heat be measured below the bonnet it will frequently be found that the air in proximity to the exhaust tract is twice as hot as that which has derived its heat from the radiator.
Concerning the increase in volume of tilr subsequent to heating, air entering at 50 degrees F. has a comparative volume of .962, being unity at 70 degrees F.; at 100 degrees F. it is 1.038, whilst at 200 degrees F. it is 1.245. It will, there. fore, be appreciated that not only has the designer to provide for the exit of that quantity of air which enters the radiator but for a larger volume.
In the case of the engine rover of a forward-control chassis there are commonly innumerable air "pockets," which hamper a free flow. These, naturally, contain hot air, which, by reason of the comparative stagnation, imparts a good deal of heat to the metal bonnet, thereby warming up the cab to an undesired extent.
To minimize this effect the bonnet, in some instances, consists of a double shell, as in certain Albion models, and this acts AS an insulator. The scheme in question B33
is of particular interest to the driver's mate, who sits in close proximity to the near side of the bonnet and often has to rest against it, so that this idea possesses an obvious merit.
Again, the exhaust pipe is commonly at the rear end of an engine tunnel having a vertical rear bulkhead, thus hampering ready egress of the heated air, which tends to rise rather than fall. It must take the latter
course to secure an outlet, unless other provision has been made. An improvement in this connection can be effected by mounting the exhaust pipe at the forward end, where it is in the slip-stream of the fan, thus being more effectively cooled.
Another case of a vertical surface acting as an impediment occurs in the case of the cowling immediately behind the radiator and in front of the driver's feet. From the off-side edge of the radiator the cowling should be swept back with a generous radius in order to prevent the formation of an air pocket, which not only overheats the driver's cab but, as we have proved by tests, has, in certain instances, caused the radiator temperature to be as much as 15 degrees higher than it need be. In one instance boiling was eliminated by rectification of this defect.
It must be remembered that the increased volume of air has to be forced backwards around many obstructions; therefore ease of exit is vital, and with a full-width cab no bonnet louvres can be provided. Where such a scheme is feasible, as in the case of a half-width cab, the problem is much less troublesome.
In the case of a passenger-carrying vehicle It. is not so simple to arrange for the easy or '13:14
streamlined outflow as in a goods machine, in which the cab and body are generally separate. In the case of the coach or bus the rear end of the engine tunnel might, with great advantage, be gradually swept down to frame level and air "pockets" be removed by the employment of a more streamlined form of casing.
To-day, when the majority of auxiliaries is grouped on the near side and the engine cover is close up to the
cylinder block on the off side, there is not much opportunity for the heated air to pass out beside the engine, but where increased lateral clearance is feasible steps should be taken to arrange for this facility.
Another point which is commonly overlooked is the need for airtight joints for the plates forming the bonnet. These should rest upon comparatively soft media which provide a bed for the metal, and the springs of the fixing devices should ensure sufficient tension to give a firm location despite road vibration.
Further assistance can be arranged in the case of a vehicle having a separately mounted gearbox by providing extractor vanes upon the periphery of the flywheel in conjunction with suitable cowling. This, of course, is not generally permissible on account of the widespread adoption of unit construction. Fumes usually emanate from two sources. One is. the crankcase breather ; this must on no account discharge directly under the bonnet. Its outlet should always be led to a point well below and to the rear of the engine, so that the slip-stream of the vehicle carries away the vitiated air.
The second source of trouble is leaky exhaust joints. Careful fitting in the maintenance shop cnsures that gaskets are airtight, but the joint between the pipe and manifold is commonly at fault, owing to the slight relative motion between the engine and the chassis—to which the exhaust pipe is generally anchored. To obviate this trouble the use of spigoted joints is advisable, in that the gas pressure is removed from the joint; this, then, merely serves as a resilient cushion or
buffer instead, in addition, of having to withstand gas pressure.
Turning now to the design of the cab, there should always be ventilators at the front which can direct streams of cool air along the sides of the bonnet casing. Their absence generally causes heated areas around the bonnet, despite any free flow of cool air in the upper part of the cab, permitted by partially opened windows.
Admission of pure air af cant-rail level also is useful, .because it is then possible adequately to ventilate a cab in the heaviest downpour of rain, which precludes . the opening of windows. In this connection, -the em.ployment of such a windscreen system as the Sytgap is an obvious suggestion, as it not only improves for ward visibility from the driver's point of view but serves as a ventilator which has an extractor effect.
A. minor point, but one of interest to maintenance engineers, is that, where " under-bonnet" temperatures are too high, the dynamo is running at an undesirable heat and its output is somewhat restricted thereby.
An interesting example of an improvement in this connection is in the latest Dennis Lance, in which the air stream passing by the exhaust is isolated and separately vented. It has been found that the temperature of the dynamo is, by this system, approximately halved.'
In the case of Crossley chassis of certain types, the dynamo is outside the bonnet area, being driven by an extended shaft on the near side and carried below the body floor.