Selected Drivers' C.M.U.A. Exam. Answers.
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It will most certainly interest many of the readers of this journal and will, we take leave to suggest, instruct not a few, to peruse a selection of the more intelligent answers which were given by competing drivers in the recent Championship examinations held by the C.M.U.A. The winners, it will be remembered, proved to be A. C. G. Hughesden (petrol); H. While (steam). We print below a selection from the written answers by various competitors. We are indebted to the Association's Inspecting Engineer, Mr. G. W. Watson, who acted as examiner, for the choice of specimens.
"H a Thing is Easier, It Stands a Setter Chance of Being Done."
Question No. 2 (petrol exuaiination).—Why arc lubricants necessary for petrol engines and other forms of machinery, and what happens when they are used? What kinds of lubricants would you use in the various parts of a chassis?
SPECIMEN ANSWER.—" The use of lubricants is most vital to any kind of machinery, and without them the machinery would soon cease to work. I use a good oil for the engine, and prefer to use oil for the gears, springs, shackles and pins, and a mixture of oil and grease for the differential gear and chains, which, in the case of my van, are enclosed. I think it would be a very good plan if all makers of commercial vehicles fitted oil caps to the steel springs and steering gear joints instead of Stauffer lubricators, for the following reasons: (a) it is easier to get a drop of oil through ; (b) it is much cleaner ; and (a) there are then no lubricator caps to be lost. A further advantage is that should the oil be at all sluggish in action, a little paraffin injected occasionally will wash out the pin bearing and help the oil through to its work again. The use of lubricants prevents the two rubbing faces of metal making actual contact with each other. I think another point in favour of the use of oil instead of grease, is that it is easier for the driver to fill up the oil caps than it is to fill up grease caps, and if a thing is easier it stands a better chance of being done."
The Last Part of the Explosion Stroke.
Question No. 5 (petrol examination).—Describe whet lakes place inside the cylinders of a petrol engine during each cycle of four strokes. State the stroke of your engine. How far from the bottom dead centre would you set the exhaust valve to open, and should it commence to open while the piston is descending or ascending? • SPECIMEN ANSWER.—" The complete cycle of an ordinary petrol engine is as follows. During first (downward) stroke mixture is drawn into the cylinder ; second (upward) stroke the charge of gas is compressed within the cylinder, both inlet and exhaust valves meanwhile being closed ; third (downward) stroke, just before the commencement of this stroke the spark occurs at the plug and the mixture is exploded; fourth (upward) stroke, exhaust valve is open and allows exploded charge to escape to the silencer. I should set the valves on my engine so that the exhaust valve would open about 40 degrees from the bottom dead centre of the crank, because the pressure of the burnt charge during the last fifth part of the explosion stroke is of no practical use to the engine, on account of the decreasing leverage of the crank, and the engine ill give its best work if the valve opens at that point and relieves hack pressure during the exhaust stroke."
The Function of an Electric Condenser.
Question No. 9 (petrolexamination).—W hat is the object of fitting a condenser in n high-tension magneto or sparking coil?
SPECIMEN ANSWER." The object of the condenser is to receive the back charge of electricity which is
generated by self-induction in t,he primary winding of . . , .
the armature, and it acts as a buffer to that charge, first taking it, and giving it out almost instantly in the right direction needed to augment the induction in the high-tension coils. By trapping the back-induced current from the primary, excessive sparking at the primary make-and-break is avoided. The condenser consists of a number of tinfoil sheets, which are interleaved with sheets of waxed tissue paper ; the tinfoil sheets are connected in two groups, and form a shunt across the primary connections."
Road Resistance and Horse-power.
Question No. 14 (petrol examination).—What horse-power would be required to drive a nwtor-wagon along a level road at 12 m.p.h., if the total weight of the wagon and its load be 1(1 tons and the road resistance amounts to 60 lb. per ton of load? You may neglect all consideration of friction in the gearing and bearings.
SPECIMEN ANSWER.—" If the total weight of a wagon and its load is 10 tons, and the road resistance amounts to 60 lb. per ton, obviously the total resistance to motion is 600 lb., and I take it that would be the constant pull required to move the vehicle along the road at 12 miles an hour. The total distance moved in one minute would, therefore, be 12 multiplied by 5'280 and divided by 60 = 1056 ft., and the foot lb. of work done in one minute would be 1056 multiplied by 600 = 633,600, and as there are 33,000 ft. lb. of work in one horse-power, the horse-power required to propel the vehicle, neglecting all friction in gearing, would be 633,600 divided by 33,000 = 19.2."
Dirty Water and Priming.
Question No. 3 (steam examination).—What will happen if oily or dirty water is admitted into the tank and allowed to pass through the pump or injectors into the boiler? Suppos ing the water also contains grit as well as oil or dirt, what ilea will it have on the cylinders, valves and glands?
SPECIMEN ANSWER..—" If oily Or dirty water is admitted into the water tank, it will pass thence to the boiler and cause it to prime. If I have picked up very dirty water during a journey, I make a point, on returning to the yard, of blowing out the boiler until only an inch or two of water shows in the gauge glass. then fill up with clean water. If the water contains grit, as well as oil and mud, particles of the grit will be carried over into the steam chest and cylinders, due to the priming, and the valve faces and cylinder walls would be badly scratched and would allow the steam to blow past the piston and cause the valves to leak. The glands and packing would become hard by the action of the water present in the steam chest, and both cylinders and valve glands would soon begin to leak."
A Good Description of an Injector.
Question No. 7 (steam examination).—Explain the action of an injector, and state why an injector will not work if the water gets very hot.
SPECIMEN ANSWER.—"An injector is an arrangement for injecting water into the boiler against the pressure of the steam within the boiler. Steam is taken at one of the highest points in the boiler, so as to be as dry as possible' and is admitted through a steam valve to the centre cone of the injector, the annular space surrounding the cone of the injector being connected to the water tank. The steam rushes through the centre cone at a terrific speed, probably some thousands of feet per second, setting up a vacuum that causes the water from the tank to follow the steam jet. The water instantly condenses the steam jet, and the energy of travel of the steam is transferred to the water ; the hot water from the injector then pas,ses at a high velocity to the boiler, the energy, due to the velocity of the water, being sufficient to overcome the pressure in the boiler. The action of the injector is continuous, but it will not
work if the feed-water connections or the water tank gets hot. The feed water must be cold enough to condense the steam."
Clearance and Cushioning.
Question No. 13 (steam examination).—What do you tmderstand by the terms " clearance" and " cushioning "? At what part of the stroke does " cushioning" OCCUI SrEermEsi .ANSWER.—" What I understand by clearance is the space between the cylinder covers and the piston at each end of the stroke, and cushioning occurs just as a piston is approaching the end of its stroke. Cushioning is caused either by admitting steam early or closing the exhaust port early • in either case there would be a certain amount of steam under pressure in the cylinder, as the piston reached the end of the stroke, and the cushioning, if not too great, would prevent knocking in the engine." Steam-engine Valve Setting.
Question No. 15 (steam examination).—How would you set the valves of your engine so as to give best running results, and would you set them while the engine is hot or cold?
SPECIMEN ANSWER,—" Before setting the valves of a steam v,sigon engine, I should get all the parts up to the working heat, so as to allow of the fullest expansion of the spindles and rods. I should then eet the high-pressure crank on the top centre, and, after locking the high-pressure valve on its spindle, I should adjust it in the following manner. After putting the reversing lever in the dead centre, I should equalize the opening of the ports by moving the reversing lever backwards and forwards until the valve opens exactly the same distance each way, and should do this either by increasing or decreasing the length of the spindle in the sleeve. I should then treat the low-pressure valve in the same way."