Motor Notes

Wanganui Chronicle, Volume LXXXI, Issue 19032, 7 June 1924, Page 9

Motor Notes

"WHYS?” OF MOTORDOM.

WHAT EVERY DRIVER SHOULD KNOW. PLAIN ANSWERS TO ’ SIMPLE QUESTIONS. Why does an engine knock when it is carbonised? The real answer is that the presence of the carbon makes the explosion more violent. The knock actually heard is the blow of the expanding gases hitting the cylinder head exactly like an ocean wave slapping up against a stone breakwater. Just as the striking of a mass of water against a wall can produce a large thud, so can the breaking of a wave of high pressure gas against an iron wall produce a sharp rap. So far, so good, but now wo approach the difficult stage. Why does the carbon so alter the nature of the explosion? First, because it is a poor conductor of heat. The excess heat ot tne combustion cannot escape through the carbon layer and be carried oft by the water in the jacket. It is retained within the burning gas, and. by superheating it makes it burn faster and ever faster.

Secondly, if there is enough carbon, the total size of the combustion space into which the charge is compressed is reduced. Quite a thin layer of carbon is enough to increase the compression ten pounds per square inch, and that in itself makes the explosion more rapid. Thirdly, the carbon being a poor conductor and getting heated itself, imparts extra heat to the new charge during compression, and for that reason intensifies the other actions. Now for a popular fallacy. Carbon does not become incandescent and cause preignition. To make such a thing possible vastly more carbon is necessary than ever is allowed to accumulate in any motor engine. That explanation is an old and utterly abandoned idea.

This explanation of the cause of carbon knock also tells whv ethvl gas and other treated petrol do not so readily knock. Fuel from certain sources, notably some Californian crudes, benzol, and other petrol mixtures and petrol treated with minute quantities of other things, like the tetraethyl lead compound, all have the characteristic of burning more slowly than ordinary straight petrol. So the effect of the carbon is offset or more carbon is needed or a higher compression before the point arrives where they, too, will knock. Dynamite explodes much more violently than gunpowder, so violently that it cannot be used in guns, but the essential exploding element of dynamite, nitro glycerine, can be made to act more gently by combination with other substances, and so become available for use in cannon. By treating petrol we do not reduce the energy or power, we merely make the release of the power more like a push and less like a kick.

THE MISFIRING OF A COLD ENGINE. Why does a cold engine spit and splutter? Because cold air and “cold petrol do not mix readily. This means that the mixture in the cylinder contains less petrol. Note the use of the word mixture, for the cylinder can be drenched with liquid fuel, and yet the air may take but little of it up into the mixture, and it is only the mixture of air and gas together that will ignite. Now. if the mixture is very weak very low in petrol, that is—instead of exploding, it burns auite slowly, more like the gas flame of a kitehen ring-, burner. This slow burning will con-' tinue all through the firing stroke, and be still going on when the intake valve opens. The intake manifold, of course, is full of uncompressed, but still burnable, petrol and air. Letting the flame out of. the cylinder into the manifold, sets alight the contents, and a puff back of flame and part air is blown out through the carburetter. This is the spitting noise. It sounds dangerous, but the actual flame seldom reached the carburetter itself, and. even if it does, everything being cold.. the chance of setting Are to the fuel in the float bowl is entirely negligible. " 6 THE CAUSE OF CARBONISATION Why does carbon form in an engine? Simply because the petrol is not fully burnt. It is exactly like the soot that collects in a chimney. Soot will burn, and so will any sort of carbon, even a diamond can be burned up without much difficulty. Petrol contains a great deal of carbon. Burning petrol first of all releases carbon, in the form of extremely fine soot, and then, if there is time enough and enough air present, the soot burns and forms compounds of carbon and oxygen that are gases. In a motor engine much of the soot burns; much of it is deposited inside the muffler, and some of it stays in the cylinder, being caught by the oil that is there. The oil itself is only responsible for a very small part of the carbon that is formed. What happens when carbon is burned out? Burning carbon out is simply doing in one operation what would have been done by each and every explosion in the engine had there been enough oxygen present to combine with all the soot. The operator drops a lighted match in the cylinder, and then turns a jet of compressed oxygen upon it. This produces intense heat at a tinv point, and sets alight the carbon. The carbon burns slowly, and as the jet of oxygen is moved.around if the operator is skilful enough all traces of deposit are removed. Only just the spot on which the jet of oxygen plays is burning at any one time, so the burning out is necessarily slow, and very little heat is produced.

is always sojne oxygen in the exhaust gas which scours past the valve. When oxygen comes against very hot iron or. steel it inevitably combines to some extent and forms an oxide. Also the principle substance formed by the burning of petrol is water. Exhaust gas consists of a little more than half water, in the form of super-heated steam. When an engine is stopped and cools down, usually a little water condenses in the cylinder, and especially in the exhaust port and manifold. This, of course, will rust the valves directly. Then, especially with low grades of fuel, there is often a trace of vitriol formed by the combustion. Sulphur in the petrol combines with the water formed, and makes sulphuric acid. The quantities that can possibly be made in this way are excessively minute, but still enough to put valves and seats after several thousands of miles. Finally, there is direct scouring action of tiny particles of dust and grit that come in with the air, and are not burned Iry the explosion. The tremendous blast of gas that rushes past |he valve when it opens carries always a tiny trifle of grit, and, after hundreds of thousands of revolutions, some wear will take place. These things explain why it is worth while to use very special sorts of steel for exhaust valves. Steels with tungsten or chromium or other rare metals in them resist heat. resist rust, and even resist mechanical wear, much better than ordinary steels. Without special alloy valve steels, the aeroplane engine would be an impossiblity.

H.T. MAGNETO TROUBLES. A trouble not at all uncommon on high-tension magnetos is that of the contact breaker arm becoming fast on the small spindle on which it rocks. This arm is bushed with a small fibre bearing. If the magneto has got damp at all, the fibre tends to swell and bind on the spindle, or the spindle becomes corroded and sticks in the bush. In either case, the rocker arm remains open, and the magneto “ceases fire.” The remedy is to reamer out the bush with a small round file. Another cause of trouble, particularly in the starting, is the sticking up of the carbon brush, which takes the current from the slip ring. If there is any wear in the armature bearings, there will be a slight up-and-down movement, so that the brush remains fast at the highest, and does not touch at the lowest point. The current would jump the gap at high speeds but not at hand cranking speeds. EMERGENCY RADIATOR HOSE. The radiator hose is one of the most neglected parts on the average motor. Placed in a position where oil and grease is continually splashed on it. it is seldom inspected, though it is known that oil rapidly decays rubber. As usual, the hose lets go many miles from nowhere, which means that some way must, be found to bring the car back to civilisation. A section of an inner tube, doubled over in each end, will answer the purpose. Though it may not be desirable to cut a tube, nevertheless, the cost of a now tube will frequently be less than the towing or services from the nearest garage, not to mention the inconvenience which may be avoided.

SELF-STARTER. AVOIDANCE OF ABUSE. The novice who has justbrought a car fitted with an electric self-starter is often guilty of over-estimating the abilities of this contrivance. Ono ma£ perhaps be excused for imagining that the self-starter wil perform its duties under all conditions, but, in fact, one has to exercise a good deal of discretion concerning its use. If allowed to stand for a long time in a cold place, the engine will become slightly gummed up, and then it will be found that the action of the starter is inadequate. The pressure on the start button will do little more than produce a few spasmodic jerks on the flywheel, and not give sufficient impetus to get a. start. When this happens, the only thing to do is to resort to the starting handle and give the engine a few good swings to ease the oil. After which the starter will take up its work without demur.

A word of warning should be given against the common fault of flooding the carburettor excessively if the engine fails to start at the first attempt. Apart from filling the Cylinders with a. mixture that is far too rich to ignite properly, it is most probable that a portion of the near petrol thus introduced will pass the piston rings and mix with the lubricating oil in the base chamber. There its presence will V’tiate the lubricating properties of the oil to an alarming extent, and render it totally unable to provide the necessary protective film on the moving parts of the engine. Several cases of seizure have been placed on record simply because owners have persisted in flooding the carburetters of their cars when attempting to start, instead nf taking steps to ensure that everything else is in perfect order.

FIRESTONE TEST In our issue of last Monday, June 2, appeared a cabled report of the result of the 500 mile motor race, held at Indianapolis on May 30. For the information of our readers we repeat it: At Indianapolis, Joe Boyer, of Detroit, driving relief, piloted L. Coran’s eight-cylinder speedster to a record-breaking victory at the twelfth annual automobile race of 500 miles. Coran drove the first portion of the race. Earl Cooper was second and Jimmy Murphy third. The winner’s time was 5 hrs. 5 mins. 234 secs., or an average of 98 miles per hour, more than 34 miles an hour faster than the previous record. Henry Ford was referee. He insured 22 drivers’ lives for 10,000 dollars each. There were no accidents.

Further particulars of this great race are contained in a cablegram received from Akron and handed us by the local agents for the popular Firestone cord tyres. It reads as follows: — “Joe Boyer, driving Duesenbcrg Special, wins 500 mile Indianapolis, race, May 30, on Firestone gumdipped cords, sets new record 98.24 miles per hour. Cooper-Studebaker Special second, Murphy-Miller Special, third. First ten cars all equipped Firestone.” Surely a most outstanding perform-

It is also interesting to note that Eddie Hearn, veteran race driver, who was, last December, awarded the American National Speedway Championship for 1923, chose Firestone gumdipped cords as his tyre equipment in all his races.

Race-driving is admittedly the severest kind of test and Firestone gum-dip-ped cords and steam welded tubes are the choice of champions. These outstanding performances are but further demonstrations and actual proof of the superiority of Firestone products. The local agents for these famous tyres are Messrs Stapleton’s Vulcanising Works, St. Hill Street. They are pardonably proud of the performances of the make of tyres they represent and will be pleased to give those interested any further information that may be desired.*