Francis John Kean

“ A single cylinder engine would only be used for a motor-cycle or a small car of low power; the vibration and noise resulting from the use of a single cylinder petrol engine of even six horse-power are most objectionable, and difficulties of starting and risk of engine unexpectedly pulling up and stopping are greatly enhanced. The two-cylinder engine offers better prospects, and was for some time considered quite good enough for most purposes, but owing to its comparatively bad balance and its low torque it has fallen into disfavour. ”

“ When the engine is at rest it is easily seen that the pressure of the air in the choke tube is atmospheric, and that the pressure above the liquid in the float chamber is also atmospheric, but when the engine is running it draws air up the choke tube at a very high speed and thus causes a partial vacuum round the petrol jet, and therefore the petrol spurts out of the jet under the pressure difference which then exists and issues in the form of a fine spray which is readily vaporized. ”

“ The problem of carburation consists in forming a mixture of the correct strength and character. Air may be carburetted by passing it over the surface of liquid petrol in a surface carburettor, or by drawing it over or among wicks saturated with liquid petrol as in the wick type of carburettor, but both these methods have been largely superseded by the use of what is now known as a jet or spray type of carburettor, in which the petrol is sprayed from a fine jet and mixes with air which is passing up rapidly round the outside of the jet. ”

“ When no high tension distributor is fitted there must be a separate coil for each cylinder, and the high tension wire runs direct from the coil to the sparking plug, so that the character of the spark as well as the exact instant at which it occurs may not be the same in each of the cylinders. If there is a high tension distributor it should be mounted on the same driving spindle as the low tension contact breaker, in order that the ignition may be synchronized, i.e., the spark will occur at the same point in the piston’s stroke for all the cylinders. ”

“ A four-cylinder engine is nearly as good as a six-cylinder of equal power, and is of course much cheaper in first cost, takes up less room, and weighs less. A good four-cylinder engine will often prove more economical in running costs than a six-cylinder, as it will probably be running a greater length of time at or near its full output, and the work done on the idle strokes of the cycle will be less owing to the smaller number of cylinders. ”

“ There is nothing to choose between poppet and sleeve valves on the score of economy in running.
Economy and Durability.—A good modern petrol engine of reasonable size—say over 3 in. bore—will give one brake horse-power for an hour from the consumption of two-thirds of a pint of petrol. This means that an engine giving 121⁄2 horse-power on the brake would use a gallon of petrol every hour. But economy in petrol consumption is not the only desirable feature of a petrol engine. There must be economy in lubricating oil and in cost of replacements or repairs. ”

“ It has long been known that work can be turned into heat, and the petrol engine is a good example of the reverse process which consists in turning heat into work. In a steam engine and boiler plant the heat of the fuel is liberated under the boiler, and then a portion of it gets transferred to the water in the boiler and forms steam, which is then taken to the engine and does work in the cylinder, the whole being a wasteful process. The petrol engine is an internal combustion engine, or one in which the fuel is burnt inside the engine cylinder itself and converted directly into work. ”

“ If too high a delivery pressure is used the fresh gas will enter in a sharp gust and get badly contaminated by mixture with the foul exhaust products instead of gently displacing them in bulk. The use of an automatic valve is very desirable for the gas inlet to the crankchamber, but unfortunately it limits the speed of the engine and also its flexibility or ability to pull well at all speeds. An engine with an automatic valve runs best at that speed for which the tension of the spring is most suitable. If the spring is weak the speed will be low. ”

“ Stiffness is due as a rule to lack of oil on the cylinder walls, caused by absence of oil in crankchamber or the film of oil on the cylinder walls having been washed off when priming the engine with petrol in attempting to start it. If a connecting rod is bent, or the crankshaft distorted or a piston ring broken, stiffness will also be noted. Very often by removing the valve caps and pouring a teaspoonful of oil or paraffin into each cylinder the engine may be freed by vigorously turning the starting handle by hand until the cylinders and pistons are well lubricated. ”

“ On the suction stroke of the engine the piston has to be pulled down, as we have already seen; on the explosion stroke the greatest pressure acts on the piston and pushes the connecting rod down. Thus sometimes the connecting rod is being pulled and at other times it is being pushed; in each case it has to overcome the resistance of the engine and drive the car. ”

“ Before closing this chapter one may add a few words on carburation and ignition for two-stroke engines. A four-cylinder two-stroke engine should have cranks at right angles to secure the maximum torque on the shaft. Looked at in end view the cranks form the four arms of a cross and thus four impulses are given every revolution, but as the ordinary magneto only gives two sparks in every revolution it must be driven at twice the crankshaft speed. ”

“ To ascertain whether the engine is firing regularly on all cylinders, or to detect which cylinder is misfiring, the best procedure is to open the compression taps in turn while the engine is running and in each case speed up the engine while you have the tap open. Cylinders which are firing well give a sharp cracking noise, those which are not firing merely give a hissing noise. If no compression taps are provided, each plug must be short-circuited to the frame in turn by the screwdriver method given above. ”

“ We have seen how the rotating parts of the engine can be balanced, but we have not considered the reciprocating parts. To understand this question of balancing we must talk about “inertia forces.” All bodies possess inertia, that is, they resent changes in their state of rest or motion. If a body is moving uniformly it tends to keep on doing so, whereas if it is at rest it tends to remain so. To start the body off from rest, or to stop the body and bring it to rest, requires a force to be exerted, and this force may be called the inertia force. ”

“ Again we make large exhaust valves and provide a free escape through the silencer for the exhaust gases, so that when we have snatched our useful one-third of the heat supply we may throw the remainder away into the atmosphere as rapidly as possible.—this part is of no use to us, we cannot turn it into work, then why let it stay here and heat our cylinder walls and piston still further? ”

“ Crankshafts for high-speed engines have always to be very carefully balanced, otherwise the engines will never run satisfactorily, the want of balance being greatly aggravated as the speed of rotation increases. Fig. 25 shows how the crankshaft of a two-cylinder engine may be balanced by drilling holes in the flywheel and fan pulley respectively, but the same effect may be produced by attaching balancing masses—this latter method would, however, be more inconvenient and expensive. ”

“ At low engine speeds there is very little suction or vacuum effect on the jet, but at high engine speeds with full throttle opening the maximum suction of the engine is exerted upon the jet. Thus at low speeds with this type of carburettor we do not get enough petrol out of the jet, and at high speeds we get too much, which results in too weak a mixture at low speeds and too rich a mixture at high speeds. ”

“ After the engine has been running for some time at a high speed this deflector becomes very hot, and as a general rule the cooling effect of the incoming gases is not sufficient to prevent it attaining a red heat on the compression stroke, thus igniting the charge before the piston reaches the top of the stroke. This defect, which is called pre-ignition, causes the engine to knock, and results in a loss of power; it may be partly overcome by admitting lubricating oil with the charge, the oil then serving to cool the deflector as the charge enters the cylinder. ”

“ When a petrol engine is running at high speed the piston has to be started and stopped at the top and bottom of its stroke every time the crankshaft revolves once, and to do this very large forces are needed, because it has to be done so quickly. ”

“ Again, when filling the radiator for a forced circulating system, it is desirable to give the engine a turn or two with the starting handle occasionally to operate the pump and prevent air locks; very often the radiator appears to be full, but as soon as the engine commences to run the water disappears owing to the system not being full, due to the above-mentioned cause. In cold or frosty weather all the water should be drained off from the circulating system when the car is in the garage, unless the garage is heated or some anti-freezing solution is used. ”

“ The float being hollow and made of very thin sheet metal, displaces a very large quantity of liquid in proportion to its own weight, and is therefore very buoyant. The buoyancy of the float will, of course, depend on the density of the liquid in the float chamber, and it will naturally sink deeper down into petrol than it would into a heavier spirit such as paraffin or benzol. ”

“ Now if we make the metal of the piston too thick so that the waste heat cannot pass quickly through it and dissipate itself at cooler parts of the engine, then the bulk of this heat will be concentrated in the piston head, causing it to expand and become a tight fit in the cylinder, as the cylinder walls are fairly thin and in contact with the jacket water which keeps them fairly cool and prevents them expanding much above their normal size. ”

“ In the new engine the charge enters at the head end of the cylinder, there is a special transverse combustion chamber, and many improvements are introduced in the scavenging and flow of gases; also there is no deflector at all on the piston head. Funds have not yet been secured to enable an experimental engine to be constructed, but it is to be hoped they will be forthcoming, for the benefit of the motor-car industry generally, as the future undoubtedly lies with the two-stroke. ”

“ As the piston moves rapidly up the cylinder on the exhaust stroke it pushes the gases in front of it out through the exhaust opening, but when it gets to the top of its stroke the piston stops and then comes down again for the suction stroke, whereas the gases will tend to keep on moving if they are not unduly restricted in their passage through the exhaust system, so that we can generally obtain some slight advantage by giving the exhaust valve a small amount of lag in closing. ”

“ A favourite and fairly successful device is to make the crankchamber gas-tight and use it as the cylinder of the pump, the underside of the engine piston then forming the pump piston which draws the charge from the carburettor into the crankchamber on its upstroke and compresses it on its downstroke, delivering it to the working cylinder through the inlet port as soon as the piston has uncovered it by its downward movement. ”

“ If a man understands, for example, the construction of the elements of a carburettor and how they ought to perform their several functions, he should have no difficulty in understanding any special type of carburettor placed upon the market. In dealing with the subject of ignition I have purposely avoided any detailed explanation of the manner in which the spark discharge is produced, because I felt that it introduces new ideas and probably causes the reader to lose sight of the fact that the magneto is only, after all, an accessory, although of course a most important one. ”

“ At high engine speeds there is great risk of the hot exhaust gases in the working cylinder setting fire to the incoming charge in the inlet ports, thus causing backfiring into the crankchamber. To avoid all possibility of backfire, some form of air scavenging must be adopted, but the general arrangement of this form of two-stroke engine does not lend itself to such an addition—it would merely reduce still further the quantity of gas reaching the cylinder. ”

“ The volatility of alcohol is very poor compared with petrol or benzol, and it generally contains some water in suspension. It will stand double the compression pressure of petrol without pre-igniting. Alcohol is not so liable to deposit carbon as petrol or benzol, but is very liable to cause rust. It is not obtainable as a fuel in Great Britain at present, owing to the high duty on it. Engines for use with alcohol ought really to be specially constructed for the purpose. ”

“ Each metal segment is electrically connected to a sparking plug lead such as L1, L2, L3, L4. The high tension electrode D is attached to a light carbon brush which presses on a gunmetal collector ring at the high tension end of the armature winding. A special terminal is provided at P, so that when a wire is attached to it and connected to the frame of the engine (usually through a switch) the low tension windings are short-circuited or closed on themselves, and the make-and-break has no effect, because there is always the path through the switch until it is opened again. ”

“ The principal method of producing petrol is by distillation of crude petroleum. The best mixture to use in a petrol engine is one composed of 2 cubic feet of petrol vapour to every 98 cubic feet of air. Petrol does not require any heat to vaporize it under ordinary atmospheric conditions. Pre-ignition of the charge is liable to occur if the compression pressure exceeds 100 lbs. ”

“ We take out the greatest amount that we can possibly turn into work and then hasten to dissipate the remaining two-thirds. We cast hollow chambers on the outside of our cylinders through which we circulate cold water to keep down the heat in the cylinder walls; if our cylinder walls and piston get too hot our engine may seize up, therefore we must cool them to ensure satisfactory running. ”

“ Thus, even allowing for the double number of power impulses resulting from the use of the two-stroke cycle, it is difficult to see how this form of engine could ever give more than about one and a quarter times the power of a four-stroke engine having the same bore and stroke even when the many difficulties experienced in the practical working of two-stroke engines have been overcome. ”

“ It can be proved that this type of engine which endeavours to draw sufficient gas to fill its working cylinder into the crankchamber by means of a piston having only the same diameter as the diameter of the working cylinder itself, and which cannot avoid some idle movement during the operation together with further loss from the exhaust opening, is incapable of retaining more than a little over one-half a cylinder full of fresh combustible gas at the instant when compression commences ”