CHAPTER II
DESCRIPTION OF A TYPICAL PETROL ENGINE
For the purpose of explaining the cycle of operations we have considered only a diagrammatic sketch of an imaginary motor-car engine, but in Fig. 7 we illustrate an up-to-date motor-car engine. In the first place we note the position and arrangement of the four water-cooled cylinders, A_{1}, A_{2}, A_{3}, A_{4}, containing their pistons and mushroom type valves. These are conveniently placed in a vertical position and mounted on top of the crankchamber C, to the bottom of which is attached the oil-base B. At the front of the engine are shown the timing wheels in their casing E, and at the rear end the flywheel F. The starting-handle connexion is at S, the fan pulley being shown at M. The high tension magneto which supplies the current to the sparking plugs is shown at H, and I is the induction pipe connected to the carburettor K. The water circulating pump is on the off side of the engine and does not appear in the illustration, but L_{1} is the inlet water pipe leading from the radiator (not shown) to the water pump, and L_{2} is the delivery pipe from the pump to the respective cylinder jackets, L_{3} being the outlet water pipe. The exhaust pipe is shown at D, and the oil pump at P. The valve springs, valve tappets and guides can also be clearly seen. In examining the several parts of the engine in detail we must not lose sight of their respective positions in the general arrangement view of Fig. 7.
=The Cylinder.=—Probably one of the most important parts of an engine is the cylinder. As we have already seen, it is inside the cylinder that the charge of petrol vapour and air is exploded and completely burnt. The heat energy of the petrol mixture which is liberated by the explosion is immediately transformed into mechanical work and propels the piston forward like a projectile from a gun. But we must also notice that our present-day arrangements (clever as they are) are by no means perfect, and we cannot, even under the most favourable circumstances, convert more than about _one-third_ of the heat energy of the petrol mixture into the mechanical energy of the moving piston. Of the remaining two-thirds of the heat, part is used up in heating the cylinder walls, the piston and the valves, and the remainder goes out with the exhaust gases to the silencer, finally escaping to the outside air. Thus two important facts are brought to our notice:—
(1) The reason why we use petrol to drive our motor-cars is because petrol (and certain other liquid fuels such as benzol, etc.) contains within itself a store of energy which can be liberated as heat when the fuel is burnt or exploded in the presence of air in the engine cylinder.
(2) At the present day, even with our most up-to-date contrivances, we cannot make use of two-thirds of the available heat in our petrol. Instead of being able to turn this heat into useful mechanical work, we are compelled to throw it away—to waste it. Further than that, we have to make special provision to ensure that it shall be wasted as quickly as possible and as easily as possible. 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. 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?
It is a good plan to extend this hollow chamber, containing the water in circulation, at least round the whole of the combustion chamber and all round the inlet and exhaust valve passages and down the barrel of the cylinder as far as the walls are likely to come into contact with the hot gases from the explosions. We refer to this hollow chamber, with its circulating water, as the _water-jacket_ of the cylinder. It is not absolutely _essential_ to have our cylinder water-jacketed, especially with small engines for motor-cycles and engines for aeroplanes which have revolving cylinders, but it is practically essential in nearly all other cases. Even in the special cases mentioned it is found necessary to form special heat radiating fins on the outside of the heated walls to assist in dissipating or getting rid of the surplus heat and preventing seizure of the piston within the cylinder. These fins are clearly seen on the cylinder of the motor-cycle engine shown in Fig. 13.
Thus we may say that motor-car engine cylinders are bound to be water-jacketed, i.e., to have a hollow space round them containing water in circulation. The cylinders themselves are nearly always made in the form of iron castings and the jacket spaces form part of the _cylinder casting_ as a general rule, but occasionally the water-jacket space is formed by attaching plates or tubes to the cylinder casting by means of bolts or screws—not an easy thing to arrange successfully, as it requires water-tight joints.
The procedure for manufacturing a motor-car cylinder is first of all to design and calculate the proportions of the various parts and get out a set of working drawings. From these drawings we get _patterns_ and _core-boxes_ made in wood. The patterns are the exact shape of the finished cylinder on the _outside_, and the core-boxes are the exact shape of the _inside_ of the finished cylinder (except in so far as allowance has to be made for parts which must afterwards be machined).
The patterns are pushed down into the moulding sand in the foundry, and when withdrawn leave their impression, thus forming _moulds_. The core-boxes are filled with sand, which when withdrawn furnishes us with masses of sand that are the counterpart of the interior of the cylinder in shape. These _cores_ are supported centrally in the mould (which is usually in halves, or _more_ than two parts), while the molten iron is poured into the intervening space to form the iron _casting_. When the casting has cooled down the sand can be cleaned off quite easily. One set of patterns and core-boxes will thus produce quite a number of cylinder castings, each being similar in every respect to the other, the process being a quick and fairly cheap method of reproduction. Later on the cylinder barrel has to be machined and bored out true to very fine limits by the use of boring tools and some kind of boring machine or lathe. The flanges or flat faces have to be planed true in a planing machine and the valve stem guides and valve seatings must be carefully and truly machined to correct size and shape.
Figs. 8 and 9 show two views of a single motor-car engine cylinder, the water-jacket forming part of the cylinder casting. In the figures C is the cylinder barrel or bore; J the water-jacket; I the inlet for the jacket water; O the outlet for the jacket water; D is for the compression tap; S for the sparking plug; V_{1}, V_{2} are the valve seats; G_{1}, G_{2} are the valve stem guides; H_{1}, H_{2} are caps which may be removed when the valves are being put in or taken out; f_{1}, f_{2}, f_{3}, f_{4}, f_{5} are called flanges. The flange f_{1} is used for attaching the cylinder to the crankchamber; while it is quite true that the force of the explosion within the cylinder drives the piston downwards, it is equally true that it also tends to force the cylinder head off or to blow the cylinder casting upwards off the crankchamber, unless it is securely fastened to it by means of screws or bolts passing through the flange f_{1}. The flanges, f_{2}, f_{3} are for the inlet and outlet water pipe attachments, and f_{4}, f_{5} are for the induction pipe and exhaust pipe connexions. Generally the pipes will have flanges and be held tight against the flanges on the cylinder casting by means of screws or studs. Figs. 10, 11, and 12 show how two metal flanges are held in contact by means of screws or studs or bolts, and they also show the _packing materials_ between the metal surfaces which keep the joint _tight_ and prevent water or gas leaking across the flanges and escaping to the outside air, or air leaking in if the internal pressure is below that of the atmosphere.
In Figs. 8 and 9 the valves are placed one on each side of the cylinder, this form of cylinder being known as a =T=-headed cylinder, but it is rather more usual here in England to place both valves on the same side of the cylinder and next to each other as indicated in Fig. 13, this form of cylinder being known as an =L=-headed cylinder. The chief object is of course to avoid the use of two valve shafts and also to produce a neater looking engine, but the _T_-headed design is better cleaned or scavenged by the passage of the inlet and exhaust gases. When a motor-car engine has two cylinders we frequently find them both in a single casting, having a common water-jacket, and then we say they are _cast in pairs_. A four-cylinder engine may thus have: (1) Cylinders _cast separately_; (2) Cylinders _cast in pairs_; (3) Cylinders _cast en bloc_; or all four in a single large casting. The third method is cheapest in first cost, but in the event of breakage will become the most expensive. The second method is a sound compromise.
An example of a _built-up_ cylinder and water-jacket is shown in Fig. 14, the cylinder barrel being of steel tube with steel flanges, and the water-jacket being formed by copper tube slipped over the outside of the steel cylinder. Its great advantage lies in the reduction of weight, and it is thus largely used for aeroplane work. The valves would then be fitted in the top cover of the cylinder and driven by overhead gearing.