The Petrol Engine / A Text-book dealing with the Principles of Design and Construction, with a Special Chapter on the Two-stroke Engine

by Francis John Kean

Fig. 7.—General Arrangement of a Modern Petrol Engine.

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.