We have already explained the necessity for cooling the cylinders of a petrol engine by means of a water-jacket, and we now proceed to show how the circulation system may be arranged. There are two forms of circulation in use: (1) Natural; (2) Forced.

Natural or Thermo-Syphon Circulation.—This system is shown in Fig. 60, and may be explained as follows:—The heat generated by the successive explosions within the cylinder causes the water at the top of the cylinder jacket A to get hot. As a column of hot water is lighter than one of cold water of equal height, the heated water rises up the pipe B and flows into the top of the radiator D, while colder water from the bottom of the radiator flows up the pipe C and into the cylinder jacket A. It is important that the height of the water in the radiator D should be at such a level that the outlet from the pipe B is submerged.

In the radiator the water falls through a series of tubes E, having gills or fins on the outside for the purpose of dissipating the heat. The cooling of the water is also assisted by the fan F, which is driven from the fan pulley G and draws air past the radiator tubes at high speed. Sometimes the water in the radiator is made to fall through a series of cells which are formed of cast aluminium; such a radiator is called a honeycomb radiator. It is important that the pipe C should not have any sharp bends and it should not rise very much in height, but the outlet pipe B may have a considerable rise with advantage. Both the inlet and outlet pipes should be of large diameter with this system of circulation, and the radiator should be so arranged that there is a good head of water above the cylinders. In the drawings H is the front cross-member of the chassis, K is the starting-handle clutch, and L is the starting handle.

Forced or Pump Circulation.—With this system the water is positively circulated through the jackets; it is drawn from the bottom of the radiator by the pump P (Fig. 61), which is mechanically driven from the valve shaft of the engine, and delivered under pressure to the jacket A. The outlet of the pipe B need not be drowned, and the pipe C may be arranged in any way most convenient to the chassis. Sometimes when a pump is fitted the pipes are arranged so that the system may be operated as a thermo-syphon in the event of a breakdown of the pump. It is not uncommon to experience trouble due to leakage at the pump gland, which results in gradual loss of water from the system, and therefore the thermo-syphon or natural circulation has much to recommend it. Also it may be said that the pump represents an additional complication to the engine and means increased first cost. Every moving part we add to the engine is of course an additional potential source of trouble, but the addition of a really first-class water circulating pump of the type shown in Fig. 58 cannot be said to be anything but a reasonable precaution. The weight and size of every part of a motor-car engine and chassis have been so much reduced recently, owing to competition with American firms, that many manufacturers who adopted the thermo-syphon principle experienced great trouble with it owing to the small size of radiator fitted, as well as faulty arrangement of the connexions. Considering any one engine, it follows that if a certain size of radiator and a given quantity of water in the circulating system will keep the engine cool when a pump is used to give a positive circulation, then a larger radiator and greater quantity of water will be required for natural circulation. Thermo-syphon circulation also means a high radiator and bonnet, which many people object to on the score of appearance, without considering its utility. With natural circulation greater care must be exercised to keep the radiator well filled, but this often leads to other difficulties on bad roads owing to the water splashing from the overflow pipe and finding its way on to ignition appliances. Before starting an engine it is always advisable to remove the radiator filling cap and examine the water level; if it should happen that at any time while the engine is running the circulating system runs quite dry, owing to a breakdown or leakage, do not attempt to pour water into the radiator, but simply raise both sides of the bonnet and leave the engine to cool down first. 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. Glycerine or alcohol added to the water will prevent it freezing, but as an additional precaution in cold countries one often sees travelling rugs strapped over the radiator and bonnet.

Occasionally one gets trouble due to the water boiling in the jackets, and on this account reasonable care should always be exercised in unscrewing the radiator filling cap if the presence of steam is suspected. An engine may have been running well for a long time without trouble and then develop symptoms of overheating in the circulation system. This overheating may be either local or general. Local overheating may result from some partial seizure of the piston in the cylinder due to dirt on the walls, or from the presence of grease on the outside of the cylinder walls, in the jacket space. If grease is suspected or there is furring up in the passages of the jacket due to bad water supply, the trouble may be cured by adding some common washing soda to the water in the radiator and running the engine with the car at standstill for half-an-hour or so. After this drain off all the water and sludge, allow the engine to cool down, and then fill up again with clean water.

General overheating may result from leaky pistons and pistons rings, or from the use of too weak a mixture in the carburettor, or from overloading the engine. If the mixture supplied to the engine is very weak, the overheating will be very marked on the exhaust side of the engine. Local overheating causes the engine to “knock” badly.

In arranging the jackets and the pipes care must be taken to arrange that a cock is placed at the lowest point in the system, so that the whole may be completely emptied, and the inlet pipe to the jacket should enter at the very bottom of the jacket chamber for the same reason. It may be thought that all that is necessary is to provide plenty of space in the jackets round the cylinders and plenty of water in the whole system, but experience shows that it is very important not to make the jacket space too large, so as to ensure positive circulation and avoid local circulation in any one portion of the jacket. When cylinders are cast in pairs the back pair have a tendency to discharge their hot water into the front pair and so back to the inlet pipe again, hence this should be guarded against in arranging the outlet pipes.

Pipes suitable for use with multi-cylinder engines are shown in Fig. 62, in which (a) is an outlet pipe for a monobloc casting, and (b) and (c) are inlet and outlet pipes respectively for engines having separate cylinders. It is advisable to modify the diameter of the branches by the insertion of metal orifice plates at the flanges to ensure an equitable distribution of the water among the several cylinders.

Fig. 62.—Forms of Water Piping.