Properties of Oils.—Owing to the very high speed at which the modern petrol engine runs great attention must be paid to lubricating the moving parts, otherwise undue wear or even seizure will result. We must be very careful to choose a suitable oil, one which is chemically pure and retains its lubricating properties at high temperatures. A considerable amount of oil finds its way into the cylinder, where it comes into direct contact with the hot gases. If an oil is heated a temperature will sooner or later be attained, when the oil will give off an inflammable vapour, i.e., one which will burn. This temperature is called the flash point of the oil. If the oil is likely to get into the cylinder of a petrol engine it should have a very high flash point; in fact, most of these oils do not flash until well over 400° Fahrenheit. Also when the oil is burnt it must not leave any appreciable residue. Some oils are very defective in this respect, and leave large quantities of carbon deposit on the metal walls of the cylinder and the valves; others again are gummy or too viscous even at high temperatures. Such oils must be avoided equally with those which lose their viscosity too much under heat.

Splash System of Lubrication.—One method of lubricating the working parts is known as the splash system. In this system oil is poured into the crankchamber and the moving parts dip into it, splashing it all over the interior of the crankcase and the lower portions of the cylinder walls. Oil holes are drilled in such positions that as the oil drops down again after being splashed upwards some of it will fall into these holes and lubricate the bearings. This is a very cheap method of lubrication in first cost, but very wasteful and unsatisfactory in regular use, hence it has practically died out. As the oil is used up a fresh supply must be admitted by some form of continuous drip-feed arrangement, the oil being forced over very often from a small tank on the footboard by means of air pressure or the pressure of the exhaust gases from the engine. It is very difficult under these circumstances to estimate how much oil is present in the crankchamber at any given instant, so that there was usually alternately too much or too little. Too little oil meant undue wear on bearings (perhaps seizure), and too much oil meant a smoky exhaust which became very obnoxious when the engine was suddenly accelerated.

Improved System of Splash Lubrication.—This is a combination of the splash system and the forced system, and is shown in Figs. 55 and 56. In these figures A₂ and A₃ represent two of the main engine bearings which support the crankshaft; C₁, C₂, C₃ are three of the crankpins; F₁, F₂, F₃ are oil troughs placed under the crankpins; D₂, D₃ are oil feed pipes to the main bearings. Generally speaking, the oil is drawn from the bottom of the crankcase by means of a pump, and this pump delivers the oil to some form of indicator mounted on the dashboard of the car. After passing through the indicator the oil flows by two main pipes, one of which feeds the main bearings by means of branches D₂, D₃, etc., and the other feeds oil troughs by means of branches such as G₂. When the troughs are full the oil overflows into the bottom of the crankchamber, and so there is always a constant depth of oil for the scoops attached to the connecting rod ends to dip into, and one great drawback to the splash system is overcome; also the main bearings are always sure of being amply supplied. The oil pump may be an ordinary plunger type pump or a rotary pump.

Forced Lubrication.—One system of forced lubrication is shown in Fig. 57. The general arrangement of the system is very similar to the preceding one, except that there are no troughs in the crankchamber and all the bearings receive an ample supply of oil under pressure so that the journals are supported in their bearings on a film of oil and the metals never come in direct contact with each other. After entering the main bearings the oil passes through holes drilled in the crankshaft and thus positively lubricates the crankpin bearing, passing up the connecting rod either internally as shown or by an external pipe it lubricates the gudgeon pin and then falls down into the crankchamber. On its way down it gets splashed about and thus lubricates the cylinder walls and piston; sometimes these are positively lubricated by leading the oil through the centre of the gudgeon pin direct to the surface of the cylinder walls—but this often gives an excess of oil and causes a smoky exhaust. In Figs. 58 and 59 we show two views of a very popular form of oil pump for forced lubrication systems. It consists of two gear wheels, one of which is driven by a spindle from the engine crankshaft, and it drives the second wheel by means of the projecting teeth. The oil is picked up by the teeth and passed round from the suction to the delivery side of the pump on the outer edge of the wheels; no liquid can pass direct across between the teeth which are in mesh, and hence the direction of rotation is as shown by the arrows.

The difficulty of securing a really good lubricant for petrol engines must be apparent from a study of the prices of the various oils. It will be observed that they are all considerably more expensive than petrol, and therefore we must economize in their use. The old splash system was very wasteful and consumed oil at the rate of one gallon every hundred miles at least, but a modern system of forced lubrication will not require more than one gallon of oil every thousand miles. Perhaps an average everyday figure for ordinary motor-car engines would be one gallon every 250 miles. The pressure of the oil in a forced feed system varies in different makes of engines from 5 up to 40 pounds per square inch—a very common figure, however, is 10 pounds per square inch. The speed of the oil pump also varies considerably, and ranges from 500 up to 2,000 revolutions per minute at normal engine speed. Generally a small relief valve is fitted in the pump casing, which returns oil to the crankchamber if the pressure tends to rise above the desired limit due to the engine speed increasing. We have mentioned already that the flash point is generally over 400° Fahrenheit when the oil is new, but after it has been in the crankcase some time and got used over and over again it is found that the petrol vapour leaking past the piston rings of the engine condenses when the engine cools down after a run and drops into the oil in the sump, thus lowering its viscosity and its flash point. According to Mr. Morcom it may come down as low as 200° Fahrenheit (about), but if the oil is heated and the petrol driven off the flash point goes up again. Therefore it is a good plan with forced lubrication systems to empty the old oil out periodically and fill up entirely with fresh oil.