Fig. 58. and Fig. 59. Two Views
of a Rotary Oil Pump for
Forced Lubrication.
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.
[CHAPTER VIII]
COOLING
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.
Fig. 60.—Thermo-syphon Water Cooling System.
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.