ments on the pump, so that the speed of the engine can be varied by the engine tender while the engine is running. An oil engine with a governor and pump built on this principle not only makes the speed of the engine steady but is very economical of fuel.

About Cooling Cylinders.-The only oil engine that I know of which is air-cooled is the little high speed Delco-light, all the others of whatever size and type, being water cooled.

In the first named a draft tube sets over the cylinder of the engine; the top of it is open and the lower part connects with a housing around the flywheel which is built like a fan. When the engine is running the fan sucks the air down through the draft tube and out into the open air.

In some oil engines the end of the cylinder in which the explosions take place, or combustion chamber, as well as the cylinder itself is jacketed, that is, they are surrounded by spaces through which the water circulates, and in this way the heat of the whole cylinder is kept down to as low a temperature as it will operate with.

Since a lot of heat is needed to convert heavy fuel oils into gas, some oil engines are made with the heads of the cylinders exposed directly to the air. In hothead engines, as these are called, the fuel oils are exploded at a very much higher temperature than in engines with water-cooled heads, and in this way the carbon is burned up.

In some of the larger engines water is injected into the cylinder, not with the idea of preventing premature explosions, but to keep down the temperature.

The circulation of water around the cylinders is done in the same ways as those used for gas engines and stationary gasoline engines.

The Lubrication of Oil Engines.-The same schemes are used for lubricating oil engines that are employed for gas and stationary gasoline engines, that is, the upright engines have the splash system and the horizontal engines are fitted with the mechanical force feed system.

Where force feed lubrication is used the oiler is worked by the camshaft so that the amount of oil supplied depends on the speed of the engine and this means economy in lubricating it.

A Word on Fuel Oils.-Kinds of Fuel Oils.Small engines run better on kerosene than on heavier oils, but the larger heavy-duty engines work well on what is called crude oils.

There are a couple of dozen names for various kinds of fuel oils, but the following will serve to show the general difference between them. Crude oil is the oil right from the oil wells, and this used to be the cheapest kind of fuel oil you could buy, but the distillates are more generally used at present.

When crude oil is refined, which means that it is distilled, a number of fuel oils are given off and the

gravity, that is, the density, of these is different, and the flash points are also different.

The name distillate is given to any kind of oil which is obtained by refining crude oil. When crude oil is refined benzine passes off first, then gasoline and next kerosene. Gasoline is called petrol in England, and on this side low grades of gasoline are called naphtha and motor spirits.

Kerosene, or coal oil, is heavier than gasoline, and when it is bleached white and has a high enough flash point it is used in lamps for lighting. When it is of poor quality and colored it is called solar oil, and when it is of a still poorer quality it is called engine distillate. Gas oil follows kerosene when crude oil is distilled and so, of course, it is heavier.

Fuel oils, in the strict meaning of the term, include all kinds of oils that do not need to be refined to burn. Tops, bottoms, residuum, stove oil and star oil are all cheap oils. Other oils made from coal tar and the by-products obtained from city gas plants can also be used for fuel for oil engines, while alcohol, where it can be bought cheaply enough, makes an admirable fuel.

Kind of Fuel Oil to Use.-Use kerosene only in engines where the makers specify this kind of fuel oil. For regular oil engines the fuel oil can be of any kind provided that it has (1) a flash-point below 275 degrees Fahrenheit, by what is known as the open cup test; (2) a specific gravity not lower than 26 degrees, Baume; (3) it should contain not

more than 12 per cent of sulphur; (4) contain not more than 1/2 per cent of water, and (5) not more than 3 per cent of coke. Finally, at least 60 per cent of the oil must pass off when it is distilled at or below 680 degrees Fahrenheit.

CHAPTER VII

THE DIESEL SELF-IGNITING ENGINE

You will remember I told you in the first and second chapters about how the Diesel oil engine fired its fuel charge by compressing the air in the cylinder, which raised its temperature, and you know that all other types of internal combustion engines use some kind of an igniter for this purpose.

Now, while this difference in the methods of ignition between the Diesel and other oil engines is the most important one, there are several other factors that are found in the former which are of interest, and these will be described in due time as we push along.

If you ever had anything to do with a gasoline or oil engine or drove a motor car you know that when the engine is running at low speed with the throttle wide open and the engine is laboring under an excessive overload it will begin to knock, and this is caused by premature ignition. Now, premature ignition has long been a great problem in building internal combustion oil engines, and it was only solved when Diesel invented the engine now under