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 be low 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 2 per cent of sulphur; (4) contain not more than 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

discussion. The Diesel engine is the most economical engine that has yet been made for producing power where crude fuel oils can be had to run it.

Diesel engines are built in both the vertical and the horizontal types, have one or more cylinders, and are made on both the two and four stroke cycle principles.

How the Diesel Four Stroke Cycle Engine Works.

In this engine the cycle of operations is as follows: (1) on the suction stroke the air inlet valve opens and air is drawn into the cylinder; at the end of the suction stroke the inlet valve closes, and (2) on the compression stroke the piston compresses the air to upwards of 500 pounds to the square inch, and this raises the temperature to about 1,000 degrees Fahrenheit, which is far above the ignition point of the heaviest fuel oils.

Just before the end of the compression stroke, the fuel valve is opened and the fuel oil, which is injected under pressure into the cylinder in a fine spray, instantly catches on fire; then (3) the power stroke begins, and the spray of fuel oil is injected during about of the length of this stroke.

Hence, the oil burns instead of exploding, as in gas and gasoline engines, and the result is that the temperature of the fuel is kept about the same and the pressure on the piston is more nearly uniform throughout the length of the stroke instead of being set up by a sudden blow, as in an ordinary gas or gasoline or oil engine. This makes the engine run