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case. On reaching its lowest dead center, that is, its lowest point, the piston opens the duct, or passage, between the crankcase and the cylinder when the compressed fuel mixture in the crankcase rushes into the cylinder above.

How a Diesel Engine Works.-The salient feature of this engine is that it does not use an igniter of any kind to fire the fuel charge, but it does this by simply compressing air in the cylinder.

It is a well-known fact that when air is compressed its temperature is raised. As a simple illustration, if you will take a toy pop-gun, plug up its free end tight and then compress the air in it by pushing the piston down quick and hard, you will find that the barrel gets quite hot. Another and more striking experiment of the same kind is to ignite a bit of tinder, or a pellet of cotton, soaked in ether by means of a simple apparatus called a fire-syringe.1

Now this principle relating to pressure and heat is taken advantage of in the Diesel engine, and in this way a high enough temperature is obtained to fire the fuel charge which is injected into the compressed air charge in the cylinder. This scheme of self-ignition does away with ignition devices entirely, and at the same time the cylinder is kept at such a high temperature that the heaviest fuel oils are instantly converted into gases that burn well when mixed with air. Diesel engines are made in

'You will find a description of this experiment in almost any elementary text-book on Physics.

both the two stroke and the four stroke cycle types. How a Semi-Diesel Engine Works.-The semiDiesel engine does not require as high a compression as the straight Diesel, but, like the latter, it burns the heaviest fuel oils and does away with complicated electric ignition devices.

The only difference between the semi-Diesel and a straight Diesel engine is that the former has a hot ball or bonnet, set in the head of the cylinder, and this must be heated by a blow-torch before the engine is started.

On the up stroke the piston compresses only air in the cylinder and when the end of this stroke is nearly reached the fuel is injected against the red-hot walls of the ball or bonnet, when it is instantly changed into gases and explodes. Semi-Diesel engines are made in both the two and four stroke cycle types and a further discussion about them will be found in Chapter VI.

How to Figure the Horse Power of an Engine.The horsepower of any internal combustion engine regardless of the kind of fuel it burns can be easily found by using the following formulae:

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where H. P. is the horsepower and is what you want to find,

D2 is the diameter of the bore of the cylinder, and this you square,

N is the number of cylinders, and

2.5 is a constant.

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CHAPTER III

THE EFFICIENT GAS ENGINE

For stationary work, where natural or artificial gas can be had, the gas engine is the most efficient kind of an internal combustion engine you can use.

Not only is it the most efficient, but since there is a constant supply of fuel at hand and it can be used directly in the cylinder of the engine without any intervening apparatus it is safer and less troublesome than an engine using liquid fuels.

Gas engines in all sizes from 1 to 150 horsepower are made with single cylinders, and from 150 to 1500 horsepower they are made with two cylinders, or twin cylinders, as they are called. Engines of from 1 to 8 horsepower have vertical cylinders, while they are made from 4 to 2000 horsepower with horizontal cylinders.

The Parts of a Gas Engine. In a gas engine of either the vertical or horizontal type there are eleven principal parts, and these are (1) the frame; (2) the cylinder; (3) the air valve; (4) the gas valve; (5) the exhaust valve; (6) the piston; (7) the connecting rod; (8) the crankshaft; (9) the

camshaft1; (10) the igniter; and (11) the governor. The Frame.-This is made of cast iron and on it rests the cylinder and crankshaft bearings. It must be heavy enough to prevent vibration and to keep the bearings from getting out of line. It is shown at A in Fig. 20.

The Cylinder and Water Jacket. The cylinder (see B) has a water jacket C fitted around it so that

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FIG. 20-A, B, C AND D. THE FRAME, CYLINDER, WATER JACKET AND CYLINDER HEAD OF AN OTTO GAS ENGINE

water can be made to circulate between them. The cylinder has a ground bore, that is, it is ground out with an emery wheel instead of being bored out, and this permits the piston to fit much closer and of course this makes the compression better.

The Cylinder Head. This is also an iron casting and all of the valves are fitted in it and the igniter is mounted on it. The combustion of the fuel charge 1A camshaft is usually called a gearshaft when on a stationary gas or gasoline engine.

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