circulated, or siphoned, by its thermal action, that is the action of the heat itself.

The Exhaust Pot and Exhaust Pipe.-When the burnt gases in the cylinder are exhausted directly into the air they sound like the staccato reports of

FIG. 35. THE EXHAUST POT OF A GAS ENGINE

a Lewis machine gun mowing down a detachment of boches.

To silence these noises an exhaust pot is used, and this is simply a large iron, or other vessel, as shown in Fig. 35, into which the burnt gases are driven by the exhaust stroke; they then expand in it until the pressure they exert is just about that of the atmosphere, when they exhaust into the open air without noise.

CHAPTER IV

STATIONARY AND PORTABLE GASOLINE ENGINES

It is clear that if you want to develop power with an internal combustion engine and natural or artificial gas is not available, or if you want an engine of this type that you can move about, or that will move you about, you must have one that will burn some kind of liquid fuel.

Now gasoline for any one of a number of excellent reasons is the ideal liquid fuel and hence, it is largely used for running stationary and portable engines up to 30 horsepower.

Stationary and Portable Engines Defined. -A stationary engine is, as everybody knows, one whose position after it is once set remains so, that is, it is intended to stay and run in a fixed place.

A portable engine is simply an engine of the stationary type which is mounted on skids or on a hand or horse-drawn truck, as shown in Fig. 36, so that it can be hauled about by some other power than its own and used wherever it may be needed.

Difference Between a Gas and a Gasoline Engine. The chief mechanical difference between a

gas and a gasoline engine of the stationary type is that in the former gas is admitted directly into the cylinder of the engine without the use of any intervening or accessory apparatus, while in the latter means must be provided for (1) storing the gasoline; (2) bringing it to the cylinder, and (3) breaking it

up into a spray so that it will mix with air to form the fuel mixture.

Further, a gasoline engine is exactly like a gas engine in that it has the same kind of inlet and exhaust valves, governor, ignition and oiling systems.

A hot-tube, or hot ball igniter must not be used on a gasoline engine, but any one of the electric igniters of the kinds described in Chapter II is well adapted to fire a gasoline fuel charge. In stationary gaso line engines the cylinder cooling systems used are identical with those employed with gas engines.

But for portable engines, a hopper, or iron box, is usually secured to the top of the engine and this holds the supply of water which is circulated by the thermo-syphon system as explained in the preceding chapter under the heading of Cooling Tanks.

EXHAUST
POT

GASOLINE
SUPPLY
TANK

HANDLE FOR CRANKING ENGINE

FIG. 37. A GRAVITY GASOLINE FEED SYSTEM

How the Gasoline System Works.-The Gasoline Supply Tank.-The tank that holds the supply of gasoline can be set either (1) above the engine when it is fed into the cylinder through the gasoline valve by gravity, or (2) below the level of the engine when it is pumped to the gasoline valve and is then drawn into the cylinder.

The Gravity Feed System.-For small engines the gasoline tank can be placed above the engine as shown in Fig. 37.

The Pump Feed System.-For larger engines the gasoline tank must be buried in the ground or placed in a brick-lined vault outside of the building containing the engine.

To make the gasoline supply perfectly safe the tank must be below the level of the gasoline pump a foot or so, and it should be at least 30 feet away from the engine. The filler should project above the ground, and the gasoline cock in the pipe line which leads to the pump must be easily get-at-able. It is shown at A in Fig. 38.