CHAPTER XXI
The Internal-combustion Engine(Cont.)

I suppose many of my readers are quite familiar with the working
of a steam-engine. Probably you have owned models of
steam-engines right from your earliest youth, and there are few
boys who do not know how the railway engine works.

But though you may be quite familiar with the mechanism of this
engine, it does not follow that you know how the petrol engine
works, for the two are highly dissimilar. It is well, therefore,
that we include a short description of the internal-combustion
engine such as is applied to motor-cars, for then we shall be
able to understand the principles of the aeroplane engine.

At present petrol is the chief fuel used for the motor engine.
Numerous experiments have been tried with other fuels, such as
benzine, but petrol yields the best results.

Petrol is distilled from oil which comes from wells bored deep
down in the ground in Pennsylvania, in the south of Russia, in
Burma, and elsewhere. Also it is distilled in Scotland from
oil shale, from which paraffin oil and wax and similar substances
are produced. When the oil is brought to the surface it contains
many impurities, and in its native form is unsuitable for motor
engines. The crude oil is composed of a number of different
kinds of oil; some being light and clear, others heavy and thick.