h a simple
test after any alterations or adjustments have been made.

Two spring balances and a rope or cord (according to the size of the
engine), fitted with a few wood blocks as shown in section, fig. 44, to
keep the rope on the rim of fly-wheel, is all that is required for this
test. The following formula may be used for arriving at the B.H.P.:--

B.H.P. = (S1 - S2) 3.14 x D x R / 33000

S1 = Reading in lbs. of spring balance No. 1.
S2 = Reading in lbs. of spring balance No. 2.
D = Diameter of fly-wheel and diameter of brake
rope in feet.
R = Revolutions of fly-wheel per minute.

As 3.14 x D / 33000 will always remain the same for any given engine and
gear, we may call that expression C; then the B.H.P. may be written--

B.H.P. = (S1 - S2) C R

CHAPTER VIII

OIL ENGINES

The small oil engine is practically the same as the gas engine, with the
addition of a vaporiser for converting the oil into gas, or vapour, to
be exploded in the cylinder; consequently the one may be converted into
the other in many cases without much trouble. The difficulty of
producing an efficient oil engine lies principally in devising a
satisfactory and reliable vapouriser--one which will work equally well
under all loads. The heat supplied to the chamber must be sufficient to
vaporise the oil, but not great enough to decompose it. There are
various methods of vaporising the oil, and many types of vaporisers are
employed to attain the same end. There are some in which a charge of oil
is drawn by suction into a hot chamber in which it is converted into
vapour and at the same time mixed with a small quantity of hot air; this
rich mixture is then passed into the combustion chamber of the engine,
in the same manner as coal-gas would be, where it is further diluted
with more air drawn in through the air valve. Other arrangements cause
a jet of oil to be injected into a chamber containing hot air, in the
form of spray, which immediately converts the oil into vapour, and is
then passed into the cylinder, compressed, and fired. Then, again, we
can pump oil through a spraying nipple into the vapouriser (which is
kept at a suitable temperature) whilst the cylinder is being filled with
air on the suction stroke. On the following compression stroke the air
is driven into the vapouriser, which communicates with the cylinder
through a narrow neck, and mixes intimately with the oil vapour.
Gradually, as the pressur