in 1861; this method is compression before ignition. Other inventors also
described very clearly the advantages to be expected from compression, but
none were able to make it commercially successful till Mr. Otto. To him
belongs the great credit of inventing a cycle of operations capable of
realizing compression in a simple manner.

Starting from the same point as inventors did to produce the free piston
engine--namely, that the more quickly the explosive force is utilized, the
less will be the loss, and the greater the power produced from a quantity
of burning gas--it is evident that if any method can be discovered to
increase the pressure upon the piston without increasing the temperature
of the flame causing this pressure, then a great gain will result, and
the engine will convert more of the heat given to it into work. This is
exactly what is done by compression before ignition. Suppose we take a
mixture of gas and air of such proportions as to cause when exploded, or
rather ignited (because explosion is too strong a term), a pressure of 45
lb. above atmosphere, or 60 lb. per square inch absolute pressure. Then
this mixture, if compressed to half volume before igniting and kept at
constant temperature, would give, when ignited, a pressure of 120 lb.
total, or 105 lb. above atmosphere, and this without any increase of the
temperature of the flame.

The effect of compression is to make a small piston do the work of a large
one, and convert more heat into work by lessening the loss of heat through
the walls of the cylinder. In addition to this advantage, greater
expansions are made possible, and therefore greatly increase economy.

The Otto engine must be so familiar in appearance to all of you, that I
need hardly trouble you with details of its external appearance. I shall
briefly describe its action. Its strong points and its weak points are