In its way the motor cycle may claim to be one of the most marvellous products of human mechanical skill. Weight has been reduced until a power equal to that of three horses can be harnessed to a vehicle which, when stored with sufficient petrol and electricity to carry it and rider 150 miles, scales about a hundredweight. It will pursue its even course up and down hill at an average of twenty or more miles an hour, the only attention it requires being an occasional charge of oil squirted into the air-tight case in which the crank and fly-wheels revolve. The consumption of fuel is ridiculously small, since an economical engine will cover fifteen miles on a pint of spirit, which costs about three-halfpence.
Practically all motor-cycle engines work on the "Otto-cycle" principle. Motors which give an impulse every revolution by compressing the charge in the crank-case or in a separate cylinder, so that it may enter the working cylinder under pressure, have been tried, but hitherto with but moderate success. There is, however, a growing tendency to compass an explosion every revolution by fitting two cylinders, and from time to time four-cylindered cycles have appeared. The disadvantages attending the care and adjustment of so many moving parts has been the cause of four-cylindered cycle motors being unsuccessful from a commercial standpoint, though riders who are prepared to risk extra trouble and expense may find compensation in the quiet, vibrationless drive of a motor which gives two impulses for every turn of the fly-wheel.
The acme of lightness in proportion to power developed has been attained by the "Barry" engine, in which the cylinders and their attachments are made to revolve about a fixed crank, and perform themselves the function of a fly-wheel. So great is the saving of weight that the makers claim a horse-power for every four pounds scaled by the engines; thus, a 3-1/2 h.p. motor would only just tip the beam against one stone. As the writer has personally inspected a Barry engine, he is able to give a brief account of its action.
It has two cylinders, arranged to face one another on opposite sides of a central air-tight crank-case, the inner end of each cylinder opening into the case. Both pistons advance towards, and recede from, the centre of the case simultaneously. The air-and-gas mixture is admitted into the crank-case through a hole in the fixed crank-spindle, communicating with a pipe leading from the carburetter. The inlet is controlled by a valve, which opens while the pistons are parting, and closes when they approach one another.