Page:Motors and motor-driving (1902).djvu/162

130 other to a greater or lesser degree, according as a greater or lesser quantity of explosive mixture is required.

The system is more economical of petrol, and gives smoother running than governing the exhaust valves.

(7) Governing both by Exhaust and Throttle.—This system is a combination of (5) and (6). The exhaust valves are governed as already described, but in addition a hand-worked throttle valve is fitted, which regulates the quantity of mixture reaching the combustion chamber. The valve generally takes the form of a plug which turns inside the supply pipe and cuts off the supply according as more or less is required. By means of this throttle petrol can be economised, noise reduced, and the smoothness of running increased.

Engines with two cylinders have the advantage of two impulses for every two revolutions of the fly-wheel, whereas a single-cylindered engine only has one impulse for every two revolutions of the fly-wheel. The timing of the firing stroke depends upon the angle at which the cranks on the crankshaft are set. This angle may be either 180 or 360 degrees. In the first, where the cranks are set at 180, though there are two firing strokes for every two revolutions of the fly-wheel, the two cylinders fire immediately after each other, so that during one revolution there are two impulses and in the next no impulse. If the cranks are set at 360, an impulse occurs every revolution, but an engine with cranks set at this angle must be balanced to counteract the extra vibration due to all the parts moving in the same direction at the same time. The diagrams will show the movements of two double-cylindered engines, with cranks set at 180 and 360.

A four-cylindered engine gives four impulses for every two revolutions of the fly-wheel, and runs even more smoothly than the tvvo-cylindered one.