All About Engines

3*8 All About Engines the number of revolutions per minute to find the work done by the engine per minute ; and that, divided by 33,000, gives the indicated horse-power of the engine. We shall return to this term: meantime there are other points of interest in the diagram. Let us follow the pencil all the way round, from the left hand near the bottom corner, up this side, along the top, round the toe, and back again along the lower line. Firstly, then, the line forming the top left-hand boundary should rise abruptly. If it does not we know that steam is admitted late. The valve is slow in opening, and if it be late ever so little the piston will have moved some distance upon its stroke before the steam exercises its full pressure. This is clearly of the greatest importance in high-speed engines. Secondly, the horizontal portion at the top repre- sents the period during which the steam exercises its full pressure, and it should be quite horizontal. If it falls, there is evidence that the steam is strangled, as it were, in the ports, and is unable to follow up the rapidly moving piston. This is called wire-dr aw- ing, because wire is made by drawing a thin rod through conical holes in a steel plate or die. Thirdly, the end of this horizontal line is the point of cut-off, usually more or less rounded, owing to wire-drawing and slowness of the valve in closing, and the curved sloping portion indicates the fall of pressure during expansion. At the end of this curve is the point of exhaust. If this exhaust port is opened too early the pressure will fall very rapidly ; if too late the toe will almost point upwards.