Locomotive Engine Running and Management

by Angus Sinclair

INSIDE LAP.

In some cases the inside edges of the valve cavity do not reach the edges of the steam-ports when the valve is on the middle of the seat, but lap over on the bridge a certain distance, as shown by the dotted lines in [Fig. 7]. This is called inside lap, and its effect upon the distribution of steam is to delay the release. By this means it prolongs the period of expansion, and hastens compression on the return stroke. Inside lap is an advantage only with slow-working engines. When high speed is attempted with engines having much inside lap, the steam does not have enough time to escape from the cylinders, and the back pressure and compression become so great as to be very detrimental to the working of the engine. As locomotive engineers have it, the engine is “logy.”