BACK PRESSURE IN THE CYLINDERS.
When, from any cause, the steam is not permitted to escape promptly and freely from the cylinder at the end of the piston stroke, a pressure higher than that of the atmosphere remains in the cylinder, obstructing the piston during the return stroke, and causing what is known as back pressure. There is seldom trouble for want of sufficient opening to admit steam to the cylinders, for the pressure is so great that the steam rushes in through a very limited space; but, when the steam has expanded two or three times, its pressure is comparatively weak, and needs a wide opening to get out in the short time allowed. This is one reason why the exhaust-port is made larger than the admission-ports. Nearly all engines with short ports suffer more or less from back pressure, but the most fruitful cause of loss of power through this source is the use of extremely contracted exhaust nozzles. Were it not for the necessity of making a strong artificial draught in the smoke-stack, so that an intense heat shall be created in the fire-box, quite a saving of power, now lost by back pressure, would be effected by having the exhaust opening as large as the exhaust-pipe. This not being practicable with locomotives, engineers should endeavor to have their nozzles as large as possible consistent with steam-making.
Engines with very limited eccentric throw will often cause back pressure when hooked up, through the valve not opening the port wide enough for free exhaust.
Locomotives suffering from excessive back pressure are nearly always logy. The engine can not be urged into more than moderate speed under any circumstances; and all work is done at the expense of lavish waste of fuel, for a serious percentage of the steam-pressure on the right side of the piston is lost by pressure on the wrong side. It is like the useless labor a man has to do turning a grindstone with one crank, while a boy is holding back on the other side. The weight of obstruction done by the boy must be subtracted from the power exerted by the man to find the net useful energy exerted in turning the grindstone. In the same way, every pound of back pressure on a piston takes away a pound of useful work done by the steam on the other side.