Piston speed

“ The constructors consider the power developed by these engines very moderate, on account of the low piston speed specified in this particular case. In some small and light engines by the same makers the piston speed is as high as 1000 ft. ”

“ They are sometimes used instead of slide valves, but are objectionable because a separate admission and exhaust valve is required at each end of the cylinder, and because at quick speeds of revolution they fall to their seats with a shock or blow which wears out both the valve and the seat. When a high piston speed is obtained by great length of piston stroke, and not by high rotative speed, their use is less objectionable. ”

“ No engine has to run at more variable speeds than a traction engine. It is very important for this reason that the steam ports should be of sufficient area to admit of a very high piston speed, and allow the steam to follow the piston at the necessary velocity. Small ports are useless, as when the link is notched up, and the travel of the valve thereby reduced, the openings are too cramped for the steam to pass in and out of the cylinder comfortably. The result is, that the slide valve is forced off its seat and the engine primes as soon as any great speed is attained. ”

“ The engine was started last year, and has worked beautifully from the first, without heating of bearings or trouble of any kind, and it gives most uniform and steady turning. It is worked now at forty-one revolutions per minute, or only 820 feet piston speed, but will be worked regularly at the intended 900 feet piston speed per minute when the spinning machinery is adapted for the increase which the four extra revolutions per minute of the engine will give; the load driven is over 1,000 horsepower, the steam pressure being 50 lb. ”

“ The obvious objection to such an arrangement is, that it allows the steam to act on the rings with its full force during slow motion, as when a train is starting, while if effective under any circumstances, it will be so only at comparatively high piston speed. The efficacy of such a remedy, if it possesses any, is in fact inversely as the piston speed. ”

“ The shank of the tool is turned to fit the hole easily, and the end of it is made hemispherical to fit the bottom of the hole, upon which the force of the reaction of the blow is received.
It would seem impossible to attain a higher degree of simplicity of form, or to construct a machine with fewer parts. The absence of a valve or striking gear of any kind ensures the utmost attainable degree of durability, and allows a high piston speed to be adopted without risk or injury. As the piston controls its own motion, there is no liability to strike [31] against the cylinder cover. ”

“ With improvements in the details of design and construction it gradually became practicable to use higher steam pressures and higher piston speeds, and consequently to Rise in Steam Pressure and in Piston Speed. obtain not only greater efficiency, but also a greater amount of power from engines of given bulk. In 1872 Sir F. I. Bramwell, describing the typical marine practice of that time, gave a list of engines, all compound, in which the boiler pressure ranged from 45 to 60 ℔, the mean piston speed was 350 ft. ”

“ The piston makes the down stroke when the air pump makes the up stroke; and it will be seen by referring to the cut that the engine does nearly all its work when making the downward stroke. When steam is acting on the top side of piston at high pressure, the vacuum at the same time is pulling on the full area of the piston underneath.
Fig. 356.
When the engine makes the up stroke, the steam at low pressure from the receiver acts to push the piston up; and as the air pump is doing no work then, being on its down stroke, the only work of the engine is to keep the machine up to speed. ”

“ When the piston is moving slowly, the mixture may be ignited toward the end of the compression stroke, for there will be sufficient time for complete combustion by the time the stroke is ended; but when moving at high speed, ignition must occur much earlier in the stroke, as otherwise the piston will have completed the compression stroke and begun to move outward on the power stroke before the mixture is entirely burned. The instant at which ignition occurs also depends on the mixture that is used, for its quality and proper combination make a difference in the rapidity with which it burns. ”

“ Since the first, second, and third operations all absorb energy, the wheel must be heavy enough to store sufficient momentum during the "power" stroke to carry the piston through all its three other duties.
Year by year, the compression of the mixture has been increased, and improvements have been made in the methods of governing the speed of the engine, so that it may be suitable for work in which the "load" is constantly varying. By doubling, trebling, and quadrupling the cylinders the drive is rendered more and more steady, and the elasticity of a steam-engine more nearly approached. ”

“ So long as engines work at a moderate speed there is little advantage in avoiding reciprocation; the alternate starting and stopping of piston and piston-rod does not affect materially the frictional efficiency, throws no deleterious strain on the joints, and need not disturb the equilibrium of the machine as a whole. ”

“ The cylinders of marine engines are always of a large diameter, 6 feet, 8 feet, or even more sometimes, and the pistons and piston-rods are necessarily of great strength and corresponding weight. Now, at every half revolution of the engines, this heavy mass of piston and piston-rod, though moving at an exceedingly high speed in the middle of the stroke, has to be brought to a standstill, and an equal velocity in the opposite direction imparted to it. A large portion of the power is therefore uselessly expended in stopping a great moving mass, and reversing its motion. ”

“ As long as that rate is not exceeded the by-pass will let out as much water as the pumps can inject into the cylinder, and the piston is not moved. But as soon as the engines begin to race, the pumps send in an excess, and the piston immediately begins to rise, closing the throttle. As the speed falls, the leak gets the upper hand again, and the piston is pushed down by a powerful spring, opening the throttle. ”

“ The loss will also be reduced by increasing the speed of rotation and velocity of piston. Where no effectual means can be found of preventing contact of the steam with a good absorbent and conductor of heat, it will be found best to sacrifice some of the efficiency due to the change of state of the vapor, by superheating it and sending it into the cylinder [476] at a temperature considerably exceeding that of saturation. With low steam and slowly-moving pistons, it is better to pursue the latter course than to attempt to increase the efficiency of the engine by greater expansion. ”

“ In designing gas engines for automobiles and other purposes requiring a high speed piston reciprocation with the accompanying development of high temperature in the explosion chamber, it is desirable to avoid thermal expansion of the close fitting piston, which will result in loss of power and possible injury to the mechanism through increased friction, which may cause the piston to seize or stick. ”

“ The highest speed recorded was 68 miles per hour, with a train of 28 tons, the speed averaging 64 miles per hour for four level miles of the line, the vacuum being 16 inches. This speed should have exhibited a resistance of about 21 lbs. per ton, or 588 lbs., as the running resistance or friction, and 645 lbs. for the resistance of the air; in all 1,233 lbs. Now, the pressure due to 16 inches of vacuum on the piston is 1,390 lbs., which gives 157 lbs. ”

“ The piston must be allowed some play in the opening of the cylinder through which it passes, or it will not be able to move and show any life. And the axles of the wheels in their sockets, and, in fact, all parts of the machine where life and movement are to occur, must have this play, this "dither." ”

“ There must, however, be momentary pauses at the ends of the strokes when the direction of movement is changed, and as the piston, though worked from an evenly rotating crank shaft, moves more quickly at the middle and slows down to no speed at the ends of its travel, there must be a considerable variation in the speed of delivery of the air. The air is therefore led from O into a large storage chamber or reservoir, whence it is again taken to the furnace ”

“ Let us take a horizontal engine of 2 feet stroke, making 200 revolutions per minute, so having a piston travel or average velocity of 800 feet per minute, which was my engine in the Paris Exposition of 1867.
We will suppose the piston to be driven through the crank, by which its motion is controlled, the power being got from some other motor, and that the cylinder heads have been removed so that the piston meets no resistance. We will also disregard the effect of the angular vibration of the connecting-rod, and assume the motion of the piston to be the same at each end of the cylinder. ”

“ It is apparent that by moving the pistons in opposite directions, the effective piston velocity is twice that of the actual velocity of either of the pistons, and that it is therefore possible to gain a high heat efficiency at high piston velocities with a low rate of rotation. ”

“ The most striking thing about all this is that when the steam goes into the cylinder where it is cooled as it expands and drives the pistons, the heat thus lost by the expanding steam is the exact equivalent of the mechanical energy realized against the piston head. Not all of the energy that is realized at the piston head is delivered to the driving shaft. Some of it is lost in the friction of the piston rings wearing against the cylinder lining; some, of course, is lost in friction at the journals connecting with the driving shaft. ”

“ Many inventors proposed to perform work upon a piston free from any direct connection with the crank or shaft of the engine; the explosion after attaining its maximum pressure expends its force in giving velocity to a piston; the velocity so acquired carries it on against atmospheric pressure until the energy is all absorbed, and a vacuum or deficit of pressure exists in the cylinder instead of an excess of pressure. The return stroke is accomplished by the atmospheric pressure, and the work is now done upon the engine shaft on the return only. ”

“ An open-ended “trunk” piston, similar in principle to that used in gas engines, is employed; and the valve is of the piston type, which is less complicated than the box form of valve, though less easily made steam-tight in small sizes. The engine is single-acting, making only one power stroke per revolution.
The cylinder is a piece of brass tubing; the piston another piece of tubing, fitting the first telescopically. Provided that the fit is true enough to prevent the escape of steam, while not so close as to set up excessive friction, a packing behind the piston is not needed ”

“ The quality of mixture that an engine will take depends on its speed. The suction of the pistons depends on the speed of the engine.
If, at ordinary speed the carbureter gives a proper mixture, the throats and passages through the pipes and manifold, as well as the valve which discharges the gasoline, may be in a prime condition to do good work; but when the pistons work at double speed the inrush of air may not carry with it the proper amount of fuel; or, under those conditions, the air may receive too great an amount of gasoline, proportionally. ”

“ Lubrication is a subject that should be studied by every gas-engine user. So far as the piston is concerned it is a matter of the utmost importance. The piston does its work under very peculiar conditions. It is driven at great linear velocities; and it is, moreover, subjected to high temperatures which have nothing in common with good lubrication if care be not exercised.
The piston is the essential, vital element of an engine. Upon its freedom from leakage depends the maintenance of a proper compression, and, consequently, the production of power and economical consumption. ”