Cutoff (steam engine)

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In a steam engine, cutoff is the point in the piston stroke at which the inlet valve is closed.

The point at which the inlet value closes and stops the entry of steam into the cylinder from the boiler plays a crucial role in the control of a steam engine. Once the valve has closed the steam in the cylinder continues to expand but its pressure drops as it does so. The later the cutoff the higher the steam pressure will be on average along the stroke of the piston delivering more force to the piston but, since there will still be unexploited pressure in the cylinder at the end of the stroke, this is achieved at the expense of engine efficiency. In this situation the steam will still have considerable pressure remaining when it is exhausted resulting in the characteristic “chuff chuff” sound of a steam engine. An early cutoff has greater thermodynamic efficiency but provides less average force on the piston rod and is used for running the engine at higher speeds. The steam engine is the only engine design that can provide its maximum torque at zero revolutions.

[edit] Explanation

Schematic Indicator diagram of pressure in a steam locomotive cylinder. Note that pressure declines only gradually, maintained by expansion, after cutoff

Cutoff is one of the four valve events. Early cutoff is used to increase the efficiency of the engine by allowing the steam to expand for the rest of the power stroke, yielding more of its energy, and conserving steam. This is known as expansive working. Late cutoff is used to provide maximum torque to the shaft at the expense of efficiency and is used to start the engine under load.

Cutoff is conventionally expressed as percentage of the power stroke of the piston; if the piston is at a quarter of its stroke at the cutoff point, the cutoff is stated as 25%.

Smaller stationary steam engines generally have a fixed cutoff point, while in large ones the speed and power output is generally governed by altering the cutoff. In steam engines for transport it is desirable to be able to alter the cutoff over a wide range. For starting and at low speed and heavy load the cylinders need steam supply at maximum pressure for almost the full length of the stroke. In a two cylinder locomotive, for example, the maximum or 'full gear' cutoff is typically about 85%. At high speeds, the cutoff may be 15% percent of the piston stroke or less.

Providing variable cutoff is an important function of the valve gear. Most valve gear designs provide it, the exception being early Stephenson valve gear.

[edit] Types of driver’s control mechanism

[edit] Reversing lever

This is the most common form of reverser. It consists of a long lever mounted, parallel to the direction of travel, on the driver’s side of the cab. It has a handle and sprung trigger at the top and is pivoted at the bottom so as to pass between two notched sector plates. The reversing rod, which connects to the valve gear, is attached to this lever, either above or below the pivot, in such a position as to give good leverage. A square pin is arranged so as to engage with the notches in the plates and hold the lever in the desired position when the trigger is released.

The advantages of this design are that change between fore and back gear can be made very quickly as is needed in, for example, a shunting engine. Disadvantages are that, because the lever must rest at one of the notches, fine adjustment of the cutoff to offer best running and economy is not possible. On large locomotives it can be difficult to prevent the mechanism from jumping into full forward gear (“nose-diving”) when adjusting the cutoff once the locomotive has gathered speed: with such engines it was the practice of drivers to select an appropriate degree of cutoff before opening the regulator, and to leave it in that position for the duration of the journey.

[edit] Screw reverser

Diagram of Stephenson valve gear controlled by a screw reverser

In this mechanism the reversing rod is controlled by a screw and nut, worked by a wheel in the cab. The nut either operates on the reversing rod directly or through a lever, as above. The screw and nut may be cut with a double thread and a coarse pitch to move the mechanism as quickly as possible. The wheel is fitted with a locking lever to prevent creep and there is an indicator to show the percentage of cutoff in use.

This method of altering the cutoff offers finer control than the sector lever, but it has the disadvantage of slow operation. It is most suitable for long-distance passenger engines where frequent changes of cutoff are not required and where fine adjustments offer the most benefit.

The two pistons of the steam reverser can be seen at the extreme left on this Bulleid Merchant Navy class

[edit] Steam reverser

This is a cutoff powered by steam. The device consists of two pistons mounted on a single piston rod. Both pistons are double-ended. One is a steam piston to move the rod as required. The other, containing oil, holds the rod in a fixed position when the steam is turned off. Control is by a small three-way steam valve (“forward”, “stop”, “back”) and a separate indicator showing the position of the rod and thus the percentage of cutoff in use. When the steam valve is at “stop” an oil cock connecting the two ends of the locking piston is also closed, thus holding the mechanism in position. The rod connects by levers to the reversing gear, which operates in the usual way, according to the type of valve gear in use. The pistons are usually installed vertically on the boiler side or horizontally between the frames.

The first locomotive engineer to fit the device was James Stirling of the South Eastern Railway in 1876. Several engineers then tried them, including William Dean of the GWR and Vincent Raven of the North Eastern Railway, but they found them little to their liking, mainly because of maintenance difficulties: any oil leakage from the locking cylinder, either through the piston gland or the cock, allowed the mechanism to creep, or worse “nose-dive”, into full forward gear while running. However Harry Wainwright of the SER’s successor company the South Eastern and Chatham Railway incorporated them into most of his designs, which were in production about thirty years after Stirling’s innovation. Later still the forward-looking Southern Railway engineer Oliver Bulleid fitted them to his famous Merchant Navy Class of locomotives, but they were mostly removed at rebuild.

Most Beyer Garratt locomotives had power reverse because unassisted links to both engine units would have been very heavy to operate. Beyer Peacock used the Hadfield system ^[1] which was capable of being worked either by steam or compressed air.

Many American locomotives were built, or retro-fitted, with power reverse, e.g. PRR K4s, PRR N1s, PRR B6, PRR L1s.

[edit] Enginemen’s terminology

In the UK, a screw reverser is called a “bacon slicer”. In the US, a reversing lever is called a “Johnson bar”.

[edit] References

[edit] Sources

• Allen, Cecil J (1949). Locomotive Practice and Performance in the Twentieth Century.. W.Heffer and Sons Ltd, Cambridge.
• Bell, A. Morton (1950). Locomotives volume one. Seventh edition. London, Virtue and Company Ltd.