Electrokinetica The Electro-mechanical Museum

The W.O. Rooper lighting plant from Firle Place

Built in 1923, this set uses a single-cylinder four-stroke engine driving a DC generator to charge a lead-acid battery. The Crossley O117 horizontal heavy-oil engine delivers 36 brake horsepower at 290 revolutions per minute, at which speed the GEC dynamo generates 17 kilowatts at 250-300 volts DC.

The Engine

Crossley O117 engine

Crossley O117 engine

The engine is closely related to the conventional Diesel compression-ignition engine, but specially designed to burn a wide variety of fuels including the low-grade heavy oils which were available at low cost. Crossley Brothers of Manchester were a large and well-known firm responsible for some of the first commercially successful internal-combustion engines (initially gas engines) built in the UK, and by the 1920s they had established a major market share in the rapidly expanding oil-engine sector. With a cylinder capacity of about 19 litres, the O117 was one of the mid-size models from a series of single and twin-cylinder horizontal engines of which the largest developed over 200 horsepower. The engine is water cooled using a closed-circuit with large evaporative tanks, designed for long runs at full power. Continuous lubrication is provided by various means to all the main parts of the engine. The cams and skew gears are lubricated by oil baths, the main bearings by rotating oil-ring pickup, the governor and big-end by gravity drip-feed and the piston pin and exhaust valve stem by pump feed. A substantial flywheel is provided to minimise the cyclic fluctuation of output that tends to occur with low speed single cylinder generating plant, as a result of the long period between successive power strokes.

Starting

Injection equipment

Injection equipment

The engine can be started either by compressed air, or (with some effort) by hand. Normally, a reservoir is kept charged with air at around 250 psi. When the engine is ready to start, a cam-operated valve lets air into the cylinder, driving the piston in the manner of a steam engine. Once the flywheel has built up momentum the air injection is shut off and normal working begins. The engine can then be used to recharge the air reservoir using energy stored in the flywheel to operate the engine as a compressor. The injector is temporarily relieved of pressure to prevent fuel injection, and the timing of the air valve altered to open on the compression stroke, delivering air to the reservoir. As the flywheel loses momentum, the controls are returned to the ‘working’ position and the engine fires again and builds up speed; this cycle is repeated until the air reservoir is fully charged. If for any reason there is inadequate pressure in the reservoir to make a start, the hand-starting method can be adopted. A small tube attached to the cylinder head is first heated to red heat using a paraffin blowlamp, to assist with the ignition of the fuel. The valvegear is then set to give reduced compression, lowering the speed at which the men must turn the flywheel (the handbook suggests at least two men on the flywheel and one to operate the controls!) Extra fuel is pumped in and everyone pulls on the flywheel spokes until the engine picks up.

Working

The engine is then set to work at constant speed regulated by a centrifugal governor, which controls the fuel delivery by actuating a relief valve in the injector feed line at a variable point during the injection pump stroke. If a viscous fuel oil is to be used, the start would preferably have been made using a small amount of light oil. As the engine warms up, the injection pump can be changed over to draw from the heated heavy oil supply. The attendant can now occupy himself with other work, checking occasionally for signs of trouble such as hot bearings and unusual sounds, while the plant charges the battery. Alternatively he might need to carry out one of the maintenance procedures detailed in the handbook to take place while the engine is running, such as washing the piston with soapy water. No, that's not a misprint; one is supposed to feed the water in through a lubricator connection and ‘wash the piston in this way for about 15 or 20 minutes, keeping a good lather on during that time and using from a few pints to a gallon of the mixture (according to the size of the engine and the state of the piston)’.

The Generator

Made by the General Electric Company in Witton, Birmingham, the dynamo is of four-pole shunt-wound design, with four commutating interpoles. The latter provide for sparkless commutation without need of attention to the brush position as the load current varies. The armature is wound with a dual-layer winding, from which the current is collected by three brushes on each of four brush arms. Oil-ring lubrication is provided to the bearings, and an additional bearing pedestal is provided between engine and machine enabling a slightly resilient coupling to be used.

GEC dynamo

GEC dynamo

Dynamo brushgear

Dynamo brushgear

Dynamo windings

Dynamo windings

The Switchboard

The large and comprehensive switchboard was manufactured by the Austin Motor Works at Longbridge. Three main functions are provided on the board; charge control, battery discharge regulation and load circuit distribution.

Charge control

Roopers / Austin switchboard

Roopers / Austin switchboard

When the engine is started and reaches working speed the dynamo self-excites and begins to generate, engaging the cutout relay as soon as the voltage is sufficient to deliver current to the battery. The rate of charge is regulated by hand, using the shunt field rheostat on the switchboard, to suit the state of charge of the battery at a given time. The cutout relay remains closed until the set is stopped or the dynamo voltage drops such that the current reverses and starts to motor the set, at which point the cutout releases to prevent the battery discharging into the dynamo. A main switch, ammeter and fuses are included for the dynamo. Voltmeter switch positions are provided for measuring the machine, battery and load voltages.

Battery discharge regulation

On typical lighting plants of this era, regular adjustment of the end-cell switch is needed, to keep the voltage reasonably constant as the battery runs down. The plant attendant is required to check the voltmeter regularly and switch-in extra end-cells to keep the voltage up until it is time to recharge the battery. It then behoves him to adjust both the charge and discharge switches while the plant is charging, in such a way that all the cells receive the proper charge without the voltage on the mains rising too high and causing damage. This switchboard incorporates a special automatic end-cell controller which maintains the voltage constant even in the absence of an attendant, both while discharging and charging. Up to fifteen cells can be put into circuit in turn, before the regulator eventually operates a switch to sound an alarm, warning that the battery requires recharging. The mechanism consists of a servo-controlled 16-position switch driven by an electric motor, under the control of a sensitive balanced armature relay. As the voltage falls below the lower limit during discharge, the relay energises a contactor to start the motor in the ‘increase’ direction. An interlock ensures that the motor advances the switch by exactly one step, and prevents it returning to its previous position. A bridging contact and resistance on the moving contact carriage ensure that no interruption occurs to the supply whilst the switch is in motion. When charging, the regulator cuts out cells by driving the switch motor in the ‘decrease’ direction, for which a corresponding upper limit relay contact and contactor are fitted. A tumbler switch can be thrown to inhibit the action of the regulator if required.

Load circuit distribution

The right hand section of the switchboard is equipped with six double pole switches and fuses for the outgoing circuits. An ammeter indicates the total consumption. The switch designation plates show the circuits as House, Stables, Lighting, Village and two others. Although ‘Village’ seems an unlikely circuit function, an investigation of the plant's original home and the layout of Firle village reveals that certain buildings there could easily have been supplied from the engine house.

Current status

The plant is generally in good condition, having been mechanically overhauled some years ago after many decades of disuse. Unfortunately the engine cooling water jacket has sustained some serious damage since that time, and will need specialist treatment before it can be run again. Both the cylinder head and the main engine casting will require welding and probably complete remachining. Installation will be quite an involved business, as the engine, outend bearing and dynamo must share a common, rigid foundation in order to limit shaft sag and flexure when running to a few thousandths of an inch. The original engine house remains unaltered at Firle Place, from which we were able to make precise measurements to enable accurate reconstruction. We look forward to the opportunity of some piston-washing!

Removing plant from beds

Removing plant from beds

Craning the flywheel

Craning the flywheel

Moving the dynamo

Moving the dynamo

Further reading

  • Williams, D.S.D. The oil engine manual. London: Temple Press Ltd, 1942
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