By small power is meant here up to about, say, 10 h.p. Many different sources may be suggested such as Windmills, Water Turbines, Steam, Electrical, Gas, Oil, Petrol Engine, etc. Taking these in order :

Windmills

The great difficulty here is due to the supply variation, it is very doubtful whether a windmill in this country could be depended upon for power for more than 25 per cent, of the time in which it was required, and this clearly puts it out of consideration except for such work as water pumping that can be done at any time of the day or night.

Water Turbine

This is dealt with in the article on p. 295.

Steam

Unless steam is wanted for other purposes at the same time that it is required to drive an engine, then it can be definitely ruled out for powers of up to 10 h.p. For one thing the steam boiler requires too much attention while the engine is running; it also requires too much attention re cleaning out periodically, and also it has to be lit up one hour or so before the engine is wanted. There are also certain regulations as regards inspection, etc., of steam boilers that have to be carried out, and which do not add to their popularity.

Electrical

This depends almost entirely upon the price that such power can be obtained at, and this depends partly upon how far the would-be consumer is from the nearest point of supply. If a supply of electricity is available at a reasonable price (and this is not always the case) then the matter is settled, especially if the Power Co. hire out electrical motors, etc. If no supply is available then whether a private electrical generating station be installed depends upon such questions as : (1) Is electrical lighting required ? and (2) can the machines that require driving be collected together and driven direct by an engine? or are they of necessity scattered at some distance? If a generating plant be installed to supply power and lighting, then a storage battery system must be also installed, in order to take care of the lights when the engine is stopped. Unless it happens that the machines to be driven cannot be driven direct from the engine, then a private generating plant is quite unfit for consideration for plants requiring up to 10 h.p. There is no useful object to be attained in converting engine power into electrical energy, and reconverting it back to mechanical power if it can be avoided.

Gas Engines

These depend upon a supply of gas from local gas mains, or from a gas producer plant on the site. The first for scattered country districts is generally not possible as no supply will be available, or it will be too costly. A suction gas plant can be installed with the engine using anthracite coal, in this country. This is burned in a closed gas producer, under the draught created by the engine suction. A supply of water is given so that the steam from it is also drawn through the burning contents of the producer. This mixture of coal gas, water vapour, and air is cleaned through a scrubber, and fired in the engine cylinder in the usual manner. The cost of running so far as materials go is low, but the producer and scrubbers require attention, and the producer fire must be started up some time before the engine is wanted, very different to other types of internal combustion engines. Also the supply of anthracite coal is not always obtainable at what many regard as a fair price. The internal combustion engine running on paraffin is therefore a severe competitor to small suction gas engines.

Oil Engines

These may be divided into Heavy Oil, or Paraffin Engines. The Heavy Oil engines include Diesel and Semi-Diesel engines. The former are not used for such low powers as 10 h.p. ; they work on the principle of compressing air only, and at the correct point the crude or heavy oil is forced in under greater pressure; the compressed air is at such a temperature that it ignites the sprayed-in oil and the explosion stroke takes place, no other igniting device is necessary. Another type termed Semi-Diesel also uses crude oils; the compressed charge is fired by allowing it at the correct moment to come into contact with a hot bulb, which ignites the charge. In some types this hot bulb has to be heated up with a lamp before starting up the engine, in others this is not necessary. There are now a number of reliable engines running on paraffin, and as paraffin is easily obtainable even in small quantities, and crude oils are certainly not yet easily obtained in small amounts, there is a decided preference for paraffin engines for many purposes. These are generally started up on petrol, and when warmed up, switched over on to paraffin. The use of paraffin is more likely to soot up the cylinder and valves than petrol; on the other hand they are cheaper to run, and the fire risk is not so great.

Petrol Engines

The use of petrol for stationary engines of over 5 h.p. is not usual; the running cost increases over and above that when using paraffin, and beyond perhaps being easier to start there are no great advantages in the use of petrol. Any of the above mentioned internal combustion types of engines may be of what are termed " four stroke " or " two stroke," and for the running of these see the article on motor cycles. All these stationary internal combustion engines require that they be water cooled, as being stationary air cooling is out of the question; this cooling is arranged in one or other of two ways, either a large tank is supplied and connected to the engine so that a water circulation is set up in the tank and engine water jackets, or else a small " hopper " of water is connected to the engine and this water gradually heats up until after some hours it must either be replaced with cooler water, or allowed to cool down. The former system costs more to supply and fit up, and it is the better; the latter takes up less room, is cheaper, but cannot allow the engine to run for such a lengthy period without overheating.