Over the years, there have been many many ways to start the engine(s) attached to an aircraft. These have included - swinging the prop by hand (still popular with DH Moth enthusiasts!), using the inertia from a manually-wound flywheel (hello, Bf109), use of a mechanical, externally driven shaft - the Hucks starter - (Po-2, Bristol Fighter), using a starter cable in flight or on the ground (B.A. Swallow), using the explosive gases from an overgrown shotgun cartridge made by Coffman (everything from later Model Spitfires to Canberras), using ground-supplied compressed air (Concorde); use of a highly toxic, semi-explosive liquid called AVPIN - isopropyl nitrate (some Hunters, Gloster Javelin), and others. However, the most efficient - until the advent of the Auxiliary Power Unit, itself started by electrical impulse - has been the electrical starter.
In many cases, aircraft were equipped with a starter motor, but often the battery power available onboard was insufficient to overcome the viscous drag of engine oil, especially when thickened by freezing weather, and often the fuel did not want to vapourize easily in the cold, either. This gave rise to desperate technical 'fixes', such as the 'dilution valve' (found on all Mosquito aircraft, by the way), whereby a proportion of petrol was admitted to the engine oil in order to thin it, and reduce the force required to turn the engine over - this was also done in multi-engined aircraft at altitude, when restarting an engine that had been shut down for some reason. This was a fairly risky business, as if too much petrol was admitted, there could well be an uncontrollable engine fire, and this re-starting procedure caused the loss of at least one Handley Page Halifax (similar to the one in the background of this photograph).
Here you can see the answer to starting many wartime aircraft - the Trolley Accumulator! Trolley accumulators were produced in several forms, but they all had a bank of lead/acid batteries contained in a covered box, which had been wired 'in series' so that each lead/acid cell (nominally producing about 2 volts) produced 12 volts and sufficient power to turn over the standard aircraft engines of the day. The reason it is called an 'accumulator' is because some early batteries could not be recharged using an electrical current and needed to have their electrolyte renewed after discharge! In 1881, the standard sulphuric acid/lead plate battery (approximately one volume of concentrated to five volumes of distilled water) was revolutionized by the use of lead grids, into which lead oxide paste was pressed. If you connect the negative terminal of an electrical circuit to one plate, and the positive to the other, then electrons will be 'forced' into the -ve plate, making it contain a higher proportion of pure lead, and 'stripped' from the +ve plate, making it contain more lead oxide. Theoretically, therefore, in a fully charged battery, you should end up with a -ve plate of pure lead (Pb, and rather nice slate blue colour), and a +ve plate of pure lead oxide (Pb02, a rather sludgy brown); the liquid electrolyte flowing around them should be a much weaker mixture of sulphuric acid. As the 'accumulator' discharges, the exact opposite of the above process happens.
When not in use these accumulators were connected to the mains electricity supply at the RAF station they were on, to build up charge (usually overnight); they would need periodic servicing, and topping up with distilled water, as they tended to lose that component of the electrolyte during the charge/discharge process. You may have observed that this 'trolley acc' has what appears to be an engine on top of it. This is because on remote sites, the small generator set/control panel mounted directly to the right of the engine's cylindrical fuel tank, and capable of producing about 300 watts, would then be able to recharge the batteries fitted below it. This was also used if the 'trolley acc' had been used a lot during the day, or in cold weather. As a rough guide, for every 1 degree Centigrade drop in temperature, you lose about 3% in maximum available electrical charge. To access the batteries, you unbolted the engine/generator set, and two men lifted it of the battery compartment lid.
The engine is one of the famous 'J.A.P.' engines - used in everything from Morgan three-wheeler sports cars, to some Aeronca aircraft. J. A. Prestwich Industries Ltd, of London, produced rugged, air-cooled engines, and this unit (which is started using a rope acting on a grooved pulley) is a single-cylinder, 412cc one, with a simple mechanical clutch between the drive shaft and the generator set. During WW2 you could find these 'trolley accs' on airfields scattered all over the British Isles. Second World War aircraft needed large amounts of power, as they had many electrical systems and literally miles of wiring - a Wellington bomber had approximately 7 miles of wiring, and the Halifax you can see to the rear of this photograph had no less than 12!
This trolley acc is connected to my favourite aircraft - the de Havilland Mosquito, of course. In this case it is Tony Agar's magnificent NF.II nightfighter, which is on display at the Yorkshire Air Museum at Elvington, Yorkshire. The work that has gone into 'HJ711' is truly monumental, with components rescued from many sites being utilized; the wonderful news is that this Mosquito is LIVE. Yes, it will actually use that trolley acc in earnest, as the engines can be 'ground run' - a wonderful sight and sound! The electrical connection on a Mosquito is to be found low on the side of the fuselage, just behind the port wing. The pilot, after receiving clearance to start engines, would have started the starboard Merlin first. This was equipped with a 24 volt, 1500 watt generator, which was driven by the Merlin when running. The power supplied from this, would then be used to start the other engine, and the trolley acc could be disconnected. Later, some WW2 aircraft had 24 volt systems, and some used the cartridge start system, so the need for the 12 volt trolley acc declined.
The trolley acc - an indispensable tool on many WW2 airfields!