in factories, theatres, lighthouses, radio stations and many other locations where a centralised electrical grid was not available. Usually, the dynamo or alternator would be driven off the engine's flywheel by a flat belt, to allow the necessary "gearing up" — making the generator turn at a faster speed than the engine. Companies such as Armstrong Whitworth and Boulton Paul manufactured and supplied complete generating sets, both the engine and generator, from the 1900s to the late 1920s, when the formation of national grid systems throughout the world and the replacement of the hot bulb engine by the diesel engine caused a drop in demand. The engines were also used in areas where the fire of a steam engine would be an unacceptable fire risk. Akroyd-Stuart developed the world's first locomotive powered by a hot bulb oil engine, the "Lachesis", for the Royal Arsenal, Woolwich, where the use of locomotives had previously been impossible due to the risk. Hot bulb engines proved very popular for industrial engines in the early 20th century, but lacked the power to be used in anything larger. Replacement From around 1910, the diesel engine was improved dramatically, with more power being available at greater efficiencies than the hot bulb engine could manage. Diesel engines can achieve over 50% efficiency if designed with maximum economy in mind, and they offered greater power for a given engine size due to the more efficient combustion method. They had no hot bulb, relying purely on compression-ignition, and offered greater ease of use, as they required no pre-heating. The hot bulb engine was limited in its scope in terms of speed and overall power-to-size ratio. To make a hot bulb engine capable of powering a ship or locomotive, it would have been prohibitively large and heavy. The hot bulb engines used in Landini tractors were as much as 20 litres in capacity for relatively low power outputs. The main limit of the hot bulb engine's power and speed was its method of combustion. In a diesel engine combustion is controlled by injecting fuel into compressed air; since no combustion can take place until fuel is injected, the timing and duration of combustion can be tightly controlled. In the hot bulb engine fuel was injected into the cylinder before compression began, and combustion would start as the air charge met the vaporised fuel in the hot bulb during the compression stroke.