This meant that combustion was difficult to control to any degree of precision. Parts of the fuel charge throughout the hot bulb would ignite at different times, often before the piston had completed the compression stroke. This is identical to preignition in a conventional spark-ignition engine and leads to uneven forces and high thermal and physical stresses on the engine's internal parts, especially the piston. In the hot bulb engine this problem could only be overcome by keeping the overall engine speeds low, the fuel quantity injected in each cycle small and the engine's components very heavily built. This resulted in a very durable engine, which was also large and heavy while producing a relatively low power output. Ideas such as water injection (to reduce preignition) and the "hot tube" engine (which allowed the volume of the vaporiser to be altered with engine speed, thus changing the overall compression ratio) added complexity and cost and still could not provide power-to-weight ratios in the same league as the rapidly developing diesel engine. To create even combustion throughout the multiple hot bulbs in multi-cylinder engines is difficult. The hot bulb engine's low compression ratio in comparison to diesel engines limited its efficiency, power output and speed. Most hot bulb engines could run at a maximum speed of around 100 rpm, while by the 1930s high-speed diesel engines capable of 2,000 rpm were being built. Also, due to the design of hot bulbs and the limitations of current technology in regard to the injector system, most hot bulb engines were single-speed engines, running at a fixed speed, or in a very narrow speed range. Diesel engines can be designed to operate over a much wider speed range, making them more versatile. This made these medium-sized diesels a very popular choice for use in generator sets, replacing the hot bulb engine as the engine of choice for small-scale power generation. The development of small-capacity, high-speed diesel engines in the 1930s and 1940s, led to hot bulb engines falling dramatically out of favour. The last large-scale manufacturer of hot bulb engines stopped producing them in the 1950s and they are now virtually extinct in commercial use, except in very remote areas of the developing world. An exception to this is marine use; hot bulb engines were widely fitted to inland barges and narrowboats in Europe. The United Kingdom's first two self-powered "motor" narrowboats—Cadbury's Bournville I