by high pressure compressed air, through an injector. The camshaft lifted a spring-loaded pin to initiate fuel delivery through the nozzle. Before World War I technology had not advanced to the point that oil engines could run faster than 150 rpm. The structure of these engines were similar to steam engines, and without pressure-fed lubrication. In hot bulb engines, fuel is injected at low pressure, using a more economical and more reliable, and simpler configuration. However, by not using compressed air injection it is less efficient. In this period diesel and hot bulb engines were four stroke. In 1902 F. Rundlof invented the two-stroke crankcase scavenged engine that went on to become the prevalent hot bulb type engine. 1910–1950s Small direct-injected diesel engines still were not practical and the prechambered indirect injection engine was invented, along with the requirement of glowplugs to be used for starting. With technology developed by Robert Bosch GmbH pump and injector systems could be built to run at a much higher pressure. Combined with high-precision injectors, high-speed diesels were produced from 1927. The hot bulbs started to develop cracks and breakups and were gradually replaced by water cooled cylinder heads with a flat hot spot. Over time the compression ratios were increased from 3:1 to 14:1. Fuel injection started from 135 degrees before top dead center with low compression down to 20 degrees before top dead center with later higher compression engines increasing the hot air factor for ignition and increasing the fuel efficiency. Glowplugs finally replaced the preheating with a blowtorch methods and engine speeds were increased, resulting in what is now classified as an indirect-injection diesel. Hot bulb or prechambered engines were always easier to produce, more reliable and could handle smaller amounts of fuel in smaller engines than the direct-injected "pure" diesels could. Production Hot-bulb engines were built by a large number of manufacturers, usually in modest series. These engines were slow-running (300-400 rpm) and mostly with cast-iron parts, including pistons. The fuel pump was usually made with a brass housing and steel plunger, operating with a variable stroke length. This resulted in a simple, rugged heavy engine. Therefore, they could be machined in an average machine shop without special tools. The Pythagoras Engine Factory in Norrtälje in Sweden is kept as a museum (the Pythagoras Mechanical Workshop Museum) and has a functioning production line and extensive factory archives. See also Crude