Hot Bulb Engine - Replacement

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). Diesel engines 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 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. 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 bulb 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 and Bournville II in 1911—were powered by 15 horsepower Bolinder single-cylinder hot-bulb engines, and this type became common between the 1920s and the 1950s. With hot-bulb engines being generally long-lived and ideally suited to such a use, it is not uncommon to find vessels still fitted with their original hot-bulb engines today.

Although many people believe that model glow engines are a variation of the hot-bulb engine, this is not the case. Model glow engines are catalytic ignition engines. They take advantage of a reaction between platinum in the glow plug coil and methyl alcohol vapour whereby at certain temperatures and pressures platinum will glow in contact with the vapour.