Generators

A wood gas generator often known as a gasifier, is a wood-fueled gasification reactor mounted on an internal combustion engine, to provide a wood gas, a form of syngas. These devices are also known as gas producers.

Gasification was an important and familiar 19th and early 20th century technology, and its potential and practical applicability to internal combustion engines were well-understood from the earliest days of their development. Town gas was produced from coal as a local business, mainly for lighting purposes, at least initially, and experience in the trade was widespread; most practicing technical people would know a good deal about it. When stationary internal combustion engines based on the Otto cycle became available, internal combustion engines began displacing steam engines as prime movers in many works requiring stationary motive power. Gaseous-fuelled internal combustion engines were commonly fueled by town gas generator during the late 19th century; however, the high price of town gas generator caused many stationary engine works to switch to using producer gas during the early 20th century. Producer gas has less heat content, but was substantially cheaper to make than town gas was to purchase, due to its generation by partial combustion of coke, a byproduct of coal, rather than through the town-gas process of destructive distillation of coal.

By the time World War II arrived in United States and Great Britain, many internal combustion engines of the Otto type were in use in automobiles; however, they were fueled by petroleum-based gasoline, not coal or wood-based town or producer gas. Due to the war, civilian uses of petroleum were sharply curtailed in both nations. In Great Britain, petroleum shortages and rationing were common; in the United States, petroleum rationing was the law of the land, as all petroleum was diverted to the war effort. Due to the lack of gasoline from petroleum, older people recalled how to build gasifiers for both wood and coal, and how to convert internal combustion engines to run on gaseous fuel, and wood gas generators were in active production. Large numbers of such generators were constructed or even improvised; commercial gas generator were in production before and after the war, for use in special circumstances or in distressed economies. Some World War II era wood gas generators were of the “Imbert” type. They were designed around 1920 by French inventor Georges Imbert.

Coal-based town gas production had been displaced by petroleum-based gas and/or natural gas. However, Great Britain continued her use of coal-based town gas until the North Sea natural gas discoveries in the 1960s and 1970s.

Usually, wood gas generator burn wood, but improvements to efficiency and energy-density are possible, by using charcoal instead, as charcoal produces a cleaner gas without the tarry volatiles and excessive water content of wood.

There was a rich literature on gas-works, town-gas, gas-generation, wood-gas, and producer gas, that is now in the public domain due to its age.

The gas generator cycle is a power cycle of a bipropellant rocket engine. Some of the propellant is burned in a gas generator and the resulting hot gas is used to power the engine’s pumps. The gas is then exhausted. Because something is “thrown away” this type of engine is also known as open cycle.

There are several advantages to the gas generator cycle over its counterpart, the staged combustion cycle. The gas generator turbine does not need to deal with the counter pressure of injecting the exhaust into the combustion chamber. This allows the turbine to produce more power and increase the pressure of the fuel and combustion chamber, thus increasing specific impulse or efficiency; this also reduces wear on the turbine, increasing its reliability, reducing its production cost and increasing its operational life-span.

The main disadvantage is lost efficiency due to discarded propellant, though this efficiency loss can be outweighed in production engines by the higher chamber pressure’s increase in net efficiency. Even so a gas generator cycle tends to have lower specific impulse than a staged combustion cycle.

As in most cryogenic rocket engines, some of the fuel in a gas generator cycle is used to cool the nozzle and combustion chamber. Current construction materials cannot stand extreme temperatures of rocket combustion processes by themselves. Cooling permits the use of rocket engines for relatively longer periods of time with today’s material technology. Without rocket combustion chamber and nozzle cooling, the engine would fail catastrophically.

Examples of gas generator engines are the Merlin rocket engine, the Vulcain engine of Snecma Moteurs used on the ESA Ariane 5 rocket, and the more recent J-2X engine to be used on the Ares I and Ares V rockets as part of NASA’s Constellation Program.