Air-blast injection

Air-blast injection is a historical direct injection system for Diesel engines. Unlike modern designs, air-blast injected Diesel engines do not have an injection pump. A simple low-pressure fuel-feed-pump is used instead to supply the injection nozzle with fuel. At injection, a blast of compressed air presses the fuel into the combustion chamber, hence the name air-blast injection.

The compressed air comes from compressed-air tanks which feed the injection nozzle. A large crankshaft-driven compressor is used to re-fill these tanks; the size of the compressor and the low rotational frequency of the engine's crankshaft means that air-blast injected Diesel engines are huge in size and mass, this, combined with the problem that air-blast injection does not allow for quick load alteration makes it only suitable for stationary applications and large watercraft.

Before the invention of precombustion chamber injection, air-blast injection was the only way a properly working internal air fuel mixture system required by a Diesel engine could be built. Rudolf Diesel was granted a patent on air-blast injection in November 1893 (DRP 82 168). By the 1920s, air-blast injection was rendered obsolete by superior injection system designs that allowed much smaller but more powerful engines.

The air-blast injection system was first used by George Bailey Brayton in 1890 for a four-stroke kerosene fueled engine. Rudolf Diesel wanted to build an engine with direct injection for which he tried using accumulating in 1893. Due to the highly viscous fuels Diesel used and thermal afterburning occurring, the principle of accumulating did not work sufficiently. Therefore, Diesel had to improve the injection system.

German engineer Friedrich Sass says that Diesel knew about Brayton's invention and that it is therefore very likely that Diesel decided to replace his own inferior injection system with an air-blast injection system similar to Brayton's. Diesel did so in February 1894, because he could not come up with a better solution.

Diesel remained determined to replace the air-blast injection system with a superior system. In 1905, an improved accumulating system which allowed for direct injection without a huge compressor was patented by Diesel and Rudolf Brandstetter. Nevertheless, this improved system was still insufficient and Diesel considered direct injection without a huge compressor "unfeasible". The precombustion chamber, which made motor vehicle Diesel engines possible, was invented in 1909. By 1915 the first working directiinjected Diesel engines which were not air-blast injected began to appear.

Initially, sieve-type atomizers were used for the injection nozzles, until the sieves were widely replaced by discs. Also, ring-type atomisers were used for some engines.

The ring-type atomiser is based on the principle of different air velocities occurring inside the nozzle, which force the fuel to mix up with air. Disc-type atomisers have small perforated discs placed above each other with small gaps in between (as seen in Fig. 6 in the sectional drawing on the right). The discs are slightly misaligned to increase constriction. Depending on the capacity of the engine and, therefore, quantity of injected fuel, either two, three or four dics are used per injection nozzle. The disc material depends on the fuel type. In general, bronze casting and phosphor bronze casting are used; for engines running on coal tar, the discs are usually made from steel.